Which device is used to adjust motor speed and torque to load?

• A. Transformer
• B. Relay
• C. Variable frequency drive (VFD)
• D. Mechanical gearbox

Answer: (C)

Controlling an AC motor’s speed and torque to match the load is done with a device that modulates the electrical supply to the motor. A variable frequency drive adjusts the frequency of the power fed to the motor and also adapts the voltage, so the motor speed can be set precisely and torque can be controlled to match the load. This enables smooth acceleration and deceleration, programmable torque limits, and efficient operation, preventing overload and stalling.

A transformer only changes voltage level and doesn’t alter motor speed or torque in response to load. A relay is merely a switch that turns circuits on or off and provides no speed control. A mechanical gearbox changes the output speed and torque through gears, but it’s a passive mechanical ratio that doesn’t actively adapt to changing load or provide electronic speed control.