ELECTRICAL MACHINE 1 Chapter 1 Electrical Machinery Generalization ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines • The two (2) most frequently used types for rotating electrical machines are: (a) Generators – mechanical energy is converted into electrical energy. (b) Motor – electrical energy is converted into mechanical energy. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines ( GENERATOR ) A + Tm S S Te N N B _ ELECTRICAL LOAD APPLICATION CONCEPT OF ALIGNMENT OF TWO MAGNETIC FIELDS ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines S ( MOTOR ) A + S DC SUPPLY v Te N B N _ TL APPLICATION CONCEPT OF ALIGNMENT OF TWO MAGNETIC FIELDS ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines • Two other types of rotating electrical machines not used so often are: rotary converters and frequency converters. Basic Operations: • Electric Generator – it is driven (rotated) by a mechanical machine usually called a prime mover ( can be a steam turbine, a gasoline engine, an electric motor, or even a hand-operated crank). A generator action can take place when, and only when, there is a relative motion between conducting wires (usually copper) and magnetic lines of force. • Electric Motor – it is supplied with electrical energy and develops torque, that is a tendency to produce rotation. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines • All rotating electric generators consists essentially of two important parts: (1) An even set of electromagnets or permanent magnets. (2) The laminated steel core containing current-carrying copper wires which is called an armature winding. • In dc-generator, the armature winding is mechanically rotated through a stationary magnetic fields created by electromagnets or permanent magnets. • In ac-generator, the electromagnets or the permanent magnets and their accompanying magnetic fields are rotated with respect to the stationary armature winding. • In dc-motor, current is sent into the armature winding, the latter is being placed inside a set of radially supported magnet poles. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines Field Structure for a dc machine Field Structure for a ac machine FIG 1: Field Structure for a dc generator (rear) and ac machine (foreground) ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines • In the illustration of previous slide: (a) dc-generator – shows many main and commutating poles bolted to the outside yoke; also visible is the brush rigging on the far side. (b) ac-machine – the two rings mounted on the shaft; the two ends of the entire field winding are connected to these rings so that stationary brushes riding on the latter can “feed” direct current into the rotating structure. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines Wound Stationary armature FIG 2: Wound Stationary armature for a low speed ac generator. A field similar to that depicted of the ac –machine in figure 1 would be rotated inside this frame. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines FIG 3: DC Motor – The armature commutator and the carbon brushes are clearly visible on the near side. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines • In an ac motor, current is sent into the armature winding, which is usually placed in a stationary laminated iron core; the rotating element may or may not be set of magnet poles. FIG 4: A rotary converter ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines • A synchronous type, in which a small dc generator is placed on the shaft extension to supply dc excitation to the rotating field. • The rotary converter, electrical energy of one form is changed into electrical energy of another, the usual arrangement is to change ac energy into dc energy although the reverse is sometimes done. FIG 5: A small frequency converter set ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines • The frequency converter, has very limited application; its function is to change ac electrical energy at one frequency into ac electrical energy at another frequency. How to do this? Two rotating machines are directly coupled together; one of them operates as an ac motor when connected to an ac source having a given frequency, while the other, driven machine functions as an ac generator to deliver electrical energy at some other frequency. The stationary part of the ac generator is also connected to an ac source, the same supply to which the motor is connected. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Rotating Electrical Machines DC MACHINES YOKE BRUSH AXIS . . S FIELD WINDING + + _ + + BRUSH + + + . . FIELD N POLES MAIN FIELD AXIS + + + ARMATURE ARMATURE CONDUCTORS APPLICATION CONCEPT OF ALIGNMENT OF TWO MAGNETIC FIELDS ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 1. BODY OR MAGNETIC FRAME OR YOKE 2. POLE CORE AND POLE SHOES 3. FIELD or EXCITING COILS 4. ARMATURE CORE 5. ARMATURE WINDING 6. COMMUTATOR ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 7. BRUSHES 8. END HOUSINGS 9. BEARINGS 10. SHAFT ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features Bearing Pulley Brush Body / Yoke End Housing Shaft Armature Commutator Field Core Brush holder Field Winding MAIN CONSTRUCTIONAL FEATURES ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features ARMATURE YOKE + SHAFT FIELD POLE & COIL BRUSH - COMMUTATOR MAIN CONSTRUCTIONAL FEATURES ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features OTHER MECHANICAL PARTS : ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 1. MAGNETIC FRAME or YOKE The outer cylindrical frame to which main poles and inter poles are fixed and by means of the machine is fixed to the foundation is called YOKE. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features MAGNETIC FRAME OR YOKE serves two purposes: a) It provides mechanical protection to the inner parts of the machines. b) It provides a low reluctance path for the magnetic flux. The yoke is made of cast iron for smaller machines and cast steel or fabricated rolled steel for larger machines. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 2. POLE CORE AND POLE SHOES The pole core and pole shoes are fixed to the yoke by bolts. They serves the following purpose : a) They support the field or exciting coils. b) They distribute the magnetic flux on the armature periphery more uniformly. c) The pole shoes have larger X- section, so, the reluctance of the magnetic path is reduced. The pole core and pole shoes are made of laminated steel assembled by riveting together under hydraulic pressure. POLE CORE ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 2. POLE CORE AND POLE SHOE ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 3. FIELD or EXCITING COILS Field coils or exciting coils are used to magnetise the pole core. Enameled copper wire is used for the construction of these coils. When direct current is passed through these coils/ winding, it sets up the magnetic field which magnetise the pole core to the reqd. flux. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 3. FIELD OR EXCITING COILS ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 4. ARMATURE CORE Armature is a rotating part of the DC machine, reversal of flux takes place, so hysteresis losses are produced. To minimize this loss, silicon steel is used for the construction. The rotating armature cuts the main magnetic field , therefore an e.m.f is induced in the armature core. This e.m.f circulates eddy currents in the core which results in eddy current loss in it. The armature core is laminated to reduce the eddy current loss. Armature core serves the following purposes: a) It houses the conductors in the slots. b) It provides an easy path for magnetic flux ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 4. ARMATURE CORE ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 5. ARMATURE WINDING The no. of conductors in form of coils placed in the slots of the armature and suitably inter connected are called winding . This is the armature winding where conversion of power takes place i.e. in case of generator, mechanical power is converted into electrical power and in case of a motor, electrical power is converted into mechanical power. ARMATURE WINDING ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 5. ARMATURE WINDINGS ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 6. COMMUTATOR It is the most important part of a DC machine and serves the following purpose : i) It connects the rotating armature conductors to the stationary external circuit through the brushes. ii) It converts alternating current induced in the armature conductors into unidirectional current in the external load circuit in generating action and it converts alternating torque into unidirectional torque produced in the armature in motoring action. The commutator is of cylindrical shape and is made of wedge shaped hard drawn copper segments. The segments are insulated from each other by a thin sheet of mica. The segments are held together by means of two V-shaped rings that fit into the V-grooves cut into the segments. Each armature coil is connected to the commutator segment through riser. COMMUTATOR ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 6. COMMUTATOR MICA INSULATION COPPER SEGMENT RISER END RING ADJUSTING NUT SHAFT METAL SLEEVE ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 6. COMMUTATOR ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 7. BRUSHES Brushes are made of high grade carbon. They form the connecting link between armature winding and the external circuit. The brushes are held in particular position around the commutator by brush holders. BRUSH + ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 6. BRUSHES ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 8. END HOUSINGS They are attached to the ends of main frame and support bearing . The front housing supports the bearing and the brush assembly whereas rear housing supports the bearing only. END HOUSING ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 9. BEARINGS The function of the bearing is to reduce friction between the rotating and stationary parts of the machines. These are fitted in the end housings. Generally, high carbon steel is used for the construction of the bearings. BEARINGS ELECTRICAL MACHINE 1 Electrical Machinery Generalization Main Constructional Features 10. SHAFT The function of shaft is to transfer mechanical power to the machine or from the machine. Shaft is made of mild steel with maximum breaking strength. All the rotating parts like armature core, commutator, cooling fan etc. are keyed to the shaft. SHAFT ELECTRICAL MACHINE 1 Electrical Machinery Generalization Understanding Armature Windings and Field Poles The armature windings of all types of motors and generators, whether dc or ac are always wound on laminated steel cores of good magnetic permeability. Alternating voltages are always generated in the windings of ac and dc generators: (a) In the ac generator, the generated alternating electromotive force (emf) is transmitted directly to the load. (b) In the dc generator, the generated alternating emf is first rectified by the commutator and its brushes, that is changed to direct current before it is transmitted to load. (c) The ac motor receives its energy directly from an ac source and, without any change whatever in form, uses it as alternating current in its winding to develop torque. