Power Electronics Circuits
A four-pole induction motor operating at 60 Hz is running with a slip of 0.05. Which of the following is the rotor speed?
- A1710 min⁻¹
- B1800 min⁻¹
- C1890 min⁻¹
Show answer and explanation
Answer: A. 1710 min⁻¹
The synchronous speed is 120 × 60/4 = 1800 min⁻¹. The slip is s = (n_s − n)/n_s, so the rotor speed is n = n_s × (1 − s) = 1800 × 0.95 = 1710 min⁻¹. The rate at which the rotor conductors cut the rotating field is proportional to the difference from the synchronous speed, so the frequency of the rotor current is s × f = 0.05 × 60 = 3 Hz. The rated slip of a typical induction motor is a few percent.
- A. Correct. 1800 × (1 − 0.05) = 1710 min⁻¹.
- B. This is the synchronous speed. An induction motor turns slower than the synchronous speed.
- C. This adds the slip to the synchronous speed. When running as a motor, it turns slower than the synchronous speed.