What is the synchronous speed of a 3-phase, 60 Hz, 6-pole motor?

What is the synchronous speed of a 3-phase, 60 Hz, 6-pole motor?

• A. 600 RPM
• B. 1200 RPM
• C. 1800 RPM
• D. 1000 RPM

Answer: (B)

The speed of a synchronous motor is set by the supply frequency and the number of poles. For a synchronous machine, the synchronous speed in revolutions per minute is ns = 120 f / P, where f is the frequency and P is the number of poles. With a 60 Hz supply and six poles, ns = 120 × 60 / 6 = 7200 / 6 = 1200 rpm. This is the rotor speed at which the magnetic field rotates; in practice, induction motors run a bit slower due to slip, but the synchronous speed here is 1200 rpm.

The speed of a synchronous motor is set by the supply frequency and the number of poles. For a synchronous machine, the synchronous speed in revolutions per minute is ns = 120 f / P, where f is the frequency and P is the number of poles. With a 60 Hz supply and six poles, ns = 120 × 60 / 6 = 7200 / 6 = 1200 rpm. This is the rotor speed at which the magnetic field rotates; in practice, induction motors run a bit slower due to slip, but the synchronous speed here is 1200 rpm.