MODULE 2
Direct-Current Generator Characteristics
INTRODUCTION
The generator performance deals primarily with the relation between excitation,
terminal voltage, and load. These relations can be best exhibited graphically by means
of curves known as generator characteristics. These characteristics show at a glance the
behavior of the generator under different load conditions. Furthermore, it is of great
importance in judging the suitability of generator for a particular purpose.
In this module, you will learn the different types of dc generators and how each type
behaves under different operating conditions. Specifically, at the end of this module, you
should be able to:



Differentiate the types of DC generators and know their operating characteristics
Solve problems related to the operating characteristics of DC generators
Evaluate the performance of DC generators based on losses and efficiency
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Direct-Current Generator Characteristics
Read the following reference/s for a detailed discussion of the
following topics:

Electrical Machines (2th edition) by C. Siskind
Chapter 4
pp. 94 – 129

A Textbook of Electrical Technology Vol. 2 by
Theraja
Chapter 28
pp. 968 - 989
Types of DC Generators
 According to the type of the main field winding used:

Series Generator: This type of dc generator uses only the series field winding.
The series field winding consists of relatively few turns of thick wires and is
joined in series with the armature. Such generators are rarely found in use
today except for special purposes.

Shunt Generator: This type of dc generator uses only the shunt field winding
which consists of many turns of small wires. The shunt field winding is
connected across or in parallel with the armature so that the full, or nearly
full, line voltage is impressed across it.

Compound Generator: This type of dc generator has two sets of field
winding. One set is made of low-resistance windings in series with the
armature (series field). The other set is made of high resistance winding and
is connected in parallel with the armature circuit (shunt field). There are two
ways of making connections for the compound generator. One way is to
connect the shunt field directly across the armature (short-shunt). The other
way is by connecting the shunt field across the armature through the series
field (long-shunt). The shunt- and series-field coils around each of the main
poles should be so connected so that they create flux in the same direction
if the tendency of the generator to lose voltage is to be counteracted.
When this done, the machine is said to be cumulative-compounded. If, for
some special reason, the action of the series field must oppose, that is,
“buck”, the shunt field, the machine is referred to as differentialcompounded.
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MODULE 2
Direct-Current Generator Characteristics
Figure 1 shows the schematic diagrams for the series and shunt types of generator,
while Fig.2 represents the short- and long-shunt connections of compound
generator.
Figure 1 Schematic diagrams for (a) Series and (b) Shunt
(a) Short-shunt connection
(b) Long-shunt connection
Figure 2 Two arrangements for compound generator operation
 According to the source of excitation for its field windings:

Separately-Excited DC Generator: Separately-excited generators are those
whose field windings are energized from a separate, source of supply. The
obvious disadvantage of a separately excited dc generator is that it
requires an external dc source of excitation. However, since the output
voltage may be controlled more easily and over a wide range (from zero
to maximum), this type of generator finds many applications.
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MODULE 2
Direct-Current Generator Characteristics
Figure 2 Separately-Excited Generator

Self-Excited DC Generator: Self-excited generators are those whose field
windings are energized by the current supplied by its own armature. This
type of generator builds up its voltage from residual magnetism.

