NEETS Module 5 − Introduction to Generators and Motors
ALTERNATING CURRENT GENERATORS
Upon completion of this chapter, you will be able to:
1. Describe the principle of magnetic
induction as it applies to AC generators.
2. Describe the differences between the two basic types of AC
3. List the advantages and disadvantages of the two types of AC generators.
Describe exciter generators within alternators; discuss construction and purpose.
5. Compare the types
of rotors used in AC generators, and the applications of each type to different prime movers.
the factors that determine the maximum power output of an AC generator, and the effect of these factors in rating
7. Explain the operation of multiphase AC generators and compare with single-phase.
8. Describe the relationships between the individual output and resultant vectorial sum voltages in multiphase
9. Explain, using diagrams, the different methods of connecting three-phase alternators and transformers.
10. List the factors that determine the frequency and voltage of the alternator output.
the terms voltage control and voltage regulation in AC generators, and list the factors that affect each quantity.
12. Describe the purpose and procedure of parallel generator operation.
Most of the electrical power used aboard Navy ships and aircraft as well as in civilian applications is
ac. As a result, the AC generator is the most important means of producing electrical power. AC generators,
generally called alternators, vary greatly in size depending upon the load to which they supply power. For
example, the alternators in use at hydroelectric plants, such as Hoover Dam, are tremendous in size, generating
thousands of kilowatts at very high voltage levels. Another example is the alternator in a typical automobile,
which is very small by comparison. It weighs only a few pounds and produces between 100 and 200 watts of power,
usually at a potential of 12 volts.
Many of the terms and principles covered in this chapter will be familiar to you. They are the same
covered in the chapter on DC generators. You are encouraged to refer back, as needed, and to refer
to any other source that will help you master the subject of this chapter. No one source meets the
complete needs of everyone.
Basic AC GENERATORS
Regardless of size, all electrical generators, whether DC or ac, depend upon the principle of magnetic
induction. An EMF is induced in a coil as a result of (1) a coil cutting through a magnetic field, or (2) a
magnetic field cutting through a coil. As long as there is relative motion between a conductor and a magnetic
field, a voltage will be induced in the conductor. That part of a generator that produces the magnetic field is
called the field. That part in which the voltage is induced is called the armature. For relative motion to take
place between the conductor and the magnetic field, all generators must have two mechanical parts - a rotor and a
stator. The ROTor is the part that ROTates; the STATor is the part that remains STATionary. In a DC generator, the
armature is always the rotor. In alternators, the armature may be either the rotor or stator.
Magnetic induction occurs when there is relative motion between what two elements?
The rotating-armature alternator is similar in construction to the
DC generator in that the armature rotates in a stationary magnetic field as shown in figure 3-1, view A. In the DC
generator, the EMF generated in the armature windings is converted from AC to DC by means of the commutator. In
alternator, the generated AC is brought to the load unchanged by means of slip rings. The rotating armature
is found only in alternators of low power rating and generally is not used to supply electric power in large
Figure 3-1. - Types of AC generators.
The rotating-field alternator has a stationary
armature winding and a rotating-field winding as shown in figure 3-1, view B The advantage of having a stationary
armature winding is that the generated voltage can be connected directly to the load.
A rotating armature
requires slip rings and brushes to conduct the current from the armature to the load. The armature, brushes, and
slip rings are difficult to insulate, and arc-overs and short circuits can result at high voltages. For this
reason, high-voltage alternators are usually of the rotating-field type. Since the voltage applied to the rotating
field is low voltage DC, the problem of high voltage arc-over at the slip rings does not exist.
stationary armature, or stator, of this type of alternator holds the windings that are cut by the rotating
magnetic field. The voltage generated in the armature as a result of this cutting action is the AC power that will
be applied to the load.
The stators of all rotating-field alternators are about the same. The stator consists of a laminated iron core
with the armature windings embedded in this core as shown in figure 3-2. The core is secured to the stator frame.
Figure 3-2. - Stationary armature windings.
Q2. What is the part of an alternator in which the output voltage is generated?
Q3. What are the two basic types of alternators?
Q4. What is the main advantage of the rotating field
The alternators described so far in this chapter are ELEMENTARY in nature; they are seldom used except
as examples to aid in understanding practical alternators.
The remainder of this chapter will relate the
principles of the elementary alternator to the alternators actually in use in the civilian community, as well as
aboard Navy ships and aircraft. The following paragraphs in this chapter will introduce such concepts as prime
movers, field excitation, armature characteristics and limitations, single-phase and polyphase alternators,
controls, regulation, and parallel operation.
