© Shutterstock UEENEEG101A Solve problems in electromagnetic devices and related circuits Unit version Document version Release date Comments/actions UEENEEG101A Copyright © TAFE Queensland 2016 Copyright protects this material. Except as permitted by the Copyright Act 1968 (Cth), reproduction by any means (photocopying, electronic, mechanical, recording or otherwise), making available online, electronic transmission or other publication of this material is prohibited without the prior written permission of TAFE Queensland. Inquiries must be addressed to the TAFE Queensland Copyright Officer, Office of the Chief Academic Officer, TAFE Queensland, PO Box 16100, CITY EAST QLD, 4002, or Email TAFEQLDIP@tafe.qld.gov.au. Disclaimer Every effort has been made to provide accurate and complete information. However, TAFE Queensland assumes no responsibility for any direct, indirect, incidental, or consequential damages arising from the use of information in this document. Data and case study examples are intended to be fictional. Any resemblance to real persons or organisations is coincidental. If you believe that information of any kind in this publication is an infringement of copyright, in material in which you either own copyright or are authorised to exercise the rights of a copyright owner, and then please advise us by contacting the TAFE Queensland Copyright Officer, Office of the Chief Academic Officer, TAFE Queensland, PO Box 16100, CITY EAST QLD, 4002, or Email TAFEQLDIP@tafe.qld.gov.au. UEENEEG101A - Solve problems in electromagnetic devices and related circuits Introduction This Learner Guide has been developed to support you as a resource for your study program. It contains key information relating to your studies including all the skills and knowledge required to achieve competence. What will I learn? By completing this unit you will be able to: • Prepare to work on electromagnetic devices and circuits • Solve electromagnetic devices/circuit problems • Complete work and document problem solving activities Are there any special requirements? Your facilitator will let you know if you need to organise and bring any additional equipment or personal protective equipment. Page 2 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits TAFE Queensland student rules TAFE Queensland student rules are designed to ensure that learners are aware of their rights as well as their responsibilities. All learners are encouraged to familiarise themselves with the TAFE Queensland student rules, specifically as they relate to progress of study and assessment guidelines. A full guide to student rules can be found at Student rules1. Information to support your learning and assessment There’s always someone to help you. Undertaking further study can bring both excitement and challenges. Our Student Services, Learning Support and Library staff can help you make the most of your time at TAFE. Callout panels A number of panels have been designed to help guide you to important information and actions throughout this Learner Guide. The full choice of panels you are likely to encounter to support you in your studies are included below. NB: not all the panels will be used in every learner guide. 1 http://tafeqld.edu.au/current-students/student-rules/ © TAFE Queensland 2016 | Page 3 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Page 4 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Contents Introduction ........................................................................................................................... 2 What will I learn? ............................................................................................................... 2 Are there any special requirements? ................................................................................. 2 TAFE Queensland student rules ........................................................................................ 3 Information to support your learning and assessment ........................................................ 3 Callout panels.................................................................................................................... 3 Contents ................................................................................................................................ 5 Welcome ..............................................................................................................................10 Required resources ..........................................................................................................10 Magnetism............................................................................................................................11 Magnetite .........................................................................................................................11 Magnetic poles .................................................................................................................13 Attraction and repulsion ....................................................................................................13 Inducing magnetism .........................................................................................................15 Magnetisation theory ........................................................................................................16 Magnetic fields .................................................................................................................17 Lines of force ....................................................................................................................18 Magnetic materials classification ......................................................................................20 Magnetic shielding ............................................................................................................21 Applications of permanent magnets ..................................................................................23 Summary of magnetism ....................................................................................................25 Electromagnetism.................................................................................................................27 Objectives ........................................................................................................................27 Introduction to right hand thumb rule ................................................................................28 Effects of current flow .......................................................................................................29 Lines of force around a conductor ....................................................................................30 Lines of force around a wire .............................................................................................31 © TAFE Queensland 2016 | Page 5 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Fields around parallel conductors .....................................................................................32 Solenoids .........................................................................................................................34 Polarity of a solenoid ........................................................................................................34 Application of electromagnetism .......................................................................................36 Summary of electromagnetism .........................................................................................37 Magnetic quantities ..............................................................................................................39 Magnetic circuit ................................................................................................................39 The relay ..........................................................................................................................40 Magnetic quantities...........................................................................................................41 Magnetomotive force (m.m.f. or fm) ..................................................................................43 Magnetising force ( h ) ......................................................................................................44 Permeability......................................................................................................................45 Forces between conductors..............................................................................................47 Magnetic flux (Ф) ..............................................................................................................47 Flux density ......................................................................................................................48 Reluctance (Rm) ..............................................................................................................49 Magnetic characteristics .......................................................................................................50 Magnetisation curve .........................................................................................................50 B-H magnetisation curve ..................................................................................................51 Magnetic core material .....................................................................................................52 Hysteresis loop .................................................................................................................54 Magnetic losses ................................................................................................................56 Electromagnetic induction ....................................................................................................58 Electromotive force ...........................................................................................................58 Relative motion .................................................................................................................59 Faraday’s law ...................................................................................................................60 Conductor length ..............................................................................................................61 Flemings right hand rule ...................................................................................................63 Lenz's law.........................................................................................................................64 Page 6 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Inductors ..............................................................................................................................65 Time constants .................................................................................................................65 The exponential curve ......................................................................................................66 Time constants .................................................................................................................67 Inductance ........................................................................................................................68 Self induction ....................................................................................................................69 Series or parallel ..............................................................................................................71 Mutual induction ...............................................................................................................71 Undesirable effects of induction ........................................................................................72 Factors affecting inductance .............................................................................................73 Types of inductors ............................................................................................................74 Magnetic devices ..............................................................................................................76 Electromagnet principles ......................................................................................................79 Generator action ...............................................................................................................79 Alternating current ............................................................................................................81 Alternator action ...............................................................................................................82 DC generator ....................................................................................................................84 Commutation ....................................................................................................................85 Generated voltage ............................................................................................................88 Voltage regulation.............................................................................................................90 Motor action......................................................................................................................92 Back or counter EMF ........................................................................................................95 Torque ..............................................................................................................................98 Calculating torque.............................................................................................................99 Rotating machines..............................................................................................................101 DC machines ..................................................................................................................101 DC machine constuction .................................................................................................102 Armature ........................................................................................................................104 Armature windings ..........................................................................................................104 © TAFE Queensland 2016 | Page 7 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Stator and field poles ......................................................................................................106 Brushes and brush gear .................................................................................................107 Armature reaction ...........................................................................................................108 Effects of commutation ...................................................................................................110 Causes of armature reaction ..........................................................................................112 Correcting armature reaction ..........................................................................................115 DC generators ....................................................................................................................118 Generators .....................................................................................................................118 Permanent magnet generator .........................................................................................119 Separately excited generator ..........................................................................................121 Series generator .............................................................................................................124 Shunt generator ..............................................................................................................126 Compound generators ....................................................................................................129 Efficiency of a DC generator ...........................................................................................133 Additional videos ............................................................................................................135 Summary ........................................................................................................................136 DC motors ..........................................................................................................................137 DC motor principles ........................................................................................................137 Permanent magnet motor ...............................................................................................139 Separately excited DC motor ..........................................................................................140 Series dc motor ..............................................................................................................141 Shunt motor ....................................................................................................................143 Compound motor ............................................................................................................146 Additional videos ............................................................................................................150 Summary ........................................................................................................................151 Efficiency ........................................................................................................................152 Speed regulation ............................................................................................................153 Specialty DC machines ..................................................................................................154 Maintenance ...................................................................................................................155 Page 8 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Glossary .............................................................................................................................157 References .........................................................................................................................158 © TAFE Queensland 2016 | Page 9 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Welcome Welcome to the unit, Solve problems in electromagnetic devices and related circuits. By completing this unit you will be able to: • Prepare to work on electromagnetic devices and circuits • Solve electromagnetic devices/circuit problems • Complete work and document problem solving activities Required resources Your facilitator will let you know if you need to organise and bring any additional equipment or personal protective