For AC motors, speed depends on which factors?

For AC motors, speed depends on which factors?

• A. Voltage and current
• B. Rotor inertia
• C. Load torque
• D. Frequency and number of poles

Answer: (D)

AC motor speed is determined by the electrical supply frequency and the motor’s pole count. The fundamental relationship is the synchronous speed, which is 120 times the frequency divided by the number of poles (Ns = 120f/P). This sets the reference speed for the rotor. In induction motors, the rotor actually runs a bit slower than this due to slip, and slip changes with load, but the baseline speed is fixed by f and P. Voltage and current influence torque and starting behavior, not the steady-state speed in a fixed-frequency, fixed-pole machine. Rotor inertia affects how quickly the motor reaches speed, not the final speed itself, and load torque changes the slip needed to deliver that torque, again affecting how close you are to synchronous speed rather than the speed formula itself. So the factors that determine speed are frequency and number of poles.

AC motor speed is determined by the electrical supply frequency and the motor’s pole count. The fundamental relationship is the synchronous speed, which is 120 times the frequency divided by the number of poles (Ns = 120f/P). This sets the reference speed for the rotor. In induction motors, the rotor actually runs a bit slower than this due to slip, and slip changes with load, but the baseline speed is fixed by f and P. Voltage and current influence torque and starting behavior, not the steady-state speed in a fixed-frequency, fixed-pole machine. Rotor inertia affects how quickly the motor reaches speed, not the final speed itself, and load torque changes the slip needed to deliver that torque, again affecting how close you are to synchronous speed rather than the speed formula itself. So the factors that determine speed are frequency and number of poles.