Rating of general-purpose induction motors

Abstract

This paper reviews principles of rating of general-purpose a-c motors, particularly in relation to overload torque and temperature limits. The overload capacity of an electric motor is limited, first, by its stalling, or breakdown torque, and, second, by its operating temperature. The breakdown torque, analogous to the stalling torque of a gas engine, is roughly proportional to the motor size and the square of its magnetic flux density. The temperature limitation is exactly analogous to the temperature limit of a transformer, the useful life of the insulation being reduced exponentially as the temperature is raised. The increased variety of motor uses in recent years, especially for automatic operation of refrigerator compressors, air conditioning, pumps, and other mechanical devices, has led to more exact methods of application, utilizing motor overload capacity and matching torque characteristics to the driven equipment. Under this economic pressure, motor overload capacities have been increased over the requirements of present standards, and small motors are now commonly used on intermittent overloads far beyond their continuous ratings. It is proposed, therefore, that American standards be revised to provide for increased values of breakdown torque and service factor in the smaller motor ratings, and that permissible intermittent duty cycles be denned, enabling the full economic life of the motor to be utilized. It is also proposed that the starting current be recognized as a convenient and accurate measure of induction-motor breakdown torque, or momentary torque capacity, and that starting current values be established on a logical and consistent basis for both single-phase and polyphase motors. Specific recommendations are given in the paper for rated characteristics and operating limits under this starting current-temperature system of rating, and the economic advantages to the industry to be gained by their adoption are pointed out, including safer wiring and control systems and a reduced variety of special motors.