Question
As the load is applied to a synchronous motor, the motor takes more armature current because
(a) The increased load has to take more current
(b) the rotor by shifting its phase backward causes motor to take more current
(c) the back e.m.f decreases causing an increase in the motor current
(d) The rotor strengthens the rotating field causing more motor current
Answer
The answer is option d. The rotor strengthens the rotating field causing more motor current.
Explanation
As the load is applied, the rotor poles will then have a larger phase angle relative to the rotating stator field, and the power factor will start to lag as well.
As we increase the load, the motor becomes more and more out of phase and draws more current as the poles move further away from the idling position until they pass a point of no return, then the motor suddenly stalls or burns up.
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