Which of the following is an important feature of a 3-phase synchronous motor?

Option 4 : No starting torque

• Synchronous motors have lots of advantages but being not self-starting unlike 3 phase induction motors is a major disadvantage.
• Synchronous motors run at synchronous speed.
• The synchronous speed of a motor depends on the supply frequency and the number of poles in the motor.
• In synchronous motors, the stator has 3 phase windings and is excited by 3 phase supply whereas the rotor is excited by DC supply.
• The 3 phase windings provide rotating flux whereas the DC supply provides constant flux.
• The torque produced on the rotor is a pulsating one and not unidirectional.
• At a particular instant rotor and stator poles might be of the same polarity (N-N or S-S) causing a repulsive force on the rotor and the very next second it will be N-S causing attractive force.
• But due to the inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition.
• Due to this, the motor cannot start on its own. The rotor of the synchronous motor has to be brought to synchronous speed by using external means.