What are the Basic Factors When Selecting AC Induction Motors?


Cutaway View of Alternating Current Induction Motor

AC induction motors are commonly used in industrial applications. The following motor discussion will center around three-phase, 460 VAC, asynchronous, induction motors. An asynchronous motor is a type of motor where the speed of the rotor is other than the speed of the rotating magnetic field. This type of motor is illustrated. Electromagnetic stator windings are mounted in a housing. 

Power connections, attached to the stator windings, are brought out to be attached to a three-phase power supply. On three-phase, dual-voltage motors nine leads are supplied for power connections. Three power connection leads are shown in the following illustration for simplicity. A rotor is mounted on a shaft and supported by bearings. On self-cooled motors, like the one shown, a fan is mounted on the shaft to force cooling air over the motor.

The nameplate of a motor provides important information necessary when applying a motor to an AC drive. The following drawing illustrates the nameplate of a sample 25 horsepower AC motor.

Nameplate of an AC Induction Motor
Photo: Siemens

Motor Connections

This motor can be used on 230 VAC or 460 VAC systems. A  wiring diagram indicates the proper connection for the input power leads. The low voltage connection is intended for use on 230 VAC with a maximum full load current of 56.8 Amps. The high voltage connection is intended for use on 460 VAC with a maximum full load current of 28.4 Amps.

Motor Speed

Base speed is the nameplate speed, given in RPM, where the motor develops rated horsepower at rated voltage and frequency. It is an indication of how fast the output shaft will turn the connected equipment when fully loaded and proper voltage is applied at 60 hertz. The base speed of this motor is 1750 RPM at 60 Hz. If the connected equipment is operating at less than full load, the output speed will be slightly greater than the base speed.

Service Factor

A motor designed to operate at its nameplate horsepower rating has a service factor of 1.0. Some applications may require a motor to exceed the rated horsepower. In these cases a motor with a service factor of 1.15 can be specified. The service factor is a multiplier that may be applied to the rated power. A 1.15 service factor motor can be operated 15% higher than the motor’s nameplate horsepower. Motors with a service factor of 1.15 are recommended for use with AC drives. It is important to note, however, that even though a motor has a service factor of 1.15 the values for current and horsepower at the 1.0 service factor are used to program a variable speed drive.

Insulation Class

The National Electrical Manufacturers Association (NEMA) has established insulation classes to meet motor temperature requirements found in different operating environments. The four insulation classes are A, B, F, and H. Class F is commonly used. Class A is seldom used. Before a motor is started, its windings are at the temperature of the surrounding air. This is known as ambient temperature. NEMA has standardized on an ambient temperature of 40° C, or 104° F for all motor classes. The temperature rises in the motor as soon as it is started. The combination of ambient temperature and allowed temperature rise equals the maximum winding temperature in a motor. A motor with Class F insulation, for example, has a maximum temperature rise of 105° C. The maximum winding temperature is 145° C (40° ambient plus 105° rise). A margin is allowed for a point at the center of the motor’s windings where the temperature is higher. This is referred to as the motor’s hot spot.

NEMA Design

The National Electrical Manufacturers Association (NEMA) has established standards for motor construction and performance. The nameplate on page 20 is for a motor designed to NEMA B specifications. NEMA B motors are commonly used with AC drives. Any NEMA design (A, B, C, or D) AC motor will work perfectly well with a properly sized variable speed drive. 


AC motor efficiency is expressed as a percentage. It is an indication of how much input electrical energy is converted to output mechanical energy. The nominal efficiency of this motor is 93.0%. 


  • Siemens

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