Question: Will you please explain the service factor rating and tell how to determine service factor amperes if it's not shown on the motor nameplate?

-Ed Swisher, Orangeville, California

Answer: The service factor of an electric motor applies to the horsepower rating of the motor. The motor may be loaded up to the rated horsepower shown on the motor nameplate multiplied b the service factor.

For example, a motor rated ¼ hp with a service factor of 1.0 can only be loaded ¼ hp. If a greater load is applied, it will be overloaded and will overheat. A motor rated ¼ hp with a service factor of 1.35 may be loaded to ¼ ´ 1.35 or approximately 1/3 hp without overloading the motor.

When a motor is operated at any service factor greater than 1.0, it will have a higher temperature rise and may have a different efficiency, power factor, and speed than it had when running at rated load. Locked rotor torque, locked rotor current, and breakdown torque will remain unchanged.

Service factor is a multiplier that is applied to the rated horsepower of a motor and gives the permissible horsepower loading that may be carried continuously under usual service conditions.

Service factor amperes is the maximum current rating that can be carried by the motor continuously without overloading it. Normally, service factor amperes are shown on the motor nameplate. If not shown on the nameplate and the motor has a service factor greater than 1.0, the service factor amperes must be obtained from the motor manufacturer. Service factor amperes cannot be determined by multiplying the rated full load current by the service factor.

-Dick Beard, PE, former manager of technical field service with General Electric