Section 1
GENERATOR FUNDAMENTALS
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 STATOR. The ROTOR is a permanent magnet which consists of a SOUTH magnetic pole and a NORTH magnetic 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 direc- tion. Current flows in the opposite direction when the magnet'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 frequency 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.
STATOR
ROTOR
MAGNETIC FIELD
Figure 1-4. – A Simple AC Generator
CURRENT VOLTAGE
(+)
0 | 180 | 360 |
ONE CYCLE
Figure 1-5. – Alternating Current Sine Wave
A More Sophisticated AC Generator
Figure 1-6 represents a more sophisticated generator. 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 regulated volt- age is induced into the STATOR. Regulated current delivered to the ROTOR is called “EXCITATION” cur- rent.
OUTPUT | 120 V | STATOR STATOR |
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AC | 120 V |
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BRUSHES |
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DC |
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RINGS
Figure 1-6. – A More Sophisticated Generator
See Figure
Generator operation may be described briefly as fol- lows:
1.Some “residual” magnetism is normally present in the Rotor and is sufficient to induce approximately 7 to 12 volts AC Into
the STATOR's AC power windings.
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