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Understanding Armature Windings and Field Poles Why Steel cores have good magnetic permeability? From magnetization curve, ELECTRICAL MACHINE 1 Electrical Machinery Generalization Understanding Armature Windings and Field Poles Cont… armature windings (d) In dc motor, however direct current is delivered to the brushes but flows as alternating current in the armature winding after passing through the brushes and commutator. • The electromagnets (field poles). There are always an even number of them in a given machine, and each one consists of a laminated steel core, of rectangular cross section surrounded by one or more copper coils. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Understanding Armature Windings and Field Poles Cont… field poles • The spread out portion of the pole core, or shoe permits the magnetic flux to enter the armature core over a wider area than would be possible with a core having straight sides. FIG 6: Pole shoe ELECTRICAL MACHINE 1 Electrical Machinery Generalization Understanding Armature Windings and Field Poles The type of field structure are: (a) Stationary field type – the electromagnets are bolted to a yoke as shown below so that they project radially inward toward the rotating armature. Interpoles FIG 7: Stationary field type of machine ELECTRICAL MACHINE 1 Electrical Machinery Generalization Understanding Armature Windings and Field Poles (b) Rotating Field Type - is driven by a slow speed prime mover, the electromagnets are bolted to a hub fastened to the shaft as shown below, so that they project radially outward toward the stationary armature core; this construction is called salient-pole field. When the alternator is driven by a high speed turbine, the field winding is placed In a slotted core; this construction is called non-salient pole field. FIG 8: Rotary field type of machine ELECTRICAL MACHINE 1 Electrical Machinery Generalization Alternating Current Generators Generally called an alternator. The speed of rotation of the field must be kept absolutely constant, since frequency of the generated voltage in cycles per second is directly proportional to the speed. The voltage developed by an ac generator varies greatly with changes with load; as the load increases, the voltage tends to fall. To maintain a constant voltage at the load, it is customary to equip an ac generator with a regulator, a device that tends to maintain the terminal voltage regardless of the load. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Alternating Current Generators There are two reasons why ac machines can be built in large sizes and made to develop high voltage: (1) No commutator is required, a commutator being a very definite limiting factor in the construction and operation of a dc generator. (2) The armature winding can be placed in the stationary part of the machine, the stator, where it is possible to provide good insulation strength for the high voltage winding. • One of the most important reasons for the use of ac generators in comparatively large power systems is that alternating current can be transformed efficiently from one voltage to another by the use of transformer. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Types and Operating Characteristics of Direct Current Motors There are three (3) general types of dc motors on the basis of the kind of excitation used. (1) Shunt Motor – if comparatively high resistance field winding of many turns of fine wires is employed for this function, it is connected in parallel with the armature. (2) Series Motor - when an extremely low-resistance field winding of very few turns of heavy wire is used, it is connected in series with the armature. (3) Compound Motor – a machine that is excited by a combination of a shunt field connected in shunt with the armature and a series field in series with the armature. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Types and Operating Characteristics of Direct Current Motors SELF EXCITED DC MOTORS ( DC SHUNT MOTORS ) F Ia If IL A M FF + E Ra V SUPPLY AA _ DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) ELECTRICAL MACHINE 1 Electrical Machinery Generalization Types and Operating Characteristics of Direct Current Motors SELF EXCITED DC MOTORS ( DC SERIES MOTORS ) Y YY Ia + A M AA_ ISE + IL E Ra V SUPPLY _ DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) ELECTRICAL MACHINE 1 Electrical Machinery Generalization Types and Operating Characteristics of Direct Current Motors SELF EXCITED DC MOTORS ( DC COMPOUND MOTORS ) ISh Y YY Z A Rsh ZZ Ia M ISE IL SUPPLY E Ra + V AA _ DIFFERENT TYPES OF EXCITATIONS ( DC MOTORS ) LECTURE 9 OF 40 ELECTRICAL MACHINE 1 Electrical Machinery Generalization Types and Operating Characteristics of Direct Current Motors The following general operating characteristics distinguish the three types of motor from one another. (1) On the basis of the same horsepower and speed rating, a series motor develops the highest starting torque and the shunt motor the least, while the compound motor falls between the first two. (2) The overload capacities follow the same general order as for starting torque. (3) The speed variation for changes in load are the least for the shunt motor, the greatest for the series motor, and somewhat larger than the shunt motor for the compound. (4) Both the shunt and compound motor operate at very definite stable speed when all mechanical load is removed, while the series motor, and somewhat larger than the shunt motor for the compound machine. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Types and Operating Characteristics of Direct Current Motors There are three methods that may be employed to alter the speed of a dc motor. (1) By changing the flux through resistance control. (2) By changing the voltage across the armature through resistance control. (3) By changing the voltage across the armature when it is supplied with power from a separate voltage-controlled generator. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Starting Direct Current Motors At the starting period, the armature current is usually much higher than normal. During this starting period, arcing at the commutator is likely to be very severe and can caused burning. What do we need to prevent burning at the commutator during start-up? An external resistance (rheostat) must be added to limit the current in the armature circuit, where the current must pass between brush and commutator and where the serious effects of poor commutation are likely to result. The value in ohms is chosen to limit the armature current for about 1.25 to twice the rated amperes at the instant the machine is started. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Starting Direct Current Motors As the motor accelerates, the armature generates an opposing emf called a counter electromotive force (counter emf) which causes the current to fall; this counter emf has the same effect in lowering the armature current as would an impressed voltage across the armature terminals. The counter emf will not exceed for about 80% to 95% of the impressed voltage. Counter emf ≠ impressed voltage Use automatic starters with relay contact to cut out the limiting resistance as the motor speed up Small machines up to ¾ HP, starters are not employed because the starting current is generally low. ELECTRICAL MACHINE 1 Electrical Machinery Generalization Commutating Poles for Direct Current Machines When current passes between brushes and commutator in dc machine, generator or motor, the ends of those coils joined the commutator segments that are bridge by the brushes are short circuited for an instant. = extremely low resistance = generated voltage is zero. If previous condition above in which the generated voltage ≠ 0, will cause excessive arcing and commutator burning which lead to motor failure. To meet the requirement Isc =0, there is a correct brush position. In order to do this, the use of special, narrow poles located between large main poles suited for variable loads called interpoles. ( see figure 7 of previous slide) ELECTRICAL MACHINE 1 Electrical Machinery Generalization References Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New York: McGraw-Hill. Norton, R. (2011). Machine Design: an integrated approach. (4th ed.). Boston: Pearson. Norton, R. (2012). Design of Machinery: an introduction to the synthesis and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill. Sharma, C.S, & Purohit, K. (2012). Design of machine elements. New Delhi: PHI Learning Private Limited. Gope, P.C. (2012). Machine design: fundamentals and applications. New Delhi: PHI Learning Private Limited. ELECTRICAL MACHINE 1 Chapter 2 Direct Current Motor Principles ELECTRICAL MACHINE 1 Direct Current Motor Principles Principle of Motor Action When an electric motor is in operation, it develops torque, which in turn can produce mechanical rotation. The principle of motor requires: (1) The presence of magnetic lines of force. (2) Current through conductors lying in the magnetic field. (3) Force, and therefore torque is produced. ELECTRICAL MACHINE 1 Direct Current Motor Principles Principle of Motor Action Illustrating how motor action is produced by the interaction of the magnetic fields created by the main poles and the current- carrying conductors ELECTRICAL MACHINE 1 Direct Current Motor Principles Principle of Motor Action Illustrating how motor action is produced by the interaction of the magnetic fields created by the main poles and the current-carrying conductors ELECTRICAL MACHINE 1 Direct Current Motor Principles Principle of Motor Action To change the direction of rotation of a dc motor can be done by either: (1) The direction of current flow through the conductors. (2) Reversing the polarity of the field. A motor will not reverse its direction of rotation if both the field polarity and the direction of the current flow through the armature changed. Interchanging the two line wires connected to a dc motor effects both these changes at once, and does not cause the motor to reverse. ELECTRICAL MACHINE 1 Direct Current Motor Principles Force and Torque Developed by Direct-Current Motors The force action exerted by a current-carrying conductor placed in a magnetic field depends on the following: (1) The strength of the main field (2) The value of the current through the conductor. Resultant Non Uniform Magnetic Field = main field +flux set up by current carrying conductor. A force of one dyne will be exerted upon a conductor 1 cm long carrying a current of 10 amp when placed under a pole the area of which is 1 sq cm and producing one line of force( flux density = one line per sq. centimeter) ELECTRICAL MACHINE 1 Direct Current Motor Principles Force and Torque Developed by Direct-Current Motors ′ 𝐹 = 𝐵′ 𝑥 𝐼 𝑥 𝑙′ 10 dynes where: B’ = flux density, lines per sq. centimeter I = current in conductor, amp l’ = length of conductor, cm If the units F, B, and l are specified in more practical terms, that is pounds, lines per square inch, and inches* respectively becomes 𝐹= 𝐵 6.45 𝑥 𝐼 (𝑙𝑥2.54) 10 𝑥 980 𝑥453.6 = 𝐵𝑥𝐼𝑥𝑙 11,300,000 lb ELECTRICAL MACHINE 1 Direct Current Motor Principles Significance of Back- EMF • In accordance with the laws of electromagnetic induction, e.m.f. is induced in them whose direction, as found by Fleming’s Right- hand Rule, is in opposition to the applied voltage. Consider a Shunt Wound DC Motor ELECTRICAL MACHINE 1 Direct Current Motor Principles Significance of Back- EMF Back EMF depends among other factors upon the armature speed. If the speed is high, back emf (Eb) is high. Hence, the armature current (Ia) is small. If the speed is less, Eb (back emf) is less. Hence, more current (Ia) flows which develops motor torque. 