Power Electronics Circuits
Which of the following is the synchronous speed of a four-pole induction motor operated from three-phase AC at 50 Hz?
- A750 min⁻¹
- B1500 min⁻¹
- C3000 min⁻¹
Show answer and explanation
Answer: B. 1500 min⁻¹
The synchronous speed is the speed of the rotating magnetic field created by the three-phase stator windings, given by n_s = 120f/p [min⁻¹] (f: frequency, p: number of poles). At 50 Hz with four poles it is 120 × 50/4 = 1500 min⁻¹. In regions with 60 Hz, the same motor has a synchronous speed of 1800 min⁻¹. The rotor of an induction motor turns slightly slower than the synchronous speed.
- A. This puts the 4 poles into 60f/p as if they were pole pairs.
- B. Correct. 120 × 50 ÷ 4 = 1500 min⁻¹.
- C. This is the synchronous speed of a two-pole motor. With four poles the field takes two supply cycles to make one revolution.