Page 238 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

Source: ELECTRICAL EQUIPMENT HANDBOOK

CHAPTER 12
SYNCHRONOUS

GENERATORS

Synchronous generators or alternators are synchronous machines that convert mechanical
energy to ac electric energy.

SYNCHRONOUS GENERATOR CONSTRUCTION

A direct current is applied to the rotor winding of a synchronous generator to produce the
rotor magnetic field. A prime mover causes the generator rotor to rotate the magnetic field
in the machine. A three-phase set of voltages is induced in the stator windings by the rotating
magnetic field.
The rotor is a large electromagnet. Its magnetic poles can be salient (protruding or stick-
ing out from the surface of the rotor), as shown in Fig. 12.1 or nonsalient (flush with the
surface of the rotor), as shown in Fig. 12.2. Two- and four-pole rotors have normally non-
salient poles, while rotors with more than four poles have salient-pole rotors.
Small generator rotors are constructed of thin laminations to reduce eddy current losses,
while large rotors are not constructed from laminations due to the high mechanical stresses
encountered during operation.
The field circuit of the rotor is supplied by a direct current. The common methods used
to supply the dc power are
1. By means of slip rings and brushes
2. By a special dc power source mounted directly on the shaft of the rotor

Slip rings are metal rings that encircle the rotor shaft but are insulated from it. Each of the
two slip rings on the shaft is connected to one end of the dc rotor winding, and a number of
brushes ride on each slip ring.
The positive end of the dc voltage source is connected to one slip ring, and the negative
to the second. This ensures that the same dc voltage is applied to the field windings regard-
less of the angular position or speed of the rotor.
Slip rings and brushes require high maintenance because the brushes must be checked
for wear regularly. Also, the voltage drop across the brushes can be the cause of large power
losses when the field currents are high.
Despite these problems, all small generators use slip rings and brushes because all other
methods used for supplying dc field power are more expensive.
Large generators use brushless exciters for supplying dc field power to the rotor. They
consist of a small ac generator having its field circuit mounted on the stator and its armature
circuit mounted on the rotor shaft.

12.1
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