In a 3-phase, 4-pole, 50 Hz synchronous motor, the frequency, pole number and load torque all are halved. The motor speed will be
A)3000 r.p.m.
B)1500 r.p.m.
C)750 r.p.m.
D)none of the above

Question

Quizplus · Accepted Answer

The speed of a synchronous motor is given by the formula $N_s = \frac{120f}{P}$, where $N_s$ is the synchronous speed in revolutions per minute (rpm), $f$ is the frequency in hertz (Hz), and $P$ is the number of poles. Initially, with a 50 Hz frequency and 4 poles, the speed is $N_s = \frac{120 \times 50}{4} = 1500$ rpm. When both the frequency and the number of poles are halved, the new speed remains $N_s = \frac{120 \times (50/2)}{(4/2)} = 1500$ rpm, because the halving of both the frequency and the number of poles cancels out, leaving the speed unchanged.

In a 3-Phase, 4-Pole, 50 Hz Synchronous Motor, the Frequency