magnetic induction. An emf is induced in a coil as a
field. At the instant the conductors are moving parallel
result of
to the magnetic field, they do not cut any lines of force.
A coil cutting through a magnetic field, or
Therefore, no electromotive force (emf) is induced in
the conductors; the meter in part A indicates zero. This
A magnetic field cutting through a coil.
position is called the NEUTRAL PLANE. As the
armature loop rotates from position A to position B, the
As long as there is relative motion between a
conductors cut through more and more lines of force at
conductor and a magnetic field, a voltage is induced in
a continually increasing angle. At 90 (position B),
the conductor. That part of a generator that produces
they are cutting through a maximum number of lines of
the magnetic field is called the field. That part in which
force and at a maximum angle. The result is that
the voltage is induced is called the armature. For
between 0 and 90, the induced emf in the conductor
relative motion to take place between the conductor
builds up from zero to a maximum value. Observe that
and the magnetic field, all generators must have two
mechanical parts--a rotor and a stator. The rotor is the
from 0 to 90 the black conductor cuts DOWN
part that rotates; the stator is the part that remains
through the field. At the same time the white conductor
stationary. In a dc generator, the armature is always the
cuts UP through the field. The induced emf in the
rotor. In ac generators, the armature may be either the
conductors are series-adding. This means the resultant
rotor or stator.
voltage across the brushes (the terminal voltage) is the
A generator that produces alternating current is
sum of the two induced voltages. The meter at position
called an ac generator (alternator). One that produces a
B reads maximum value.
direct current is called a dc generator. However, both
As the armature loop continues to rotate from
types operate by the induction of an ac voltage in the
position B (90) to position C (180), the conductors
coils (conductors). The major difference between an ac
that were cutting through a maximum number of lines
generator and a dc generator is in the method by which
of force at position B now cut through fewer lines. At
the electrical energy is connected and applied to the
external circuit.
position C they are again moving parallel to the
magnetic field. They no longer cut through any lines of
Automotive ac generators are normally called
force. As the armature rotates from 90 to 180, the
alternators. But, when used to supply external power
induced voltage decreases to zero in the same manner
on the MEPPs, they are usually called ac generators.
as it increased from 0 to 90. The meter again reads
The ac generators used on the MEPP are usually larger
zero. From 0 to 180, the conductors of the armature
in size, heavier in weight, and have a heavier power
output than the automotive type.
loop moved in the same direction through the magnetic
field. Therefore, the polarity of the induced voltage
remained the same. This is shown by positions A
through C. As the loop starts rotating beyond 180,
A dc generator is similar to an ac generator with
from position C, through D, to position A, the direction
one major exception. An ac generator has slip rings,
of the cutting action of the conductors through the
while a dc generator has commutator segments.
magnetic field reverses. Now the black conductor cuts
The purpose of the commutator is to rectify the ac
UP through the field. The white conductor cuts DOWN
waveform. The segments of the commutator are
through the field. As a result, the polarity of the
insulated from each other and the armature shaft.
induced voltage reverses. If you follow the sequence
Wires from the armature are connected to the
shown in positions C through D and back to position A,
commutator segments. Armature current is conducted
the voltage is in the direction opposite to that shown
from the commutator to the load by carbon brushes.
from positions A, B, and C. The terminal voltage is the
Figure 7-24 shows a graph of the output of a simple
same as it was from A to C, except for its reversed
dc generator. This waveform is for a simple dc
polarity (as shown by meter deflection in D). The
generator, one that has only a single coil armature and a
voltage output waveform for the complete revolution
two-section commutator. It is not the waveform
of the loop is as shown on the graph.
generated by a practical dc generator, which should
Regardless of size, all electrical generators,
produce output with very little fluctuation. To help
whether dc or ac, depend upon the principle of
produce a smoother dc output, more coils and
7-15
