Page 231 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

SYNCHRONOUS MACHINES

11.6 CHAPTER ELEVEN

the field current to the exciter must affect the exciter output before it can vary the voltage
at the terminals of the main machine, delays occur in the variations of the voltage from
the main machine. A typical response time of a rotating exciter is around 0.5 to 1.0 s. The
response time required to change the voltage of a machine using slip rings and brushes is
around 0.2 s.
The rotating rectifier machines cannot be used in large modern power plants due to the
requirement of short response time of the excitation system. This is necessary to be able to
vary the voltage quickly to stabilize the power out of the machine.

Series Excitation
The excitation of a synchronous machine may also be derived from the output power of
the main machine. This type of excitation is accomplished in conjunction with a sepa-
rate excitation system that originates from a separate source, such as a battery bank.
The series excitation is appropriate for isolated synchronous alternators which are used
to start motors requiring high inrush current. This type of excitation is rarely used with large
synchronous machines.

NO-LOAD AND SHORT-CIRCUIT VALUES

The no-load or open-circuit voltage is generated in the armature windings of a synchronous
machine when the armature terminals are open-circuited and the rotor is rotating at syn-
chronous speed while the field winding is energized. The induced open-circuit voltages in
the three phases are given by
e E cos t
a m
e E cos ( t 120°)
b m
e E cos ( t 120°)
c m
where E K
m
K constant that depends on generator size and design
rotational speed of rotor
flux created by field current
Figure 11.5 illustrates a typical open-circuit saturation curve of a synchronous machine.
It represents the no-load characteristics of the machine. The air gap line represents the
extension of the straight-line portion of the saturation curve. Figure 11.5 also illustrates a
typical short-circuit saturation characteristic of a synchronous machine. It is obtained by
shorting the terminals of the armature together while the rotor is rotating at synchronous
speed and the field current is increased from zero to a small value. The short-circuit satu-
ration curve is a plot of the variations of the current in the shorted armature windings ver-
sus the field current.
The zero power factor saturation characteristic is obtained by overexciting the machine
while it is connected to a highly inductive load or an idle running synchronous motor. The
terminal voltage of the machine being tested is varied while its armature current is held con-
stant at the rated value by adjusting the excitation of the load and the machine being tested.
This characteristic is important for the analysis of a synchronous motor.

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