Question 6A346

The speed of a synchronous motor is precisely determined by the frequency of the AC power supply and the number of magnetic poles built into the motor. The synchronous speed ($N_s$) can be calculated with the formula: $N_s = (120 \times f) / P$, where $f$ is the line frequency in Hertz and $P$ is the total number of poles. This means the speed is directly proportional to the line frequency (B) and inversely proportional to the number of poles (or pairs of poles) (A). Therefore, both A and B are correct determinants. Unlike induction motors, a synchronous motor operates at a constant speed, synchronized with the rotating magnetic field of the stator, independent of the mechanical load (C), as long as the load does not exceed its pull-out torque. If the load is too high, the motor will stop, rather than just slow down.