What is the primary purpose of a bimetallic overload relay?

• A. Protect motor by opening circuit during overload
• B. Regulate motor speed
• C. Filter electrical noise
• D. Monitor power factor

Answer: (A)

The main purpose is to protect the motor by opening the circuit when it overheats due to an overload. A bimetallic overload relay uses a strip made of two metals with different thermal expansion rates. When current heats the strip, it bends after reaching a set temperature, which trips the relay contacts or de-energizes the contactor coil. That interruption removes power to the motor, preventing damage from overheating. There’s usually a built-in time delay to avoid nuisance trips from brief surges. This device protects against overload and stall, not speed control, noise filtering, or power-factor monitoring.