equipment. Page 10 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Magnetism Magnetite Natural permanent magnets exist in a material called magnetite, which is the crystalline form of iron oxide and which attracts iron (ferrous) objects. Fragments of magnetite called lodestones were used as magnetic compasses by ancient mariners as early as the 12th century. Magnets will magnetise other magnetic materials that lie within their magnetic field. Magnetite © Chisholm Video Watch the full video titled “Magnetism”. https://www.youtube.com/watch?v=ehcOvU0Rixs Source: CHMnanoed: Magnetism: Introduction [Accessed from YouTube: 30th May 2016] Theory of magnetism All metals will exhibit magnetic properties to some degree. The extent or amount of magnetism within metals depends on the group to which they belong: Diamagnetic - is repelled by an external magnetic field (i.e. has a negative susceptibility) and is extremely weak when compared to ferromagnetism. Paramagnetic - is attracted by an external magnetic field and is also weak in comparison to ferromagnetism. Ferromagnetic - has a very high paramagnetic behaviour such that some actually become magnets and do not rely on an external magnetic field. © TAFE Queensland 2016 | Page 11 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Magnet names Permanent magnets are generally named after their shape: Bar Magnets Horseshoe Magnets © Chisholm © Chisholm Disk Magnets Toroidal Magnets © Chisholm © Chisholm Page 12 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Magnetic poles The North pole of a magnet (e.g. compass needle) is a North seeking pole and is attracted to Earth’s magnetic North pole. Like poles (N-N) repel and unlike poles (NS) attract. Magnetic Poles © Chisholm Video Watch the full video titled “DC Electronics Theory Lesson 7 Segment 2A Basic Concepts of Magnetism” - ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=IW7BCTQDY_g Source: Venturecaplaw: DC Electronics Theory Lesson 7 Segment 2A - Basic Concepts of Magnetism [Accessed from YouTube: 30th May 2016] Magentic poles are called North or South and they are the points at which the magnetic lines of force originate and return. Flux density is at its highest at the poles. The end which points North is sometimes called the North Seeking Pole (North pole for short). Magnetic Poles © Chisholm © TAFE Queensland 2016 | Page 13 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Attraction and repulsion Like poles repel When two poles of the same type are brought into close proximity to each other, they interact by repelling one another. Magnetic Poles Repel © Chisholm Unlike poles attract When two opposite poles are brought into close proximity to each other, the suspended magnet will swing towards the fixed magnet. That is, unlike poles attract. Magnetic Poles Attract © Chisholm Page 14 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Inducing magnetism Permanently magnetising materials Several techniques can be employed to align the domains and permanently magnetise hard magnetic materials (those that remain permanently magnetised). Video Watch the full video titled “Methods of Magnetisation and Demagnetisation” – ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=Dka-cROHxBY Source: TutorVista: Methods of Magnetisation and Demagnetisation [Accessed from YouTube: 30th May 2016] Stroking method Stroking a hard ferromagnetic material with a permanent magnet. Stroking Method © Chisholm © TAFE Queensland 2016 | Page 15 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Passing current (solenoid) method Passing a current through a conductor wound around a hard ferromagnetic material. Passing Current Method © Chisholm Magnetisation theory Domain theory Basic theory of magnetism can be explained using the domain theory. In ferromagnetic materials, the electron spins around atoms, which combine and form what is called a dipole. These dipoles are randomly arranged in tiny groups known as domains within un-magnetised ferromagnetic material. When the domains align and the electron spins of the majority of the dipoles are all in the same direction, the result is a magnetic field. The greater the number of domains that align, the stronger the magnetic field. The combined affect is the formation of North seeking and South seeking magnetic poles at the ends of the material. If the material opposite were to be broken in half along a horizontal line, then 2 magnets would be formed, both with North and South poles. Page 16 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Video Watch the full video titled “Physical Science 6.7b - Magnets and Magnetic Domains” – ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=Z-arJTDUIo0 Source: Derek Owens: Physical Science 6.7b - Magnets and Magnetic Domains [Accessed from YouTube: 30th May 2016] Un-magnetised ferromagnetic material Randomly arranged diploes Magnetised ferromagnetic material - Diploes spin into the same direction © Chisholm © Chisholm Magnetic fields Magnetic flux A magnetic field is energy that takes the form of a series of magnetic lines of force that are collectively called magnetic flux, with the following characteristics: The magnetic lines of force the name given to the magnetic field lines that originate from poles in North to South closed loops. They reduce in strength, the further they are away from the magnet. Magnetic flux is concentrated at the poles. Lines of force that act in the same direction will oppose each other. Lines of force will follow lines of least resistance (reluctance) and do not cross each other. © TAFE Queensland 2016 | Page 17 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Video Watch the full video titled “Physical Science 6.7a - Magnetic Fields” - ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=OQ6m_w0TS_A Source: Derek Owens: Physical Science 6.7a - Magnetic Fields [Accessed from YouTube: 30th May 2016] Lines of force Flux lines - bar magnet Iron filings (ferrous) can be sprinkled on a non-ferrous sheet (e.g. plastic) held over the magnet. This demonstrates the lines of force and magnetic flux for the particular magnet. International Agreement says that magnetic lines of force (or flux) flow from the north pole to the south pole externally to the magnet. Internally within the magnet, the lines of force (or flux) flow from the south pole to the north pole. Magnetic Flux © Chisholm The direction of the magnetic field is shown with arrows. The path taken by flux or lines of force depends on the shape of the magnet and any other ferromagnetic materials that may be placed within the magnetic field. Ferromagnetic material will undergo magnetic induction and distort the magnetic field as it becomes a magnet in its own right during the induction process Page 18 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Characteristics of magnetic lines of force Magnetic lines of force are continuous and will always form closed loops. Magnetic lines of force will never cross one another Parallel magnetic lines of force oriented in the same direction repel one another. Parallel magnetic lines of force oriented in opposite directions tend to unite with each other and form into single lines oriented in a direction determined by the magnetic poles creating the lines of force. Magnetic lines of force tend to take the shortest path. Therefore, the magnetic lines of force existing between two unlike poles, cause the poles to be pulled together. Magnetic lines of force pass through all materials, both magnetic and nonmagnetic but are diverted by magnetic materials. Magnetic lines of force always enter or leave a magnetic material at right angles to the surface. Patterns for lines of force © Chisholm © Chisholm © Chisholm © Chisholm © TAFE Queensland 2016 | Page 19 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits © Chisholm © Chisholm Magnetic materials classification Magnetic materials have two distinct classes: 1. Permanent or Hard 2. Temporary (non-permanent) or Soft Hard magnetic materials Retain their magnetism indefinitely unless subjected to heat or jarring. They are “hard” to induce magnetism into but once done, will retain their magnetism. Hard Magnetic Materials © Chisholm Page 20 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Soft magnetic materials Can easily have magnetism induced into them but they will quickly lose their magnetism Usually made of soft iron or mild steel. Often associated with coil windings and the magnetic properties depend on the magnetic effect of the electric current in the coil. Soft Magnetic Materials Magnetic shielding © Chisholm Two known characteristics of lines of force are 1. They will pass through any substance. 2. Flux lines will take the path of least resistance, eg. through steel rather than air as previously shown. It is desirable to shield electrical measuring instruments from the Earth's field and stray fields. This is achieved by surrounding the instrument with an iron shell. The shell bypasses practically the entire flux. A Leakage Flux is the name given to that portion of the magnetic field which does not pass along the desired path One method of protecting delicate instruments from stray magnetic fields is to surround the equipment with a soft iron ring, as illustrated: Any stray magnetic fields are then diverted around the sensitive equipment by the soft iron ring. Magnetic shielding © Chisholm © TAFE Queensland 2016 | Page 21 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Soft magnetic materials (i.e. high permeability) allow magnetic flux lines to readily flow through them. They are used as a shield against the intrusion of a magnetic field. Shielding protects devices that may be damaged or are sensitive to magnetic fields (e.g. magnetic tape or disc records, watch hair spring, or cathode ray tube). Storing permanent magnets Storing Permanent Magnets © Chisholm To keep a permanent magnet at peak performance it is necessary to provide a path for the magnetic flux lines to flow through whilst the magnet is not in use. Placing a soft metal, called a keeper, across the poles of a magnet provide a closed circuit for the flux lines and helps preserve its strength. Page 22 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Applications of permanent magnets Some applications for permanent magnets are: Magnetic Signs Door Alarm Switches (Reed Switch) © Chisholm © Chisholm Refrigerator Door Seals Navigation Compass © Chisholm © Chisholm Magnetic Chucks Audio Speakers © Chisholm © Chisholm © TAFE Queensland 2016 | Page 23 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Small Electric Motor © Chisholm Page 24 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Summary of magnetism 1. A magnet is any solid which has the property of attracting to it certain substances which are called magnetic substances. The property of attracting in this way is called magnetism. 2. Lodestone, an oxide of iron, is a natural magnet, that is, it is found in nature as a ore that exhibits magnetic characteristics. 3. Artificial magnets acquire the property of magnetism by being stroked with a magnet. They may also be magnetised by electrical means. 4. A temporary magnet can be magnetised readily but loses practically all of its magnetism immediately the magnetising influence is removed. 5. A permanent magnet is one that retains its magnetism for an indefinite period after having been once magnetised. 6. The poles of a magnet are points near the ends of the magnet where the magnetic forces are concentrated. 7. If a bar magnet is freely suspended, the end which points northwards is referred to as the north pole (N-pole) and the end which points southwards the south pole (S-pole). 8. A magnetic field may be regarded as everywhere transversed by what are called lines of magnetic force. 9. Lines of force have the following characteristics: o They are completely closed curves. o They have a definite direction. o They may be distorted or bent to any shape, but they do not cross each other. o The lines are perfectly elastic. They may be extended to any length, or close up and vanish when the magnetising force is removed. o The lines pass unaffected through any non-magnetic substance. 10. When the unlike poles of two bar magnets are brought near each other their fields combine and a force of attraction is exerted between the unlike poles due to the characteristic of perfect elasticity of the lines of force by which they always tend to shorten themselves as much as possible. 11. When the like poles of two bar magnets are brought near each other the resultant field is distorted out of its shape and the reaction set up results in a force of repulsion between the like poles. © TAFE Queensland 2016 | Page 25 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits 12. The law of magnetic poles states: o Like poles repel each other, o Unlike poles attract each other. 13. Repulsion is the test for permanent magnetism, as a magnet will attract a magnet substance 14. If a pole of a magnet is brought near a magnetic substance it induces a pole of opposite polarity in that portion of the substance nearest to it. 15. The magnetism remaining in a magnet after the magnetising force has been removed is called residual magnetism. 16. Permeability may be defined as the ease with which a magnetic flux may be established in a substance. 17. The reluctance of a substance is its ability to oppose the passage of magnetic lines of force. Iron has a much lower reluctance than air. 18. Whenever permanent magnets are removed from apparatus - for example, instruments, magnetos, etc. - magnetic keepers should be applied immediately. 19. Instruments etc, may be shielded from the effects of magnetism by providing a path of low magnetic reluctance around the article to be protected, the action being referred to as magnetic screening. 20. If a magnetic substance is stroked by a magnet, the end of the substance which the magnet leaves last is of opposite polarity to that of the stroking pole. Page 26 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Electromagnetism Objectives At the conclusion of this topic, you should understand: Magnetic field patterns around a straight current carrying conductor and a solenoid. Direction of the magnetic field around a straight current carrying conductor. Direction of the north pole of a solenoid. Factors effecting the force and direction between adjacent current-carrying conductors. Video Watch the full video titled “Physics - Electromagnetism: Magnets and Electricity” - ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=bSge-qDcS4Y Source: EducationCommonsRW's channel: Physics - Electromagnetism: Magnets and Electricity [Accessed from YouTube: 30th May 2016] Electro-magnetism When current flows through a conductor, a magnetic field is formed around that conductor (also called an electro-magnetic field) along its full length. The field is always at right angles to the axis of the conductor. To monitor the presence of a field, a compass can be positioned over or along side the conductor. When the switch is open, there will be no magnetic field Switch Open © Chisholm © TAFE Queensland 2016 | Page 27 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits When switch is closed and conventional current flows through conductor, the compass needle (located above conductor with Red indicating North) will swing 90o to the conductor to line up with the field produced. Switch Closed © Chisholm Introduction to right hand thumb rule Introduction to right hand thumb rule (or right hand curl rule) The direction of the field can be determined without the aid of a compass by applying the Right Hand Thumb Rule. This rule states: “for conventional current flow, the palm of the right hand is placed on the conductor with the thumb pointed in the direction of current flow. The fingers, when wrapped around the conductor, will indicate the direction of the magnetic field”. Video Watch the full video titled “The Right Hand Curl Rule” - ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=jCcUochTbVo Source: Matrix Education: The Right Hand Curl Rule [Accessed from YouTube: 30th May 2016] Page 28 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Right Hand Thumb Rule © Chisholm Effects of current flow Increased current flow If current through the conductor doubles, the field strength also doubles (up to a point) There is a limit to the amount of current that can flow through a given conductor. This limit is set by the conductors cross sectional area. Increasing current beyond this limit will overheat the conductor and cause damage to its insulation. Increased Current Flow © Chisholm © TAFE Queensland 2016 | Page 29 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Lines of force around a conductor Indicating current flow Current flowing towards the observer (out of the page). © Chisholm When current is flowing away from the observer (into the page) the flights of the arrow are seen as a cross. © Chisholm Page 30 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Lines of force around a wire © Chisholm Using the right hand rule, when viewed with current flowing towards the observer, the magnetic lines of force will circle the conductor in a counter clockwise direction. © Chisholm When viewed with current flowing away from the observer, the magnetic lines of force will circle the conductor in a clockwise direction. © TAFE Queensland 2016 | Page 31 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Fields around parallel conductors Fields around parrallel conductors © Chisholm When current flows through both conductors in the same direction, there is mutual attraction between the two fields. They will combine into a single field and the two conductors will be forced toward each other. Fields around parrallel conductors © Chisholm Page 32 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits If current through each conductor flows in the opposite direction, the fields will have the same polarity and will oppose one another. This will result in a force that pushes the two conductors apart. Fields around parrallel conductors © Chisholm When a number of current carrying conductors pass through a metal plate, this could be a consumer unit or the edge of some metal trunking etc, eddy currents occur. As current passes through the conductors, a magnetic field is induced in the steelwork. This current circulates continually unless stopped. Fields around parrallel conductors © Chisholm There are two ways of stopping eddy currents. One is to make sure that conductors of opposing current flow are placed in the same hole. The second is to cut a slot in the metalwork so that the eddy current cannot circulate. © TAFE Queensland 2016 | Page 33 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Solenoids Once a straight conductor has been formed into a loop, it becomes a basic solenoid. A solenoid produces more magnetic field strength and can also be used to produce linear movement of a soft iron armature (i.e. ferromagnetic material) within its core. Basic solenoid Types of solenoids © Chisholm © Chisholm Polarity of a solenoid The direction of the field (or its polarity) can be determined using the right-hand solenoid rule. If the fingers of the right hand wrap around the coil in the direction of conventional current flow, the thumb points to the north pole of the field. © Chisholm Page 34 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Video Watch the full video titled “Magnetic Field Due to a Solenoid” – ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=EsJXZLwSCdA Source: cstephenmurray:Magnetic Field Due to a Solenoid [Accessed from YouTube: 30th May 2016] © Chisholm © TAFE Queensland 2016 | Page 35 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Application of electromagnetism Electric motors (AC &DC) Generators Industrial electromagnets for clamping, sorting and movement of materials Relays, contactors and solenoids in electrical control systems Communications systems Entertainment equipment Test instruments Alarm bells and buzzers Medical equipment Page 36 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Summary of electromagnetism 1. A magnetic needle, when placed close to a current-carrying conductor will be deflected and remain so deflected until the wire no longer carries current, whereupon the needle will resume its normal N-S direction. 