How does a motor overload relay protect an induction motor, and what parameter does it monitor?

How does a motor overload relay protect an induction motor, and what parameter does it monitor?

• A. By sensing overload current (or temperature) and interrupting current when excessive.
• B. By monitoring ambient room temperature of the motor installation.
• C. By switching off the motor during peak load times.
• D. By reducing motor torque continuously.

Answer: (C)

Overload protection for a motor works by watching how hard the motor is being asked to run. An overload relay senses when the motor current is higher than it should be for longer than the allowed time, which means the windings are overheating due to excessive load. In many relays there’s also a temperature sensing element tied to the winding heat, so the device can trip if the winding gets too hot. When the sensed current or temperature exceeds the preset limit, the relay trips and interrupts power to the motor, preventing damage. The key parameter it monitors is the motor current (and, in devices with thermal sensing, winding temperature). Ambient room temperature or peaking at certain times isn’t what the relay uses to decide to trip, and it doesn’t continuously reduce torque as its primary function.

Overload protection for a motor works by watching how hard the motor is being asked to run. An overload relay senses when the motor current is higher than it should be for longer than the allowed time, which means the windings are overheating due to excessive load. In many relays there’s also a temperature sensing element tied to the winding heat, so the device can trip if the winding gets too hot. When the sensed current or temperature exceeds the preset limit, the relay trips and interrupts power to the motor, preventing damage.

The key parameter it monitors is the motor current (and, in devices with thermal sensing, winding temperature). Ambient room temperature or peaking at certain times isn’t what the relay uses to decide to trip, and it doesn’t continuously reduce torque as its primary function.