A motor overload relay consists of a current-sensing unit and a mechanism activated by the sensing unit that
a) dissipates excess current
b) decreases motor speed
c) trips an alarm
d) opens the motor circuit
The correct answer and explanation is :
Correct Answer: d) opens the motor circuit
Explanation (Approx. 300 words):
A motor overload relay is a vital protective device used in electric motor circuits to prevent damage caused by excessive current. The correct function of the overload relay is to open the motor circuit when an overload condition is detected, making option (d) the correct answer.
Electric motors are designed to operate within a specific current range. When a motor draws more current than its rated capacity for an extended period—due to factors like mechanical overload, stalled rotor, or phase loss—it heats up. Excessive heat can degrade motor insulation, shorten the motor’s lifespan, or even cause a fire. To prevent this, an overload relay monitors the current flowing to the motor.
The current-sensing unit of the relay continuously checks the motor current. When it detects an overload (typically a sustained overcurrent condition rather than a short-circuit), it activates a mechanism that triggers a disconnect—typically by opening a set of contacts that cut power to the motor. This effectively interrupts the current flow, protecting the motor and connected equipment.
Overload relays come in different types, such as thermal, magnetic, and electronic. Thermal overload relays use a bimetallic strip that bends when heated by excessive current, triggering the circuit to open. Magnetic relays use an electromagnet activated by high current, and electronic versions use sensors and microcontrollers for precise monitoring.
Importantly, overload relays do not act instantly like circuit breakers. They are designed to tolerate momentary surges, such as during motor startup, and only trip when high current persists beyond a set time. This delayed action ensures reliability while still providing protection.
In summary, a motor overload relay protects the motor from overheating by interrupting power during prolonged overcurrent situations, hence the correct answer is (d) opens the motor circuit.