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How To Control 3 Phase Induction Motors

Three phase induction motors are the workhorse of industry.  It has been estimated that 70% to 80% of all electricity in the world is consumed by these motors.  They are truly elegant machines in that there are no moving parts except the rotor, and there are no brushes, commutators, or slip rings to wear out.

This computer program explains How To Control 3 Phase Induction Motors in easy to understand text, graphics, and pictures. The program explains:

The program is recommended for electricians, technicians, engineers, and anyone else interested in How To Control 3 Phase Induction Motors.

This program requires no installation on your computer.   Simply double-click on the Table of Contents.html file or any other .html file on the CD and your web browser will run the program.

Example screen showing Table of Contents of the program.

Table of Contents


1.  Introduction

2.  Why Not Just Use a Switch?

3.  Manual Motor Starters

4.  NEMA Size Motor Starters

5.  Magnetic Motor Starters

6.  Motor Starting Circuit

7.  Motor Starting Circuit Explained

8.  Reversing Motor Starters

9.  Reversing Motor Starter Schematic

10.  Controlling Motor Speed

11.  Multi-Speed Motor Starters

12.  Variable Speed Motor Starters

13.  Variable Speed Drives (VFDs)

14.  Variable Speed Drive Features

15.  Motor Starting Characteristics

16.  Reduced Voltage Starters

17.  Primary Resistor Starters

18.  Autotransformer Starters

19.  Wye-Delta Starters

20.  Part Winding Starters

21.  Solid State Starters


 Example screen showing magnetic motor starting circuit.

Electrical drawings usually show schematic diagrams, not pictorial diagrams.  The pictorial diagram below is the same as the one seen on the previous page.


Pictorial Diagram

The schematic diagram below is how the same circuit would be shown on an electrical drawing.  In addition, a control transformer has been added to the circuit so that the pushbutton and coil of the motor starter are 120VAC instead of 480VAC, because it is generally considered safer to have 120VAC on a pushbutton than 480VAC.  

Schematic Diagram 



Example screen showing reversing motor control schematic. 

When the Forward pushbutton is pressed, the F coil is energized, sending power to the motor through the green power contacts. Phases A, B, and C are applied to the motor as shown below:



When the Reverse pushbutton is pressed, the R coil is energized sending power to the motor through the red power contacts. Phases A, B, and C are applied to the motor as shown below, reversing the direction of the motor rotation.







Example screen showing Variable Frequency Drives

Another way to vary the speed of the motor is to vary the frequency of the power applied to the motor.

Packaged drives, called VFDs (variable frequency drives) like the ones shown below from Allen Bradley  provide variable frequency power to induction motors so that the speed of the motors can be varied.  Click here to see more variable speed drives from Allen Bradley.


These drives have become available in only the last few years due to the development of  IGBTs (insulated gate bipolar transistors) like the one shown below.



The IGBT is essentially a power transistor capable of switching at very high speeds.


Hardware/Software Requirements:

  • IBM.® 486 or higher compatible computer
  • Web browser such as Internet Explorer®, Firefox®, Chrome®, Edge®, etc. 
  • 800 x 600 or higher screen video capability.