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GEH-5201, Synchronous-motor Control
SECTION 3 - Description of Synchronous-motor Controllers
3.2.4 Solid-state field exciter
General Electric Synchronous-motor Controllers may be
supplied with either; (1) diode-rectifier/tapped transformer field
exciter or (2) Silicon Controlled Rectifier exciter. Of course,
provisions can be made for remotely supplied dc field power.
These two exciter types are described as follows:
1. The diode-rectifier/tapped-transformer exciter utilizes a
full wave bridge rectifier (either single or three phase) to obtain
the required dc field supply. A tapped transformer provides a
wide range of available secondary voltages so that the desired
field current may be obtained by proper choice of the
transformer tap. The motor must be shut down before the
transformer tap can be changed.
2. The Silicon Controlled Rectifier (SCR) Exciter is
available when anyone of the following features is required.
8
A. ON LINE FIELD ADJUSTMENT. A lockable control
knob is provided either on the door or base mounted
inside the enclosure. Adjustment of the knob
increases or decreases the field current
conveniently while motor is running.
B. FIELD CURRENT REGULATOR. As a synchronous-
motor field heats up while running, its resistance
increases causing the field current to drop. This
condition causes the power factor to vary, resulting
in undesirable voltage fluctuations or nuisance
tripping from synchronous motor pull-out. A field
current regulator senses the field current and
causes the SCR to either increase or decrease its
output in order to maintain a constant field current
regardless of field winding temperature.
C. POWER FACTOR REGULATOR. Power factor
regulation is useful in those applications where
motors are subjected to high-level transient impact
loads (such as chipper drives). The PF regulator
senses the power factor dip that occurs when the
motor is loaded and causes the SCR Exciter to
respond with a boosted output. As a result, the pull-
out torque of the synchronous motor is increased
for the duration of the transient load. After the load
subsides, the regulator senses an excessive
leading power factor and causes the SCR to reduce
its output. This automatic boosting of field current to
avoid pull-out is called field forcing. The Power
Factor regulator thus provides automatic boosting
when field forcing is required and economical low
field operation when the motor is idling.
Another application of the power factor regulator is
to control power factor swings that result from
various levels of loading so as not to cause
fluctuations in the, plant system voltage.
The JlSPM provides the.control signal to the SCR
exciter when PF regulation is required. See
Section 5.7.
3.3 Synchronous-motor field
panel
The Synchronous-motor Field Panel contains at least the
features listed in the previous section that are unique to the
Synchronous-motor Controller. Additionally, the Synchronous-
motor Field Panel may include stator protective relaying and
start-stop control as well. However, the field panel will not switch
or handle any of the line power to the synchronous motor stator
windings.
Hence, the field panel becomes a piece of control equip-
ment that can interconnect with either a separate contactor or
circuit breaker to form a complete Synchronous-motor Controller
or feeder.
Field panels may be supplied with solid-state field exciters.
See Section 3.2.4.
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