Which statement best describes the difference between an AC drive and a servo drive?

• A. AC drives control torque; servo drives control only speed.
• B. AC drives require more maintenance than servo drives.
• C. AC drives control motor speed; servo drives can also control position and movement.
• D. Servo drives are cheaper and simpler than AC drives.

Answer: (C)

The key idea is how each drive controls the motor. An AC drive (VFD) mainly regulates speed by varying the supply frequency and voltage to the motor, so you set how fast the shaft turns and, with some control modes, the torque as well. It doesn’t inherently track or hold a specific position. A servo drive, by contrast, uses a feedback sensor on the motor (like an encoder or resolver) and a closed-loop control system to regulate not only speed and torque but also the exact position and movement of the shaft. This lets the system move to a commanded position, follow a trajectory, and stop precisely at the target. So the best description is that AC drives control motor speed, while servo drives can also control position and movement.