2. Lines of force encircle a current-carrying conductor at right angles to the conductor and remain so suspended as long as the current remains constant. When the current ceases, the lines are no longer present. 3. The conventional method of representing the direction of current in a conductor towards an observer is by a dot in the centre of a circle, and away from an observer by a cross within a circle. 4. The Right Hand Rule states: "Place the right hand with the palm facing the conductor, fingers slightly closed to encircle the wire, the thumb outstretched and at right angles to the fingers; when the thumb points in the direction of the current, the fingers will point in the direction of the "magnetic flux". 5. The use of the Right Hand Rule enables the direction of the magnetic field of a current-carrying conductor to be determined, provided that the direction of the current is know conversely, to find the direction of the current when the direction of the magnetic field is known. 6. If two conductors, carrying current in the same direction are near and parallel to each other, their electromagnetic fields link up and encircle the two conductors. If the conductors are free to move, each will be drawn towards the other. 7. When two conductors, near and parallel to each other, carry current in opposite directions, their electromagnetic fields exert mutually repelling actions, tending to force the wires apart. In electrical machines carrying large currents, these repelling forces assume very high values When a conductor is wound into several loops the resulting coil is called a solenoid. 8. A current-carrying solenoid behaves like a powerful bar magnet, with a N-pole at one end of the solenoid and a S-pole at the other end. 9. The polarity of a current-carrying solenoid may be deduced by adapting the Right Hand Rule as follows: "Place the right hand with the palm on the outside of the solenoid so that the fingers indicate the direction of the current; the thumb at right angles to the fingers, points to the N-pole of the solenoid. 10. A solenoid with a magnetic-material core is called an electromagnet. The strength of an electromagnetic field depends upon three factors, the current value, the number of turns in the coil and the material used in the core. © TAFE Queensland 2016 | Page 37 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits 11. There are three principle types of electromagnets: 1. The attractive type, in which the electromagnet attracts an armature to which mechanism is attached: for example, a relay or contactor where the attraction and release of the armature controls the closing and opening of contacts: 2. The solenoid type, in which the coil surrounds a sliding plunger which is drawn into the coil; and 3. The lifting type, in which the electromagnet poles attract magnetic material for transportation. 12. A magnetised substance may be demagnetised by placing it across the poles of an electromagnet supplied with alternating current and gradually decreasing the current to zero. Page 38 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Magnetic quantities Magnetic circuit Lines of magnetic flux form closed paths; as shown in the diagrams below. The dotted lines represent the flux passing through the iron core Flux passing through the iron core © Chisholm An air gap in a magnetic circuit simply introduces a higher reluctance path but does not stop the flow of magnetic flux Flux passing through the iron core © Chisholm © TAFE Queensland 2016 | Page 39 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits The relay In the diagrams below, the relay is shown with the armature open and then with the armature closed. The flux produced by the coil takes the shortest magnetic path, which when the armature is open, is the core of the coil, the air gap, armature and the metal frame of the relay. When the armature is closed, there is no air gap. Relay with the armature open and the armature closed © Chisholm The reluctance of the magnetic path therefore drops when the armature is closed, as the high reluctance of the air-gap is no longer in the path. This means more flux .is produced by the coil, because there is less reluctance in the magnetic circuit. For this reason, the m.m.f. (and therefore the current in the coil) to make the coil operate is greater than that needed to hold the relay closed. Page 40 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Magnetic quantities There are a number of similarities between magnetic and electrical circuits: Magnetic Circuit Term Symbol Unit Magnetomotive Force Fm Ampere-Turn Reluctance Rm Ampere-turn/weber Magnetic Flux Wb Weber Term Symbol Unit Electromotive Force E or V Volt Resistance R Ohm Current I Ampere Electric Circuit © TAFE Queensland 2016 | Page 41 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits 1. Electromotive Force (EMF) - Volts. Magnetic equivalent is Magnetomotive Force (MMF). Quantity symbol = Fm, Units = Amp Turns (At). 2. Current Flow (I) - Amps. Magnetic equivalent is Magnetic flux. Quantity symbol = Φ (Phi), Units = Weber (Wb) (pronounced vayber). 3. Resistance (R) - Ohms. Magnetic equivalent is Reluctance. Quantity symbol = Rm, Units = Amp Turns per Weber (At/Wb). © Chisholm © Chisholm © Chisholm Page 42 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Magnetomotive force (m.m.f. or fm) In previous studies it was stated that in an electric circuit, an electric current is due to the existence of an electromotive force. By analogy, it may be stated that in a magnetic circuit, the magnetic flux is due to the existence of a magnetomotive force (mmf) which is caused by a current flowing through one or more turns. The value of the m.m.f. is proportional to the current and to the number of turns. In pure SI units, the m.m.f. unit is the ampere because the number of turns in a coil or solenoid is considered dimensionless. For practical calculations, the number of turns need to be known so the unit ampere/turn is used, abbreviated IN for the m.m.f. Where a number of units is specified, the abbreviation "At" is used. To determine how much magnetomotive force is creating a magnetic flux, the following formula is used: Fm = I x N where - Fm = magnetomotive force in ampere turns I = current flowing in amperes N = number of turns in coil Example If a current of 5 A is flowing in a coil of 120 turns, find the value of m.m.f. creating a magnetic flux: Fm = I x N = 5 X 120 = 600At The capacity of a coil to produce magnetic flux is called its Magnetomotive Force (Fm), measured in Ampere-turns (A/t). © TAFE Queensland 2016 | Page 43 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Example Calculate the magnetomotive force created in a 960 turn coil when it carries 0.25 amperes. Fm = I x N = 0.25 x 960 = 240 A/t Magnetising force (h) If a magnetic circuit is homogeneous, i.e. made of one material and of uniform cross sectional material, the magnetomotive force per metre length of the magnetic circuit is termed the magnetising force or magnetic field strength and is represented by the symbol H. Thus if the mean length of the magnetic circuit of the diagram is 1 metres, then magnetic field strength can be determined by using the formula: Formula: H = Fm / ℓ Remember that Fm = I N Therefore H = IN/ ℓ where ℓ= length of magnetic circuit in metres, N = number of turns in the coil H = magnetising force measured in ampere turns per metre current in the coil Magnetising Force © Chisholm I = current in coil The unit of magnetising force is Amp turns per metre (At/m). The longer the magnetic circuit the weaker the magnetising force. The m.m.f required to magnetise a unit length of magnetic circuit is called the Magnetising force and brings length into the Fm equation. The magnetising force must not be confused with the magnetomotive force. Page 44 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits One is simply magnetomotive force (= IN), while the other expresses the m.m.f. per metre of the magnetic circuit. Example The magnetising force excerted on a 150mm core by the mmf found in the previous example (the magnetomotive force created in a 960 turn coil when it carries 0.25 amperes). may be calculated thus: H = IxN/ℓ = 0.25x960/0.15 = 1600 At/m (sometimes written as A/m) Permeability Permeability (μ) is the ease at which a magnetic material allows magnetic lines of force to travel through it. This is similar to conductivity in an electrical circuit. Permeability is measured in the Henry/metre. Permeability of free space (µo) refers to the ability of air or a vacuum to conduct magnetic lines of force. Air or a vacuum is a poor conductor of magnetic lines of force and has a constant value equal to: µo = 4π X 10-7 Relative permeability Relative permeability (µr) refers to the ability of a magnetic material to conduct magnetic lines of force compared to air. A material with a relative permeability of 5 means that the material can conduct magnetic lines of force 5 times easier than air. © TAFE Queensland 2016 | Page 45 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Actual permeability Actual permeability refers to the overall ability of a material to conduct magnetic lines of force and can be calculated by multiplying the permeability of air (μo) by the relative permeability (μr) of the material. μ = μ r x μo Where µ = actual permeability µr = relative permeability µo = permeability of free space Permability 1 © Chisholm Another way of calculating actual permeability is to use the formulae: µ = B/H Where μ = actual permeability B = flux density H = magnetising force Page 46 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Forces between conductors Forces acting on conductors: F=BxIxl Where: F = force in newtons B = flux density I = current l = length in metres Example Cable installed on a tray develops a flux density of 0.75 teslas. Calculate the force exerted on 35 metres of cable that carries 40 amps. F = 0.75 x 40 x 35 = 1050N or 1.05kN Magnetic flux (Ф) The amount of Magnetic Flux (Ф) in a circuit is measured in Webers. One Weber is equal to 100,000,000 (10 ) lines of force: Φ = Fm/Rm Where Φ= the magnetic flux in webers Fm = the m.m.f in Ampere-turns Rm = the reluctance in Ampere-turns per weber © TAFE Queensland 2016 | Page 47 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Flux density Flux density is a measure of the strength of a magnetic field. Quantity symbol = B. The unit = Tesla (T) and is derived from Wb/m2. The unit of flux density is taken as the density of magnetic flux such that a conductor carrying one ampere at right angles to that flux has a force of 1 newton per metre acting upon it. This unit is termed a Weber per square metre (Wb/m2). Density refers to the number of lines of force per unit area. The general symbol for flux density is B, and one Weber per square metre is called a tesla (T). Low and High Flux Density © Chisholm Flux density is the number of flux lines in an area. When the total flux and the area of the magnetic path are known, the flux density may be calculated using the formula: B = Ф/A Where Ф = webers B = flux density in teslas ( Wb/m) A = area in m2 The amount of Magnetic flux (Ф) in a circuit is measured in Webers. One Weber is equal to100,000,000 (10 ) lines of force. However, knowing the total flux present may not be as important as knowing the flux density on the face of a magnet's pole. Page 48 of 160 | © TAFE Queensland 2016 Flux Density © Chisholm UEENEEG101A - Solve problems in electromagnetic devices and related circuits Example A magnetic circuit with a cross-sectional area of 120mm2 carries a total flux of 0.24 Wb. Calculate the flux density in the core. B = Ф/A = 0.24 /120 x 10-6 = 2000 Wb/m2 Example A magnetic circuit has a cross sectional area of 100mm2 and a flux density of 0.00IT. Calculate the total flux in the circuit. By transposition Ф = BxA = 0.001 x (100 x 10-6) = lxlO-7 Wb Reluctance (Rm) Reluctance (Rm) is the opposition offered by a material to the passage of flux through it. Just like resistance in an electrical circuit, reluctance depends on the length of the magnetic circuit (l), the cross-sectional area of the circuit (A) and the permeability of the circuit material The reluctance of a magnetic circuit depends on three things: 1. Length - the longer the magnetic circuit, the higher the reluctance 2. Cross Sectional Area - the smaller the CSA, the higher the Reluctance 3. Permeability - the lower the permeability, the higher the reluctance © TAFE Queensland 2016 | Page 49 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Example The total mean length of a path of an iron core is 200mm. The core is rectangular in cross section with dimensions of 15mm X 10mm. If the core has a relative permeability of 830 at a certain flux density, calculate the reluctance of the core. Rm = l / (µr µo A) = 0.2m / [830 x (4π x 10-7)x (0.015m x 0.010m)] = 1.278 x 106 ampere-turns/Wb Magnetic characteristics Magnetisation curve Field strength further defined When current flows through a coil, a magnetic field is produced. The field strength = (B) = μ x H. Since μ = μr x μo ,the above formula can be restated as B = μr x μ o x H Since H = I x N / ℓ μ, the above formula can be restated as B = μr x μ o x I x N / ℓ So it can be seen that field strength depends on: The amount of current that flows I The number of turns N The magnetic core/circuit length ℓ The type of core material used μr Page 50 of 160 | © TAFE Queensland 2016 Magnetisation Curve © Chisholm UEENEEG101A - Solve problems in electromagnetic devices and related circuits If the air-core is replaced with a material in the ferromagnetic group, the permeability of the core material will also affect the field strength. Ferromagnetic materials will not support an unlimited flux density. The material will saturate. Air or Non-magnetic material core in a Coil Since μr = 1 for air or a non-magnetic material, B = μo x I N / ℓ . Current (I) cannot increase without limit, the CSA of the conductor and its total length will limit the maximum current that can flow without causing damage to the winding. Magnetisation Curve © Chisholm Also, air cannot be saturated so the magnetising force (H) is theoretically only limited by coil current. B-H magnetisation curve A B-H curve is a plot that shows the relationship between flux density (B) and magnetising force (H) for a given core material. B=μH = μ0 μr H The magnetisation curve for non-magnetic material If current is increased through an air-cored coil, the magnetising force produced is limited only by the coil current. Air cannot be saturated, so a graph of magnetising force (H) and flux density (B) will produce a linear plot. The B-H Curve For Air or Non-Magnetic Material © Chisholm © TAFE Queensland 2016 | Page 51 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Magnetic core material A coil with soft magnetic material is one that magnetises readily when current flows through the coil but does not retain the magnetism once the current stops flowing. If the current through the coil is increased slowly from zero to a maximum value, the flux density of the field will not be proportional to the current. The plot on any B-H graph in this situation will not be linear. It will depend on the type of material and its relative permeability, and the level at which the material “saturates”. Saturation means that additional magnetising force adds very little flux density the rest of the energy is given off as heat. Different core material permeability will give different curve shapes. B = μ0 μr H The Magnetisation Curve Magnetic Core Material © Chisholm Page 52 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Magnetisation Curves for Different Core Materials © Chisholm © TAFE Queensland 2016 | Page 53 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Hysteresis loop Hysteresis The word hysteresis is derived from a Greek word meaning coming after or lagging behind. In electromagnetic circuits it refers to the characteristic where a change in the direction of a magnetic field lags behind a change in the direction of the current producing that field. Hysteresis loop is a B-H graph that compares magnetising force (H) with flux density (B), but the current is made to flow through a coil firstly in one direction and then it is reversed. The resulting graph forms a closed loop called a hysteresis loop. Hysteresis Loop Hysteresis Loop © Chisholm © Chisholm Analysis of the hysteresis loop 1. As a ferromagnetic material is subjected to an increasing magnetising force (AH) the flux density (B) increases until the material is saturated (curve AB). 2. If the magnetising force (B) is then returned to zero, the magnetism in the material does not return to zero but lags behind the magnetising force (segment BC). 3. The lagging (fall behind) of the magnetisation behind the magnetising force is known as hysteresis. The greater the lag the greater is the residual magnetism retained by the material (coordinate AC) 4. The flux density and thus the residual magnetism, can be reduced to zero (segment CD) only by reversing the magnetic field and building up the magnetising force (coercive force) in the opposite direction (AD). 