In a motor starter assembly, which device switches the motor circuit on and off?

• A. A device that provides overload protection.
• B. A device that switches the motor circuit on and off.
• C. A device that measures motor RPM.
• D. A device that converts AC to DC.

Answer: (B)

The part that actually turns the motor on and off is the contactor in the motor starter. A contactor is a heavy-duty switch controlled by a low-voltage control circuit. When the control coil is energized, the main contacts close and power is connected to the motor, starting it. When the coil is de-energized, the contacts open and the motor is disconnected.

The other piece in a motor starter is the overload protection device. It sits in the same assembly to protect the motor by opening the circuit if current exceeds safe levels, but it doesn’t perform the normal on/off control. Measuring motor RPM or converting AC to DC serve different purposes (speed sensing and power conversion, respectively) and aren’t the devices that switch the motor circuit on and off.

So, the device that performs the on/off switching of the motor circuit is the contactor.