Functions of ALTERNATOR COMPONENTS
a typical rotating-field AC generator consists of an alternator and a smaller DC generator built into a single
unit. The output of the alternator section supplies alternating voltage to the load. The only purpose for the DC
exciter generator is to supply the direct current required to maintain the alternator field. This DC generator is
referred to as the exciter. a typical alternator is shown in figure 3-3, view A; figure 3-3, view B, is a
simplified schematic of the generator.
Figure 3-3. - Ac generator pictorial and schematic drawings.
The exciter is a DC, shunt-wound, self-excited generator. The exciter shunt field (2) creates an area of
intense magnetic flux between its poles. When the exciter armature (3) is rotated in the exciter-field flux,
voltage is induced in the exciter armature windings. The output from the exciter commutator (4) is connected
through brushes and slip rings (5) to the alternator field. Since this is direct current already converted by the
exciter commutator, the current always flows in one direction through the alternator field (6). Thus, a
fixed-polarity magnetic field is maintained at all times in the alternator field windings. When the alternator
field is rotated, its magnetic flux is passed through and across the alternator armature windings (7).
armature is wound for a three-phase output, which will be covered later in this chapter. Remember, a voltage is
induced in a conductor if it is stationary and a magnetic field is passed across the conductor, the same as if the
field is stationary and the conductor is moved. The alternating voltage in the AC generator armature windings is
connected through fixed terminals to the AC load.
Q5. Most large alternators have a small DC generator built into them. What is its purpose?
All generators, large and small, AC and DC, require a source of
mechanical power to turn their rotors. This source of mechanical energy is called a prime mover.
movers are divided into two classes for generators-high-speed and low-speed. Steam and gas turbines are high-speed
prime movers, while internal-combustion engines, water, and electric motors are considered low-speed prime movers.
The type of prime mover plays an important part in the design of alternators since the speed at which the rotor is
turned determines certain characteristics of alternator construction and operation.
There are two types of rotors used in rotating-field alternators. They are called the
turbine-driven and salient-pole rotors.
As you may have guessed, the turbine-driven rotor shown in figure
3-4, view A, is used when the prime mover is a high-speed turbine. The windings in the turbine-driven rotor are
arranged to form two or four distinct poles. The windings are firmly embedded in slots to withstand the tremendous
centrifugal forces encountered at high speeds.
Figure 3-4. - Types of rotors used in alternators.
The salient-pole rotor shown in figure 3-4, view B, is used in low-speed alternators. The salient-pole
rotor often consists of several separately wound pole pieces, bolted to the frame of the rotor.
could compare the physical size of the two types of rotors with the same electrical characteristics, you would see
that the salient-pole rotor would have a greater diameter. At the same number of revolutions per minute, it has a
greater centrifugal force than does the turbine-driven rotor. To
reduce this force to a safe level so that the windings will not be thrown out of the machine, the
salient pole is used only in low-speed designs.
ALTERNATOR Characteristics and Limitations
Alternators are rated according to the voltage they are designed to produce and the maximum current they are
capable of providing. The maximum current that can be supplied by an alternator depends upon the maximum heating
loss that can be sustained in the armature. This heating loss (which is an I2R power loss) acts to heat the
conductors, and if excessive, destroys the insulation. Thus, alternators are rated in terms of this current and in
terms of the voltage output - the alternator rating in small units is in volt- amperes; in large units it is
When an alternator leaves the factory, it is already destined to do a very specific job. The speed at which it is
designed to rotate, the voltage it will produce, the current limits, and other operating characteristics are built
in. This information is usually stamped on a nameplate on the case so that the user will know the limitations.
Q6. How are alternators usually rated?
Q7. What type of prime mover requires a specially designed
Q8. Salient-pole rotors may be used in alternators driven by what types of prime
A generator that produces a single, continuously
alternating voltage is known as a Single-Phase alternator. All of the alternators that have been discussed so far
fit this definition. The stator (armature) windings are connected in series. The individual voltages, therefore,
add to produce a single-phase AC voltage. Figure 3-5 shows a basic alternator with its single-phase output
Figure 3-5. - Single-phase alternator.
The definition of phase as you learned it in studying AC circuits may not help too much right here.
Remember, "out of phase" meant "out of time."
Now, it may be easier to think of the word phase as meaning
voltage as in single voltage. The need for a modified definition of phase in this usage will be easier to see as
we go along.
Single-phase alternators are found in many applications. They are most often used when the loads being
driven are relatively light. The reason for this will be more apparent as we get into multiphase alternators (also
Power that is used in homes, shops, and ships to operate portable tools and small
appliances is single-phase power. Single-phase power alternators always generate single-phase power. However, all
single-phase power does not come from single-phase alternators. This will sound more reasonable to you as we get
into the next subjects.
Q9. What does the term single phase indicate?
