If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then

it runs at a slower speed
the motor stops
it continues to run at the same speed
it runs at a very high speed

The correct answer is: C. it continues to run at the same speed.

When the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, the motor will continue to run at the same speed. This is because the speed of a synchronous motor is determined by the frequency of the power supply and the number of poles on the stator. The open-circuited field does not affect the speed of the motor.

The other options are incorrect because:

  • Option A is incorrect because the motor will not run at a slower speed.
  • Option B is incorrect because the motor will not stop.
  • Option D is incorrect because the motor will not run at a very high speed.

I hope this explanation is helpful. Let me know if you have any other questions.