What happens to the synchronous motor when the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected?

What happens to the synchronous motor when the excitation of an unloaded salient-pole synchronous motor suddenly gets disconnected?

Right Answer is:

Run as a variable reluctance motor

SOLUTION

When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected the motor run as a variable reluctance motor.

  • Reluctance torque is the torque generated because the motor is moving to a position where the reluctance seen by the armature flux is declining.
  • Reluctance exists only when there is an unsymmetrical air gap.
  • Therefore it exists only for Salient pole synchronous machines and does not exist for the cylindrical machine.
  • Reluctance torque comes into play when any winding is disconnected i.e. filed winding or armature winding in the running conditions.
  • Reluctance torque also produces reluctance power which makes the machine more stable.
  • The machine will continue to run at synchronous speed due to reluctance torque (i.e. even when field winding or armature windings are disconnected).
  • The rotor will remain in synchronism for a short time ie.. until the residual flux of the rotor decreases below a certain value.
  • Variable reluctance motor behaves as similar to silent pole synchronous motor unexcited.
  • And after the loss of residual flux, it will run as a reluctance motor due to the generation of reluctance torque.

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