Page 230 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

SYNCHRONOUS MACHINES

SYNCHRONOUS MACHINES 11.5

varnish and baked or thermally cured for rigidity and high insulation resistance. These coils
are known as random-wound, or mesh, coils.
Distributed windings are used as armature windings and as field windings in cylindrical
machines.
Solenoid Windings. This type of winding is made of a multilayer of conductors sep-
arated by insulating strips. Hard electrical insulation separates the pole from this winding.
This type of winding is used in salient-pole and dc machine fields, electromagnets, and
power relays.

Damper Windings. The damper windings are installed at the outer surface of the rotor in
a similar arrangement to the squirrel-cage bars in an induction machine.

FIELD EXCITATION

The field windings of a synchronous machine are supplied from a dc source. It originates
from batteries, solar converters, dc generators, or a rectified ac source. This is called the
excitation of the machine. Permanent magnets are also used to excite synchronous machines.
The excitation current controls the terminal voltage, power factor, short-circuit current,
torque, and transient response of the machine.
A dc or ac exciter is used for large synchronous machines. The exciter is normally
mounted on the same shaft as the synchronous machine. The ac exciter is normally a con-
ventional synchronous machine similar to the main machine. The three-phase ac output of
the exciter is rectified and fed to the field winding of the main machine. Slip rings and
brushes are also used to supply the dc field directly to synchronous machines. The excita-
tion is normally controlled by varying the dc field by using a variable resistor.
Most excitation currents (except the ones supplied from batteries) contain ripple. This
is highly undesirable because it increases harmonics* in the power output from the main
machine. Thus, a filter is required to reduce the ripple in the excitation system of the
machine to an acceptable value.

Rotating Rectifier Excitation
Small synchronous machines use “brushless” excitation known as rotating rectifier instead
of slip rings and brushes. The reason for this is that synchronous machines using slip rings
and brushes require high maintenance that includes replacement of the carbon brushes
on power.
The excitation system of a rotating rectifier machine uses an exciter mounted on the shaft
as the main synchronous machine. The field of the exciter is on the stator, and the armature
is on the rotor. The ac power output from the exciter armature is rectified and fed to the field
of the main machine. The excitation of the main machine is controlled by controlling the
field in the stator of the exciter.
The main disadvantage of the rotating rectifier machine is the long response time
required to change the voltage at the output from the main machine. Since the variation of
*Harmonics are ripples that distort the ac voltage and current in the power output. They are normally signals that
are superimposed on the main (60- or 50-Hz) signal. They normally have a frequency equal to a multiple of the
main frequency. For example, if the main frequency is 60 Hz, the harmonic signal will have a frequency of 5 60 or
8 60 Hz. Harmonics are highly undesirable because they increase the heat losses and electromagnetic interference
from the machine.

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