Chapter 3, Problem 1RQ

A single-phase, 120 volt motor is controlled by a manual motor starter. The motor is not running, and the switch handle on the starter is found to be in the center position. What does this indicate?

To determine

The indication of center position of switch handle of a manual motor starter.

Explanation of Solution

Manual motor starter is simple and common motor starter. Operator involvement is must to change the state of a manual motor starter. A manual motor starter is looks like a simple toggle switch. In addition, the manual motor starters are come up with overload protectors. Thus, the manual motor starter has three positions namely ON, OFF, and over load protection.

The two end positons of the toggle lever of a manual motor starter indicates ON and OFF positions. The center position indicates that the motor is tripped due to over load. Whenever the current flows in a circuit, a proportionate heat is generated in a conductor. When a motor is over loaded, more current will flow, heat generation is high. If this continues, the motor will fail. Thus, the overload heater (protector) is involved. The over load protector is designed for normal operating condition. When a motor runs abnormally (over loading), the over load protector stops the power supply to the motor. Thereby, the toggle lever (switch handle) moved to center position.

Hence, the center position of switch handle of a manual motor starter denotes that the motor is tripped due to over load.