Short-circuiting the field winding by low resistance

- When starting a synchronous motor, it is often necessary to use an induction motor action to bring the motor up to synchronous speed.
- This is because a synchronous motor requires a rotating magnetic field to operate, and the field winding of the motor cannot produce this rotating field when the motor is stationary.
- By using the induction motor action, the motor can be brought up to speed and then switched over to synchronous operation.
- During this starting process, it is important to ensure that the field winding of the synchronous motor is short-circuited by a low resistance.

Reasoning behind short-circuiting the field winding

- The field winding of a synchronous motor is responsible for producing the magnetic field that interacts with the stator winding to generate torque and rotation.
- When the field winding is short-circuited by a low resistance, it effectively becomes an electromagnet that is magnetically coupled to the stator winding.
- This allows the motor to start up as an induction motor, with the field winding producing a magnetic field that rotates at a speed slightly below synchronous speed.
- The induction motor action then brings the rotor up to speed, and once the motor is running close to synchronous speed, the field winding can be switched back to its normal operating mode.

Benefits of short-circuiting the field winding

- Short-circuiting the field winding by a low resistance during the starting process has several benefits:
- It allows the motor to start up as an induction motor, which typically has a higher starting torque compared to a synchronous motor.
- It ensures that the field winding does not produce a stationary magnetic field, which would prevent the motor from starting and cause excessive heating.
- It allows the motor to synchronize with the power system and operate in synchronous mode once it reaches synchronous speed.
- The low resistance in the short-circuiting circuit helps to limit the current flowing through the field winding during the starting process, preventing excessive heating and potential damage to the winding.

In conclusion, short-circuiting the field winding of a synchronous motor by a low resistance during the starting process is necessary to allow the motor to start up as an induction motor, bring the rotor up to speed, and then switch over to synchronous operation. This method provides several benefits, including higher starting torque and preventing excessive heating of the field winding.