DC Generators

1
Unit 30

• DC Generators

2
Objectives

• After studying this unit, you should be able to
• Discuss the theory of operation of DC generators
• List the factors that determine the amount of
  output voltage produced by a generator
• List the three major types of DC generators
• List different types of armature windings

3
Objectives (contd.)

• Describe the differences between series and shunt
  field windings
• Discuss the operating differences between
  different types of generators
• Draw schematic diagrams for different types of DC
  generators
• Set the brushes to the neutral plane position on
  the commutator of a DC machine

4
Preview

• Most of the electric power generated throughout
  the world is AC
• DC is used for some applications
• Industrial plants use it to produce the power
  needed to operate large DC motors

5
Preview

• DC motors
• Have characteristics that make them superior to
  AC motors for certain applications
• Also used in diesel locomotives

6
What Is a Generator?

• Device that converts mechanical energy into
  electrical energy
• Loops of wire cut through magnetic flux lines
  when the shaft is mechanically turned
• Commutator is used to change the alternating
  voltage produced in the armature windings into
  direct voltage before it leaves the generator

7
FIGURE 301 A voltage is induced in the conductor
as it cuts magnetic lines of flux.
8
FIGURE 302 The loop is parallel to the lines of
flux, and no cutting action is taking place.
9
FIGURE 303 Induced voltage after 90 degrees of
rotation.
10
FIGURE 304 Induced voltage after 180 degrees of
rotation.
11
FIGURE 305 The negative voltage peak is reached
after 270 degrees of rotation.
12
FIGURE 306 Voltage produced after 360 degrees of
rotation.
13
FIGURE 307 The commutator is used to convert the
AC voltage produced in the armature into DC
voltage.
14
FIGURE 308 The loop is parallel to the lines of
flux.
15
FIGURE 309 The loop has rotated 90 degrees.
16
FIGURE 3010 The loop has rotated 180 degrees.
17
FIGURE 3011 The commutator maintains the proper
polarity.
18
FIGURE 3012 The loop completes one complete
rotation.
19
FIGURE 3013 Increasing the number of turns
increases the output voltage.
20
FIGURE 3014 Increasing the number of loops
produces a smoother output voltage.
21
FIGURE 3015 The loops of wire are wound around
slots in a metal core.
22
Armature Windings

• Include
• Lap-wound armatures
• Wave-wound armatures
• Frogleg-wound armatures

FIGURE 3017 Lap-wound armatures have their
windings connected in parallel. They are used in
machines intended for high-current and
low-voltage operation.
23
FIGURE 3018 Wave-wound armatures have their
windings connected in series. Wave windings are
used in machines intended for high-voltage,
low-current operation.
FIGURE 3019 Frogleg-wound armatures are
connected in series-parallel. These windings are
generally used in machines intended for medium
voltage and current operation.
24
Brushes

• Ride against the commutator segments
• Used to connect the armature to the external
  circuit of the DC machine
• Made from a material that is softer than the
  copper bars of the commutator
• Permits the brushes to wear instead of the
  commutator
• Brush leads generally marked A1 and A2
• Referred to as armature leads

25
Pole Pieces

• Located inside the DC machine housing
• Provide the magnetic field necessary for the
  operation of the machine

FIGURE 3020 Pole pieces are constructed of soft
iron and placed on the inside of the housing.
26
Field Windings

• Two types of field windings are used
• Series field windings
• Shunt field windings

FIGURE 3023 Both series and shunt field windings
are contained on each pole piece.
27
Generators

• Three basic types of DC generators
• Series contains only a series field connected in
  series with the armature
• Additional consideration connecting load to the
  series generator
• Shunt contain only a shunt field winding
  connected in parallel with the armature
• Additional considerations field excitation
  current and generator losses
• Compound contain series and shunt fields

28
FIGURE 3025 The series field is connected in
series with the armature.
29
FIGURE 3030 Shunt field windings are connected
in parallel with the armature.
30
FIGURE 3040 Schematic drawing of a short shunt
compound generator.
FIGURE 3039 Schematic drawing of a long shunt
compound generator.
31
Compounding

• Amount of compounding
• Determined by the relationship of the strengths
  of the two fields in a generator

FIGURE 3041 Characteristic curves of compound
generators.
32
Compounding (contd.)

• Controlling compounding
• Controlled by connecting a low-value variable
  resistor in parallel with the series field
• Cumulative compound
• Shunt and series fields are connected so when
  current flows through them they aid each other in
  the production of magnetism
• Differential-compound
• Fields connected so they oppose each other in the
  production of magnetism

33
Countertorque

• Measure of the useful electric energy produced by
  the generator
• Turning resistance
• Must be overcome by the device used to drive the
  generator

FIGURE 3045 A magnetic field is produced around
the armature.
34
Armature Reaction

• Twisting or bending of the magnetic lines of flux
  of the pole pieces

FIGURE 3046 Armature reaction changes the
position of the neutral plane.
35
Armature Reaction (contd.)

• Corrected in several ways
• One method is to rotate the brushes an equal
  amount to the shift of the neutral plane

FIGURE 3047 In a generator, the brushes are
rotated in the direction of armature rotation to
correct armature reaction.
36
Setting the Neutral Plane
FIGURE 3052 Setting the brushes at the neutral
plane.
37
Paralleling Generators
FIGURE 3053 The equalizer connection is used to
connect the series fields in parallel with each
other.
38
FIGURE 3054 One generator takes all the load,
and the other becomes a motor.
39
Summary

• Generator
• Converts mechanical energy into electric energy
• Operates on principle of magnetic induction
• AC is produced in all rotating armatures
• Commutator changes the AC produced in the
  armature into DC
• Brushes are used to make contact with the
  commutator and to carry the output current to the
  outside circuit

40
Summary (contd.)

• Armature types
• Lap-wound, wave-wound, and frogleg-wound
• Series field windings
• Made with a few turns of large wire and have a
  very low resistance
• Connected in series with the armature
• Marked S1 and S2

41
Summary (contd.)

• Shunt field windings
• Made with many turns of small wire
• Connected in parallel with the armature
• Marked F1 and F2
• Factors that determine voltage produced by a
  generator
• Number of turns of wire in the armature
• Strength of magnetic field of pole pieces
• Speed of the armature

42
Summary (contd.)

• Series generators
• Increase output voltage as load is added
• Decrease output voltage as load is added
• Voltage regulation of a DC generator
• Proportional to the resistance of the armature
• Compound generators
• Contain both series and shunt field windings

43
Summary (contd.)

• Armature reaction
• Twisting or bending of main magnetic field
• Caused by interaction of magnetic field produced
  in the armature
• Proportional to armature current
• Interpoles
• Small pole pieces connected between main field
  poles
• Help correct armature reaction
• Connected in series with the armature

44
Summary (contd.)

• Generator supplies current to a load
• Countertorque is produced
• Makes the armature harder to turn
• Proportional to armature current if the field
  excitation current remains constant
• Measure of the useful electric energy produced by
  the generator