INTRODUCTION TO AC GENERATORS
Most of the electrical power used aboard military 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 to any other source that will help you master the subject of this section. No one source meets the complete needs of everyone.
Regardless of size, all electrical ac 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.
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 the illustration below, 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.
Types of ac generators.