𝑇 = 𝑘 𝑥 Ø 𝑥 𝐼𝐴 lb-ft where: T = torque develop, lb-ft Ø = magnetic flux, maxwells Ia = armature current ELECTRICAL MACHINE 1 Direct Current Motor Principles General Voltage Equation for Direct Current Motors and Generators Factors to consider: (1) number of poles (2) number of armature conductors (3) number of parallel paths (4) armature speed (5) flux cuts for each conductor ------------------------------------------= Generated Voltage and Back EMF 𝐸= Ø 𝑥 𝑃 𝑥 𝑟𝑝𝑚 𝑥 𝑍 𝑎 𝑥 60 𝑥 10−8 , volts ELECTRICAL MACHINE 1 Direct Current Motor Principles General Voltage Equation for Direct Current Motors and Generators where: Eg or Eb = total generated voltage or back emf Ø = flux per pole, maxwells P = number of poles, an even number rpm = speed of armature, revolutions per minute Z = total number of armature conductors effectively used to add to resulting voltage a = number of armature paths connected in parallel (determined by type of armature windings) Note: 1 weber = 10−8 maxwell ELECTRICAL MACHINE 1 Direct Current Motor Principles General Voltage Equation for Direct Current Motors and Generators Cont…. For a simplex wave-wound No. of parallel paths = 2 No. of conductors (in series) in one path = Z/2 For a simplex lap-wound No. of parallel paths = P No. of conductors (in series) in one path = Z/P ELECTRICAL MACHINE 1 Direct Current Motor Principles ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) Problem 1: A conductor is 8 in long and carries a current of 140 amp when placed perpendicularly to a magnetic field the intensity of which is 58,000 lines per square inch. Calculate the force exerted by the conductor. Problem 2: The armature of a dc motor has 648 conductors 65 percent of which are directly under the poles where the flux density is 48,000 lines per square inch. If the core diameter is 7 in and its length 4 in and the current in each conductor is 20 amp. Calculate (a) the total force tending to rotate the armature (b) the torque exerted by the armature in pound feet. Problem 3: A dc motor has an armature containing 192 conductors, 70 percent of which lie directly under the pole faces at any given instant. If the flux density under the poles is 52,000 lines per square inch and the armature diameter and length are 12 in. and 4.5 in respectively, calculate the current in each armature conductor for a torque of 120 lb-ft. ELECTRICAL MACHINE 1 Direct Current Motor Principles ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) Problem 4: A four pole machine generates 250 volts when operated at 1500 rpm. If the flux per pole 1.85 x 10^6 maxwells, the number of armature slots is 45, and the armature winding is simplex wound. Calculate: (a) the total number of armature conductors (b) the number of conductors in each slot. ELECTRICAL MACHINE 1 Direct Current Motor Principles Commutation of Direct Current Motors Sketch showing the directions of the currents in the conductors of four pole motor for clockwise rotation. Note the dividing axes midway between adjacent north and south poles. ELECTRICAL MACHINE 1 Direct Current Motor Principles Commutation of Direct Current Motors Commutator and brushes act as an inverter, that is to change direct current to alternating current, because the current in the armature conductors must be alternating if rotation in the same direction is to continue. Note: 1. In the dc generator the commutator and brushes function to change internally generated alternating current to a load applied direct current. 2. In the dc motor, the commutator and brushes perform an inverse function by changing the externally applied direct current to alternating current flowing in the armature conductor. ELECTRICAL MACHINE 1 Direct Current Motor Principles References Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New York: McGraw-Hill. Norton, R. (2011). Machine Design: an integrated approach. (4th ed.). Boston: Pearson. Norton, R. (2012). Design of Machinery: an introduction to the synthesis and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill. Sharma, C.S, & Purohit, K. (2012). Design of machine elements. New Delhi: PHI Learning Private Limited. Gope, P.C. (2012). Machine design: fundamentals and applications. New Delhi: PHI Learning Private Limited. ELECTRICAL MACHINE 1 Chapter 3 Direct Current Generator Principles ELECTRICAL MACHINE 1 Direct Current Generator Principles Principle of Generator Action If it is assumed that the magnetic lines of force leave a cylindrically shaped pole core, faces and and pass across the air space (called the air gap) and hence into the rotating armature, it is clear that the moving conductors cut the lines of force as they are rotated mechanically. = generated voltage. Principle of Generator requires: (1) The presence of magnetic lines of force. (2) Motion of conductor cutting the flux. (3) Voltage is generated. ELECTRICAL MACHINE 1 Direct Current Generator Principles Principle of Generator Action From Faraday’s Law, The magnitude of the generated voltage is directly proportional to the rate at which a conductor cuts magnetic lines of force. It implies that higher voltages may be generated by moving conductors more rapidly across lines of flux, by increasing the number of flux lines across which the conductors move, or by increasing both the speed of the conductors and the flux across which they move. 1 volt is generated for every 100,000,000 (10^8) lines cut per second. ELECTRICAL MACHINE 1 Direct Current Generator Principles Principle of Generator Action ELECTRICAL MACHINE 1 Direct Current Generator Principles Principle of Generator Action 𝐸𝑎𝑣𝑒. = Ø 𝑡 𝑥 108 , volts where: Eave. = average generated voltage in a conductor. Ø = total flux cut t = time, seconds, during which the cutting takes place Note : 1 weber = 10−8 maxwell The number of parallel paths, determines the current rating of the generator, whereas the number of series conductors per path is a measure of the terminal voltage of the machine. ELECTRICAL MACHINE 1 Direct Current Generator Principles Principle of Generator Action Example: 120 cells,1.5 volts each with 5 amps /path No. of parallel paths E, volts I, amps P, watts 2 90 10 900 4 45 20 900 6 30 30 900 8 22.5 40 900 10 18 50 900 12 15 60 900 Therefore: The power rating is independent of the manner in which the conductors are connected. ELECTRICAL MACHINE 1 Direct Current Generator Principles ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) Problem No.1: A four-pole d-c generator has an armature winding containing a total of 648 conductors connected in two parallel paths. If the flux per pole is 0.321 𝑥 106 maxwells and the speed of rotation of the armature is 1,800 rpm, calculate the average generated voltage. Problem No.2: In the first problem, calculate the rated current in each conductor per path if the power delivered by the armature is 5kw. Problem No.3: A magnetic coil produces 100,000 maxwells with 2,000 turns and with a current of 2 amperes. The current is cut-off and flux collapses in 0.01 sec. What is the average voltage that will appear across the coil? ELECTRICAL MACHINE 1 Direct Current Generator Principles General Voltage Equation for Direct Current Generator Factors to consider: (1) number of poles (2) number of armature conductors (3) number of parallel paths (4) armature speed (5) flux cuts for each conductor ------------------------------------------= Generated Voltage 𝐸𝑔 = Ø 𝑥 𝑃 𝑥 𝑟𝑝𝑚 𝑥 𝑍 𝑎 𝑥 60 𝑥 10−8 , volts ELECTRICAL MACHINE 1 Direct Current Generator Principles General Voltage Equation for Direct Current Generator where: Eg = total generated voltage Ø = flux per pole, maxwells P = number of poles, an even number rpm = speed of armature, revolutions per minute Z = total number of armature conductors effectively used to add to resulting voltage a = number of armature paths connected in parallel (determined by type of armature windings) For a simplex wave-wound generator No. of parallel paths = 2 No. of conductors (in series) in one path = Z/2 For a simplex lap-wound generator No. of parallel paths = P No. of conductors (in series) in one path = Z/P ELECTRICAL MACHINE 1 Direct Current Generator Principles ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) Problem No.1: An 85 kw six-pole generator has an armature containing 66 slots, in each of which of 12 conductors. The armature winding is connected so that there are six parallel paths. If each pole produces 2.18 𝑥 106 maxwells and the armature speed is 870 rpm, determine the generated voltage. Problem No.2: How many armature conductors are there in a generator given the following information: Ø = 2.73 𝑥 106 maxwells; P= 4; rpm = 1,200; a = 2; Eg = 240 V? Problem No.3: A four-pole generator, having wave-wound armature winding has 51 slots, each slot containing 20 conductors. What will be the voltage generated in the machine when driven at 1500 rpm assuming the flux per pole to be 7.0 mWb ? ELECTRICAL MACHINE 1 Direct Current Generator Principles Direction of a Generated Voltage The direction of the generated voltage in a conductor as it is rotated to cut the lines of force produced by the electromagnets in a generator, will depend upon two factors: (1) direction of the flux, which is determined by the magnet polarity. (2) the direction of motion of the conductor or coil. Lenz’s law states, the direction of the generated voltage in the coil is such that it tends to produce a current flow opposing a change of flux through the coil. ELECTRICAL MACHINE 1 Direct Current Generator Principles Direction of a Generated Voltage MAGNETIC FIELD A B Q A A P B LOAD 0o MAGNETIC FIELD A B A + Q P B _ e LOAD 30o t MAGNETIC FIELD A B + Q A B P _ e LOAD 60o t MAGNETIC FIELD A B + Q A B P _ e LOAD 90o t MAGNETIC FIELD A + Q BA A B P _ e LOAD 120o t MAGNETIC FIELD B A + BA Q A P _ e LOAD 150o t MAGNETIC FIELD B A + Q B A P A e LOAD 180o t MAGNETIC FIELD B B + A Q P A _ e LOAD 210o t MAGNETIC FIELD B A + Q B A P _ e LOAD 240o t MAGNETIC FIELD B A + Q B A P _ e LOAD 270o t MAGNETIC FIELD B + Q AA B A P _ e LOAD 300o t MAGNETIC FIELD B + Q AA B A P _ e LOAD 300o t MAGNETIC FIELD A B + A A Q B P _ e LOAD 330o t MAGNETIC FIELD A B Q A A P B e LOAD 360o t ELECTRICAL MACHINE 1 Direct Current Generator Principles Direction of a Generated Voltage Thus, it is seen that for a clockwise rotation the side of the coil under north pole will always have a voltage direction away from the observer, while the side of the coil under south pole will always have a voltage direction toward the observer. ELECTRICAL MACHINE 1 Direct Current Generator Principles The Elementary Alternating-current Generator Elementary two-pole ac generator ELECTRICAL MACHINE 1 Direct Current Generator Principles The Elementary Alternating-current Generator In general terms, the frequency of the alternating current in cycles per second is P/2 x revolutions per second. 𝑓= 𝑃 2 𝑋 𝑟𝑝𝑚 60 = 𝑃𝑥 𝑟𝑝𝑚 120 where: f = frequency P= number of poles rpm= speed in revolutions per minute , cps ELECTRICAL MACHINE 1 Direct Current Generator Principles The Elementary Alternating-current Generator Illustrating the relations between the number of poles and the generated ac frequency in cycles per revolution. ELECTRICAL MACHINE 1 Direct Current Generator Principles ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) Problem 1: An ac generator has six poles and operates at 1,200 rpm. (a) What frequency does it generate? (b) At what speed must the generator operate to develop 25 cycles? 