5. The reverse magnetising force (A-H), if increased enough, causes the material to reach saturation again (E), but with its poles reversed (segment DE). Page 54 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits 6. In order to remove residual magnetism it is necessary to use a force which acts in the opposite direction to the original magnetising force. This force used to overcome residual magnetism is known as the coercive force (AD) and (AG) 7. Magnetisation of the material follows the closed loop (BE-EB), a curve called a hysteresis loop. 8. The area enclosed by a hysteresis loop gives an indication of the quantity of energy that is dissipated in taking a ferromagnetic substance through a complete cycle of magnetisation. In the operation of many electric devices, this energy is wasted and appears as heat. 9. The area within the hysteresis loop is called the total hysteresis loss. The wider the hysteresis loop, generally the more will be the hysteresis loss. 10. Hysteresis loss is also called an “iron loss” and will vary with the permeability (μ) of the material (high permeability = low loss). Hysteresis Loop © Chisholm © TAFE Queensland 2016 | Page 55 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Magnetic losses Hysteresis losses Changing the magnetic flux causes changes in the alignment of molecules in a magnetic material. This results in the generation of heat within the material. Such heat generation in a magnetic material represents a loss of energy which is known as hysteresis loss. The amount of hysteresis loss for a particular material varies directly as the area within the hysteresis loop for that material. It follows that the hysteresis loop gives an important indication of the suitability of a magnetic material for a particular application. Eddy currents These are currents that circulate within the molecular structure of the iron core, serving no useful purpose as they produce heat in the core, due to the low magnetic resistance of the core. No useful work is done and the heat produced is a source of loss and is measured in watts. These current can be minimized by using cores made up of thin laminated sheets, Residual magnetism can be undesirable in applications such as electromagnets, motors, relays, solenoids and contactors. They require core material having very low residual magnetism (i.e. magnetically soft). Residual magnetism Residual magnetism in a relay causes it to become sluggish when energising. This is because there is still some magnetic effect remaining in the core, which tends to delay or in extreme cases prevent the release of the armature when current stops flowing through the coil. Magnetic fringing losses Fringing losses occur across an air gap in a magnetic circuit because the flux density in air is less than the flux density in the magnetic medium. As the flux lines leave the high density magnetic medium and enter the air gap, they bow out at the edges of the gap Magnetic Fringing © Chisholm Page 56 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Magnetic leakage losses Leakage is flux that does not travel in the magnetic circuit. It may return to the magnetic circuit at some point or it may not. Magnetic leakage is a loss that is largely due to poor design. It occurs in magnetic circuits where small gaps are left between laminations at the point where one lamination butts up against the next. A certain amount of leakage will also occur where the grain of the laminated steel changes orientation. Magnetic Leakage © Chisholm © TAFE Queensland 2016 | Page 57 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Electromagnetic induction Electromotive force Objectives At the conclusion of this topic, you should understand: Factors required to induce an emf in a conductor. Faraday’s Law. Direction of induced voltage in a moving conductor in a magnetic field. Relationship between the forces acting on a closed conductor when an emf is induced in it. (Lenz’s law) Electromagnetic induction When current is passed through a conductor, a magnetic field is formed around it. This process is reversible. That is, when a magnetic field is moved past a conductor, an e.m.f is induced into the conductor. The production of electricity using this method is called electromagnetic induction. “Inductance is the ability of a conductor to have an e.m.f. induced in it by a changing magnetic field” To induce an emf three factors must be present simultaneously. There must be: 1. A conductor 2. A magnetic field 3. Relative motion between the conductor and the field that will result in a changing magnetic field strength Note The motion may be either of the conductor or the field. Page 58 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Faraday’s laws Michael Faraday discovered that if you plunge a magnet into coil, an e.m.f is generated in that coil. Faraday's first law When relative motion exists between a magnetic field and a conductor, an e.m.f. is induced in the conductor. Faraday's second law The magnitude of the induced e.m.f. is proportional to the rate of change in flux linkage. Combining both the above laws: The value of an e.m.f. induced in a coil depends on the number of conductors in the circuit and the rate of change of the magnetic flux linking the conductors. Relative motion Relative motion means that the conductor can be stationary and the field is made to move, or the field is stationary and the conductor is made to move. The greater the rate of motion between the field and conductor, the larger the e.m.f induced. Relative Motion © Chisholm © TAFE Queensland 2016 | Page 59 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Faraday’s law The polarity of the induced e.m.f. depends on the polarity of the magnetic field and its direction of motion relative to the coil. Video Watch the full video titled “Physics - Electromagnetism: Faraday's Law” – ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=S0wbEl7caTY Source: EducationCommonsRW's channel: Physics - Electromagnetism: Faraday's Law [Accessed from YouTube: 30th May 2016] Faraday's Law © Chisholm Page 60 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Faraday’s Law Induced Voltage Faraday's law reveals that the induced voltage will depend on: The total length of conductor exposed to the field (relates to number of turns if conductor coiled) The strength of the magnetic field The rate at which the conductor cuts the flux lines The angle at which the conductor cuts the flux lines (max. E.m.f induced at 90o to flux) Video Watch the full video titled “Electromagnetism 3 - Faraday' s Law” - ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=wGfVVGjGVB8 Source: David Williams:Electromagnetism 3 - Faraday' s Law [Accessed from YouTube: 30th May 2016] Conductor length Because conductors are usually coiled, the induced e.m.f is also related to the number of turns in the coil (more turns means more length). If coiled, the total length refers only to that part of the conductor exposed to the field. Induced Voltage Formula Where V = induced voltage (V) N = No. of turns ∆Ø ∆𝑡 © Chisholm = rate of change of flux (Wb/s) © TAFE Queensland 2016 | Page 61 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Example Calculate the value of e.m.f induced into a coil of 1 000-turns cutting a magnetic field of 80 x 10-5 Wb in 1.5 ms. V = N x ΔΦ/Δt V = 1000turns X (80 x 10-5 ) / (1.5 x 10-3 ) V = 533.3 Volts For a straight conductor moving through a magnetic field, the induced emf is given by: V=Bxlxv where V = induced voltage (V) B = flux density of field (T) l = total length of conductor in field (m) v = velocity of relative motion (m/s) Example Calculate the value of e.m.f induced into a straight conductor with a length of 150 mm moving at a speed of 20 m/s within and at right angles to a magnetic field with a flux density of 0.01 T. V=Bxlxv V = 0.01 x 0.15 x 20 V = 30mV If the conductor does not move at right angles (90°) to the magnetic field then the angle θ° will be added to the above expression giving a reduced output as the angle increases. V = B x l x v x sin Θ Page 62 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Flemings right hand rule Fleming’s Right Hand Rule for Polarity of Induced E.M.F The polarity of the induced E.M.F. depends on the relative directions of the magnetic field and the movement of the conductor through it. The E.M.F. direction can be determined using Fleming’s Right Hand Rule – for Generators. Fleming's Right Hand Rule © Chisholm Video Watch the full video titled “Flemings right hand generator rule” – ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=Uc2R35K1Ak0 Source: RoutledgeElectrical:Flemings right hand generator rule [Accessed from YouTube: 30th May 2016] If the thumb, forefinger and second finger of the right hand are held at right angles to each other, and the thumb points in the direction that the conductor moves, and the forefinger points in the direction of the magnetic field (North to South), then the second finger indicates the direction of the induced e.m.f. © TAFE Queensland 2016 | Page 63 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Lenz's law Lenz's Law, a law of electromagnetic induction formulated in 1833 by the German physicist H. F. E. Lenz. "The polarity of the induced e.m.f is such that any current resulting from it will develop a flux which tends to oppose the change in the original flux creating the induced E.M.F.” Lenz's law applies particularly to electric generators. When a generator induces an electric current, the direction of the current is such as to oppose the rotation of the generator. Therefore, the more electrical energy a generator delivers, the more mechanical energy is required to turn it. Video Watch the full video titled “Physics - Electromagnetism: Lenz's Law” - ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=uGUsTWjWOI8 Source: EducationCommonsRW's channel: Physics - Electromagnetism: Lenz's Law [Accessed from YouTube: 30th May 2016] Page 64 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Inductors Time constants The LR time constant When a current is applied to an inductor it takes some time for the current to reach its maximum value, after which it will remain in a "steady state" until some other event causes the input to change. The time taken for the current to rise to its steady state value in an LR circuit depends on: The resistance (R) This is the total circuit resistance, which includes the DC resistance of the inductor (RL) itself, plus any external circuit resistance. The inductance of L Which is proportional to the square of the number of turns, the cross sectional area of coil and the permeability of the core An Inductor opposes changes in current flow When the circuit above is switched on current changes rapidly from zero, this sudden change creates a rapidly expanding magnetic field around the coil, and in doing so induces a voltage back into the coil. This induced voltage (called a back EMF) creates a current flowing in the OPPOSITE direction to the original current. The result of this is that the initial rate of change of the circuit current is reduced. An Inductor opposes changes in current flow © Chisholm If this initial rate of change were to continue in a linear fashion, the current would reach its maximum or steady "state value" in a time given by: T = L/R seconds. T is the TIME CONSTANT and is measured in seconds L is the INDUCTANCE and is measured in Henrys R is the TOTAL CIRCUIT RESISTANCE and is measured in Ohms. © TAFE Queensland 2016 | Page 65 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Seconds and Henrys are usually far too large for most electronics measurements, and milli and micro units are commonly used, but remember when calculating to convert any of these sub units to seconds or Henrys for use in formulae. The rise in current is not linear however, but follows a curved "exponential" path, and in one time constant the current will have only risen to 63.2% of its maximum (steady state) value. After five time constants it will reach 99.5%, which is regarded as its maximum value The exponential curve © Chisholm The change of current in an inductor in response to a step change in input is exponential. For a series of equal time periods, the current charges the inductor towards its maximum value, by a percentage of the remaining difference between the present and maximum values. So although this difference continues to shrink, the extra charge built up during each time period also shrinks. The outcome of this is that the current can never ever reach the maximum. Page 66 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Time constants Discharge If the circuit is switched off, current does not immediately fall to zero, it again falls exponentially, and after one time constant period will have reached 36.8% of the previous steady state value (i.e.the steady state value -63.2%). It is considered to reach zero in five time constant periods. Why 63.2%? If the current never reaches its steady state value, this presents a problem of how to measure the time taken to fully charge. This is why the idea of a time constant, (the time it takes to charge by 63.2%) is used. Why choose 63.2% when there are easier numbers such as 50% that could be used? Well 50% would be nice but would create an awkward formula with which to calculate the time taken. It's simple It so happens that using 63.2% (which is not too different from 50%) results in a nice simple formula of L/R for the inductor time constant, and CR for the capacitor time constant. This greatly simplifies calculations, and because the current will have reached 99.5% of the steady state value after 5 time constants, this is near enough in practice to consider that the maximum value has been reached. © TAFE Queensland 2016 | Page 67 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Inductance Electromagnetic induction Whenever a conductor cut by an external magnetic field has an e.m.f induced into it, the process is called electromagnetic induction. Inductance is the property of a circuit that allows it to have an e.m.f induced in it by a magnetic field. The quantity symbol = L. The unit = Henry. The unit symbol = H. One Henry is that inductance which will induce an e.m.f of one Volt if the current in a circuit changes at the rate of one Ampere per second. Inductors – are components such as conductors wound into coils. They commonly have cores of air; ferrite; or iron and will store energy in the magnetic field. The three requirements for electromagnetic induction to occur are: A conductor A magnetic field Relative movement between them, resulting in a changing magnetic field strength Increasing any of the above will increase the level of the induced e.m.f The value of induced EMF is calculated using this formula: v = L x (Δl / Δt) Page 68 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Self induction When a conductor is cut by its own changing magnetic field (due to changing current flow) an e.m.f is produced. This is called self-induction. The change in current, (either rise or fall) produces relative motion of its own magnetic field, inducing an e.m.f back into itself.In fact, self-induced e.m.f is often called back e.m.f. Consider the change in magnetic field due to current flow on the top part of the coil. Change in magentic field due to current flow © Chisholm For clarity, only the magnetic field around the left-hand part of this loop is shown below. (An identical field would also appear around the right-hand section of the loop). Assume that current is building up in the conductor and that field is expanding from its centre outward so that it cuts through the right hand part of this loop. Magnetic field around left-hand section © Chisholm If the vertical field is moving to the right, this is the same as saying the adjacent conductor is moving to the left in a stationary vertical field. We can apply Fleming’s right hand rule to find the direction (polarity) of the induced e.m.f. © TAFE Queensland 2016 | Page 69 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Applying Fleming's right hand rule to find the direction of the induced e.m.f © Chisholm As a result, we find that a characteristic of self-induction (or back e.m.f) is that it opposes the change of current flow that resulted in the self-induced (or back) e.m.f.In other words. The induced e.m.f. is of such polarity that it opposes the change in the original current. This is called Lenz’s Law. Page 70 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Series or parallel Inductors connected in Series or Parallel. Inductors may be connected in series or parallel circuits. The following laws apply when calculating the total inductance for a circuit. Series and parallel combination © Chisholm Mutual induction This is the induction of an e.m.f in one circuit by a changing induction current in another circuit. This happens because the magnetic lines of flux of one circuit, cut the conductors of the neighbouring circuit. The result is current flow in the second circuit even though it is not connected to an external source. The coil connected to the source is called the “primary”, the coil connected to the load, is called the “secondary”. © TAFE Queensland 2016 | Page 71 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Mutual induction © Chisholm If the flux linking the coils passes only through air, then maximum coupling occurs when the coils are aligned and in close proximity. Alternatively the coils can be wound on a common ferromagnetic core which means all the flux is linked via the magnetic circuit irrespective of position or orientation. Power transformers consist of primary and secondary windings on a laminated iron core and operate on the principle of mutual induction. Undesirable effects of induction Mutual inductance is when the flux of one cable cuts another. This effect can be unwanted in the case of a cable carrying current that is run or installed beside a cable requiring a very small current, e.g. computer or television cable. In other cases the current induced into a cable could be large enough to hold a relay in the energised state. The undesired effect of self induction is the high induced voltages that occur on the circuit as the coil field collapses. When current through an inductive circuit is abruptly turned off, the energy released as the field collapses induces a large Back e.m.f, which is of such a polarity that it will keep the current flowing in the same direction as the applied current. This induced e.m.f can be many times greater than the applied e.m.f and must be suppressed to a safe level because it can. Arc across the switch burning its contacts, Damage conductor insulation Affect other components (particularly semiconductor devices) in nearby circuits. Page 72 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Factors affecting inductance The factors affecting a long solenoid coil’s inductance (L) in Henries. Are: Number of turns (N) Core permeability (µ) Wb/At.m Core length (l) m Cross sectional area of the core (A) m2 and be calculated using the following formula: L = N2 x μ x A / l Low inductance Higher inductance © Chisholm © Chisholm Low inductance - showing Air Core Low inductance - showing Ferromagnetic core © Chisholm © Chisholm Low inductance Higher inductance © Chisholm © Chisholm © TAFE Queensland 2016 | Page 73 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Low inductance Higher inductance © Chisholm © Chisholm Types of inductors A practical inductor is a complex component because it has a number of properties: Inductance because of the magnetic field that exists when current flow through it Resistance because of the conductive material used its coil Capacitance because each coil loop is insulated from the next but is laying in parallel with it. The proportion of each property a given coil has is dependent on its physical