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SSeeccttiioonn 11
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A SIMPLE AC GENERATOR
Figure 1-4 shows a very simple AC Generator. The
generator consists of a rotating magnetic field called
a ROTOR and a stationary coil of wire called a STA-
TOR. The ROTOR is a permanent magnet which con-
sists of a SOUTH magnetic pole and a NORTH mag-
netic pole.
As the MOTOR turns, its magnetic field cuts across
the stationary STATOR. A voltage is induced Into the
STATOR windings. When the magnet's NORTH pole
passes the STATOR, current flows in one direction.
Current flows in the opposite direction when the mag-
net's SOUTH pole passes the STATOR. This con-
stant reversal of current flow results in an alternating
current (AC) waveform that can be diagrammed as
shown in Figure 1-5.
The ROTOR may be a 2-pole type having a single
NORTH and a single SOUTH magnetic pole. Some
ROTORS are 4-pole type with two SOUTH and two
NORTH magnetic poles. The following apply:
1. The 2-pole ROTOR must be turned at 3600 rpm to produce an
AC frequency of 60 Hertz, or at 3000 rpm to deliver an AC fre-
quency of 50 Hertz.
2. The 4-pole ROTOR must operate at 1800 rpm to deliver a 60
Hertz AC frequency or at 1500 rpm to deliver a 50 Hertz AC
frequency.
Figure 1-4. – A Simple AC Generator
Figure 1-5. – Alternating Current Sine Wave
A MORE SOPHISTICATED AC GENERATOR
Figure 1-6 represents a more sophisticated genera-
tor. A regulated direct current is delivered into the
ROTOR windings via carbon BRUSHES AND SLIP
RINGS. This results in the creation of a regulated
magnetic field around the ROTOR. As a result, a reg-
ulated voltage is induced into the STATOR.
Regulated current delivered to the ROTOR is called
“EXCITATION” current.
Figure 1-6. – A More Sophisticated Generator
See Figure 1-7 (next page). The revolving magnetic
field (ROTOR) is driven by the engine at a constant
speed. This constant speed is maintained by a
mechanical engine governor. Units with a 2-pole rotor
require an operating speed of 3600 rpm to deliver a
60 Hertz AC output. Engine governors are set to
maintain approximately 3720 rpm when no electrical
loads are connected to the generator.
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