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AC Generators and Motors
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INTRODUCTION
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
as those covered in the chapter on dc generators. You are encouraged to refer back, as needed, and to refer
3-2
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.
ROTATING-ARMATURE ALTERNATORS
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 the
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
quantities.
ROTATING-FIELD ALTERNATORS
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.
The 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.
PRACTICAL ALTERNATORS
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.
PRIME MOVERS
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.
Prime 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.
ALTERNATOR ROTORS
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.