construction. Common inductor cores and symbols Common core materials are: Air cored – offer low L; high ƒ (<150 MHz) Iron cored – offer high L; low ƒ (<20 kHz) Ferrite cored – offer high L; med ƒ (50 k – 1 MHz), are smaller and more efficient. Page 74 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits 6 symbols for core materials © Chisholm © Chisholm © Chisholm © Chisholm © Chisholm © Chisholm © Chisholm © TAFE Queensland 2016 | Page 75 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Magnetic devices Solenoid valve The solenoid valve uses electromagnetism to open or close a valve port by means of a coil and a soft iron-cored slug that is free to move under spring tension. Solenoid Valve © Chisholm Relays and contactors Relay and contractor © Chisholm The coil current creates a magnetic field and induces magnetism into the ferromagnetic core which magnetises and attracts the armature to it. The armature carries a set of contacts which operate when the coil is energised. N/O and N/C contacts are usually provided. Page 76 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Trembler bell A normally closed contact and a coil are connected in series. This will switch on power to the coil. The electromagnetic field created attracts a soft iron armature towards it and this opens the series contact. This opens the switch, interrupts current to the coil, which then releases the armature and re-closes the contact. The process starts again. Trembler Bell © Chisholm P.A. system P.A.system © Chisholm A coil attached to the speaker cone can move within a magnetic field. Current from the amplifier is sent to the voice coil and causes an electromagnetic field to interact with the permanent magnetic field. The interaction results in back and forth movement of coil and diaphragm. Causing the speaker’s cone to produce air compressions we hear as sound. © TAFE Queensland 2016 | Page 77 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Other applications Some further applications include: Protection devices o Magnetic overload o Circuit breaker Sensor devices o Proximity detector (ferromagnetic) o Inductive pulse generator (position sensing) Automotive / transport o Ignition coil o Magneto Mechanical drives o Brakes & clutches (eddy current / hysteresis / magnetic particle) Page 78 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Electromagnet principles Generator action The most convenient way to produce electric current using electromagnetic induction is by rotating either a conductor or a magnetic field relative to each other in a generator. With a permanent magnet field, the only way in which the level of that e.m.f can be controlled is by varying the speed of rotation. This is not practical for most applications so is usually replaced with a winding to produce an electromagnetic field. Video Watch the full video titled “Electric Motors: "DC Motors and Generators" pt1-3 1961 US Army Training Film” - ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=obP1TQh1e2w Source: Jeff Quitney: Electric Motors: "DC Motors and Generators" pt1-3 1961 US Army Training Film [Accessed from YouTube: 30th May 2016] Permanent magnet generator Permanent Magnet Generator © Chisholm © TAFE Queensland 2016 | Page 79 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Speed variation is not a practical method of voltage control for any but the simplest of applications. One such application is a bicycle generator. Variable field generator Variable Field Generator © Chisholm A proven and practical method of controlling output level is by varying the strength of the field. The electromagnetic field strength is varied by varying the current through a field coil. Page 80 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Alternating current Alternating Current © Chisholm The e.m.f induced into the conductor using these is method is alternating. Often, generators refer to DC output, alternators refer to AC output. The method used (i.e. commutator or slip ring) to take the e.m.f from the winding will determine whether AC or DC will flow in the external circuit. If the output from the rotating coil is connected to the external circuit by means of slip rings mounted on the armature shaft, with one attached to each end of the coil, then AC will flow in the external circuit. © TAFE Queensland 2016 | Page 81 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Slip rings and carbon brushes © Chisholm Slip rings are brass rings insulated from each other and from the shaft. The coil ends are connected to these rings and current is removed from the coil via carbon brushes that slide on them. Alternator action The simple alternator An electrical generator in its simplest form is a single loop of wire that rotates within a stationary magnetic field (Remember that a magnetic field rotating in a loop of wire will produce the same effect). The following diagrams show how an alternator produces an EMF as the armature rotates clockwise. In the starting position section a and b of the coil are not cutting any of the flux lines. Therefore no EMF is generated. © Chisholm Page 82 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits As the coil turns point a is in influence of the south pole and point b the north the EMF increases as the angle at which the coil cuts the flux lines increases (remember that e = B x l x v x sin Θ ). © Chisholm At 90o the voltage is at its peak value because the flux lines are being cut at right angles. © Chisholm As the coil turns from 90o to 180o the the EMF decreases as the angle at which the coil cuts the flux lines dereases. At 180o the EMF is zero because no lines of flux are being cut by the coil. © Chisholm As the coil turns from 180o to 270o point b is now under the influence of the south pole and point a the north pole. This reversal causes the EMF to be the opposite polarity to that in the first 90o of the revolution. As the coil turns from 270o to 360o the negative EMF dereases as the angle at which the coil cuts the flux lines decreases. © Chisholm © TAFE Queensland 2016 | Page 83 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits DC generator Commutator By replacing the slip rings with mechanical switching mechanism called a commutator, will enable the current in the external circuit to flow in one direction only. A commutator consists of copper segments insulated from one another, formed into a cylinder and mounted on the rotating shaft. The ends of the rotating coil are connected to the segments. Often, 2 coil ends will share one segment, so the number of segments = the number of coils. Spring loaded carbon or graphite brushes make contact with the commutator and remove electric current from the rotating member (called the armature or rotor). Commutator © Chisholm The purpose of a commutator is to: Connect the rotating coils to the stationary external circuit Provide a switching mechanism so current flow through the external circuit is always in one direction (D.C.) Video Watch the full video titled “Commutators: Basics on AC and DC Generation” – ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=ATFqX2Cl3-w Source: EdisonTechCenter: Commutators: Basics on AC and DC Generation [Accessed from YouTube: 30th May 2016] Page 84 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Although only one loop of the coil is shown in the diagram above, in practice there will be many turns. There will be as many segments to the commutator as there are coils. Commutation Action of the commutator To convert the "alternator to a DC generator", the slip rings are replaced with a "commutator" which is, in effect, a slipring that is split in two. The commutator changes (or rectifies) the ends of the loop from one brush to the other, as the sides of the loop move from one magnetic pole to the other. This means that the current flow at each brush is always in the same direction so that each brush has a fixed polarity, i.e. one is always positive and the other is always negative. That is the current is Direct Current (DC). The action of the commutator in converting the AC generated in the loop to DC at the brushes illustrated below. © Chisholm In the starting position section a and b of the coil are not cutting any of the flux lines. Therefore no EMF is generated. © Chisholm © TAFE Queensland 2016 | Page 85 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits As the coil as rotated clockwise to 90o point a is under the south field pole and the point b is under the north pole. Using Flemings Right Hand Rule, we can deteimine the direction of the induced EMF (voltage) which produces current flow in the loop. The direction of cmrent flow in the loop is clockwise, as shown by the arrows near the coil sides, resulting in positive current flow out of the bottom brush through the voltmeter, and retuming to the loop via the top brush. The bottom brush is therefore positive and the top brush is negative. © Chisholm As the coil turns from 90o to 180o the the EMF decreases as the angle at which the coil cuts the flux lines dereases. At 180o the EMF is zero because no lines of flux are being cut by the coil. Note that point b is now at the top of the diagram. The diagram above shows the loop has rotated from 180o to 270o point b is now under the influence of the south pole and point a the north pole, © Chisholm The current in each coil side is now in the opposite direction,but the commutator has swapped the brush connectioas so that the ends of the loop are now connected to the opposite brush. This results in current still flowing out of the bottom brush and into the top brush. The commutator therefore acts like an automatic switch which reverses the coil connections at the brushes when the current in the coil is reversed. This means the current in the external circuit is always in the same direction. The wave form show that the voltage, and hence the current, is always in the same direction. © Chisholm Page 86 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits As the coil turns from 270o to 360o the negative EMF decreases as the angle at which the coil cuts the flux lines decrease. © Chisholm The output of the single loop generator with commutator is a ‘pulsed DC’ waveform. © Chisholm The output is smoothed by having in practice multiple coils and commutator segments. © TAFE Queensland 2016 | Page 87 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Generated voltage Value of a generated dc voltage In the previous section we looked at the basic D.C. Generator, consisting of a single loop of wire rotating in a permanent magnet field. A practical D.C. generator consists of many coils or loops, mounted on the armature, to produce a smoother D.C. current . These coils have many turns of copper wire. All of these coils are connected to the commutator, which will have many segments to accommodate all of the coil connections. The magnetic field may have two or more poles, which are normally energised by a D.C. current to produce the magnetic field but sma]ler generators may have permanent magnet poles. The value of voltage generated in. the windings of the armature of a D.C. generator depends on three factors: 1. The strength of the main magnetic field Determined by the strength of the field magnets in a “permanent magnet” machine, or by the number of turns of wire on the, field coils and the current through the coils in a “wound field” machine. 2. The number of armature conductors connected in series, which cut the main magnetic field Determined by the number of turns on armature coils and whether the armature is lap or wave wound, which determines the number of armature conductors connected in series. 3. The speed at which the armature conductors cut the main magnetic field The faster the armature cuts the magnetic field; the higher will be the value of the voltage generated in the machine. Page 88 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Calculating the value of a generated voltage The value of a generated voltage for a D.C. generator is found from the following formula: Vg = (pФnz) / 60a Vg = the generated voltage in the armature windings p = the number of poles in the main field Ф = the magnetic field flux per pole in webers n = the armature speed in revolutions per minute 1/60 = a constant to convert “n” to revolutions per second a = the number of parallel current paths in.the armature Z = the number of effective armature conductors Note The number of current paths in parallel depends on whether the armature is “lap” or “wave” wound. A “lap wound” armature has the same number of parallel current paths as there are poles in the magnetic field of the machine. A “wave wound” armature has only two parallel current paths regardless of the number of poles in the magnetic. Therefore in a wave wound armature “a” always equals 2. © TAFE Queensland 2016 | Page 89 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Example 1.Calculate the value of voltage generated (Vg) in a lap wound armature of a 4 pole (p) D.C. generator. The magnetic field flux is 004 webers per pole (4) and the armature has 400 conductors (z) rotating at 1,000 r.p.m(n) NOTE: As this is a 4 pole lap winding there are four (4) current paths so a = 4 Vg = (pФnz) / 60a Vg = (4 x 0.04 x 1000 x 400) / (60 x 4) Vg = 266.67 V 2.Calculate the value of voltage generated ( (Vs) in a wave wound armature of a 4 pole (p) D.C. generator. The magnetic field flux is 0.04 webers per pole (4) and the armature has 400 conductors(z) rotating at 1,000 r.p.m.(n) NOTE: As this is a wave winding there are only two (2) current paths so a = 2 Vg = (pФnz) / 60a Vg = (4 x 0.04 x 1000 x 400) / (60 x 2) Vg = 533.33 V Voltage regulation Voltage regulation of a DC generator is the ratio, expressed as a percentage of the difference between the no-load and full load voltage. The regulation percentage indicates the generators ability to maintain the terminal voltage over a full range of loads and can calculated using the following formula. Vreg % = {(VNL - VFL) / VFL}x 100 where Vreg % = voltage regulation VNL = no load terminal voltage VFL = full load terminal voltage Page 90 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Example Calculate the voltage regulation of a generator rated at 240 V if the voltage at no load was measured to be 260V Vreg % = {(VNL - VFL) / VFL} x 100 Vreg % = {(260 - 240) / 240} x 100 Vreg % = {20 / 240} x 100 Vreg % = 8.3 % Example The greater the percentage voltage regulation the greater the variation in voltage between no load and full load. A well designed generator, depending on its application, would normally have a very low percentage regulation figure, indicating that there is only a small variation as the load increases. A generator 200V at no load, but at full load the voltage falls to 175V. Determine the percentage voltage regulation. Vreg % = {(VNL - VFL) / VFL} x 100 Vreg % = {(200 - 175) / 175} x 100 Vreg % = {25 / 175} x 100 Vreg % = 14.29 % © TAFE Queensland 2016 | Page 91 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Motor action When current is passed through a conductor (that is in the main field), it will generate a magnetic field around the conductor which will interact with the main field. The conductor will tend to move because a force due the magnetic flux directions is exerted on the conductor. This is called motor action. The magnitude of the force on a conductor carrying current in a magnetic field is given by: F=BxlxI where F = force in Newton (N) B = flux density in Tesla (T) (=Wb/m2) l = length of the conductor in magnetic field in metres (m) I = current in the conductor (A) Video Watch the full video titled “Flemings Left Hand Rule” – ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=vkZtsrgso2A Source: RGPhysics: Flemings Left Hand Rule [Accessed from YouTube: 30th May 2016] Page 92 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Fleming’s left hand rule for motors © Chisholm The fingers still represent the same items as the Right Hand for Generators, however now using the right hand, the second finger has the e.m.f in the opposite direction. If the current through the conductor is reversed,the direction of the force acting on the conductor will also reverse. © Chisholm Field interaction in a motor © Chisholm © TAFE Queensland 2016 | Page 93 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits The coil in a vertical plane has no clockwise rotational forces acting on it. The commutator has just switched the current direction to that shown opposite. Movement is due to field interaction of other coils (not shown) In this diagram, repulsion forces would push the 2 conductors toward each other. © Chisholm As the armature rotates due to repulsion force of the field reaction. The field reaction provides increased force is it approaches the horizontal plane. Most force (and therefore torque) is provided with the conductor moving at 90o to the magnetic lines of force. © Chisholm Note Note the lines of forces now have a distinct S shaped field around the two conductors. Page 94 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Back or counter EMF Back EMF By applying the Left and Right Hand Rules, the presence of back EMF can be seen. When the armature rotates in the magnetic field, an EMF is induced in it. This induced EMF is in opposite polarity to the armature supply voltage. From this polarity comes the back EMF. The effective armature voltage is the difference between the armature supply and the back EMF. The value of the back EMF depends on the strength of the field and the speed of the armature. Back EMF never reaches the amplitude of the supply voltage as some power is required to drive the shaft. If back EMF was equal to EMF, no torque would be developed. As motor speed and back EMF varies according to the load, we find that motors tend to have a definite speed/load relationship: ie. An increase in load reduces speed, which reduces back EMF. Generated EMF in a motor Calculate the current taken by the motor armature (Neglect field R): © Chisholm Approx. working I=5A 𝑉 𝑅 200 0.5 I= = = 400A © TAFE Queensland 2016 | Page 95 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits This value of current is unreasonable as the current taken by a 3.5kW motor on 200V does not normally exceed 5A. As the armature conductors in a motor rotate they are in actual fact cutting field flux. This action induces an EMF into these conductors. The direction of the induced EMF is found by using Fleming's R.H.R. and is in direct opposition to the applied EMF This generated EMF in a motor armature is known as "back EMF or "counter EMF". At armature standstill there is no back EMF but, as the armature starts to rotate the back EMF builds-up and limits the armature current. The value of back EMF generated is proportional to the strength of field flux and the speed of armature rotation. The *effective voltage across the armature conductors is the resultant of the applied EMF minus the back EMF * the voltage causing a current flow through the armature. © Chisholm Lenz’s law tells us that the induced EMF will be acting in an opposite direction to the Applied EMF Page 96 