Page 240 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

SYNCHRONOUS GENERATORS

SYNCHRONOUS GENERATORS 12.3

The exciter generator output (three-phase ac) is converted to dc power by a three-phase
rectifier circuit also mounted on the rotor. The dc power is fed to the main field circuit. The
field current for the main generator can be controlled by the small dc field power of the
exciter generator, which is located on the stator (Figs. 12.3 and 12.4). A brushless excita-
tion system requires much less maintenance than slip rings and brushes because there is no
mechanical contact between the rotor and the stator.
The generator excitation system can be made completely independent of any external
power sources by using a small pilot exciter. It consists of a small ac generator with per-
manent magnets mounted on the rotor shaft and a three-phase winding on the stator.
The pilot exciter produces the power required by the field circuit of the exciter, which
is used to control the field circuit of the main generator. When a pilot exciter is used, the
generator can operate without any external electric power (Fig. 12.5).
Most synchronous generators that have brushless exciters also use slip rings and brushes
as an auxiliary source of field dc power in emergencies. Figure 12.6 illustrates a cutaway
of a complete large synchronous generator having a salient-pole rotor with eight poles, and
a brushless exciter.

THE SPEED OF ROTATION OF A SYNCHRONOUS
GENERATOR

The electrical frequency of synchronous generators is synchronized (locked in) with the
mechanical rate of rotation. The rate of rotation of the magnetic fields (mechanical speed)
is related to the stator electrical frequency by:

n m P
f
e 120

where f electrical frequency, Hz
e
n mechanical speed of magnetic field, r/min ( speed of rotor for synchronous
m
machines)
P number of poles
For example, a two-pole generator rotor must rotate at 3600 r/min to generate electricity at
60 Hz.

THE INTERNAL GENERATED VOLTAGE OF A
SYNCHRONOUS GENERATOR

The magnitude of the voltage induced in a given stator phase is given by
E K
A
where K is a constant that depends on the generator construction, is the flux in the
machine, and is the frequency or speed of rotation. Figure 12.7a illustrates the relation-
ship between the flux in the machine and the field current I . Since the internal generated
F
voltage E is directly proportional to the flux, the relationship between E and I is similar
A A F
to the one between and I (Fig. 12.7b). The graph is known as the magnetization curve
F
or open-circuit characteristic of the machine.

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