Which device is typically used to protect a motor by interrupting current if the motor overheats?

• A. A timer relay.
• B. A transformer.
• C. A lighting ballast.
• D. An overload protector.

Answer: (D)

When protecting a motor from overheating, you want a device that reacts to excessive current or temperature and stops the current flow. An overload protector is built for that purpose. It monitors the motor’s current (and can sense winding temperature) and trips to open the circuit as soon as the safe limit is exceeded, preventing insulation damage and potential fire. These protectors can be thermal (often using a bimetal element) or electronic and may reset automatically or manually after cooling.

The other devices aren’t meant for motor overheating protection: a timer relay focuses on controlling when the circuit is on or off, not on protecting against overheating; a transformer changes voltage; a lighting ballast limits current for lamps.