50 cycles? Problem 2: How many poles are there in a generator that operates at a speed of 240 rpm and develops a frequency of 60 cycles? ELECTRICAL MACHINE 1 Direct Current Generator Principles ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) Problem No. 3: Thirty six dry cells are connected in four parallel groups of nine cells in series per group. If the voltage and current rating of each cell is 1.45 volts and 4 amp respectively, (a) calculate the voltage, current and power rating of the entire combination (b) Recalculate the problem for nine parallel groups of four cells in series per group. Problem No.4: A six dc pole generator has an armature winding with 504 conductors connected in six parallel paths. Calculate the generated voltage in this machine if each pole produces 1.65 𝑥 106 maxwells and the armature speed is 1,800 rpm. ELECTRICAL MACHINE 1 Direct Current Generator Principles ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) Problem No. 5: Calculate the voltage generated by a four pole dc machine given the following particulars: number of slots in the armature=55; number of conductors per slot=4; flux per pole= 2.62𝑥 106 maxwells; speed = 1,200 rpm, number of parallel paths in armature = 2. Problem No. 6: A four pole machine generates 250 volts when operated at 1,500 rpm. If the flux per pole is 1.85 𝑥 106 maxwells, the number of armature slots is 45, and the armature winding has two parallel paths, calculate (a) the total number of armature conductors (b) the number of conductor in each slot. ELECTRICAL MACHINE 1 Direct Current Generator Principles ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) Problem No. 7 : The speed of the generator in Problem No. 4 is decreased to 1,350 rpm. (a) What will be the generated voltage if the flux per pole is maintained at the same value, i.e. 1.85 𝑥 106 maxwells? (b) To what value of flux per pole should the excitation be adjusted if the generated voltage is to remain the same, i.e. 250 volts? ELECTRICAL MACHINE 1 Direct Current Generator Principles The Commutation Process of DC Generator The elementary two pole dc generator with two segment commutator. ELECTRICAL MACHINE 1 Direct Current Generator Principles The Commutation Process of DC Generator • As an emf’s in the conductors mn and pq reverse direction, the semi rings, to which they are connected, automatically change places under stationary brushes. It follows therefore, that the polarity of the brushes does not change. • The magnitude of the current change, but there will be no reversal of current through the load. • The process performed by rectifying the alternating current – changing the internal alternating current to an external direct current – is called commutation. ELECTRICAL MACHINE 1 Direct Current Generator Principles The Commutation Process of DC Generator Illustrating how the rectified current pulsates in two, four and six pole generators. ELECTRICAL MACHINE 1 Direct Current Generator Principles The Commutation Process of DC Generator Illustrating the combined additive effects of two or three coils. It shows how the voltage is increased and how the waveform tends to become smoother. ELECTRICAL MACHINE 1 Direct Current Generator Principles References Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New York: McGraw-Hill. Norton, R. (2011). Machine Design: an integrated approach. (4th ed.). Boston: Pearson. Norton, R. (2012). Design of Machinery: an introduction to the synthesis and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill. Sharma, C.S, & Purohit, K. (2012). Design of machine elements. New Delhi: PHI Learning Private Limited. Gope, P.C. (2012). Machine design: fundamentals and applications. New Delhi: PHI Learning Private Limited. ELECTRICAL MACHINE 1 Chapter 4 Direct Current Motor Characteristics ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Operating Differences between Motors and Generators GENERATORS MOTORS 1. When in operation, it is driven by mechanical machine such as engine, water turbine or even electric motors producing a current in an electric circuit. 1. When in operation, it is “fed” by an electric current from an electrical source of supply develop a torque produces mechanical rotation. 2. The load on the generator constitutes those electrical devices that convert electrical energy into other forms of energy. 2. The load on the motor constitutes a force that tends to oppose rotation and is called a counter torque. 3. The voltage of a generator tends to change when the load changes. 3. The speed of rotation tends to change as the load varies. 4. The voltage of the generator can always be adjusted by doing either or both of two things: a) changing the speed b) changing the strength of the magnetic field. 4. The speed of rotation can be changed by varying either or both of two things: a) the strength of the magnetic field b) the voltage impressed across the armature terminals. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Operating Differences between Motors and Generators GENERATORS MOTORS 5. The speed of rotation is usually quite constant, 5. The impressed emf across the motor terminals since the speed of the mechanical prime mover is substantially constant, except in the case of is generally fixed by the governor controls. special motors in which the power supply constitutes a separate source. 6. Frequently operated in parallel with others to supply power to a common load. 6. Usually operate as single independent units to drive their individual loads although they may be connected in parallel or in series for the purpose of particular jobs at varying speeds. 7. Are started without electrical loads; the procedure is to bring them up to speed, adjust the voltage and then close the main switch that permits the machine to deliver current. 7. May not have a mechanical load when they are started. It is quite customary for a motor to start a load that is often equal to or greater than the rated name plate value. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Classification of Direct Current Motors There are three general types of motors: series, shunt and compound. For the purposes of classification, it is convenient therefore to indicate how a motor behaves between no load and full load by using such a term as constant speed and variable speed. In general, if a change from no mechanical load to full load causes the speed to drop approximately 8 percent or less, the motor is said to be of the constant speed classification. (shunt) ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Classification of Direct Current Motors Motors in which the speed changes by greater values than indicated as falling into variable speed classification. (series and compound) Whenever the speed of a motor can be controlled by an operator who makes a manual adjustment/automatic control equipment, it is said to be adjustable-speed type. The speed changes inherently as a result of a modification of the loading conditions and referred to as a variable speed type. Constant speed-adjustable speed – shunt motor with field rheostat control. Variable speed–adjustable speed – series motor with a field rheostat. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Counter Electromotive Force (Counter EMF) Voltage Generated by a Motor With the armature rotating as a result of motor action, the armature conductors continually cut through the resultant stationary magnetic field, and because of such flux cutting voltages are generated that experience force action. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Counter Electromotive Force (Counter EMF) Voltage Generated by a Motor The generated voltages are indicated by crosses and dots below the circles and are in directions opposite to the flow of current. This is called counter emf or back emf. This counter emf can never be equal to, and must always be less than, the voltage impressed across the armature terminals. This can mean only that the armature current is controlled and limited by the counter emf. By Ohms Law, ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Counter Electromotive Force (Counter EMF) Voltage Generated by a Motor Formula: 𝐼𝐴 = 𝑉𝐴 −𝐸𝐶 𝑅𝐴 , amp where: 𝐼𝐴 = armature current 𝑉𝐴 = impressed voltage across the armature winding 𝐸𝐶 = counter emf generated in armature 𝑅𝐴 = resistance of armature Counter emf depends upon two factors: (1) flux per pole Ø (2) the speed of rotation S in revolutions per minute. if, Ec = kØS ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Counter Electromotive Force (Counter EMF) Voltage Generated by a Motor where k = is a proportionality constant that depends upon the number of armature conductors, the type of armature winding and the number of poles becomes, 𝐼𝐴 = 𝑉𝐴 −𝑘Ø𝑆 𝑅𝐴 Ec is directly proportional to Ø and S. amp ELECTRICAL MACHINE 1 Direct Current Motor Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM No.1 : A 115 volt shunt motor has an armature whose resistance is 0.22 ohm. Assuming a voltage drop across the brush contacts of 2 volts, what armature current will flow (a) when the counter emf is 108 volts? (b) if the motor is increased so that the counter emf drops to 106 volts? PROBLEM No.2 : A compound motor operates at a speed of 1,520 rpm when the voltage impressed across the armature terminals is 230. If the flux per pole is 620,000 maxwells and the armature resistance is 0.43 ohm, calculate: (a) the counter emf and the (b) armature current (Assume a value of k = 2.2 x 10^-7 and a brush drop of 2 volts) ELECTRICAL MACHINE 1 Direct Current Motor Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM No. 3: If the load on the motor in problem No. 2 is increased so that the armature current rises to 64 amp. What will be the speed of the motor, assuming that the flux increases by 6 percent? Note: The counter emf developed in the armature of a motor is usually between 80 and 95 percent of the voltage impressed across the armature terminals: • Ec is high percentage of the Va, will operate most efficiently. • Ec is small percentage compared with Va, will have low efficiency. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) • A motor will develop the greatest power when the counter emf Ec is a maximum. PROBLEM No. 4: The armature of the 230-volt motor has a resistance of 0.312 ohm and takes 48 amp when operating at a certain load. (a) Calculate the counter emf and the power developed by the armature. (b) If the armature resistance had been 0.417 ohm, the other conditions remaining the same, what would have been the values of Ec and the power developed in the armature? (Assume a brush of 2 volts in both cases) ELECTRICAL MACHINE 1 Direct Current Motor Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) Note: In starting a dc motor, if Ec is zero or very small, as the motor is coming up to speed, a resistance must be inserted to take place of Ec, as the speed increases, resistance may be cut out gradually because Ec rises; finally when the motor has attained normal speed, all resistance can be cut out of the armature circuit. PROBLEM No. 5: The armature of a 230 volt shunt motor has a resistance of 0.18 ohm. If the armature current is not exceed 76 amp, calculate: (a) the resistance that must be inserted in series with the armature at the instant of starting; (b) the value to which this resistance can be reduced when the armature accelerates until Ec is 168 volts; (c) the armature current at the instant of starting if no resistance is inserted in the armature circuit. (Assume a 2-volt drop at the brushes) ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Starting Resistors Connection Connection of Starting Resistor ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Starting Resistors Connection In the case of very small motors, usually up to about ¾ hp, no starting resistor is necessary. There are two reasons for this: (1) The resistance and the inductance of the armature winding are generally sufficiently high to limit the initial rush of current to values that are not particularly serious. (2) The inertia of a small armature is generally so low that it comes up to speed very quickly, thereby minimizing the serious effect that might otherwise result from a high sustained current. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Starters for Shunt and Compound Motors Their primary function is to limit the current in the armature circuit during the starting or accelerating period. There are two standard types of motor starter for shunt and compound motor: (a) three point type (b) four point type ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Starters for Shunt and Compound Motors Three point starters are not completely satisfactory when used with motors whose speeds must be controlled by inserting resistance in the shunt field. Why? ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Starters for Shunt and Compound Motors Schematic Wiring Diagram (Three-point starter connected to shunt motor) • If sufficient resistance is cut in by the field rheostat, so that the holding coil current is no longer able to create sufficient electromagnetic pull to overcome the spring tension the starter arm will fall back to the OFF position. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Starters for Shunt and Compound Motors (Four point starter connected to compound motor) If this starter is compared with the three point type, it will be observed that one important change has been made: the holding coil has been removed from the shunt field circuit and in series with a current protecting resistor r, has been placed in a separate circuit in parallel with armature and field. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Starters for Shunt and Compound Motors Schematic Wiring Diagram (Four point starter connected to compound motor) • This starter are divided into three parts: (1) The main circuit is through the starting resistor r, the series field and the armature. (2) The second circuit is through the shunt field and its field rheostat. (3) The third circuit is through the holding coil and the current protecting resistor r. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Controllers for Shunt and Compound Motor Schematic Wiring Diagram (Controllers for Shunt and Compound Motor) To eliminate providing a separate mounting for field rheostat, controllers are available that incorporate both starting resistors and field rheostats in single, panel mounted units and make it impossible to start shunt or compounding motors with any resistance cut into the field rheostat. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Automatic Starter for Shunt and Compound Motor The counter emf type of starter , a number of relays are connected across the armature where the counter emf increases as the motor accelerates, and the former are adjusted to pick up the predetermined values of voltage. The time limit starter, a group of relays are timed to operate at preset intervals of time, by means of devices that function mechanically, pneumatically or electrically. The current limit starter, the relays are designed so that they are sensitive to current changes in the armature circuit. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Automatic Starter for Shunt and Compound Motor Wiring Diagram of a Counter EMF Starter connected to shunt motor ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Automatic Starter for Shunt and Compound Motor Wiring diagram of a time limit acceleration starter connected to a compound motor. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Automatic Starter for Shunt and Compound Motor Wiring diagram of a current limit acceleration starter connected to a compound starter. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Loading a Motor-Effect upon Speed and Armature Current If the mechanical load on the motor changes, either the torque or the speed, or both, must change. The control of the speed of the motor is generally exercised through the medium of flux adjustment or control. When a load is applied to a motor, the natural tendency is to slow down because of the opposition to motion called counter torque. The reduction 𝑉 −𝐸 of speed results in the increase of the armature current, since I = 𝐴 𝐶 . 𝑅𝐴 This means that any increase in mechanical driving power must be met by a corresponding increase in electrical power input to the armature. Loading a motor always results in two changes: (1) reduction in speed ; (2) an increase in armature current. Vice-versa will occur when the motor is unloaded. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Loading a Motor-Effect upon Speed and Armature Current For shunt motor, when the mechanical loading completely removed, it will operate at a speed only slightly higher than the normal speed, between 2% and 8% higher than the normal speed. For compound motor, when the mechanical loading completely removed, will result in a rise in speed of about 10% to 25% . For series motor, when the mechanical loading completely removed, does attempt to race, or operate at a very high speed. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Torque Characteristics of Direct-Current Motors The torque developed by a motor, the tendency of the motor to produce rotation depends on two factors: (1) The flux created by the main poles. (2) The current flowing in the armature winding. The torque is independent of the speed of rotation. 𝑇 = 𝑘 𝑥 Ø 𝑥 𝐼𝐴 lb-ft where: T = torque (usually in lb-ft) k = proportionality constant Ø = flux per pole in maxwells Ia = total armature current = 0.1173 108 x 𝑃𝑥 𝑍 𝑎 ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Torque Characteristics of Direct-Current Motors hp 2𝜋 𝑥 𝑟𝑝𝑚 𝑥 𝑇 = 33,000 where: rpm = speed of the motor T = torque in lb-ft Note: just multiply the formula above by 746 watts when converted to watts unit. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Torque Characteristics of Direct-Current Motors • The developed torque of a motor varies with the armature current Ia of the motor: (1) The torque of a shunt motor depends only upon the armature current; assuming that the shunt field current is not changed by field rheostat adjustment, the torque is independent of the flux. Therefore, a graph indicating the relation between torque and load should be straight line (T= k1Ia). (2) The torque developed by a series motor depends upon the armature current and the flux that this current produces in passing through the series field. T= k2Ia^2 when the motor series field is not saturated. (3) The torque of compound motor combines the torque load characteristic of shunt and series motors. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Torque Characteristics of Direct-Current Motors Characteristic Torque vs. armature current curves for three types of dc motor. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO. 1 – (a) Calculate the torque in pound feet developed by a dc motor, given the following particulars: poles = 4; total number of armature conductors = 828; flux per pole = 1.93 x 10^5 maxwells; total armature current = 40 amp; winding = wave. (b) What will be the horsepower of the motor when operating at a speed of 1,750 rpm? PROBLEM NO. 2 – A four pole shunt motor develops 20 lb-ft of torque when the flux per pole is 700,000 maxwells. If the armature winding has 264 conductors and is wound wave, calculate the total armature current. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO. 3- To change the shunt motor of example no. 2 into a compound motor, the main poles are wound with a series field. What torque will this motor develop when the armature carries 65 amp if the series field increases the flux by 22 percent ? PROBLEM NO. 4 – A series motor develops 62 lb-ft of torque when the current is 48 amp. Assuming the flux varies directly with the current, calculate the torque if the load increases so that the motor takes 56 amp. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO. 5 – The speed of the 50 HP (37.3 kW) series motor working on 500V supply is 750 rpm at full-load and 90% efficiency . If the load torque is made 350 N-m and a 5 ohm resistance is connected in series with the machine, calculate the speed at which the machine will run. Assume the magnetic circuit to be unsaturated and the armature and field resistance to be 0.5 ohm. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Torque Characteristics of Direct-Current Motors The torque developed by a motor at the instant of starting is called starting torque. Among the three types of motors, in terms of percentage: (1) Series motor have approximately 500% starting torque (2) Compound motor have approximately 250% starting torque (3) Shunt motor have approximately 125% starting torque ELECTRICAL MACHINE 1 Direct Current Motor Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) • From previous lecture, s= Va −Ia Ra , 𝑟𝑝𝑚 kØ PROBLEM NO.1 – The armature of a 230 volt shunt motor has a resistance of 0.30 ohm and takes 50 amp when driving its rated load at 1500 rpm. At what speed will the motor operate if the load is completely removed, when it is running idle, a condition under which the armature current drops to 5 amp? Assume that the flux remains constant and that the brush drops at full load and no load are 2 volts and 1 volt, respectively. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO. 2 – A 220 volt long shunt compound motor has an armature resistance of 0.27 ohm and a series field resistance of 0.05 ohm. The full load speed is 1,400 rpm when the armature current is 75 amp. At what speed will the motor operate at no load if the armature current drops to 5 amp and the flux is reduced to 90 percent of its full load value? Assume brush drops to be the same as in problem no. 1. PROBLEM NO. 3 – A 25 hp 240 volt series motor takes 93 amp when driving its rated load at 800 rpm. The armature resistance is 0.12 ohm, and the series-field resistance is 0.08 ohm. At what speed will the motor operate if the load is partially removed so that the motor takes 31 amp.? Assume that the flux is reduced by 50 percent for a current drop of 66 2/3 percent and that the brush drop is 2 volts at both loads. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Speed Characteristics of Direct-Current Motors Characteristic speed vs. horsepower output curves for three types of dc motor: ELECTRICAL MACHINE 1 Direct Current Motor Characteristics Speed Regulation of Direct-Current Motors The change is speed of a shunt or compound motor between full load and no load is referred to as regulation. When this regulation is referred to the full load or rated speed of the motor expressed in percent , it is called a percent speed regulation. 𝑆𝑁𝐿 − 𝑆𝐹𝐿 % 𝑠𝑝𝑒𝑒𝑑 𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = X 100% 𝑆𝐹𝐿 ELECTRICAL MACHINE 1 Direct Current Motor Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) Problem 1: The no-load speed of the compound motor is 2,200 rpm. At what speed will it operate at full-load if the percent speed regulation is 18%? Problem 2: A 550 V long shunt compound motor has an armature resistance of 0.815 ohms and a series field resistance of 0.15 ohm. The full-load speed is 1,900 rpm when the armature current is 22 amp. (a) At what speed will the motor operate at no-load if the armature current drops to 3 amperes with the corresponding drop in flux to 88 percent of the full-load value? (Assume a brush drop of 5 volts at full-load and 2 volts at no-load. (b) Calculate the percent speed regulation of the motor. ELECTRICAL MACHINE 1 Direct Current Motor Characteristics References Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New York: McGraw-Hill. Norton, R. (2011). Machine Design: an integrated approach. (4th ed.). Boston: Pearson. Norton, R. (2012). Design of Machinery: an introduction to the synthesis and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill. Sharma, C.S, & Purohit, K. (2012). Design of machine elements. New Delhi: PHI Learning Private Limited. Gope, P.C. (2012). Machine design: fundamentals and applications. New Delhi: PHI Learning Private Limited. ELECTRICAL MACHINE 1 Chapter 5 Direct Current Generators Characteristics ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Types of Direct Current Generators • There are three general types of self-excited dc generators: (1) Shunt Generator – if the excitation is produced by a field winding that is connected to full, or nearly full line voltage. (2) Series Generator – if the excitation originates in the field winding connected in series with the armature, so that the flux depends upon the current delivered to the load. (3) Compound Generator – if the excitation is produced by two field windings, one connected to the full, or nearly full, line and the other excited by the line or armature current, a comparatively large current. