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Lesson Objectives :

Explain the principle of electromagnetic induction upon which dc generator
operate

Describe the construction and properties of dc generator

Describe the uses and operating characteristic of various types of dc generators
3.1
Introduction
In DC machinery principle, generator converts mechanical energy to dc electric energy.
Whenever we use mechanical rotation to generate electricity, the output is always an ac
voltage. This is true for any conceivable arrangement of coils, flux and type of rotation.
The only way to obtain a dc voltage is to periodically reverse the terminals of the coil in
which the voltage is induced. Thus, whenever the polarity is about to change, we reverse
the terminals so that the output is always positive. This reversing action is called
commutation and the device that achieves the reversing is called a commutator .
Today, electronic rectifiers that convert ac power into dc are often used instead of dc
generators. These rectifiers are more efficient and in addition they have no moving
parts. Why, then, we study dc generators? The reason is that many dc motors operate
briefly as generators. Furthermore, dc motor and generators are built the same way and
share the same properties.
3.2 Generating a dc voltage
An ac voltage is induced in a coil when it rotates between the poles of a permanent
magnet. This simple generator is shown in Figure 3-1.
Figure 3-1
Simple ac generator producing an output voltage between terminal a & b
However, we have added slip rings and brushes so that the ac voltage is available
between the stationary terminals a and b. The output voltage Eab has the general shape
given in Figure 3-2.
Figure 3-2
Waveshape of the ac output voltage and corresponding position of the coil in Fig 3-1
In order to obtain a dc output, the negative half of the ac voltage must be rectified.
Thus, when Eab is rectified, we obtain the pulsating voltage Exy shown in Figure 3-3.
Figure 3-3
Waveshape of Eab in Figure 3-2 after rectification
A simpler way to rectify the voltage is to use a ring that is cut in half, so as to form two
segments. The split ring is mounted on the shaft but insulated from it. The segments are
directly connected to the two ends of the coil, as shown in Figure 3-4.
Figure 3-4
A split ring commutator acts as a reversing switch to rectify the ac voltage induced in the coil.
Two brushes enable us to bring the voltage to stationary terminals x and y. The
combination of the split ring and brushes is equivalent to the switch. Note that the split
ring is mounted in such a way that the switching action takes place when the coil is in
the vertical position. The split ring is called a commutator.
3.3 Armature of a dc generator
We can obtain a higher dc voltage, and one that is much smoother, by increasing the
number of coils and the number of segments on the commutator. The coils are identical
and evenly spaced around a cylindrical iron core, see Figure 3-5.
Figure 3-5
Components and construction of a dc armature
The coils are embedded in slots and insulated from the core by appropriate slot
insulation. The coils are then connected to adjacent segments, and so there are as many
segments as there are coils.
The assembly composed of the commutator, coils, and iron core is called the armature.
The core is made of thin laminations stacked together to form a single block, see figure
3-6. The laminated core reduces the eddy current losses when the armature rotates.
Figure 3-6
Armature core is made of stacked laminations insulated from each other.
The dc output voltage is obtained by means of carbon brushes that ride on the
commutator surface. In two-pole generators, the brushes are spaced 1800 apart.
To summarize, when the armature of Figure 3-5 rotates, the armature coils cut across
the flux lines created by two stationary magnets (not shown). This action induces an ac
voltage in each coil. The sum of the ac voltages induced in the coils is rectified by means
of a commutator. The output voltage fluctuates slightly, see figure 3-7, therefore it is
not as steady as the voltage produced by a battery. However, this so-called
“commutator ripple” is unimportant in most industrial applications.
Figure 3-7
The dc output voltage of a multi-segment commutator contains only a small ripple
Figure 3-8 and 3-9 show the armature of a commercial dc generator
Figure 3-8
Armature of a commercial dc generator
Figure 3-9
Armature of an 1150kW, 750V, 1000 r/min dc generator
3.4 Field of a dc generator
The magnetic flux in a generator is created by its “field”. Here, the term field means all
the components needed to create the flux in the machine. Figure 3-10 shows the typical
construction of the field of a two-pole dc generator
Figure 3-10
Field structure of a two-pole dc generator
A circular cast-steel frame supports the salient poles and also provides a low-reluctance
path for the magnetic flux. Mounting feet and lifting lugs are usually welded to the
frame.
Two identical pre-wound coils are slipped over the poles and are then connected in
series. These so-called shunt coils usually have many turns of small wire, and produce
the mmf that creates the flux in the machine. The two field leads are brought out to
terminals F1 and F2, see figure 3-11.
