at the start of the motor, the average torque on the rotor is zero. This is because when a DC supply is applied to the stationary rotor, the unlike poles try to attract each other, causing the rotor to be subjected to an instantaneous torque in one direction. However, the rotor's inertia prevents it from rotating, and as the stator poles continue to rotate, the direction of the torque on the rotor changes. This cycle continues, resulting in an average torque on the rotor of zero, so an external force is required to bring the motor up to the synchronous speed.

Ways to start a synchronous motor:

Pony Motor

• A smaller auxiliary motor (the "pony motor"), either AC or DC, is mechanically coupled to the synchronous motor. The pony motor accelerates the synchronous motor to a speed slightly above synchronous speed. Once this speed is reached, the pony motor is disconnected, and the synchronous motor's field is energized, allowing it to lock into synchronism with the AC supply.

Induction Motor Starting (Damper Windings)

• The rotor of the synchronous motor can be equipped with a "cage winding," essentially an embedded squirrel cage. This cage winding enables the motor to operate as an induction motor during the starting phase. The induction motor action accelerates the rotor up to near synchronous speed. Once close to synchronous speed, the DC field is applied, pulling the rotor into synchronism and allowing it to operate as a synchronous motor

Variable Frequency Drive (VFD)

• A VFD gradually increases the supply frequency from zero, enabling the synchronous motor to accelerate smoothly without additional starting mechanisms. This method provides precise control over the motor's acceleration and is commonly used in modern applications.