Which option among the following is correctly associated with the ‘Auxiliary motor starting’?

Synchronous Motor

• A synchronous motor is an AC motor where the rotation of the rotor (or shaft) is synchronized with the frequency of the supply current. That is, the rotation period of the rotor is equal to the rotating field of the machine it is inside.
• The speed of operation is in synchronism with the supply frequency and hence for constant supply frequency, they behave as constant speed motors irrespective of load condition.
• Synchronous motors are inherently not self-starting. They require some external means to bring their speed close to synchronous speed before they are synchronized.
• A small direct-coupled induction motor called a pony motor, is used for the starting purpose of the synchronous motor.
• In this method of starting a synchronous motor, the external motor brings the synchronous motor to synchronous speed and then the synchronous motor is synchronized with the AC supply as a synchronous generator. Then the prime mover is disconnected. Once synchronized, the synchronous machine will operate as a motor.
• Now, the mechanical load can be connected to the shaft of the synchronous motor. Since the load is not connected to the synchronous motor before synchronising, thus the prime mover motor has to overcome only the inertia of the synchronous motor at no-load. Consequently, the rating of the prime mover or starting motor is much smaller than the rating of the synchronous motor.