Figure 3-11
Shunt field coils are mounted on the pole-pieces and connected in series.
3.5 The construction of dc generator
All generators whether ac or dc consist of a rotating part and a stationary part. In most
dc generator the coils that the output is taken from is mounted on the rotating part-the
rotor, which is referred to as the armature. The coils that generate the magnetic field are
mounted on the stationary part-the stator, which is referred to as the field.
Since the generator supplies electrical power to a load, mechanical power must be put
into generator to cause the rotor to turn and to generate electricity. The generator
converts mechanical power into electrical power. Consequently, all generators must have
machines associated with them that will supply the mechanical power necessary to turn
the rotors. These machines may be steam, gasoline, or diesel engines; electric motors;
or steam turbines actuated by the heat given off in the combustion of coal or oil, or
nuclear fission; or turbines driven by water power.
The relationship of the various components making up a dc generator is shown in Figure
3-12
Figure 3-12
Assembly of a dc generator
Generator design varies depending on the sizes, type and manufacture but the general
arrangement of parts is as shown in Fig 3-12
Main Frame: The main frame is sometimes called the yoke. It is the foundation of the
machine and supports the other components. It is also serves to complete the magnetic
field between the pole pieces.
Pole pieces: The pole pieces like transformer cores, are usually made of many thin
laminations of iron, joined together and bolted to the inside of the frame. These pole
pieces provide a support for the field coils and are designed to produce a concentrated
field. By laminating the poles, eddy currents are reduced.
Field winding: The field windings mounted on the pole pieces form electromagnets,
which provide the magnetic field necessary for generator action. The windings and pole
pieces together are often called the field. The windings are coils of insulated wire wound
to fit closely around pole pieces. Current flowing through these coils generates the
magnetic field. In some small generators, permanent magnets replace the field coils.
A generator may have only 2 poles, or it may have a number of pairs of poles.
Regardless of the number of poles, alternate poles will always be of opposite polarity.
Field windings can be connected either in series or in parallel (or shunt, as the parallel
connection is often called) with the armature. Shunt field windings consist of many turns
of fine wire, while series windings are composed of fewer turns of fairly heavier wire.
End Housing: These are attached to the ends of the main frame and contain the bearings
for the armature. The rear housing usually supports only the bearing, whereas the front
housing also supports the brush assemblies.
Brush Holder: This component supports the brushes and their connecting wires. The
brush holders are secured to the front end housing with clamps. On some generators,
the brush holders can be rotated around the shaft for adjustment.
Figure 3-13
The assemblies of armature and brush
Armature assembly: In practically all dc generators, the armature rotates between the
poles of the field. The armature assembly is made up of a shaft, an armature core,
armature windings, and a commutator. The working principle of the armature is already
stated in section 3.3. The shaft supports the entire armature assembly and rotates in the
end bearing. There is a small air gap between the armature and pole pieces to prevent
rubbing between the armature and pole pieces during rotation. This gap is kept to a
minimum in order to keep the field strength at a maximum.
Brushes: The brushes ride on the commutator and carry the generated voltage to the
load. They are usually made of a high grade of carbon, or a carbon-copper mixture, and
are held in place by brush holders. The brushes can slide up and down in their holders so
that they are able to follow irregularities in the surface of the commutator. A flexible
braided conductor, called a pigtail, connects each brush to the external circuit.
3.6
Generator voltage output
The amount of emf induced into a conductor depends upon the speed at which it is
cutting the flux. When a conductor cuts flux at the rate of 108 lines of force per second,
1 volt is induced. By utilizing this fact it is possible to derive an equation that will give
the average emf produced by generator. The equation is as follows:
VG = PZ N / 108(60b)
3-1(a)
Where
VG = generated emf, which at no load is the same value as the
terminal voltage, in volts (V)
P = number of poles in the main field
= number of lines of force per pole, flux per pole in weber (Wb)
N = armature speed, in revolutions per minute (r/min)
b = number of parallel paths through the armature
Z = total number of conductors (inductors) on the armature.
Because there are 2 inductors per turn, the total number of
Inductors is equal to 2 times the number of turns.
Example 3.1
Determine the average voltage generated in a 6-pole machine, running at 900 r/min, if
the armature has 300 conductors cutting the field. The flux per pole is 5 X 106 magnetic
lines. The armature has 6 parallel paths.
