What is the function of a contactor in a motor control circuit?

• A. It is an electrically operated switch that closes and opens the main power circuit to a motor when its coil is energized by a control circuit.
• B. It regulates motor speed by varying voltage across the motor.
• C. It provides thermal overload protection by sensing current.
• D. It acts as a safety interlock preventing motor start unless enclosure is closed.

Answer: (A)

A contactor is an electrically operated switch that closes and opens the main power circuit to a motor when its coil is energized by a control circuit. This lets a low-voltage control signal remotely start or stop a high-current motor, keeping the control and power sides isolated. When the control circuit energizes the coil, the contactor’s main contacts close and supply power to the motor; when the coil is de-energized, the contacts open and cut power. They often include auxiliary contacts for signaling or control purposes and are built to handle the heavy current in motor circuits.

Other options describe different devices: regulating motor speed by varying voltage is done by variable-frequency drives or similar speed-control devices, not a contactor. Providing thermal overload protection by sensing current is the job of an overload relay. Acting as a safety interlock to prevent starting unless an enclosure is closed is a door interlock or safety switch.