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Types of Direct Current Generators Series field Shunt field ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Types of Direct Current Generators DC Generators are divided into: (1) Self-Excited – the shunt field of a shunt or compound generator may be excited by current supplied to its own armature. (2) Separately-Excited – the shunt field of a shunt or compound generator may be excited by or connected to an outside, i.e. separate source of supply. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics No Load Characteristics of Generators • When a shunt or compound generator operates without load – that is, when it is driven by a prime mover, is properly excited, and has none of the load switches closed - a voltage will appear at the terminals that are normally connected to the electrical devices, provided two factors are being considered: (1) the speed of rotation and (2) the flux. Separately excited shunt generator connections to determine experimentally the no-load characteristics ELECTRICAL MACHINE 1 Direct Current Generators Characteristics No Load Characteristics of Generators The graph shown below is the direct proportionality relationship between the no-load generated voltage Eg and the speed RPM with constant excitation. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics No Load Characteristics of Generators To determine the relationship of the no-load generated voltage Eg and field current, it is important to note that this relationship is not a direct one for all changes in excitation, because magnetic saturation sets in after the field is increased beyond a certain value. E1 Eh E E2 El Saturation Curve or Magnetization Curve ELECTRICAL MACHINE 1 Direct Current Generators Characteristics No Load Characteristics of Generators Thus if E = 100 volts at 1,500 rpm, Eh will be 110 volts at 1,650 rpm [(1,650/1500) x 100= 110] and El will be 90 volts at 1,350 rpm [(1,350/1500) x 100= 90] If saturation curve is determined by experiment by one speed, it is quite possible to calculate values for another such curve at any other speed by the simple method of proportionality. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Building Up the Voltage of a Self-excited Shunt Generator Self excited shunt generator ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Building Up the Voltage of a Self-excited Shunt Generator • Four Requirements for Build Up: (1) The first requirement for the build up process is a small voltage resulting from residual magnetism. A generator will not build up if the residual flux is insufficient; in 110 and 220 volts generator, this flux should be such a magnitude that about 4 to 10 volts is developed. Flashing the field is the solution for the new or one that has lost its residual flux because of a long period of idleness must be separately excited to create necessary magnetism. (2) A second requirement for build up is a field circuit resistance that is less than the so-called “critical value” for the speed used in operating the generator. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Building Up the Voltage of a Self-excited Shunt Generator (3) A third requirement is that the speed be high enough for the shunt field resistance used. It should be stated that a generator is usually operated at some definite speed originally fixed by the manufacturer. (4) Finally, there must be a proper relation between the direction of rotation and the field connection armatures. If a generator will not build up when operated clockwise, for example, it will build up when the direction of rotation is counterclockwise, assuming other conditions to be satisfied. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Brush Contact Drop It is the voltage drop over the brush contact resistance when current passes from commutator segments to brushes and finally to the external load. Its value depends on the amount of current and the value of contact resistance. This drop is usually small and includes brushes of both polarities. However, in practice, the brush contact drop is assumed to have following constant values for all loads. 0.5 V for metal-graphite brushes. 2.0 V for carbon brushes. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) Problem 1: A short-shunt compound generator delivers a load current of 30 A at 220 V, and has armature, series-field and shunt-field resistances of 0.05 Ω , 0.30 Ω and 200 Ω respectively. Calculate the induced e.m.f. and the armature current. Allow 1.0 V per brush for contact drop.. Problem 2: A long-shunt compound generator delivers a load current of 50 A at 500 V and has armature, series field and shunt field resistances of 0.05 Ω , 0.03 Ω and 250 Ω respectively. Calculate the generated voltage and the armature current. Allow 1 V per brush for contact drop. Problem 3:In a long-shunt compound generator, the terminal voltage is 230 V when generator delivers 150 A. Determine (i) induced e.m.f. (ii) total power generated and (iii) distribution of this power. Given that shunt field, series field, diverter and armature resistance are 92 Ω , 0.015Ω , 0.03 Ω and 0.032 Ω respectively. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Behavior of a Shunt Generator Under Load One of the most important characteristics of any generator is its behavior with regard to the terminal voltage when the load current is increased. For shunt type generator, the voltage always falls as more current is delivered to the load. There are three reasons for this: ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Behavior of a Shunt Generator Under Load (1) As more current is delivered by the armature, the voltage drop in the armature Ia Ra increases, thus making a lower emf available at the load terminals. (2) When the armature terminal voltage falls, the field winding suffers a corresponding reduction in current, which in turn, reduces the flux; the latter further reduces the generated emf. (3) When the armature winding carries increasing values of load current, the armature core becomes an electromagnet, apart from the effect of the main poles; this electromagnet action of the armature core reacts with the main field flux further to reduce the flux, the result being that the generated emf suffers an additional drop. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Behavior of a Shunt Generator Under Load It should be clearly understood that the generated voltage, which depends upon the flux(other factors remaining unchanged), is always greater than the terminal voltage or load voltage by exactly the amount of the voltage drop in the armature circuit. This is why it is important to keep the armature circuit resistance as low as possible. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Behavior of a Shunt Generator Under Load A convenient standard of reference used to measure the performance of a generator is referring the change in voltage between full load and no load (VNL to VFL) to the full load voltage VFL. This is called percent voltage regulation. Percent regulation = 𝑉𝑁𝐿 − 𝑉𝐹𝐿 𝑉𝐹𝐿 𝑋 100% ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO. 1: A d.c. shunt generator has an induced voltage on opencircuit of 127 volts. When the machine is on load, the terminal voltage is 120 volts. Find the load current if the field circuit resistance is 15 ohms and the armature-resistance is 0.02 ohm. Ignore armature reaction. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO. 2: An 8-pole d.c. shunt generator with 778 wave-connected armature conductors and running at 500 r.p.m. supplies a load of 12.5 Ω resistance at terminal voltage of 250 V. The armature resistance is 0.24Ω and the field resistance is 250 Ω . Find the armature current, the induced e.m.f. and the flux per pole. PROBLEM NO. 3: The voltage of a 100 kw 250 volt shunt generator rises to 260 volts when the load is removed. What full-load current does the machine deliver, and what is its per cent regulation? ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO. 4: A 25 kW 230 volt shunt generator has a regulation of 8.7 percent. (a) What will be the terminal voltage of the generator at no load? (b) If the change in voltage is assumed to be uniform between no-load and full-load kilowatts, calculate the kilowatt output of the generator when the terminal voltages are 240 and 235 volts. PROBLEM NO. 5: A 4-pole, lap-wound, d.c. shunt generator has a useful flux per pole of 0.07Wb. The armature winding consists of 220 turns each of 0.004 Ω resistance. Calculate the terminal voltage when running at 900 r.p.m. if the armature current is 50 A. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Controlling the Terminal Voltage of Shunt Generators It is possible to prevent the terminal voltage V from changing as the load changes by merely adjusting the field rheostat as the voltage changes in accordance with the load. This can be done manually or by automatic regulators. Automatic regulators are designed and constructed in several ways, but all operate on the fundamental principle that a terminal voltage change is accompanied by an inverse change in flux. Some of these are: (1) Tirrill Regulators – a pair of short circuiting contacts are connected across a portion of the field rheostat and are actuated by the electromagnetic action of a relay. Note: In practice, the field rheostat was usually adjusted so that terminal voltage was about 25 to 35 percent below rated value when the machine was delivering full load current with relay contacts held open. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics A Tirrill Regulator ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Controlling the Terminal Voltage of Shunt Generators (2) Diactor (direct-current) unit – the same functions are accomplished w/o vibrating contacts but by special rheostatic elements that are acted upon directly by a unique design of torque elements; the latter, energized by the varying potential is made of tilt stacks of plates at different angles so that the proper values of resistance are connected across a manually set rheostat in series with the shunt field. (3) Saturable Reactor – adapted to voltage regulators for generators. This extremely magnetic amplifier is made to act upon a special control field wound directly over the main field of the generator. In this way the total flux may be adjusted to the proper value without the necessity for making changes directly in the main field. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Saturable Reactor (Magnetic Amplifier) ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Controlling the Terminal Voltage of Shunt Generators The tendency on the part of a shunt generator to lose voltage with increasing values of load may be minimized by operating the machine at a lower speed. When this is done, the field flux must be increased which means that the iron portions of the magnetic circuit are more highly saturated. