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

An AC drive controls the rotational speed of an AC motor primarily by varying the frequency of the electrical supply to the motor. In AC motors, the speed of the motor is directly related to the frequency of the alternating current supplied to it. As the frequency increases, the speed of the motor increases, and conversely, as the frequency decreases, the speed of the motor decreases. This principle is based on the relationship between the synchronous speed of the motor and the frequency of the AC current, described by the formula:

Synchronous Speed (RPM) = (120 × Frequency) / Number of Poles.

By using an AC drive to adjust the frequency, one can achieve precise control over the motor's speed, making this method essential for applications that require variable speed control. While voltage, current, and power also play roles in motor operation, it is the frequency that directly influences the rotational speed in this context.