A. First, starts as induction motor then run as synchronous motor
B. Not start
C. Motor will burn out
D. Run as induction motor
Answer: D. Run as induction motor
Explanation:
If voltage is impressed on the armature winding when the motor is at standstill with field winding unexcited, the revolving field produced by stator currents will cut across the field winding, thereby inducing a high voltage in the field winding.
This induced voltage would be dangerous and would often result in the breakdown of insulation of field winding.
Therefore, in starting a synchronous motor, the field winding is shorted through a suitable resistance.
Consequently, the induced voltage is distributed throughout the whole winding and no part is subjected to high voltage.
The resistance is removed and excitation is applied to the rotor when the motor attains 90-95% of synchronous speed.
In the case of an induction motor, the field winding is permanently short-circuited by end rings.
In a Synchronous motor, the rotor winding is supplied with DC excitation. Now, if we eliminate the DC excitation, there are two ends of the wire available.
Let short circuit the two ends of the wire (or connect the two ends of variable resistors).
Now the rotor winding of the synchronous motor is analogous to that of an induction motor.
Therefore if we short circuit the field winding of a synchronous motor then it will behave as an induction motor.