AC Motor Selection and Application Guide

Polyphase Or Single-Phase Motor?

A power system can be either single-phase or polyphase. Figure 1 illustrates single-phase power, which is most commonly found in homes, rural areas and in small commercial establishments.

A polyphase power system consists of 2 or more alternating currents of equal frequency and amplitude but offset from each other by a phase angle. Figure 2 illustrates a three-phase power system having phases A, B and C. Each phase is offset by 120 degrees, 360 degrees being the span of one complete cycle.

* For motors, an advantage of three-phase power is simpler construction which requires less maintenance. Also, a more powerful machine can be built into a smaller frame and will generally operate at a higher efficiency than single-phase motors of the same rating.

Motor Output Rating

Speed

The speed at which an induction motor operates is dependent upon the input power frequency and the number of electrical magnetic poles for which the motor is wound.

To determine the synchronous speed of an induction motor, the following equation is used:

Actual full-load speed (the speed at which an induction motor will operate at nameplate rated load) will be less than synchronous speed. The difference between synchronous speed and full-load speed is called slip. Percent slip is defined as follows:

*Induction motors are built having rated slip ranging from less than 5% to as much as 20%.

A motor with a slip of less than 5% is called a normal slip motor. Motors with a slip of 5% or more are used for applications requiring high starting torque (conveyor) and/or higher than normal slip (punch press) where, as the motor slows down, increased torque allows for flywheel energy release.

Source: electrical-engineering-portal.com