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

The correct response indicates that an AC drive controls the torque of an AC motor by varying the voltage applied to the motor. While voltage is indeed a factor in motor operation, it’s important to understand that the fundamental mechanism of controlling torque in AC motors, especially through variable frequency drives (VFDs), primarily involves changing the frequency of the power supply.

By varying the frequency of the alternating current supplied to the motor, an AC drive effectively alters the speed of the motor. The relationship between frequency and motor speed is direct: as the frequency increases, so does the speed of the motor, and therefore, the torque can also be adjusted. Although voltage adjustments will affect the motor's torque indirectly through back EMF and other factors, the most direct and effective method by which an AC drive controls torque is through frequency modulation.

Using voltage alone without frequency manipulation could lead to insufficient torque control since the motor's performance is tied closely to both these electrical parameters. Understanding the role of frequency is crucial for effectively controlling motor performance in various applications.

Thus, while voltage is a component of motor control, frequency is the key variable in controlling the torque effectively in AC motors.