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The starting torque of 3-phase synchronous motor is
1. Very high
2. Low
3. Zero
4. None of these above

1 Answer

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Correct Answer - Option 3 : Zero
  • 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.

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