AC MOTOR TYPES
Starting a Synchronous Motor
A synchronous motor may be started by a DC motor on a common shaft. When the motor is
brought to synchronous speed, AC current is applied to the stator windings. The DC motor now
acts as a DC generator and supplies DC field excitation to the rotor of the synchronous motor.
The load may now be placed on the synchronous motor. Synchronous motors are more often
started by means of a squirrel-cage winding embedded in the face of the rotor poles. The motor
is then started as an induction motor and brought to ~95% of synchronous speed, at which time
direct current is applied, and the motor begins to pull into synchronism. The torque required to
pull the motor into synchronism is called the pull-in torque.
As we already know, the synchronous motor rotor is locked into step with the rotating magnetic
field and must continue to operate at synchronous speed for all loads. During no-load conditions,
the center lines of a pole of the rotating magnetic field and the DC field pole coincide (Figure
8a). As load is applied to the motor, there is a backward shift of the rotor pole, relative to the
stator pole (Figure 8b). There is no change in speed. The angle between the rotor and stator
poles is called the torque angle (a).
If the mechanical load on the motor is increased to the point where the rotor is pulled out of
Figure 8 Torque Angle
synchronism (a@90o), the motor will stop. The maximum value of torque that a motor can
develop without losing synchronism is called its pull-out torque.