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Back EMF calculation The amount of back EMF generated in a DC motor can be found with the aid of: Ia = (V - Vg) / Ra Where; Ia = Armature Current V = Applied Voltage Vg = Back EMF Ra = Armature Resistance Note: Field resistance is ignored in back EMF calculations. Example A 200V, DC motor draws an armature current of 5A on full load. Find the value of the back EMF if the armature resistance is 0.5Ω. By transposition: Ia = (VS-Vg) / Ra therefore: Vg = VS - (Ia x Ra) Vg = 200 - (5 x0.5) = 197.5V Video Watch the full video titled “Back Emf of a DC Motor” - ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=qwtiX3pFCIE Source: Mike Richardson: Back Emf of a DC Motor [Accessed from YouTube: 30th May 2016] © TAFE Queensland 2016 | Page 97 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Torque Torque developed by a motor The figure below shows a rectangular coil of a single turn whose plane lies parallel to a magnetic field. Due to direction of current, the left hand conductor tends to move upwards with a force and the right hand conductor tends to move downwards with an equal force. © Chisholm As the value of the current in each of the conductors is the same and they lie in the same magnetic field. Both forces,are equal and opposite in direction, and act to develop a torque, which tends to turn the coil. The operation of an electric motor depends upon the principle that a conductor carrying current in a magnetic field tends to move at right angles to the field. The figure below shows armature conductors carrying current and the crowding of the magnetic field to one side of the conductor. This crowding tends to turn the armature in a clockwise direction. Page 98 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits © Chisholm Calculating torque There is a definite relationship between the direction of the lines in a magnetic field, the direction of the current in the conductor and the direction in which the conductor tends to move. This relationship is expressed in Fleming's Left Hand Rule. The value of torque produced in a DC motor an be determined by the formula: T = (P x Φ x I x Z) / (2π x a) Where T = torque produced in newton metres P = the number of poles in the stator Φ = the flux/pole strength in webers I = the current in the armature a = the number of parallel paths in the armature note a = number of poles for lap wound armature or 2 for a wave wound armature 2π = a constant (2 x 3.14) © TAFE Queensland 2016 | Page 99 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Example A 6 pole/lap wound armature has 60 coils, each coil has 30 conductors. Calculate the torque produced by the motor if the armature current is 19A and the flux per pole is 0·02 Wb. T = (P x Φ x I x Z) / (2π x a) T = (6 x 0.02 x 19 x 30) / (2π x 6) T = 1.8 Nm The torque developed by the armature is proportional to the pole flux and the armature current. The relationship between torque and speed can be determined from the equation. T = (60 x P) / (2π x n) Where T = torque produced in newton metres P = power mechanical power n = motor speed in rpm Example Calculate the power output of a DC shunt motor that develops a torque of 50 Nm at 1500 rpm. by transposition Pout = (2π x n x T) / 60 Pout = (2 x 3.14 x 1500 x 50) / 60 Pout = 7850 watts Page 100 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Rotating machines DC machines A "Direct Current Generator" is an electro-mechanical machine that converts "mechanical energy" into "electrical energy" The mechanical energy is supplied by, for example, a petrol engine that drives the generator shaft and produces direct current energy at the terminals of the generator. © Chisholm A "Direct Current Motor" is an electro-mechanical machine that converts "electrical energy" into "mechanical energy". The electrical energy from a DC generator is supplied to the motor terminals, turning the motor shaft that then supplies mechanical energy to drive a mechanical load. © Chisholm © TAFE Queensland 2016 | Page 101 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits DC machine constuction Generally speaking DC machines can be used as either a motor or a generator, so their construction is basically identical. A DC machine consists of: Frame or Yoke Field Poles End Shields Field Windings Armature Commutator Brush Gear Cooling Fan DC Machine © Chisholm Page 102 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Schematic drawing of a DC Machine © Chisholm Video Watch the full video titled “Construction and Working of DC Motor Electrical” – ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=IC-PWxtcirI Source: sharpedgelearning's channel: Construction and Working of DC Motor Electrical [Accessed from YouTube: 30th May 2016] © TAFE Queensland 2016 | Page 103 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Armature In most DC machines the armature is the rotating portion, fixed on the drive shaft, which passes through its centre. The armature has a "stack" of circular, slotted laminations, held on the shaft. Armature coils are made of insulated copper wire and are securely held in the slots by insulating wedges or steel wire binders on higher speed machines. The coil ends are connected to the commutator risers, which are made of copper segments, insulated from each other. Armature Armature windings © Chisholm A 4 pole lap wound armature with 4 current paths . Coil leads connect to adjacent commutator bars. © Chisholm Page 104 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits A 4 pole wave wound armature with 2 current paths Coil leads connect to commutator bars a "double pole pitch" apart © Chisholm The previous figures show examples of 4 pole lap and wave windings. The major difference between the two winding types is the relative positions where the leads for each coil connect to the commutator. In a lap windiag the leads of each con connect to adjacent segments.In the wave winding they connect some distance away from each other,normally a double pole pitch. If you trace the current paths from the positive brushes, through the coils to the negative brushes for each winding, you will find that the lap winding has four current paths, the same number as there are magnetic poles and there are two coils connected in series in each path. The wave winding has only two current paths, but has three coils connected in seriesn each path, coils 1, 3 and 5 are short circuited by the brushes and have no current flow in them. © TAFE Queensland 2016 | Page 105 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Stator and field poles The Stator of a DC machine is the stationary part of the machine and consists of the: Field poles and field windings which are fixed The cast iron or steel casing (frame/yoke) that also forms a major part of the magnetic circuit. DC Machine – Stator The Field poles © Chisholm Are made from laminated silicon steel to reduce iron losses The field flux is concentrated in this area A few dc machines have permanent magnet poles Contains residual magnetism required at start up. Wound-field DC motor field pole construction © Chisholm Page 106 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Field pole - coils Wound around the field pole with insulated copper wire Provide flux when current is passed through them. Coils are connected either in parallel (shunt) or series or both (compound) with the armature windings. Shunt Windings – wired in parallel and have many turns of fine gauge wire as they have much less current going through them than the series winding. Series Windings – wired in series and have few turns of heavy gauge wire as they have much more current going through them than the shunt winding. Most machines incorporate both series and shunt windings on the same pole = Compound Coil. Brushes and brush gear Brushes are made from graphite or carbon and provide electrical connection between the armature windings and the external circuit via the commutator. Brush Holders are made from brass or steel and hold or support the brushes and brush springs. © Chisholm © Chisholm Brush springs are made of spring steel and keep the brush in firm contact with the commutator. © TAFE Queensland 2016 | Page 107 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Armature reaction Commutation In the preceding notes, it was shown that the voltage generated in a DC generator is in fact AC and the current flowing in the armature is also AC. We also saw that the commutator converted this alternating current to direct current by acting as a "reversing switch" at the brushes. This switching process is called - Commutation. It will be obvious that the bushes short circuit coils in the commutation process if you examine the following diagram which shows part of a DC generator armature winding and the portion of the commutator to which it is connected. Fig 1 : Before commutation current flow is to the right in the heavily lined coil © Chisholm At the instant shown in Fig. 1 the heavily lined coil is approaching the brush and current in that coil is flowing to the right and entering the brush to flow out to the external circuit. Page 108 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Fig 2 : During commutation the heavily lined coil is short circuited and no current flows in the coil © Chisholm In Fig.2, we can see that the heavily lined coil has moved to the right and is now short circuited by the brush. As the coil is in the Neutral Plane position at this time, there will be little or no current flowing in it and so there will be no sparking at the brush. Fig 3 : After commutation current in the heavily lined coil has reversed direction © Chisholm © TAFE Queensland 2016 | Page 109 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Fig.3 shows that the heavily lined coil has now moved another position to the right and is no longer short circuited by the brush. The current in the coil is now in the opposite direction to that in Fig. 1, before commutation. The commutator has acted as a reversing switch by changing the brush connection from one end of the coil to the other, thus ensuring that the output current from the coil is maintained in the one direction, known as Direct Current. Effects of commutation The switching action of the commutator and brushes occurs at high speed and happens many times per revolution of the armature. The current being switched may be relatively high and parking may be present at the brushes causing heating and burning of the commutator surface and brushes,especially as the load increases. There may be a number of reasons for this: 1. Incorrect brush type; 2. Brushes not in the correct position when short circuiting the coils. The first fault may be remedied by choosing a brush with a higher resistance, but it may be easier and quicker to consult a brush manufacturer for specialist advice. The second fault is discussed below and is more complex as a number of factors can govern the ideal brush position. Brush position for no load conditions When a coil is short circuited by the brush, it is vital that the coil has as little voltage and current as possible being induced in it, to minimise sparking at the brushes. To achieve this the brushes are placed so that they short circuit coils which are not cutting flux in the magnetic field, the coils are, in fact, travelling parallel to the field.This position is called the Neutral Plane and is illustrated for "no load'' conditions in Fig. 4 below Page 110 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Fig 4 : Coils in "no load" neutral plane are short circuited by brushes during commutation © Chisholm Fig. 4 above shows a generator under no load eonditions, meaning that the current in the armature is nearly zero so there is virtually no field produced around the armature by armature current The two heavily lined coils are located in the neutral plane and therefore are not cutting the magnetic flux when they are being short circuited by the brushes. They therefore do not have an e.m.f. induced in them and so are carrying minimum current, thus minimising the sparking between commutator and brushes. Therefore, the brushes are located so that they short circuit coils which are located in the neutral plane. © TAFE Queensland 2016 | Page 111 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Causes of armature reaction When load is applied to the generator terminals the neutral plane will move from the position shown in Fig. 4 on the previous page due to a factor called Armature Reaction which is caused by the current flowing n the armature producing a magnetic field around theanaature, which reacts with and distorts the main magnetic field produced by the stator poles, which moves the neutral plane from its no load position. This results in the brushes short circuiting coils which are no longer in the neutral plane, but are under the influence of the main magnetic field and are having an e.m.f. induced in them and therefore have quite substantial current flowing in them, resulting in sparking at the brushes and commutator. Figs. 5(a),(b) and (c) below show the effect of load on the main magnetic field, i.e. armature reaction. Fig. 5(a) shows the main magnetic field at no load,'i.e. no armature current and the magnetic flux is flowing in straight parallel tines across the air gap from the north pole to the south pole of the field, through the armature conductors. The neutral plane is at 90° to the main field. Fig 5(a): At "no load" - Neutral Plane at 90o to the main magnetic field © Chisholm Page 112 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Hatched contlucton the in the neutral plane Fig5(b) : With load connected- Current in armature conductors produces a field around the armature - North at top and South at bottom © Chisholm Fig. 5(b) shows the armature rotating in a clockwise direction and using Fleming's Right hand rule to determine the direction of the induced current in the armature conductors, the current in the conductors under the north pole is away from us, while the current in the conductors under the south pole is toward us. This produces a field around the conductors in the directions shown, producing a north pole under the armature and a south pole on top of the armature. This field is at 90° to the main field and we will see in Fig. 5 (c) how this armature will distort the main field resulting in movement of the neutral plane from the original no load position. © TAFE Queensland 2016 | Page 113 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Fig5(c) : Resulttant main magnetic field caused by armature reaction © Chisholm Neutral plane is shifted in the direction of armature rotation. From Fig.5(c) it can be seen that the field created around the armature by the armature current has distorted the main magnetic field and has twisted it in the direction of armature rotation so that the hatched conductors may now be cutting the field and so have voltage induced in them. The neutral plane has also shifted in the direction of rotation so that it is still at 90° to the main field in its new position. Any increase in load would mean more current flowing in the armature which would make the armature field stronger further twisting the main field and shifting the neutral plane further around in the direction of rotation. In a DC generator armature reaction causes the main field to twist in the direction of armature rotation. Page 114 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Correcting armature reaction Armature reaction can be corrected or compensated for in following methodes Moving brush position In this method, the brushes are set so that they will short out coils which are in the full load neutral plane which means that there may be sparking at light loads, but not at full oad when the machine is delivering maximum current. Uses This method is satisfactory for small machines running at relatively constant loads but not those with constantly or suddenly changing loads. Interpoles Magnetic poles, smaller than the main field poles, are placed between the main poles.These interpoles are connected in series with the armature and so carry full armature current.Their polarity and strength is such as to cancel the armature field and therefore reset the main field to its original position. Fig.6 (below) shows the placement of the interpoles and their polarity. They produce a magnetic field which is of equal strength and opposite polarity to the armature field, thus cancelling it out. Advantages Because the interpole method is more effective than the brush shifting method, interpole machines can be made smaller than non-interpole machines. Disadvantages Not suitable for rapidly fluctuating loads Uses Used for small to medium size machines where loads may vary © TAFE Queensland 2016 | Page 115 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Fig 6 : Interpoles produce a field equal and opposite to the armature field © Chisholm Compensating windings Compensation windings are windings wound into the faces of the main field poles to compensate for armature reaction. They, like the interpoles, are connected in series with the armature and carry full armature current. This is produces a field equal in trength and opposite in direction to the armature field, thereby cancelling out the armature field and restoring the main field to its correct position. Page 116 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Fig 7: Compensation windings © Chisholm © TAFE Queensland 2016 | Page 117 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits DC generators Generators Watch part 2 of this excellent old video from 1961. The name given to a DC machine whether a motor or generator is derived from the way in which the main magnetic field is produced or the type of field winding connection used with respect to the armature, i.e. the field connected in series or parallel with the armature. Video Watch the full video titled “Electric Motors: "DC Motors and Generators" pt2-3 1961 US Army Training Film” - ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=9E-TSnMCj9A Source: Jeff Quitney: Electric Motors: "DC Motors and Generators" pt2-3 1961 US Army Training Film [Accessed from YouTube: 30th May 2016] Field connections There are two methods of producing the main magnetic field in a DC machine; these are: (1) Use of permanent magnets In this machine the pole pieces consist of permanent magnets. (2) Excitation of the field coils with DC Methods used are: (a) Separate excitation In separately excited machines the field windings are supplied by an external source such as, batteries or a DC generator etc. (b) Self excitation Page 118 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits When a field winding is internally connected to the armature and the machine used as a generator it will produce its own magnetic field and is said to be self excited. The field windings in a D.C. machine can be connected to the armature in 3 ways, these are: (1) Shunt (Parallel) (2) Series (Series) (3) Compound (Series/Parallel) Permanent magnet generator In this machine the pole pieces consist of permanent magnets. In recent