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.
Q1. Magnetic induction occurs when there is relative motion between what two elements?
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.
