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SECTION 1 - Introduction
1.1
General
These application notes and instructions apply to General
Electric Synchronous-motor Controllers and field panels that
employ a CR192 Microprocessor-based, Synchronous-motor,
Starting and Protection Module (IlSPM). These instructions also
apply to the CR1921lSPM when it is supplied as a separate
component. If the complete controller is not supplied, the
purchaser is requested to interpret these instructions for
applicability to his particular assembly by referring to the
diagrams shown in Figures 10 and 23 to develop his diagrams
for his particular equipment.
Brushless synchronous motors can also be controlled by
the proper CR192IlSPM. See Section 9 for discussion of the
Brushless Synchronous-motor Control.
SECTION 2 - Starting Synchronous Motors
2.1
General
The most attractive and widely applied method of starting
a synchronous motor is to utilize squirrel cage windings in the
pole faces of the synchronous motor rotor. The presence of
these windings allows for a reaction (or acceleration) torque to
be developed in the rotor as the ac excited stator windings
induce current into the squirrel cage windings. Thus, the syn-
chronous motor starts as an induction motor. These rotor
windings are frequently referred to as damper or amortisseur
windings. The other major function of these windings is to
dampen power angle oscillations after the motor has synchro-
nized. Unlike induction motors, no continuous squirrel cage
torque is developed at normal running speeds. See Figure 1.
When the motor accelerates to near synchronizing speed
(approximately 95 percent synchronous speed), dc
Stator
Slip
rings
Rotor
Figure
1.
Salient-pole synchronous motor
Stator
core
Slots
for ac
winding
To dc
current is introduced into the field windings in the rotor. This
dc current creates constant-polarity poles in the rotor which
cause the motor to operate at synchronous speed as the
rotor poles "lock" onto the rotating ac stator poles.
Torque at synchronous speed is derived from the
magnetic field produced by the dc field coils on the rotor
linking the rotating field produced by the ac currents in the
armature windings on the stator.
Magnetic polarization of the rotor iron is due to phYSical
shape and arrangement of the rotor plus constant potential
direct current in coils looped around the circumference of
the rotor.
Synchronous motors possess two general categories of
torque characteristics. One characteristic is determined by
the squirrel-cage design, which produces a torque in relation
to "slip" (some speed other than synchronous speed), and
only in relation to slip since it can develop no torque unless
there is slip. The other characteristic is determined by the
flux in the salient field poles on the rotor as it runs at syn-
chronous speed. The first characteristic is starting torque,
while the second characteristic is usually referred to as syn-
chronous torque.
In the starting mode, the synchronous-motor salient poles
are not excited by their external dc source. If the poles were
excited there would be no useful torque developed by them. The
reason for this is that the average torque due to field excitation
during Slip would be a negative or braking torque that would
result in reducing the total amount of acceleration torque. In
addition, there is a very large oscillating component of torque at
slip frequency produced by field excitation which could result in
damage to the motor if the full field current was applied during
the whole starting sequence. Therefore, application of direct
current (dc) to the field is usually delayed until the motor reaches
a speed from which it can be pulled into synchronism without
slip.
At synchronous speed, the ferro-magnetic rotor
poles become magnetized, resulting in a small torque
(reluctance torque) which enables the motor to run at
3
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