years, a few quite large DC machines have been made using permanent magnets to provide the field flux. © Chisholm The performance of a DC machine when a load is applied dictates its characteristics, which in turn determines the most appropriate use for the machine. Permanent magnet generator characteristics The voltage generated by this machine drops steadily as the load is increased. This is due to factors such as armature resistance and armature reaction. © Chisholm © TAFE Queensland 2016 | Page 119 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits The output voltage of the permanent magnet generator is directly proportional to its speed, which means a 10% increase in speed produces a 10% gain in output voltage. © Chisholm Application Because of its linear output voltage to speed the most common use is a tachogenerator. Small versions of this generator find use as magnetos and bicycle generators. Page 120 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Separately excited generator The separately excited generator requires an outside source of DC power to supply the field current, as well as a prime mover to drive the armature. © Chisholm Separately excited generator characteristics The excitation curve indicates the variation in the generated voltage for different values of field current. © Chisholm At zero field current point (i) there is a small voltage generated due to residual magnetism in the poles.In the point (ii) region, small increases in field current produce larger changes in the output voltage until saturation or the point (iii) region is reached, in this region large changes in field current are required to produce small changes in output voltage. © TAFE Queensland 2016 | Page 121 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Operation © Chisholm Suppose that the generator is driven at a constant speed, and the field current is varied by means of the field regulator as shown in the circuit above. The magnetic flux in the field system will follow the same pattern as the magnetisation curve for the poles and frame. The generated voltage (Vg) will change when the field current (If) is varied as shown in above graph. (i) Voltage due to residual magnetism (ii) Field flux increases as If increases (iii) Iron becoming saturated Note The excitation curve is similar to the BH curve it follows. Page 122 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Output voltage versus speed © Chisholm If the field current (If) increases, the output voltage (Vout) increases. If the speed of rotation increases, the output voltage (Vout) also increases If the field current (If) is held constant, then the output voltage is proportional to rpm.ie. straight line. The output voltage of the separately excited generator drops as the load is increased due to factors such as armature resistance and armature reaction. If speed and excitation remain constant and load is placed on the output, the output voltage decreases due to IaRa drop (Volt drop of Armature) and armature reaction. © Chisholm © TAFE Queensland 2016 | Page 123 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Application Where quick and definite response to control is important, such as in process work and machine control. Note A small change in field input causes a large change in output voltage. Series generator Since the series field also draws load current, its windings must be of heavy gauge conductors with few turns per coil, so as to create the field without limiting the current to the load. © Chisholm © Chisholm Characteristics © Chisholm Page 124 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits At NO load, there is no load current (IL) hence no field current (If) or armature current (Ia). There is a small voltage generated due to residual magnetism. (point `a' on the graph) If a load is connected then IL increases and thus If increases. This causes the output voltage (Vo) (point `b' on the graph) to increase more or less proportionally. As the load increases, Vo increases until saturation is reached (point `c' on the graph). If more load is applied past saturation, then Vo decreases due to Ia increasing and armature reaction (point `d' on the graph) Output voltage versus speed © Chisholm If the load resistance is constant and speed is increased, then Vo increases, this causes If to increase causing a further increase in Vo, hence Vo increases non-linearly with speed. Control of output voltage For fixed load and rpm, the only way to alter field flux is to connect a variable R in parallel with the series field. This "field diverter" causes current to be diverted past the field, hence weakening the field. Current is established mainly by the load. © Chisholm © TAFE Queensland 2016 | Page 125 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits If the field controller increases the field resistance, then If will decrease, resulting in an increase in Vg and Vo If Vout does not build up, then: there is no residual magnetism or the field windings are reversed or the prime mover is reversed or there is no load connected. Application Output depends on load so they are only suitable if the load is constant. Generators with only series fields have very limited practical use. Supply arc lamps (early street lighting) Series boosters in D.C. supply mains. Shunt generator Operation The self excited generator uses some of its own output power to effectively magnetise its field system. The simplest form of self excited generator has the field winding shunt connected with the armature.There must be residual magnetism in the field poles from previous use. When the armature is driven from an external mechanical drive, a small voltage (Vg) is generated. Vg then causes current to flow in the field (If) and armature (Ia) coils If then creates field flux that should assist the residual magnetism.Therefore Vg increases, causing an increase in If and Ia The shunt field ceases to build up when magnetic saturation is reached, and Vout reaches a steady value. The generator will fail to build up an output voltage if: There is NO residual magnetism. Or the field windings are reversed. Or the armature is driven in the reverse direction. Page 126 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits To remedy NO output voltage: Provide momentary separate d.c. Excitation (flash the fields with battery). Or reverse field winding. Or reverse drive direction. The shunt field windings consist of many turns of fine wire connected in parallel with the armature. (The field has a high resistance, the armature a low resistance). © Chisholm © Chisholm Characteristics As the armature begins to rotate a small voltage is generated shown at point (i) on the excitation curve below. This causes a small current to flow from the armature through the shunt connected field, resulting in an increase of field strength and thus the voltage generated. This process is continuous until the field poles reach saturation point. At this point the generated voltage stabilises. © Chisholm © TAFE Queensland 2016 | Page 127 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits The fall in voltage on a shunt generator as the load is increased is similar to that of a separately excited machine, but is more pronounced at full load. This is because the load is also, connected in parallel with the field and a decreasing output voltage means a decreasing field current, which results in a decrease in the output voltage.If there is a "dead short", then only residual magnetism remains to cause a relatively small circulating current through the armature short circuit path. No damage is done. © Chisholm Output voltage control This is usually achieved by varying If with a variable field resistance connected in series with the shunt winding. © Chisholm If the variable field resistance control is increased the field current If will decrease causing the generated voltage Vg and the output voltage to decrease and vice-versa. Page 128 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Application Constant load/speed applications eg car generators, magnetic chucks. Small power generators operated by inexperienced personnel (almost foolproof).Process control. Compound generators The compound, self excited generator has both series and shunt windings in the one machine and wound on the same poles. Connections may be short or long shunt as shown. © Chisholm Most compound generators are connected so the shunt and series fields assist each other. These are called Cumulatively Compounded generators (Compounded) or subtractive (Differentially Compounded). Cumulatively compounded The series field of a cumulatively compounded machine does not produce any flux until load is applied to the machine. The flux produced by the series field adds to the flux of the shunt field and, depending on the amount of turns in the series field, the voltage will: 1. Rise as load is added (Over compounded) 2. Remain steady (Level compounded) 3. Fall slightly (Under compounded) © TAFE Queensland 2016 | Page 129 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Cumulative compounding is achieved when the magnetic fields produced by the shunt and series windings aid each other. By adjusting the number of turns in the series field which changes the strength of the magnetic field, the machine can be made to be flat, over or under compounded. Short Shunt Long Shunt © Chisholm © Chisholm In many compound machines, the series field is just sufficient to compensate for the usual voltage drop typical of the shunt generator. This is called flat or level compounding. Used where Vout needs to be constant at various loads. In some cases a stronger series field is used to give a higher Vout on load ie. to compensate for voltage drop (Vd) in long distance DC transmission (overcompounding). If the series field is very weak, then "undercompounding" occurs. © Chisholm Page 130 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Flat or level compounded When the series field slightly compensates for the voltage losses associated with the shunt field, the output voltage will remain almost constant from no load to full load. The change between no load and full load will be less than 5 percent in a flat compound generator. Application Can be used for varying loads relatively close to the generator, for example, ship board installations, rolling mills and multiple-lift installations. Over compounded If the series field is constructed with extra turns then the magnetic field strength will increase. The generator output voltage will increase with increasing load current over the normal operating range. Application To supply loads that vary and are some distance from the generator. The rising voltage characteristic compensates for the line voltage loss across the cables. Under compounded With less turns in the series field than are needed to obtain a flat compounded machine, the magnetic field is reduced as the load is increased; therefore the output voltage is reduced. Application Has little practical use. Differentially compounded In this machine the residual magnetism of the shunt field will allow the machine to excite and produce an operating EMF at the terminals. When load is applied the current passing through the series field will produce flux that will oppose the shunt field flux and the output voltage will fall. Further increases in load will further reduce the output voltage. These machines are unsuitable for use as power supplies, they are primarily used as welders. © Chisholm © TAFE Queensland 2016 | Page 131 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits If the magnetic fields produced by the series and shunt windings oppose each other, the machine is differentially compounded. Under these conditions the output voltage will decrease rapidly as the load current is increased. Short shunt Long shunt © Chisholm © Chisholm Application Main use is in D.C. welders. Also standby generators and home lighting sets. Page 132 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Efficiency of a DC generator In every machine there are always losses, for example, if the mechanical input to a machine is 2,000 watts, the output would always be less, such as 1, 750 watts. This is because some of the input power bas been lost due to a number of different factors. In electrical machines such as generators, the losses are due to four main factors: 1. Friction Friction Is present in all rotating machinery due to the friction of the bearings and any mechanical parts rubbing against each other. 2. Windage Windage Is present due to the resistance of the air on various rotating components such as fans which are used to force air through the stator windings and over the surface of the motor frame for cooling purposes. 3. Copper losses Copper Losses Copper power losses are due to the resistance of the electrical windings and the power loss in copper conductors varies as the square of the current flowing (P = I2R). For example, with light loads (low current), the small current flow means the copper loss is at a minimal. If the armature current is doubled the copper loss becomes four times greater,therefore four times the heat is generated which must be removed by air circulation.This represents a comsiderable power loss within the machine. 4. Iron losses lronLosses The iron losses are almost constant from no load to full load and they consist of both hysteresis losses (i.e. power used to magnetise the iron) and magnetic leakage. Calculating effciency of a generator The efficiency of a generator is expressed as a percentage, i.e. 75%; 800/o; 100% and can be easily calculated using the following formulae: Efficiency = (output power / input power) X 100 or Efficiency = {output power / (output power + losses)} X 100 © TAFE Queensland 2016 | Page 133 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Example 1 The input power (mechanical) to a DC generator is 3,000 watts. If the output electrical power is 2,850 watts, determine the efficiency of the generator. © Chisholm Efficiency = (output power / input power) X 100 Efficiency = (2850 / 3000) x 100 = 95% Example 2 The output power of a DC generator is 2,650 watts. The losses are fiction 250W, iron 125W, copper 690W. Determine the efficiency of the machine. © Chisholm Efficiency = {output power / (output power + losses)} X 100 first add the losses to obtain the total losses (250 + 125 + 690 = 1065 watts) Efficiency = {2650 / (2650 + 1065)} x 100 Efficiency = 71.33 % Page 134 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Additional videos Video Watch the full video titled “DC SHUNT GENERATOR 00” – ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=VnkdXfmvOvg Source: kn6f: DC SHUNT GENERATOR 00 [Accessed from YouTube: 30th May 2016] © TAFE Queensland 2016 | Page 135 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Summary This table is a summary of the different connection types and characteristics of DC generators. © Chisholm Page 136 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits DC motors DC motor principles If the armature of a dc machine has a potential connected across it such that armature current flows from an outside source, then the field flux and the armature flux react.The resultant combined flux thus exerts a rotating force on the conductors of the armature.The armature is free to rotate and delivers mechanical energy at the shaft. ie:Electrical input gives mechanical output. Note If either field polarity or armature current is reversed, then direction of rotation is reversed. Video Watch the full video titled “Electric Motors: "DC Motors and Generators" pt3-3 1961 US Army Training Film” - ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=Lx45QwT39EI Source: jeff Quitney: Electric Motors: "DC Motors and Generators" pt3-3 1961 US Army Training Film [Accessed from YouTube: 30th May 2016] Operation of motor When the armature is stationary and supply is switched on the back EMF (Vg) = 0 volts, therefore the armature current (Ia) is high on starting. © TAFE Queensland 2016 | Page 137 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits © Chisholm As armature speed increases, Vg increases causing Ia to decrease until there is just enough torque to overcome friction, losses and normal running load. If the load increases the speed will decrease causing Vg to decrease this will result in Ia increasing to give more torque to overcome the load. If field flux decreases, Vg will decrease therefore Ia and speed will increase until stability is achieved. Torque produced by motor equals torque required by load - this is obtained by Vg controlling Ia. The sequence of changes which determine the behaviour of a motor when the load torque, supply voltage or field strength is varied, is shown in the following diagram: Page 138 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Permanent magnet motor The permanent magnet DC motor consists of an armature winding as in case of an usual motor, but does not necessarily contain the field windings. The construction of these types of DC motor are such that, radially magnetized permanent magnets are mounted on the inner periphery of the stator core to produce the field flux. The rotor on the other hand has a conventional dc armature with commutator segments and brushes. The diagrammatic representation of a permanent magnet dc motor is given below. © Chisholm The speed of a permanent magnet motor is almost constant between no load and full load. © Chisholm © TAFE Queensland 2016 | Page 139 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Torque is proportional to the load current throughout the normal load range. This produces the linear characteristic shown below: © Chisholm Application Battery powered portable tools are a common use. Small versions are found in battery powered toys whilst, larger versions are used for steel mill table drives etc. Separately excited DC motor As the name suggests, in case of a separately excited DC Motor the supply is given separately to the field and armature windings. The main distinguishing fact in these types of dc motor is that, the armature current does not flow through the field windings, as the field winding is energized from a separate external source of dc current as shown in the figure. © Chisholm Page 140 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Characteristics The characteristics of a separately excited motor are similar to that of a permanent magnet motor. Note: This motor will excessively over speed if the field circuit is opened. Application Used mainly for drive motors that needs to respond quickly and precisely to operational commands, e.g. conveyors,steel reduction mills, extruders. Series dc motor The series motor has a high initial torque, due to the initial absence of back EMF. Until the back EMF builds up, the field and armature currents are very high, giving high torque. Speed of a series motor decreases dramatically with increased load (or increased output torque requirement). As the armature slows down. back EMF decreases, causing more field and armature current, therefore giving higher torque. Because of its dramatic speed