Solution
VG = PZ N / 108(60b)
VG = (6 X 300 X 5 x 106 X 900) / (108 X 60 X 6)
VG = 225V
For a specific generator, all the factors in the equation are fixed values with the
exception of the speed (N) and the flux per pole ( ). Therefore, the letter K may be
substituted for all value except N and . The equation now becomes
VG = K N
3-1(b)
Where K = combined result of all the fixed values or
A constant that depends upon the winding on the armature
Example 3.2
Determine the value of K for example 1
K = PZ / 108(60b)
K = 6 X 300 / (108 X 60 X 6)
K = 5 X 10-8
The formula VG = K N can be useful for designing generators. In practice, however,
generators usually operate at a fairly constant speed and the only variable is the field
flux ( ). A rheostat or a similar device is frequently installed in the field circuit to
provide a means of controlling the field current. Varying the field current will vary the
flux and thus the generated emf.
3.7
Schematic diagram of a dc generator
A schematic diagram of the generator is given in Fig 3-14.
Figure 3-14
Schematic diagram of a dc generator. The single pole represents 2 or more poles
It shows the field flux
created by field current If. Only one pole is shown, but it is
understood that it represents both poles. The circles inside the armature represent the
conductors that are lodged in the slots. The diagram can be further simplified to that
shown in Fig 3-15
Figure 3-15
Schematic diagram of a separately excited dc generator using graphic symbols recommended by the
Institute of Electrical and Electronic Engineers
3.8
Current in the armature: armature reaction
Referring back to Fig 3-5, when current flows into brush y and out of brush x, it flows in
all the armature coils. The armature then behaves like an electromagnet producing a
mmf and corresponding N and S poles, see Fig 3-16.
Figure 3-16
The armature produces a substantial mmf and flux when it carries its rated current
The armature mmf is almost as strong as the field mmf when the armature is carrying
its rated current.
The direction of the armature mmf depends upon the position of the brushes. If the
brushes are shifted 30˚, the mmf shifts 30˚. We are assuming that the mmf acts along
the axis of the brushes as shown in Fig 3-16. In practice, it could be at any angle to the
brush axis as shown in Fig 3-17.
Figure 3-17
The armature mmf acts at an angle to the brush axis
The direction of the mmf with respect to the brushes depends upon how the armature
coils are connected to the commutator segments.
When the armature begins to rotate the mmf remains unchanged as long as the
armature current Ia is the same. Thus, the mmf remains fixed in spaced whether or not
the armature is turning. However, if the armature current reverses, the mmf reverses,
as well as the N and S poles of the armature.
When the armature is mounted inside the generator, its mmf will have important
magnetic effects. These effects are grouped under the general term armature reaction.
In Fig 3-18, the field mmf acts from left to right, but the direction of the armature mmf
depends upon the brush setting.
Figure 3-18
Direction of the armature and field mmfs when the brushes are in neutral position and the armature is
turning ccw
It so happen that when the brushes are in the neutral position, the armature mmf in a
two-pole machine always acts at right angles to the field mmf. In a multi-pole machine
the axis of the armature mmf is midway between adjacent poles.
3.9 Separately excited dc generator
The flux in a dc machine is created by the field current If. When this current is supplied
by an independent dc source, such as a battery, the generator is said to be separately
excited. Figure 3-19 shows the schematic diagram of such a machine.
Figure 3-19
Separately excited generator using a potentiometer in order to vary the field current from zero to maximum
The field current can be varied from zero to maximum by moving the potentiometer
wiper from point A to point B. We assume the armature is driven by an internal
combustion engine or electric motor.
When such a generator operates at no-load, with the armature terminal x and y open
circuit, the armature voltage Exy depends upon the speed of rotation and the field
current If.+
If the speed of the driving motor is increased, the armature voltage increases in direct
proportion to the armature speed. In effect, the armature conductors cut the flux lines
more quickly, which raises the induced voltage in every conductor, according to the
equation e = BLv. At no-load, the total induced voltage Eo appears as Exy across the
armature terminals.
On the other hand, if we increase the field current while keeping the speed constant, the
flux
will increase. The increase in flux will produce a proportional increase in the flux
density B, and hence in the induced voltage Eo. However, as we continue to raise If, the
iron in the magnetic circuit begin to saturate. This means that for a large increase in If,
the flux will increase only slightly. Consequently, the induced voltage Eo and the
corresponding output voltage Exy will tend to flatten with increasing If.
If we plot Exy versus If, we obtain a curve such as the one shown in Fig 3-20.
