How do thermal overloads protect motors and what characteristic can trigger them?

• A. They monitor motor current or temperature and trip if the current or temperature exceeds the rated limit for a period.
• B. They monitor ambient temperature and trip on high humidity.
• C. They monitor motor vibration and trip on excessive vibration.
• D. They trip only when manually reset.

Answer: (A)

Thermal overloads protect motors by watching the heat generated in the windings from the current the motor is drawing. If the current is higher than what the motor is designed to carry for too long, or if the winding temperature gets too high, the protective element trips and disconnects power to prevent insulation damage or overheating. The important part is the time element: a short spike doesn’t trip, but sustained overcurrent or rising temperature beyond the rated limit will. This protection can be provided by a bimetal element that heats up with current or by electronic sensors that monitor current and temperature, both aiming to prevent damage by keeping the winding within safe temperatures.

What triggers it is the current or winding temperature exceeding the rated limit for a period of time. Ambient conditions or vibration aren’t the signals these protections use, and while some overload devices can be auto-reset or require manual reset after tripping, the trigger remains the overheating condition, not the reset method.