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UEENEEG101A
Solve problems in electromagnetic devices
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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.
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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/
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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
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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
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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
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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
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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.
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Magnet names
Permanent magnets are generally named after their shape:
Bar Magnets
Horseshoe Magnets
© Chisholm
© Chisholm
Disk Magnets
Toroidal Magnets
© Chisholm
© Chisholm
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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
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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
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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
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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.
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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.
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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
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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
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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
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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
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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.
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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
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UEENEEG101A - Solve problems in electromagnetic devices and related circuits
Small Electric Motor
© Chisholm
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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.
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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.
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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]
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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
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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
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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
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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
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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
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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.
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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.
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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
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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.
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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
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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
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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).
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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.
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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.
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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
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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
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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.
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Flux Density
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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
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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
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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
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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
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Magnetisation Curves for Different Core Materials
© Chisholm
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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).
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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
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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
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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
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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.
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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
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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
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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
Ư
∆𝑡
© Chisholm
= rate
of change of flux (Wb/s)
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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
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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.
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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]
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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.
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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.
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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)
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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.
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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.
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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
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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.
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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
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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.
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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.
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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
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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
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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.
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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.
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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
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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
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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]
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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
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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
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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
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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.
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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.
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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
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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 %
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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]
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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
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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.
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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
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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
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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]
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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.
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© 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)
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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
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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
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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
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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]
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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
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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.
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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
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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.
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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.
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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
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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
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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.
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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
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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.
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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.
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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
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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.
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Fig 7: Compensation windings
© Chisholm
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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
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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
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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.
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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
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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.
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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
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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
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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
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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.
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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
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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.
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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)
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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
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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
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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.
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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
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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 %
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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]
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Summary
This table is a summary of the different connection types and characteristics of DC generators.
© Chisholm
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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.
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© 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:
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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
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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
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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
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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.
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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
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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
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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
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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.
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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
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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
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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
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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]
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Summary
This table is a summary of the different connection types and characteristics of a DC motor.
© Chisholm
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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
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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 %
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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).
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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.
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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.
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Glossary
Term
Meaning
EMF
Electromotive Force
MMF
Magnetomotive Force
DC
Direct Current
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References
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