Q10. In single-phase
alternators, in order for the voltages induced in all the armature windings to add together for a single output,
how must the windings be connected?
Two phase implies two voltages if we apply our new definition of phase. And, it's that simple. a two-phase
alternator is designed to produce two completely separate voltages. Each voltage, by itself, may be considered as
a single-phase voltage. Each is generated completely independent of the other. Certain advantages are gained.
These and the mechanics of generation will be covered in the following paragraphs.
Figure 3-6 shows a simplified two-pole, two-phase alternator. Note that the windings of the two phases are
physically at right angles (90º ) to each other. You would expect the outputs of each phase to be 90º apart,
which they are. The graph shows the two phases to be 90º apart, with a leading B. Note that by using our original
definition of phase (from previous modules), we could say that a and B are 90º out of phase. There will always be
90º between the phases of a two-phase alternator. This is by design.
Figure 3-6. - Two-phase alternator.
Now, let's go back and see the similarities and differences between our original (single-phase)
alternators and this new one (two-phase). Note that the principles applied are not new. This alternator works the
same as the others we have discussed.
The stator in figure 3-6 consists of two single-phase windings
completely separated from each other. Each winding is made up of two windings that are connected in series so that
their voltages add. The rotor is identical to that used in the single-phase alternator. In the left-hand
schematic, the rotor poles are opposite all the windings of phase A. Therefore, the voltage induced in phase a is
maximum, and the voltage induced in phase B is zero. As the rotor continues rotating counterclockwise, it moves
away from the a windings and approaches the B windings. As a result, the voltage induced in phase a decreases
from its maximum value, and the voltage induced in phase B increases from zero. In the right-hand schematic, the
rotor poles are opposite the windings of phase B. Now the voltage induced in phase B is maximum, whereas the
voltage induced in phase a has dropped to zero. Notice that a 90-degree rotation of the rotor corresponds to
one-quarter of a cycle, or 90 electrical degrees. The waveform picture shows the voltages induced in phase a and B
for one cycle. The two voltages are 90º out of phase. Notice that the two outputs, a and B, are independent of
each other. Each output is a single-phase voltage, just as if the other did not exist.
advantage, so far, is that we have two separate output voltages. There is some saving in having one set of
bearings, one rotor, one housing, and so on, to do the work of two. There is the disadvantage of having twice as
many stator coils, which require a larger and more complex stator.
The large schematic in figure 3-7 shows
four separate wires brought out from the a and B stator windings. This is the same as in figure 3-6. Notice,
however, that the dotted wire now connects one end of B1 to one end of A2. The effect of making this connection is
to provide a new output voltage. This sine- wave voltage, C in the picture, is larger than either a or B. It is
the result of adding the instantaneous values of phase a and phase B. For this reason it appears exactly half way
between a and B. Therefore, C must lag a by 45º and lead B by 45º , as shown in the small vector diagram.
Figure 3-7. - Connections of a two-phase, three-wire alternator output.
Now, look at the smaller schematic diagram in figure 3-7. Only three connections have been brought out
from the stator. Electrically, this is the same as the large diagram above it. Instead of being connected at the
output terminals, the B1-A2 connection was made internally when the stator was wired. a two- phase alternator
connected in this manner is called a two-phase, three-wire alternator.
The three-wire connection makes
possible three different load connections: a and B (across each phase), and C (across both phases). The output at
C is always 1.414 times the voltage of either phase. These multiple outputs are additional advantages of the
two-phase alternator over the single-phase type.
Now, you can understand why single-phase power doesn't
always come from single-phase alternators. It can be generated by two-phase alternators as well as other
multiphase (polyphase) alternators, as you will soon see.
The two-phase alternator discussed in the
preceding paragraphs is seldom seen in actual use. However, the operation of polyphase alternators is more easily
explained using two phases than three phases. The three-phase alternator, which will be covered next, is by far
the most common of all alternators in use today, both in military and civilian applications.
determines the phase relationship between the voltages in a two-phase AC generator?
Q12. How many
voltage outputs are available from a two-phase three-wire alternator?
Q13. What is the relationship of
the voltage at C in figure 3-7 to the voltages at a and B?
The three-phase alternator, as the name implies, has three single-phase windings spaced such that the voltage
induced in any one phase is displaced by 120º from the other two. a schematic diagram of a three-phase stator
showing all the coils becomes complex, and it is difficult to see what is actually happening. The simplified
schematic of figure 3-8, view A, shows all the windings of each phase lumped together as one winding. The rotor is
omitted for simplicity. The voltage waveforms generated across each phase are drawn on a graph, phase-displaced
120º from each other. The three-phase alternator as shown in this schematic is made up of three single-phase
alternators whose generated voltages are out of phase by 120º . The three phases are independent of each other.
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