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3 SYNCHRONOUS MOTOR APPLICATIONS
3.1 OVERVIEW
3 SYNCHRONOUS MOTOR APPLICATIONS 3.1 OVERVIEW
3.1.1 GENERAL
The most attractive and widely applied method of starting a synchronous motor is to utilize squirrel cage wind-
ings 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 synchronous 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 synchronized. Unlike induction motors, no continuous
squirrel cage torque is developed at normal running speeds. Examine the figure below:
3
Figure 3–1: SALIENT POLE SYNCHRONOUS MOTOR
When the motor accelerates to near synchronizing speed (about 95% synchronous speed), DC current is intro-
duced into the rotor field windings. This current creates constant polarity poles in the rotor, causing 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 link-
ing the rotating field produced by the AC currents in the armature windings on the stator.
Magnetic polarization of the rotor iron is due to the rotor's physical shape and arrangement and the constant
potential DC in coils looped around the circumference of the rotor.
Synchronous motors possess two general categories of torque characteristics. One characteristic is deter-
mined by the squirrel-cage design, which produces a torque in relation to "slip" (some speed other than syn-
chronous speed). The other characteristic is determined by the flux in the salient field poles on the rotor as it
runs at synchronous speed. The first characteristic is referred to as starting torque, while the second character-
istic is usually referred to as synchronous torque.
In starting mode, the synchronous motor salient poles are not excited by their external DC source. Attempting
to start the motor with DC applied to the field does not allow the motor to accelerate. In addition, there is a very
large oscillating torque component at slip frequency, produced by field excitation, which could result in motor
damage if full field current is applied during the entire starting sequence. Therefore, application of DC to the
field is usually delayed until the motor reaches a speed where it can be pulled into synchronism without slip.
At synchronous speed, the ferro-magnetic rotor poles become magnetized, resulting in a small torque (reluc-
tance torque) which enables the motor to run at very light loads in synchronism without external excitation.
Reluctance torque can also pull the motor into step if it is lightly loaded and coupled to low inertia.
It is convenient to make an analogy of a synchronous motor to a current transformer for the purpose of demon-
strating angular relationship of field current and flux with rotor position.
GE Multilin
SPM Synchronous Motor Protection and Control
3-
1
Courtesy of store.ips.us
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