electrical machines and energy conversion unit 1 deck 2 dc generator basics

Electrical Machines and

Energy Conversion

Unit 1 Deck 2DC Generator Basics

FIGURE 4-1 Schematic diagram of an elementary ac generator turning at 1 revolution per second.

Theodore WildiElectrical Machines, Drives, and Power Systems, 6e

Copyright © 2006 by Sperika Enterprises and published by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458All rights reserved.

FIGURE 4-2 Voltage induced in the ac generator as a function of the angle of rotation.




FIGURE 4-3 Voltage induced as a function of time.




FIGURE 4-4 Elementary dc generator is simply an ac generator equipped with a mechanical rectifier called a commutator.




FIGURE 4-5 The elementary dc generator produces a pulsating dc voltage.




FIGURE 4-6 The three armatures (a), (b), and (c) have identical windings. Depending upon how they are connected (to slip rings or a commutator), an ac or dc voltage is obtained.




FIGURE 4-7 Schematic diagram of a dc generator having 4 coils and 4 commutator bars. See Fig. 4.9.




FIGURE 4-8 The voltage between the brushes is more uniform than in Fig. 4.5.




FIGURE 4-9 The actual physical construction of the generator shown in Fig. 4.7. The armature has 4 slots, 4 coils, and 4 commutator bars.




FIGURE 4-10 Position of the coils when the armature of Fig. 4.9 has rotated through 45.




FIGURE 4-14 Magnetic field produced by the current flowing in the armature conductors.




FIGURE 4-15 Armature reaction distorts the field produced by the N, S poles.




FIGURE 4-16 Commutating poles produce an mmfc that opposes the mmfa of the armature.




FIGURE 4-17 Separately excited 2-pole generator. The N, S field poles are created by the current flowing in the field windings.




FIGURE 4-18a Flux per pole versus exciting current.




FIGURE 4-18b Saturation curve of a dc generator.




FIGURE 4-19 a. Self-excited shunt generator. b. Schematic diagram of a shunt generator. A shunt field is one designed to be connected in shunt (alternate term for parallel) with the armature winding.




FIGURE 4-20 Controlling the generator voltage with a field rheostat. A rheostat is a resistor with an adjustable sliding contact.




FIGURE 4-21 The no-load voltage depends upon the resistance of the shunt-field circuit.




FIGURE 4-22 Equivalent circuit of a dc generator.




FIGURE 4-23 Separately excited generator under load.




FIGURE 4-24 Load characteristic of a separately excited generator.




FIGURE 4-25 a. Compound generator under load. b. Schematic diagram.




FIGURE 4-26 Typical load characteristics of dc generators.




FIGURE 4-27 Cross section of a 2-pole generator.




FIGURE 4-28 Cutaway view of a 4-pole shunt generator. It has 3 brushes per brush set.




FIGURE 4-29 Adjacent poles of multipole generators have opposite magnetic polarities.




FIGURE 4-30 Armature of a dc generator showing the commutator, stacked laminations, slots, and shaft. (Courtesy of General Electric Company, USA)




FIGURE 4-31 Armature laminations with tapered slots.




FIGURE 4-32 Crosssection of a slot containing 4 conductors.




FIGURE 4-33 Commutator of a dc machine.




FIGURE 4-34 a. Brushes of a 2-pole generator. b. Brushes and connections of a 6-pole generator.




FIGURE 4-35 a. Carbon brush and ultraflexible copper lead. b. Brush holder and spring to exert pressure. c. Brush set composed of two brushes, mounted on rocker arm. (Courtesy of General Electric Company, USA)




FIGURE 4-36 Sectional view of a 100 kW, 250 V, 1750 r/min 4-pole dc generator. (Courtesy of General Electric Company, USA)




FIGURE 4-37 This direct-current Thompson generator was first installed in 1889 to light the streets of Montreal. It delivered a current of 250 A at a voltage of 110 V. Other properties of this pioneering machine include the following:Speed 1300 r/minTotal weight 2390 kgArmature diameter 292 mmStator internal diameter 330 mmNumber of commutator bars 76Armature conductor size # 4Shunt field conductor size # 14A modern generator having the same power and speed weighs 7 times less and occupies only 1/3 the floor space.




FIGURE 4-38a Schematic diagram of a 12-pole, 72-coil dc generator.




Electrical Machines and Energy Conversion

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electrical machines and energy conversion unit 1 deck 2 dc generator basics

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