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Small Motor Control (1/4 HP - 200 HP)

Thomas Mason, P.E.

Course Outline

This two hour online course discusses methods of motor control and representation in motor design. It is directed towards small AC induction motors in the range of 1/4 HP to 200 HP. It is important to understand these principles because automatic control technology has been advancing so rapidly that basic safety and simple manual functions may be overlooked. This course will enable the supervising engineer or submittals approval officer to recognize safety and functional design errors.

This course includes a multiple-choice quiz at the end.

Learning Objective

At the conclusion of this course, the student will:

Course Introduction

Small AC induction motors (1/4 HP to 200 HP) are simple and reliable. They are almost always started by connecting power to the terminals. If a plug is inserted into a receptacle to start a motor, a substantial electrical arc is produced by the inrush current. For this reason, a switch or magnetic relay contact is used, which shields the arc from the operator. As motors increase in size from fractional horsepower, it becomes economic to provide motor protection, so that the motor can be stopped and repaired before catastrophic failure. As motor control becomes more complicated, standard designs must be followed to assure operator and maintenance safety and to comply with government regulations. OSHA and local building Codes require compliance with NEC 70, the National Electric Code. Beyond this minimum, NFPA 79, Electrical Standard for Industrial Machinery, is required for certain controls but is an excellent guide for all controls.

Course Content

The course content is in a PDF file (89 KB) Small_Motor_Control.pdf. You need to open or download this document to study this course.

Course Summary

Motors in the range ¼ - 200 HP are almost universally controlled by motor starters. The majority of the presently installed motor starters utilize electro-mechanical components. This course demonstrates graphic representation of simple controls and discusses the trend towards solid-state controls. A brief discussion of testing for response to open circuits and short circuits is included.

Related Links

For additional technical information related to this subject, please visit the following websites or web pages:

NFPA 79 - Electrical Standard for Industrial Machinery

General Electric Motor Control
General Electric : Products & Solutions : Business Products AZ

Cutler-Hammer Motor Control
Cutler-Hammer Home Page

Square D Motor Control
Square D - Electrical Distribution, Control and Power Management

Siemens Motor Control
Siemens Distribution Equipment SPEEDFAX 1999

Automation Direct
Motor Contactors -

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

Take a Quiz

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered professional engineer. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.