Water Distribution Manager (WDM) Greenbook 2 Practice Exam

In a synchronous motor, the rotor speed is

The rotor turns at the same speed as the rotating magnetic field produced by the stator. Slightly slower due to slip. The rotor turns faster than the rotating magnetic field. The rotor remains stationary.

Correct Answer:

The rotor turns at the same speed as the rotating magnetic field produced by the stator.

Explanation:

In a synchronous motor the rotor speed is synchronized with the stator’s rotating magnetic field. The stator creates a magnetic field that rotates at the synchronous speed, which is determined by the supply frequency and the number of poles (Ns = 120 f / P). The rotor—fed with DC excitation or equipped with permanent magnets—locks to that rotating field and follows it in steady operation, so there is essentially no slip between rotor and field. This means the rotor turns at the same speed as the rotating magnetic field, not slower, faster, or stationary. For example, at 60 Hz with four poles, the synchronous speed is 1800 rpm, so the rotor runs at about 1800 rpm.

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