Duty Cycles and Motor Rating

Abstract

The horsepower rating of a motor carries a dual implication¿first, torque ability; second, temperature rise. The two are frequently confused. In selecting motors for duty-cycle jobs the two concepts should be considered separately. The motor rating and type should be chosen to fit the torque requirements of the job. The proper time rating or service factor to associate with this horsepower rating to insure satisfactory insulation life can be determined from the duty cycle. Use of oversize continuously rated motors instead of short-time-rated motors to secure high torque ability on variable, load jobs imposes an economic loss. Frequent starting and reversing or starting high inertia loads imposes a temperature hazard frequently greater than heavy overloads, Horsepower rating as now understood is not a satisfactory criterion of a motor's reversing ability. These topics are developed by means of simple hydraulic analogies.