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Controlling the Terminal Voltage of Shunt Generators The result is that the rise in voltage between full load and no load is not so great at the reduced speed. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Controlling the Terminal Voltage of Shunt Generators An objection to operation at lower speed, however, is that the machine is likely to overheat because: (1) The copper loss in the field is increased. (2) Ventilation, that is cooling, is not as good because the fanning action of the armature is reduced. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Compound Generator Operation under Load The shunt field excitation is usually more or less steady and is affected only slightly as the terminal voltage fluctuates. The effect of the series field is quite variable, however, since its ampere turns depend upon the load current; when the load current is zero, it produces no component of flux and when the load current is high, it creates appreciable component of flux. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Compound Generator Operation under Load Cumulative Compounded - the shunt and series field coils around each of the main poles should be connected that they create flux in the same direction if the tendency of the generator to lose voltage is to be counteracted. Differential Compounded – the action of the series field must oppose, that is ‘buck’ the shunt field. Testing of a compound generator is the same as the procedure for testing the shunt machine. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Compound Generator Operation under Load ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Compound Generator Operation under Load ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Compound Generator Operation under Load (1) Flat-compound – if the terminal voltage at no load and full load are equal. Transmission distance between the generator and the load is short. (2) Over compound – if the full load voltage is higher than the no-load value. Few series field ampere turns vs shunt field ampere turns. (3) Under compound- if the full load voltage is less than the zero load voltage. Many series field ampere turns vs shunt field ampere turns. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Degree of Compounding Adjustment By connecting a very low resistance shunt directly across the series field, the no load voltage may be brought up to almost any desired value to meet individual demands. The low resistance shunt is called diverter because it diverts, or bypasses, part of the load current through a section that creates no flux. Thus, the series field is less effective in creating flux to boost the generated emf to an extent determined by the diverted current. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Degree of Compounding Adjustment Then, the relationship between the diverter resistance and series field resistance to their respective currents are shown below: 𝐼𝑆𝐸 𝐼𝐷 = 𝑅𝐷 𝑅𝑆𝐸 𝐼𝐿 = 𝐼𝑆𝐸 + 𝐼𝐷 It follows that: 𝐼𝑆𝐸 = 𝐼𝐿 ∗ 𝑅𝐷 𝑅𝐷 +𝑅𝑆𝐸 ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Degree of Compounding Adjustment Where 𝐼𝑆𝐸 𝐼𝐷 𝑅𝑆𝐸 𝑅𝐷 = series-field current = diverter current = series field resistance = diverter resistance • In practice, the diverter material is manganin, german silver, or any other high-resistivity material with a low temperature-resistance coefficient. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO.1: The series field of a compound generator has a resistance of 0.018 ohm. If the full-load current is 120 amp and it is necessary to divert 36 amp so that the full-load voltage will be brought down to the desired value, calculate: (a) the value of the diverter resistance, (b) the length of a square manganin wire (resistivity =265) whose cross sectional area is 15,616 circular mils. PROBLEM NO.2: Each of the series-field coils of a four-pole 50 kw 250 volt compound generator has 6 ½ turns of wire. The resistance of the entire series field is 0.012 ohm, and the diverter resistance is resistance is 0.036 ohm. Calculate the number of ampere-turns of each series-field coil at full load. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO.3: A long shunt compound generator has a shunt field winding of 1000 turns per pole and series field winding of 4 turns per pole and resistance 0.05 Ω . In order to obtain the rated voltage both at no-load and fullload for operation as shunt generator, it is necessary to increase field current by 0.2 A. The full-load armature current of the compound generator is 80 A. Calculate the divertor resistance connected in parallel with series field to obtain flat compound operation. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Series Generator Behavior under Load ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Series Generator Behavior under Load When a generator has a single field that is connected in series with the armature, the load current is simultaneously the excitation current; called series generator, its voltage will depend upon the current delivered to the load. On open circuit, when the load is zero, the series field ampere turns is likewise zero and the generated voltage is that due to cutting of the residual flux , the residual value Er. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Series Generator Behavior under Load The terminal voltage Vt will rise as the load amperes are increased and continue to do so as long as the resultant generated voltage rises more rapidly than those factors already noted tend to reduce it. At loads that are considerably above normal values, the iron portions of the magnetic circuits become highly saturated, under which condition the subtractive effects exceed the slowly rising generated emf; the terminal voltage begins to drop. Some applications of dc generator: used in the dc system for voltage boosting purposes or to minimize the leakage currents in grounded dc systems so that electrolytic action in underground structures may be reduced; in Europe it is sometimes employed in the Thury high-voltage dc systems for transmission of electrical energy. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Armature Reaction in DC Generators When the armature of a dc generator carries a load current, it becomes an electromagnet, apart from the magnetic effect induced in it by the main poles. (a) Main pole flux distribution (b) Distribution of emfs and currents in the armature windings ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Armature Reaction in DC Generators If load is delivered by the armature, the current directions in the armature winding will be exactly the same as those indicated for the emfs if the brushes are positioned on the commutator so that they are on the exact neutral plane with respect to the armature conductors being short circuited. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Armature Reaction in DC Generators In (a) the ends of the armature coil connect to commutator segment midway between the coil sides, the brush neutral lines up with the center of the poles. In (b) the ends of the armature coil connect to commutator segments in line with one of the coil, the brush neutral lines up with the magnetic neutral of the main poles. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Armature Reaction in DC Generators (a) Chamfered pole shoes (b) pole lamination with one pole slip Summary: Armature reaction is: (1) Produced by the load current in the armature conductors that results in the magnetic field whose direction is displaced 90 electrical degrees with respect to the main field. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Armature Reaction in DC Generators (2) Depends upon and directly proportional to the load current. (3) Shifts the neutral plane in the same direction as that of the rotating armature. (4) Reduces the total flux. (5) Can be counteracted by the used of specially shaped pole lamination. Interpoles of DC Generators These are narrow poles placed exactly halfway between the main poles and directly in line with the load magnetic neutral, usually called the mechanical neutral. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Interpoles of DC Generators The exciting winding for these poles are always permanently connected in series with the armature because interpoles must produce fluxes that are directly proportional to the armature current. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Compensating Windings of DC Generators In some large machines, and in those in which the fluctuations are violent, the cross magnetizing action – armature mmf distorts the main field, can become severe enough to cause flashover between plus and minus of brushes. It maintains a uniform flux distribution under the faces of the main poles. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) A 2,500 kW, 600 volt 16 pole generator has a lap wound armature with 2,360 conductors. If the pole faces cover 65% of the entire circumference, calculate the number of pole face conductors in each pole of a compensating winding. . ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Need for Operation of Generators in Parallel Power plants will generally be found to have several small generators rather than single units capable of taking care of the maximum peak loads (both dc and ac). The several units can then be operated singly or in various parallel combinations, on the basis of the actual load demand. (Reason: keep maximum efficiency, continuity of service, maintenance and repair problems, and additions to plant capacity as the service demands change.) Uninterrupted service recognized as economic necessity. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Operation of Shunt Generators in Parallel The shunt generators have “drooping” voltage vs. load characteristics, i.e. the voltage drops as the load increases. For two shunt generators have identical external characteristics: if the voltage changes in both by exactly the same amount for the same percent of change in load, then the two machines will divide the total load in proportion to their relative capacities. For two shunt generators not similar external characteristics, then if both have been adjusted for the same voltage at rated load currents, one generator will deliver a larger percent of its rated capacity than the other. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Operation of Shunt Generators in Parallel Characteristics voltage vs. percent rated current of two shunt generators operated in parallel with external characteristics not the same ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Operation of Shunt Generators in Parallel Sample Problems As the load increases, it is necessary to: (1) Connect a larger generator than A in parallel with the latter, after which the smaller machine, when gradually unloaded, is disconnected from the line. (2) Connect another generator in parallel with A and have two machines operate jointly to supply the total load. The procedure are the following: (a) Generator B is brought up to speed by its prime mover. (b) Field switch B will closed, whereupon the voltage will build up. Caution: It is important that the polarity of the incoming generator be exactly the same as that of the line polarity. “plus to plus and minus to minus”. (c) After proper adjustments are made, quickly close switch SB. This places generator B in parallel with generator A. