An AC drive controls the torque of an AC motor by varying the ___ applied to the motor.

• A. Voltage
• B. Frequency
• C. Current
• D. Phase angle

Answer: (A)

Torque in an AC motor comes from the current in the stator windings. A variable-frequency drive controls that current primarily by adjusting the voltage applied to the motor (within the proper voltage–frequency relationship). Increasing the applied voltage raises the stator current and, thus, the torque, up to the motor’s limits. Frequency mainly sets speed, while maintaining the V/Hz ratio to keep the flux and current behaving predictably. Phase angle isn’t the direct lever for torque in this control scheme.