Which factors determine the speed of an AC motor?

• A. Number of poles and supply frequency
• B. Voltage and current
• C. Load and torque
• D. Temperature and cooling

Answer: (A)

AC motor speed is set by the rotating magnetic field, which runs at the synchronous speed determined by the supply frequency and the motor’s number of poles. The rotor tries to catch that field but can’t quite reach it, so it runs a little slower by slip. The basic speed relationship is Ns = 120 f / P, so higher frequency or fewer poles means higher speed, while more poles or lower frequency means lower speed. Voltage and current influence flux and torque but don’t set the speed; load and torque affect how much slip occurs, so speed can vary under load, but the fundamental determinant is frequency and pole count. Temperature and cooling affect performance and longevity, not the speed setting.