For a four-pole, 60 Hz motor, the synchronous speed is approximately:

For a four-pole, 60 Hz motor, the synchronous speed is approximately:

• A. 900 RPM
• B. 1800 RPM
• C. 3600 RPM
• D. 7200 RPM

Answer: (B)

The rotating magnetic field speed (synchronous speed) is determined by Ns = 120 f / P, where f is the supply frequency and P is the number of poles. For a four-pole machine at 60 Hz, Ns = (120 × 60) / 4 = 1800 rpm. This is the speed of the stator field; the rotor actually runs a bit below this due to slip, producing torque. The other speeds correspond to different pole counts at 60 Hz (e.g., fewer poles gives higher speed, more poles gives lower speed), so they don’t apply to a four-pole motor.

The rotating magnetic field speed (synchronous speed) is determined by Ns = 120 f / P, where f is the supply frequency and P is the number of poles. For a four-pole machine at 60 Hz, Ns = (120 × 60) / 4 = 1800 rpm. This is the speed of the stator field; the rotor actually runs a bit below this due to slip, producing torque. The other speeds correspond to different pole counts at 60 Hz (e.g., fewer poles gives higher speed, more poles gives lower speed), so they don’t apply to a four-pole motor.