Synchronous motor - ques 1

 What will happen if  the excitation of normally operating unloaded salient pole synchronous motor suddenly gets disconnected?

                When the excitation of normally operating unloaded salient pole synchronous motor suddenly gets disconnected, induction torque disappears but reluctance torque independent on excitation , hence rotor remain in synchronism due to reluctance torque alone based on reluctance principle .So synchronous motor continue to run as a variable reluctance motor.

What is Reluctance torque?

         Reluctance torque is created due to the saliency of the rotor.

Reluctance in a magnetic circuit is analogous to resistance in Electric circuit. It is like the resistance offered to the magnetic flux lines by the medium.

Metals like Iron provide a low reluctance path and the flux lines 'like' to pass through the least reluctance path. The least reluctance path is along the pole axis where the air gap is minimum. So the flux lines will exert force on the rotor to align with the flux lines. This torque is created additional to the torque produced by the locking of excited poles with synchronously rotating magnetic field. This effect is absent in cylindrical rotor machine as the air gap is uniform.

This mechanism is the working principle behind reluctance motor, switched reluctance motor etc.

Salient pole motor may continue to run even when excitation fails but it depends on the load. If the power produced by reluctance mechanism is not sufficient enough to drive the load, the motor will get pulled out of synchronism. On no load it is highly probable that the reluctance torque will maintain the synchronism when the field excitation fails. Reluctance power contribute 5-15% of the rated power.