Figure 3-20
No-load saturation curve of a separately excited dc generator
For low values of If, Exy increases linearly with If. The reason is that the reluctance of the
magnetic circuit is then mainly due to the air gaps. The flux in air is directly proportional
to the mmf. But, as If is increased and the iron begins to saturate, the corresponding
increase in flux is small, causing the curve to level off. This is called the no-load
saturation curve of the generator
If we reverse the direction of If, the flux will reverse, and this will reverse the polarity of
Exy. Finally, if we reverse the direction of rotation, the polarity of Exy will reverse.
3.10 Separately excited generator under load
When a dc generator delivers power to a load as shown in fig 3-21, several factors must
be considered.
Figure 3-21
Separately excited dc generator under load
1.
The flux
2.
The armature has a low, but not negligible, resistance Ra. It
causes an internal voltage drop IaRa, similar to that in a battery.
Consequently, the voltage Exy across the generator terminals is
less than the induced voltage Eo.
3.
The armature current Ia produces an armature mmf that acts at
right angles to the field mmf. The resultant mmf is therefore
inclined at an angle, as shown in fig 3-22. This causes the flux to
move diagonally across the armature. Unfortunately, this
increases the flux density in pole tips a and b, and so they
become saturated. This, in turn, reduces the total flux
that
crosses the armature. As a results, the induced voltage Eo is less
at full-load than at no-load.
induces a voltage Eo in the armature
Figure 3-22
The flux crossing the armature become distorted and smaller because of armature reaction
The overall effect of armature resistance and armature reaction is that the armature
voltage Exy decreases with increasing load current, as shown in fig 3-23.
Figure 3-23
Voltage regulation curves of separately excited and self-excited dc generators starting from the no load
voltage. To obtain rated voltage at rated current, the no-load excitation of the self-excited generator must
be raised
The same voltage drop takes place in a self-excited shunt generator, but is accentuated.
The reason is that as the armature voltage decrease (due to the effects just mentioned),
there is a corresponding drop in field current. Therefore, the flux
decrease even more
with increasing load than it did before. Thus, the voltage regulation curve of a selfexcited generator drops more steeply than that of a separately excited generator.
3.11 Shunt generator
A self-excited shunt generator is one that provides its own field excitation in which the
field and armature are connected in parallel.
The field coils are constructed of many turns of small wire. By using small wire and
many turns, a strong field can be produced while keeping the current to a minimum.
This result is more economical and compact construction, and also improves the
efficiency of the machine.
Because the armature supplies the emf for both the load and the field, all the current
must flow through the armature. Figure 3-24 is a schematic drawing for a shunt
generator. The arrows indicate the current flow.
Figure 3-24
Shunt generator
The armature current increases as a load are added to a shunt generator. An increase in
the armature current causes an increase in the effects of armature reaction. In other
words, as the load current increases, the generated emf decreases. This decrease in the
generated emf results in a lower output voltage. Another factor that affects the output
voltage of a generator is the IR (voltage) drop in the armature.
Because the field receives its voltage from the armature, any decrease in terminal
voltage results in fewer fields current. The reduction in field current causes a weakening
of the main field flux. The weaker field results in a further decrease in the output
voltage.
It may appear from this information that the generated emf would quickly drop to zero
but this does not occur, due to the internal design of the generator. Figure 3-25 is shown
the load/voltage characteristics of a shunt generator.
Figure 3-25
Load/voltage characteristics of a shunt generator
It is generally desirable that the voltage across the load be a constant value. However,
shunt generators still are used in industry because they are economical and provide
efficient means for supplying power to constant loads. Well-designed generators do not
have more than an 8-percent decrease in voltage from no load to full load. These
machines are suitable where slight voltage fluctuations are not a problem.
One advantage of the load/voltage characteristics of the shunt generator is selfprotection. A short circuit on the load will cause the output voltage drop rapidly as
shown in fig 3-26.
Figure 3-26
Voltage curve of a shunt generator when a short circuit occurs across the armature
This sharp drop in voltage will reduce the current to a minimum and thus prevent
overheating.
3.12 Series generator
A series generator, as the name implies, has the field connected in series with the
armature. Figure 3-27 shows these connections. The arrows indicate the current flow.
Figure 3-27
Schematic diagram of a series generator
Notice that the entire load current flows through the field windings. For this reason, the
field coils must be wound with wire large enough to carry the full-load current of the
generator.
In a series generator, an increase in load causes a similar increase in the field current.
