Question

A 3-phase, 400V, synchronous motor is providing load at 0.8 p.f. lagging. If the field current of the motor is continuously increased, then the _____________

(a) power factor increases and then decreases

(b) power factor is not affected

(c) power factor decreases upto a certain value of field current and then increases

(d) power facor increases simply

I got this question in an interview for job.

This intriguing question originated from Starting of Synchronous Motors in section Synchronous Motors of Electrical Machines

Answer 1

Right answer is (a) power factor increases and then decreases

For explanation I would say: The magnetization current is increased as we increase the excitation, so the power factor will increase upto the saturation and then again it will decrease.