Page 250 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

SYNCHRONOUS GENERATORS

SYNCHRONOUS GENERATORS 12.13

The output power equation shows that the power produced depends on the angle
(torque angle) between V and E . Normally, real generators have a full-load torque angle
A

of 15 to 20°. The induced torque in the generator can be expressed as
kB
B
ind R S
or as
kB
B
ind R net
The magnitude of the expressed torque is
kB B sin
ind R net
where (the torque angle) is the angle between the rotor and net magnetic fields.
An alternative expression for the induced torque in terms of electrical quantities is

3V E A sin
ind
X
m S

THE SYNCHRONOUS GENERATOR
OPERATING ALONE

When a synchronous generator is operating under load, its behavior varies greatly depending
on the power factor of the load and if the generator is operating alone or in parallel with
other synchronous generators.
Throughout the upcoming sections, the effect of R is ignored, and the speed of the
A
generators and the rotor flux will be assumed constant.

The Effect of Load Changes on a Synchronous Generator Operating Alone
Figure 12.17 illustrates a generator supplying a load. What are the effects of load increase
on the generator? When the load increases, the real and/or reactive power drawn from the
generator increases. The load increase increases the load current drawn from the generator.
The flux is constant because the field resistor did not change, and the field current is
constant. Since the prime mover governing system maintains the mechanical speed con-
stant, the magnitude of the internal generated voltage E K is constant. Since E is
A A
constant, which parameter is varying with the changing load?
If the generator is operating at a lagging power factor and an additional load is added at
the same power factor, then the magnitude of I increases, but the angle
between I and
A
A

FIGURE 12.17 A single generator supplying a load.

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