For a fixed number of poles in an AC motor, which factor determines its speed?

• A. Frequency
• B. Voltage
• C. Load
• D. Efficiency

Answer: (A)

The speed of an AC motor with a fixed number of poles is set by how fast the stator magnetic field rotates, which depends on the supply frequency. The synchronous speed is determined by the formula Ns = 120 f / P, where f is frequency and P is the number of poles. With a fixed pole count, changing the frequency changes the rotational speed of the magnetic field, and thus the motor’s speed.

In practice, the rotor can’t perfectly keep up, so the actual speed is slightly less than the synchronous speed by an amount called slip, and slip grows with load. But the fundamental factor that fixes the speed is the supply frequency. Voltage affects torque, load affects how much slip occurs, and efficiency relates to losses, not the speed setting. For example, at 60 Hz with 4 poles, the synchronous speed is 1800 rpm (actual speed a bit below that depending on load). At 50 Hz, it drops to 1500 rpm.