Dual-Excited DC Generator: The sources of excitation for the field windings
are from the armature and a separate source. This applies to compound
generators.
Figure 3 Dual-Excited Generator
No-load (Open-Circuit) Characteristics of DC 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. This
generated voltage will depend, for a given machine, upon two factors:
a) the speed of rotation
b) the flux
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MODULE 2
Direct-Current Generator Characteristics
E g  N
E g  kN
 If the flux is kept constant while the speed is increased or decreased, the voltage
will rise or fall, respectively, in direct proportion to the change in speed. This may
be shown to be true experimentally by driving a separately excited generator over
a wide range of speed as possible while the field current is kept absolutely
constant. The set-up for this experiment is shown in Fig. 4. To perform such an
experiment, it will be desirable first to adjust the generator speed to its highest
permissible value and at the same time to set the field current If so that a high
voltmeter reading is recorded.
Figure 4 Separately excited shunt generator connections to determine
experimentally the no-load characteristics
As the speed is gradually lowered without changing the field excitation, lower
readings of Eg are recorded. A plot of Eg vs. rpm will yield a straight line as depicted
in Fig. 5.
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MODULE 2
Direct-Current Generator Characteristics
Figure 5 Direct proportionality relationship between the no-load generated
voltage Eg and the speed rpm with constant excitation
 Similarly, if the speed is held constant while the flux (not the field current) is varied,
the voltage will change in direct proportion to the change in magnetism. However,
to show that the generated voltage is directly proportional to the flux is much more
difficult because magnetism measurements are not made as readily as are those
of amperes and volts or rpm. From a practical point of view, it is more desirable to
know how the no-load generated voltage is affected by changes in field current.
This can also be demonstrated experimentally with the same set-up in Fig. 4. In
performing this experiment, the generator is run at its normal speed. The field
current If is adjusted from zero in steps and the corresponding values of generated
emf are recorded. A so-called saturation curve (or magnetization curve) can then
be plotted to show the relationship between the generated voltage and the field
current (Fig. 6).
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MODULE 2
Direct-Current Generator Characteristics
Figure 6 Saturation curve for dc shunt generator operating at constant speed
The following important points may be noted from the saturation curve:
1. It should be observed that the initial voltage is not zero at zero field current.
The initial value, usually low, is due to residual magnetism.
2. The relationship between the no-load voltage and the field current is linear
up to the so-called “knee”.
3. After the knee, the curve departs from the straight line. This means that the
generated voltage does not increase as rapidly as the field current.
4. In the upper part of the curve, the voltage is leveling off. This where
magnetic saturation of the poles sets in.
5. The intersection of the saturation curve and the excitation line is the value
of voltage to which the generator will build up. The excitation line is the plot
of field voltage, Vf, versus field current, If.
NOTE:
The saturation curve emphasizes the extremely important fact that the
generated voltage is directly proportional to the flux and not the field current.
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MODULE 2
Direct-Current Generator Characteristics
Building Up the Voltage of a Self-excited Shunt Generator
To build up means to rise from its residual voltage, E r, to its normal operating value. The
self-excited shunt generator cannot build up its voltage unless all the conditions for
building up have been fulfilled. There are four requirements for build-up as discussed
below:
1. The machine must develop a small voltage resulting from residual magnetism. The
voltage of a self-excited shunt generator will not rise much above an extremely
low residual value if the residual flux is insufficient. Generators that are expected to
operate at voltages up to 250 V should have residual values of flux so that 4 to 10
residual volts are developed. A new machine or one that has lost its residual flux
because of a long period of idleness must be separately excited to create the
necessary magnetism. This is usually done while the armature is at rest by
connecting the shunt field only to a separate dc source for a few seconds. This
practice is generally referred to as flashing the field.
2. The total field resistance must be lower than the so-called critical resistance. A
generator will fail to build up if the slope of excitation line (i.e.,
Vf
 R f ) is about
If
equal to or greater than the straight-line portion of the magnetization. An example
should make this clear. Suppose the build-up point is 300 V. As shown in Fig. 7, if
the total field resistance is 180 Ω, the generator cannot build up to the required
voltage. But if the total field resistance is lowered to 125 Ω, the excitation line will
cross the saturation curve at the build-up point. The so-called critical resistance is
defined as the resistance below which machine will build up and above which it
will not.
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MODULE 2
Direct-Current Generator Characteristics
Figure 7 Effect of Field Resistance in the Build-up process of Self-excited shunt dc
generator
3. The speed of the armature must be above the so-called critical speed. A
generator will fail to build up if, for a given field resistance, the speed is below the
so-called critical speed. The critical speed is defined as the speed above which
build-up will occur and below which it will not. As shown below, the generator will
build up to 300 V at the speed of 1800 rpm. Below 1800 rpm, it will not build up to
the desired voltage.
Figure 8 Effect of generator speed in the Build-up process of Self-excited shunt dc
generator
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Direct-Current Generator Characteristics
The critical speed may be determined experimentally. If, starting from rest, the
armature speed is gradually increases, the critical speed will be indicated by a
sudden rapid rise in voltage.
The generator is usually operated at some definite speed originally fixed by the
manufacturer. However, should it be desirable to operate at some higher or lower
speed, the field rheostat must be adjusted for the new speed.
4. There must be a proper relation between the direction of rotation and the
connections of the field to the armature terminals. Thus, if a generator fails to build
up, and other conditions have been fulfilled, the difficulty may be corrected by:
a) reversing the direction of rotation; or
b) interchanging the field terminals with respect to the armature terminals
EXAMPLE 1
If the no-load voltage of a separately excited shunt generator is 110 V at 1350 rpm, what
will be the voltage if the speed is increased to 1600 rpm? Assume constant field
excitation.
Solution
E g  kN
Eg1
N1
Eg2
Eg1


Eg2
N2
N2
N1
 1600 
E g @ 1600  
 110  130.37 V.
 1350 
EXAMPLE 2
A self-excited shunt generator develops 230 V when the field current is 3.6 A. What will be
the open-circuit voltage of this machine when the field resistance is reduced until the
field current rises to 4 A. Assume that the flux changes half as much as the field current.
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MODULE 2
Direct-Current Generator Characteristics
Solution
E g  kN
Assume constant speed :
Eg2
Eg1

2
1
 2  1

1
 2  1

1
2
19

1
18
Eg2
Eg1

1  4  3.6 


2  3.6 
1
18
19
18
 19 
E g 2   230  242.78 V
 18 
EXAMPLE 3
The following data were obtained for the magnetization curve of a 4-pole shunt
generator.
If
E
If
E
If
E
0
6
0.8
160
1.56
260
0.1
20
1.0
200
1.92
280
0.4
80
1.14
220
2.40
300
0.6
120
1.32
240
3.04
320
(a) Draw the magnetization curve.
(b) If the total shunt field resistance (including the field rheostat) is 125 Ω, determine the
voltage to which the machine will build up as a self-excited generator.
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MODULE 2
Direct-Current Generator Characteristics
Solution
If
0
0.1
0.4
0.6
0.8
1.0
1.14
1.32
1.56
1.92
2.40
3.04
Vf  If Rf
0
12.5
50
75
100
125
142.5
165
195
240
300
380
The build-up voltage is 300 V and it occurs when If = 2.40 A.
Behavior of a Shunt Generator under Load
After a self-excited shunt generator builds up to a required voltage, a no-load voltage, it
is ready to supply power to a number of electrical loads up to, and a little above, its rated
capacity. One of the most important characteristics of any generator is its behavior with
regard to the terminal voltage when the load current is increased. In the shunt type of
generator, the voltage always falls down as more current is delivered to the load. There
are three reasons for this:
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84