SOLUTION

A synchronous motor is switched on to supply with its field windings short-circuited. It will run as an Induction motor.

The procedure to start a synchronous motor is as follows:

1. The field winding of the motor is short-circuited.
2. Stator winding is applied a reduced voltage using an auto-transformer when the motor is starting up.
3. Once the motor speed reaches a steady state, a weak DC excitation is applied by removing the short-circuited field winding.

It is practically impossible to start a synchronous motor with its D.C. field energized. Even when left de-energized, the rapidly rotating magnetic field of the stator will induce extremely high voltages in the many turns of the field winding. It is customary, therefore, to short-circuit the D.C. field winding during the starting period; whatever voltage and currents are induced in it may then aid in producing induction motor action.

• The short-circuited winding acts as a squirrel-cage rotor winding of an induction motor.
• Upon excitation of the stator by a three-phase supply, the motor starts rotating as an induction motor at sub-synchronous speed and the DC supply is excited to the field winding.
• Once the field winding is excited, the motor is pulled into synchronism and starts rotating at its synchronous speed. During this condition, the relative motion between the damper winding and the rotating magnetic field becomes zero.
• Now, the motor behaves like a synchronous motor and it does not induce any emf in the damper winding.
• Therefore, at the time of starting, the damper winding is used to run the motor as an induction motor, and afterward, it goes out of the circuit.
• While starting a synchronous motor as an induction motor, the field winding is short-circuited by low resistance in this method. The number of poles in the Induction motor is less than the number of poles in the synchronous motor.

How synchronous motor acts as induction motor during the starting period

• 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.

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