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Operation of Shunt Generators in Parallel Sample Problems (d) To shift the load from A to B, it is necessary merely to manipulate both field rheostats simultaneously, cutting in resistance in the field of A and at the same time cutting out resistance in the field of B. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO.1: In the figure of the previous slide, generator A has a rating of 150 kW and generator B has a rating of 200kW, both at 230 Volts. Calculate the kilowatt output of each machine and the total kilowatt load when the terminal voltage is 240 Volts. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO.2: Two shunt generators each with an armature resistance of 0.01 Ω and field resistance of 20 Ω run in parallel and supply a total load of 4000 A. The e.m.f.s are respectively 210 V and 220 V. Calculate the bus-bar voltage and output of each machine.. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM NO.3: Two shunt generators, each with no-load rating of 140 volts, are operated in parallel. Both machines have external characteristics which are straight lines over their operating ranges. Machine 1 is rated at 350 kW, and its full load voltage is 135 volts. Machine 2 is rated 400 kW at 127 volts. Calculate the operating voltage when two machines supply a total current of 6000 A. How is the load divided between the two. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Operation of Compound Generators in Parallel When two compound generators are to be operated in parallel, it is necessary to use essentially the same wiring as that employed for shunt machines, except that an equalizer connection must be added. An equalizer is a very low-resistance copper wire that joins together identical ends of the series fields, the other ends of which are connected together after the main switches have been closed. If the equalizer is removed, instability would result if machines are over compounded because any tendency on the part of one machine to assume larger share of the total load than it should is immediately accompanied by an increased in its generated emf, the other machine in drooping load, generates less voltage because the series field flux is increased in the first and decreased in the second. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics Operation of Compound Generators in Parallel The equalizer connects the series fields in parallel, and this results in a division of the total current in a ratio inversely proportional to the resistance values of the two fields. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM : Two compound generators A and B, fitted with an equalizing bar, supply a total load current of 500 A. The data regarding the machines are : A B Armature resistance (ohm) 0.01 0.02 Series field winding (ohm) 0.004 0.006 Generated e.m.fs. (volt) 240 244 Calculate (a) current in each armature. Shunt currents may be neglected. ELECTRICAL MACHINE 1 Direct Current Generators Characteristics ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) 244 ELECTRICAL MACHINE 1 Direct Current Generators Characteristics References Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New York: McGraw-Hill. Norton, R. (2011). Machine Design: an integrated approach. (4th ed.). Boston: Pearson. Norton, R. (2012). Design of Machinery: an introduction to the synthesis and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill. Sharma, C.S, & Purohit, K. (2012). Design of machine elements. New Delhi: PHI Learning Private Limited. Gope, P.C. (2012). Machine design: fundamentals and applications. New Delhi: PHI Learning Private Limited. ELECTRICAL MACHINE 1 Chapter 6 Efficiency, Rating, and Application of Dynamos ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos Power Losses in Dynamo The power received by a dynamo is called its input. In generator, it is a mechanical power, while in a motor it is electrical power. The power delivered by a dynamo is called its output. In a generator, it is electrical power, while in a motor is it mechanical power. The difference between the power input to a machine (watts) and its power output (in watts) is called the power loss. There are two general classifications of power losses in electric machines: (1) Those that are caused by the rotation of the armature (rotational losses) (2) Those that result from a current flow in the various parts of machine (electrical losses) ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos Power Losses in Dynamo A. Rotational losses are divided into five parts: (1) Bearing friction (2) Brush friction (3) Wind friction, usually called windage. (4) Hysteresis (5) Eddy Currents Hysteresis loss takes place in the revolving armature core because the magnetic polarity in the iron changes in step with the changing positions of the magnetic material under the various poles. In modern dynamo, it depends upon the flux density in the armature core iron, the speed of rotation, and the quality of the magnetic iron. ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos Power Losses in Dynamo Eddy Current loss result because the generated voltage in the iron near the outside surface are greater than those closer to the center shaft because of the higher speed; the difference in potential then causes of current to flow in the iron. To minimize eddy current, laminating or slicing the armature core and then coating each lamination with a high resistance varnish. B. Copper losses occur when there is a current flow through the various copper circuits. (1) Armature winding loss (𝐼𝑎2 𝑅𝑎) (2) Brush contacts between copper commutator and the carbon brushes. (3) Shunt field (4) Series field ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos Power Losses in Dynamo (5) Interpole field (6) Compensating winding field C. Stray-Load loss (1) The distortion of the flux because of the armature reaction. (2) Lack of uniform division of the current in the armature winding through the various parts and through individual conductors of large crosssectional area. (3) Short circuit currents in the coils undergoing commutation. Reasonable value arbitrarily : 1% of the output of the machine with the rating 150kW (200HP) or more. For smaller rating machines, the stray load loss is generally neglected when efficiency calculations are made without much loss of accuracy. ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos Power Losses in Dynamo ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos Efficiency of DC Motors The efficiency of dc motor is the ratio of the mechanical power output, converted to watts, to the electrical power input. ℎ𝑝 𝑜𝑢𝑡𝑝𝑢𝑡 𝑥 746 𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 𝑥 100% 𝑤𝑎𝑡𝑡𝑠 𝑖𝑛𝑝𝑢𝑡 Since watts input = (hp output x 746) + watts losses, 𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = ℎ𝑝 𝑜𝑢𝑡𝑝𝑢𝑡 𝑥 746 𝑥100% ℎ𝑝 𝑜𝑢𝑡𝑝𝑢𝑡 𝑥 746 + 𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠 or written in another way, 𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠 𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 1 − 𝑥 100% ℎ𝑝 𝑜𝑢𝑡𝑝𝑢𝑡 𝑥 746 + 𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠 ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos Power Stages Motor Over all or commercial efficiency = C/A Electrical Efficiency = B/A Mechanical Efficiency = C/B ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM 1: A 15 –hp motor has a total loss of 1,310 watts when operating at full load. Calculate the percent efficiency. PROBLEM 2: A 25 - hp motor has an efficiency of 84.9 percent when delivering three-quarter of its rated output. Calculate the total losses at this load. PROBLEM 3: A d.c. shunt machine while running as generator develops a voltage of 250 V at 1000 r.p.m. on no-load. It has armature resistance of 0.5 Ω and field resistance of 250 Ω . When the machine runs as motor, input to it at no-load is 4 A at 250 V. Calculate the speed and efficiency of the machine when it runs as a motor taking 40 A at 250 V. Armature reaction weakens the field by 4 %. (ch29-1024) ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM 4: A 20-hp (14.92 kW); 230-V, 1150-r.p.m. 4-pole, d.c. shunt motor has a total of 620 conductors arranged in two parallel paths and yielding an armature circuit resistance of 0.2 Ω . When it delivers rated power at rated speed, it draws a line current of 74.8 A and a field current of 3 A. Calculate (i) the flux per pole (ii) the torque developed (iii) the rotational losses (iv) total losses expressed as a percentage of power. (ch29-1025) ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos Efficiency of Direct Current Generators The efficiency of a dc generator is the ratio of the electrical power output 𝐸𝑇 𝑥 𝐼𝐿 to the mechanical power input converted to watts. As a percentage, this statement may be written as: 𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 𝑤𝑎𝑡𝑡𝑠 𝑜𝑢𝑡𝑝𝑢𝑡 𝑤𝑎𝑡𝑡𝑠 𝑖𝑛𝑝𝑢𝑡 𝑥 100% Since watts input = watts output + watts losses, 𝑤𝑎𝑡𝑡𝑠 𝑜𝑢𝑡𝑝𝑢𝑡 𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 𝑥 100% 𝑤𝑎𝑡𝑡𝑠 𝑜𝑢𝑡𝑝𝑢𝑡 + 𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠 𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠 𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 1 − 𝑥 100% 𝑤𝑎𝑡𝑡𝑠 𝑜𝑢𝑡𝑝𝑢𝑡 + 𝑤𝑎𝑡𝑡𝑠 𝑙𝑜𝑠𝑠𝑒𝑠 ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos Power Stages Generator A B C Over all or commercial efficiency = C/A Electrical Efficiency = C/B Mechanical Efficiency = B/A ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos Efficiency of Direct Current Generators The two methods of determining the efficiency of a generator are: (1) by directly measuring the total power output and the total power input. (2) by making certain necessary tests from which the various power losses are determined, then applying the formula of efficiency. Procedure in determining the conventional efficiency of the generator: (1) Measure the resistance of the armature. (2) Measure the resistance of the interpole field. (including compensating winding if there is) (3) Measure the resistance of the series field. (4) Measure the shunt field resistance. (5) Measure the rotational (stray-power loss) = 𝐸𝐴 𝑥 𝐼𝐴 ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos ILLUSTRATIVE PROBLEMS: ( Instructor actual discussion) PROBLEM 1: The following information is given in connection with a 10kW 250 Volt long shunt flat-compounded generator: shunt field resistance = 125 ohms; armature and interpole resistance = 0.4 ohm; series field resistance= 0.05 ohm; stray power loss = 540 watts; brush drop at full load (assumed) = 2 volts. Calculate the efficiency of the generator at full-load. PROBLEM 2: A 312.5 kW, 550 volts DC shunt generator has an armature resistance of 0.05 ohm and field resistance of 55 ohm. The stray power loss for this machine at rated voltage and speed are 5000W. Neglecting the brush loss, calculate the following: (a) the generated voltage at rated output (b) The generated power at rated output (c) The electrical efficiency at rated output (d) The conventional efficiency at rated output (e) The load current at which the maximum efficiency will result (f) The load power at maximum efficiency. ELECTRICAL MACHINE 1 Efficiency, Rating, and Application of Dynamos References Chapman, S. (2012). Electric Machinery Fundamentals. (5th ed.). New York: McGraw-Hill. Norton, R. (2011). Machine Design: an integrated approach. (4th ed.). Boston: Pearson. Norton, R. (2012). Design of Machinery: an introduction to the synthesis and analysis of mechanisms and machines. (5th ed.). Boston : McGrawHill. Sharma, C.S, & Purohit, K. (2012). Design of machine elements. New Delhi: PHI Learning Private Limited. Gope, P.C. (2012). Machine design: fundamentals and applications. New Delhi: PHI Learning Private Limited.