variations with load changes, a series motor should never be run without a load connected as the tendency to ‘run away’ will cause such a high rotation rate that the motor could disintegrate. For the same reason, series motors should never be connected to the load by a belt drive. If the belt breaks, the motor has no load and could ‘run away’. © Chisholm The main advantage of a series motor is its initial high starting torque. Speed versus load current With a steady load, the motor will run at a steady speed, at which the back EMF emf Vg is almost equal to the applied voltage Vs. If more mechanical load is placed on the motor, more current is drawn through the field windings, therefore the load current IL, armature current Ia and the field current If will increase causing the field flux Φf and Vg to increase. The speed will instantly reduce to lower Vg © Chisholm © TAFE Queensland 2016 | Page 141 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits The required Vg is now obtained at a lower speed therefore motor slows down considerably. If less mechanical load, is placed on the motor, less current is drawn through the field windings:therefore the load current IL, armature current Ia and the field current If will decrease causing the field flux Φf and Vg to decrease. The speed will instantly increase to increase Vg If load is removed entirely, speed which tends to become inversely proportional to applied load. ie. 1/2 load ® 2 x speed, 1/10 load ® 10 x speed At no load - speed is dangerously high Portable electric tools and household appliances with series motors usually have a small cooling fan on the armature shaft which also limits the speed when the appliance is running on NO-LOAD. Torque versus load current As I load increases, If and Φf and Ia and Φa increase because of this double effect of increasing both field and armature flux, torque is proportional to I2 © Chisholm Speed control For speeds below normal, reduce Ia by weakening series field with increased Series Resistor Increasing series R will decrease Ia and If and therefore torque and speed. For speeds above normal, weaken series field with decreased field diverter R. Page 142 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Decreasing field divider R will decrease If causing Vg to decrease so that Ia, torque and speed increase to restore Vg. © Chisholm To reverse direction of rotation To reverse direction of the motor, reverse either series field or armature BUT not both. Applications Anything that starts with a heavy load e.g. Hoists, trams, trains, cranes, rolling mills, starter motors, elevators etc. Also portable electric tools. Coupling to the motor should be direct (e.g. geared) rather than using a belt, which if snaps will result in motor over-speeding. Shunt motor A characteristic of this type of motor is that, unlike the series motor, speed regulation for a shunt motor is good.The field current is constant, unless the supply volts vary, independent of variations in armature current and load. Because the field of the shunt motor is weak compared to the series motor, it has a low starting torque and should not be used to start heavy loads, as damage to the armature windings can occur. This is due to the motor carrying high initial current until the back EMF has built up with armature speed. © Chisholm © TAFE Queensland 2016 | Page 143 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits The shunt motor may be run without load as its speed depends on the field strength, hence it will settle down to run at its nominal speed since field strength is constant. The speed may be controlled and varied by using a variable resistor in series with the field winding. When load is applied to the motor it slows down only slightly, causing a minimal reduction in back EMF, which causes more armature current to flow due to the constant field strength, giving more torque. The armature current and speed will settle at a value to give just enough torque to drive the load. The motor will then run at this speed until the load is varied.If the load decreases the speed will build up slightly, giving more back EMF, less armature current and less torque. Again, just enough to drive the load. The main advantage of the shunt wound motor is that speed is relatively constant and independent of load. As the speed loss at full load is only about 10% of the no load speed, the shunt motor is considered to be a constant speed machine. Like the separately excited motor, this motor also has a definite no load speed and will over speed dangerously if the field becomes open circuited. Speed versus load current An increase in load causes the speed to decrease, this will result in a decreased back EMF (Vg) The decrease of Vg causes the armature current (Ia) and torque to increase to match the load requirements (Note: Ia2Ra loss also increase). The speed VS load curve is the same for separately excited or permanent magnet motor. © Chisholm Page 144 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Torque versus load current Torque is the "turning moment" or force x radius and the unit is the Newton-metre (Nm). T = F x r Nm The torque VS load curve is the same for separately excited or permanent magnet motor. v To reverse direction of rotation Change polarity of shunt winding (most common) Change polarity of armature and interpoles NOT both shunt winding and armature © Chisholm If there are no interpoles, the brushes may have to be shifted. Speed control Speeds above normal are obtained by weakening the field flux i.e.increasing field regulator resistance. If the field regulator resistance is increased the field current (If) will decrease, causing the back EMF (Vg) to decrease. © Chisholm © TAFE Queensland 2016 | Page 145 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits The decrease of Vg will result in the armature current Ia and the torque to increasing causing the speed to increase until Vg is restored and speed stabilised Speeds below normal are normally obtained by strengthening the field flux If the field regulator resistance is decreased, If will decrease causing the back EMF (Vg) to increase. The increase of Vg will result in the armature current Ia and the torque to decreasing causing the speed to decrease until Vg is restored and speed stabilised. Applications Used where reasonable constant speed is required. eg. motor-generator sets, hydraulic pump drives, trucks, compressors Compound motor A compound motor uses both Series and Shunt field windings.This means that a compound motor can be made to produce various combinations of series and shunt characteristics, by varying the size and polarity of the winding combination. © Chisholm The two common types of compound motor are the cumulative compound and the differential compound. In cumulative compounding the series and shunt fields aid each other. The main features of this method of winding are that it does not run away with no load connected and it can handle large sudden changes in load. Differentially compounded motors have the series and shunt windings opposing each other and the resultant field is the difference between the two fields, giving different characteristics. Page 146 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Cumulative compound These are motors in which the shunt field and the series field assist each other Short Shunt (cumulatively connected) © Chisholm If the shunt field is connected directly across the armature so that the current flows through it in the same direction as the series field and the magnetism from the series field assists the shunt field,the motor is known as a ''short-shunt Long Shunt (cumulatively connected) © Chisholm If the shunt field is connected across both the armature and series field and current flows through it in the same direction as the series field and the magnetism from the series field assists the shunt field,the motor is known as a ''long-shunt © TAFE Queensland 2016 | Page 147 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Differentially compound Short Shunt (differentially connected) © Chisholm The figure above shows that the shunt field is connected across the armature, but with the differential connection the shunt field is reversed, that is, the current through the shunt field is opposite in direction to that through the series field This means that as the load increases the current drawn through the series field increases. increasing the magnetic strength of the series field, which tends to demagnetise or weaken the shunt field. If the motor is overloaded the series field can overcome the shunt field and the motor may try to reverse direction, thereby damaging the motor shaft. Differential connections have little practical use because of this. Long Shunt (differentially connected) © Chisholm Page 148 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Characteristics As the load is applied, the series turns increase the flux, causing the torque for any given current to be greater than it would be for the shunt motor. On the othei- band, this increase of flux causes the speed to decrease more rapidly than it does in the shunt motor. The figures below shows the torque and the speed characteristics of shunt and compound motors. © Chisholm Speed versus Load Current Combined field flux increases with load. Speed varies more than that of a shunt motor but less than that of a series motor. © Chisholm © TAFE Queensland 2016 | Page 149 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Torque versus Load Current As the load current increases the armature current,armature flux and series fielld flux will increase, hence combined field flux will increase and hence torque will increase (i.e. torque between that for a shunt and a series motor) Reversing rotation Easiest way is to reverse the "armature-interpole" section.Reversing both shunt and series winding will work but if an error is made (e.g. differentially compounding the windings), consequences can be dramatic. Applications Cumulatively compounded motors are used where: Both high starting torque is required and "No-load" conditions may occur.Constant speed is not necessary. Examples include motors used in Punches, Shears, Rolling mills Differentially compoundedmotors are dangerous. An increase in load causes shunt field flux to decrease (and even reverse) and series field strengthens, so torque and speed are unstable. The motormay even stall and abruptly reverse direction as load is applied above a certain level. Additional videos Video Watch the full video titled “DC MOTOR CALCS 1” – ask your teacher for the local location of the videos. Alternatively, view the video from the below YouTube link: https://www.youtube.com/watch?v=RmGBWWNFYTs Source: kn6f: DC MOTOR CALCS 1 [Accessed from YouTube: 30th May 2016] Page 150 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Summary This table is a summary of the different connection types and characteristics of a DC motor. © Chisholm © TAFE Queensland 2016 | Page 151 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Motor reversal The direction of rotation of a DC motor can be reversed by: a. Reversing the field connections, or b. Reversing the armature connections….but not both! Efficiency From previous work studied output power of a motor. Efficiency = (output power / input power) x 100 Therefore, Output power of a motor = Efficiency x input power Output power = kW Input power = kW Small machines may be rated in watts. © Chisholm Example Calculate the output power of a 200V DC motor if, when fully loaded,it draws 7·46 amperes. The efficiency of the machine is 85% Efficiency% = (output power / input power) x 100 Therefore 85 \ 100 = output power / input power (note input power = V Input x I Input ) .85 = output power / (200v x 7.46A) output power = 200v x 7.46A x .85 = 1270 watts or 1.27 kW Page 152 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Speed regulation The percentage speed regulation gives an indication of a motor's ability to maintian its speed from no load to full load. The speed regulation can be calculated from the equation % regulation (reg) = {(nn/l - nf/l) / nf/l} x 100 Where nn/l = armature speed at no load nf/l = armature speed at full load Example Calculate the full load percantage speed regulation of a DC shunt motor when the no load speed of 1500 rpm drops to 1440 rpm at full load. % regulation (reg) = {(nn/l - nf/l) / nf/l} x 100 % reg = {(1500 - 1440) / 1440} x 100 % reg = (60 / 1440) x 100 % reg = 4.17 % © TAFE Queensland 2016 | Page 153 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Specialty DC machines There are specialised D.C. machines available which are used for specific applications. Four of these machines are: Tachogenerators Servomotors Steppermotors Tachogenerator A typical tachogenerator (sometimes referred to as a tachometer) has the same basic construction as previously considered permanent magnet generator, only on a smaller scale.Use of a permanent magnet instead of electromagnet eliminates the need for a DC source and makes the device extremely reliable and stable.Usually connected to high impedance indicators, minimising load resistance affecting output linearity (often 0-10 V DC). The output is directly proportional to the shaft RPM.Tachogenerators also indicate direction of rotation by the polarity of the output voltage. Tachogenerators are frequently used to measure the speeds of electric motors, engines, and fitness equipment.Their output signal is often used as a feedback signal in process control systems to control the speed of various machines such as pumps, motors, mixers and variable speed drives used in paper and steel processing plants. Servo motor The shaft of the servo can be positioned to specific angular positions by sending a pulse or coded signal. As long as the coded signal exists on the input line, the servo will maintain the angular position of the shaft. If the coded signal changes, then the angular position of the shaft changes.The width of the servo pulse signal dictates the range of the servo's angular motion. For example, a servo pulse of 1.5 ms width will set the servo to its "neutral" position, or 90°. Then a servo pulse of 1.25 ms could set the servo to 0° and a pulse of 1.75 ms could set the servo to 180°.The physical limits and timings of the servo hardware varies between brands and models, but a general servo's angular motion will be around 180° - 210° . Applications vary from industrial, chart recorder and other high quality instrumentation applications to robotics and remote control hobby type applications. A very common use of hobby type servos is in Radio Controlled models like cars, aeroplanes and robotics. Servos are rated for Speed and Torque. Normally there are two servos of the same kind, one geared towards speed (sacrificing torque), and the other towards torque (sacrificing speed). Page 154 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Electronic servomotor control modules designed to provide accurate position or bi-directional speed control is available for this type of motor Stepper motor A motor that takes electrical pulses and changes them into mechanical movement, one step or partial turn per pulse, is called a “stepper (or stepping) motor ”stepper motors use a magnetic field to move a rotor in small angular steps or fractions of steps. Stepping can be done in full step, half step or other fractional step increments.Voltage is applied to poles around the rotor. The voltage changes the polarity of each pole, and the resulting magnetic interaction between the poles and the rotor causes the rotor to move. A “permanent magnet stepper motor” is a common type of this motor. In its simplest form, it consists of a two pole permanent magnet rotor revolving within a four pole slotted stator. Maintenance Introduction The key to minimizing motor problems is scheduled routine inspection and service.A motor may require additional or more frequent attention if a breakdown would cause health or safety problems, severe loss of production, damage to expensive equipment or other serious losses.Written records indicating date, items inspected, service performed and motor condition are important to an effective routine maintenance program. A routine inspection and servicing can generally be done without disconnecting or disassembling the motor. Check the 4 following items: Dirt and Corrosion Dirty motors run hot when thick dirt insulates the frame and clogged passages reduce cooling air flow. Lubrication Lubricate the bearings only when scheduled or if they are noisy or running hot. Do NOT over-lubricate. Excess grease and oil creates holds dirt and can damage bearings. Winding Insulation When records indicate a tendency toward periodic winding failures in the application, check the condition of the insulation with an insulation resistance test. Heat, Noise and Vibration Feel the motor frame and bearings for excessive heat or vibration. Listen for abnormal noise. All indicate a possible system failure. © TAFE Queensland 2016 | Page 155 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Vibration can cause damage in several ways. It tends to shake windings loose and mechanically damages insulation by cracking, flaking or abrading the material. Noise and vibrations can be caused by a misaligned motor shaft or can be transmitted to the motor from the driven machine or power transmission system. They can also be the result of either electrical or mechanical unbalance in the motor Overheating may be caused by: WRONG MOTOR: It may be too small or have the wrong starting torque characteristics for the load. POOR COOLING: Accumulated dirt or poor motor location may prevent the free flow of cooling air around the motor. Dirt on the frame may prevent transfer of internal heat to the cooler ambient air. OVERLOADED DRIVEN MACHINE: Excess loads or jams in the driven machine force the motor to supply higher torque, draw more current and overheat. Brushes and commutators (DC motors) Observe the brushes while the motor is running. The brushes must ride on the commutator smoothly with little or no sparking and no brush noise (chatter). Stop the motor and check: The brushes move freely in the holder and the spring tension on each brush is about equal. Every brush has a polished surface over the entire working face indicating good seating. The commutator is clean, smooth and has a polished brown surface where the brushes ride. There is no grooving around the circumference of the commutator. Black spots on the slip rings must be removed by rubbing lightly with fine wet and dry. If not removed, these spots cause pitting. An imprint of the brush, signs of arcing or uneven wear indicate the need to remove the motor from service and repair or replace the rings. Page 156 of 160 | © TAFE Queensland 2016 UEENEEG101A - Solve problems in electromagnetic devices and related circuits Glossary Term Meaning EMF Electromotive Force MMF Magnetomotive Force DC Direct Current © TAFE Queensland 2016 | Page 157 of 160 UEENEEG101A - Solve problems in electromagnetic devices and related circuits References Page 158 of 160 | © TAFE Queensland 2016