This results in an increase in the generated emf and a higher output voltage. The
increase in voltage with increases in load will continue until the magnetic circuit of the
field is saturated. An increase in load beyond this point will cause the output voltage to
decrease. The decrease is a result of IR drop and armature reaction. Figure 3-28 is
shown the load/voltage characteristics of a series generator.
Figure 3-28
Field strength versus output voltage for a series generator
In the past, series generators were used extensively for series arc lighting, particularly
for street lighting. Later, when the arc lamps were replaced by series tungsten lamps,
the series generator still supplied the power. Most tungsten lamps used for this purpose
have been replaced by mercury vapor or high-pressure sodium lamps. These lamps
require alternating current. Modern technology has practically phased out the series
generator. However, it is sometimes found in remote areas or in applications where it is
supplying constant loads.
3.13 Compound generator
In industry, most loads require a constant value of voltage, but the load on the
generator fluctuates considerably when a plant is in operation. This type of demand
makes both the shunt and series generators undesirable for most conditions of use.
However, it is possible to maintain a constant voltage under varying loads by combining
certain features of the shunt and series generators into one machine. This type of
machine is called a compound generator, as shown in figure 3-29.
Figure 3-29
Schematic diagram of a compound generator
The load/voltage characteristics of a compound generator depend upon the number of
turns on the series field. With many turns on the series field, the machine will begin to
assume the characteristics of a series generator. Few turns will result in predominantly
shunt characteristics.
The number of turns on the series field determines the amount of compounding. A
machine with many turns on the series field is called an over-compounded generator. A
generator with just enough series turns to maintain a steady voltage from no load to full
load is a flat-compounded generator. One with fewer turns is said to be an undercompounded generator. Most compound generator are designed for over-compounding.
The degree of compounding is determined by a diverter (a resistance of specific value)
connected in parallel with the series field, see figure 3-30.
Figure 3-30
Compound generator with series field diverter
The load/voltage curves for over-, under-, and flat-compounded generators are depicted
in figure 3-31
Figure 3-31
Load/voltage characteristics of compound generators
In practice, flat compounding does not actually produce a flat curve. The nearest
approach is to adjust the machine so that the terminal voltage will rise slightly and then
drops again, reaching the same value at full load as at no load.
Flat-compound generators are used when the load is located near to generator. If the
load located some distance away, an over-compounded machine is used. The overcompounding will compensate for the voltage drop in the line wire. The amount of overcompounding used depends upon the type of service. For generators where the load is a
great distance away, 10 percent over-compounding is common. Generators supplying
power to street railway, monorail, and subway systems fall into this category.
The differential-compound generator is another type of compound generator. This
machine is connected in such a way that the series field mmf opposes the shunt field
mmf. The voltage drops off sharply as load is added. This type of machine is used most
commonly in arc welding. When a heavy load occurs, the series field flux tends to
neutralize much of the shunt field flux. The result is a decrease of the overall flux. The
induced emf drops, reducing the armature current and preventing the armature from
overheating.
How much mechanical power is needed to drive the compound generator of figure 3-32?
Figure 3-32
Compound dc generator under load
First, all the electric power that is consumed by the load, the shunt field, the series field,
the armature resistance, and the shunt field rheostat must be equal to the total
generated power, which is equal to EoIa.
Second, this generated power is entirely produced by the internal mechanical power Pmi
that is needed to drive the armature. Thus, we can write;
Pmi = EoIa
3-2
Where
Pmi = mechanical power needed to produce the electrical output, in watts (W)
Eo = induced voltage, in volts (V)
Ia = armature current, in amperes (A)
It takes slightly more mechanical power than Pmi to drive the generator because of the
windage and friction losses, and the hysteresis and eddy-current losses in the armature
core. Note that Eq. 3-2 applies to any dc generator-compound, shunt or series.
Example 3.3
The following information is given for the two-pole compound generator shown in fig 333.
Figure 3-33
shunt field resistance = 40Ω
series field resistance = 0.04Ω
armature resistance = 0.16 Ω
field rheostat resistance = 10 Ω
terminal voltage Exy = 250 V
load current IL = 90A
windage and friction losses = 200 W
iron losses in the armature = 300 W
turns on the shunt field = 240
turns on the series field = 4
Calculate:
1.
2.
3.
4.
5.
6.
the
the
the
the
the
the
Solution:
shunt field current If
voltage across the shunt field coils
power loss in the series field
value of the induced voltage Eo
mechanical power needed to drive the generator
total mmf of the shunt and the series windings, per pole