What is the safety device that protects a motor from overload, and how does it operate?

What is the safety device that protects a motor from overload, and how does it operate?

• A. A thermal fuse that permanently opens on overheating
• B. An overload relay detects excessive current and interrupts the circuit
• C. A pressure sensor that stops the motor on high pressure
• D. A soft-start device that reduces voltage gradually

Answer: (B)

The idea being tested is how motors are protected from drawing too much current. The device that does this is an overload relay, which is part of the motor starter. It watches the current flowing to the motor and keeps the circuit energized as long as the current stays within the safe range. If the motor experiences overload — for example due to a jam, a mechanical binding, or a stalled rotor — the current rises above the set limit. The overload relay then trips, opening the control circuit to the motor contactor and cutting power to stop the motor, preventing winding damage and insulation failure. Many overload relays use a thermal element: the excess current heats a bimetal strip that bends and trips after a short delay. Others are magnetic or electronic and can react even faster, often with adjustable trip settings. After the fault is cleared, the relay can be reset and the motor can be restarted, with the trip setting sized to the motor’s full-load current to avoid nuisance trips. Why the other choices aren’t the best fit: a thermal fuse opens permanently when overheated and isn’t designed for resetting after an overload; a pressure sensor protects against high pressure in a hydraulic or pneumatic system, not electrical motor overload; a soft-start device gradually lowers inrush at startup but doesn’t provide a protective trip if the motor is already overloaded.

The idea being tested is how motors are protected from drawing too much current. The device that does this is an overload relay, which is part of the motor starter. It watches the current flowing to the motor and keeps the circuit energized as long as the current stays within the safe range. If the motor experiences overload — for example due to a jam, a mechanical binding, or a stalled rotor — the current rises above the set limit. The overload relay then trips, opening the control circuit to the motor contactor and cutting power to stop the motor, preventing winding damage and insulation failure.

Many overload relays use a thermal element: the excess current heats a bimetal strip that bends and trips after a short delay. Others are magnetic or electronic and can react even faster, often with adjustable trip settings. After the fault is cleared, the relay can be reset and the motor can be restarted, with the trip setting sized to the motor’s full-load current to avoid nuisance trips.

Why the other choices aren’t the best fit: a thermal fuse opens permanently when overheated and isn’t designed for resetting after an overload; a pressure sensor protects against high pressure in a hydraulic or pneumatic system, not electrical motor overload; a soft-start device gradually lowers inrush at startup but doesn’t provide a protective trip if the motor is already overloaded.