AC Motor Selection and Application Guide // by General Electric
AC Motor Selection and Application Guide // by General Electric

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.

Alterations- per-second of an alternating current
Figure 1 – Alterations- per-second of an alternating current

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.

Three-phase Power
Figure 2 – Three-phase Power

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


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.

The higher the frequency, the faster the motor runs. The more poles the motor has, the slower it runs. The speed of the rotating magnetic field in the stator is called synchronous speed.

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

Synchronous Speed

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:

Percent slip

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.

Title:AC Motor Selection and Application Guide – General Electric
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AC Motor Selection and Application Guide // by General Electric
AC Motor Selection and Application Guide // by General Electric

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Page edited by E.C. (Google).


  1. priya
    Apr 30, 2015

    Is it possible to run motor on direct 3phase supply without using any drive..
    and if possible at what speed it will run

  2. Hermanus Hadjon
    Jan 08, 2015

    All materis very help me in do my job, thank you very must

  3. Raul Suarez
    Dec 29, 2014

    I want to fit the EEP portal. Greetings.

  4. ben manesh
    Dec 29, 2014

    I have a question for you: What if speed of induction motor reaches synchronous motor?

    • senthil kumar
      Jan 01, 2015

      Hi Ben
      Induction motor never rotate in synchronous speed, I hope that U know about slip, induction motor reach the synchronous speed means the slip is zero,if slip is zero means induction motor speed also zero.but it not possible induction motor catch the synchronous speed

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