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Determining Motor Load & Efficiency from Measured Data

A. Bhatia, B.E.

Course Outline

Those familiar with motors know that just because a motor has a nameplate rating of 50 horsepower doesn't mean that it actually delivers 50 horsepower to its load. The load itself determines the horsepower needed to drive it, and whatever motor is connected to that load will try to deliver the power that the load demands. Spotting overloaded motors can help you avoid motor burnouts whereas spotting underloaded motors can lead to downsizing and reduced operating costs.

This 3-hr course presents a brief overview of some of the different ways to determine a motor's load from in-situ measurements. This course shall be useful for field engineers to select or to establish a proper efficiency evaluation method by understanding the theories and error sources of the methods.

The course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the reader will:

Intended Audience

This course is aimed at students, electrical engineers, service technicians, energy auditors, operational & maintenance personnel, sales & marketing personnel, and general audience.

Course Introduction

There have been many articles concerning energy efficiency of induction motors but in general, an energy efficiency improvement program includes development of a motor management plan that focuses on development of a plant motor inventory and an evaluation of motor performance for large or critical motors. There are a variety of motor performance measurements that can be made (e.g., speed, voltage, current, etc.) and often a facility has logged database measurements of motor current. Several questions arise, such as: Which load estimation method should an energy manger use? Which is most accurate?

What data is required for the different measurements? What data should be taken during an energy audit? What do you do if you do not have a complete set of data--are there still ways to determine the motor's load?

In this course, the physical basis of each basic method is described in terms of how the efficiency is obtained and of the potential errors associated with it.

Course Content

The course content is in a PDF file Determining Motor Load & Efficiency from Measured Data. You need to open or download this document to study this course.

Course Summary

There are a variety of different measurements that can be made to assess a motor's performance. The basic methods are nameplate method, slip method, current method, segregated loss method, equivalent circuit method and computational techniques. A field evaluation method can consist of a single basic method or can be built using a combination of different basic methods. The intrusiveness or cost and accuracy are the major considerations for selecting any of the field efficiency evaluation method.

The slip method using a tachometer is usually faster and easier than the wattmeter method. Assuming an accurate tachometer, the method would be accurate if the rated speed on the motor nameplate represented the exact speed at which full load power is delivered. Unfortunately, the number on the nameplate is just an approximation, usually rounded off to the nearest 5 rpm and subject to further error due to testing tolerances and differences in motor voltage. Naturally, if you take measurements using both techniques and they agree, the tachometer can be used from then on to check the performance of that particular motor.

The slip and current methods show greater than 20% discrepancies from the true efficiency. The Equivalent Circuit Method has a very low level of intrusiveness (only a measurement of speed is required), but relies on statistical data heavily, gives good results for loads above 50%. It is suitable for a targeted group of motors.


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 PDH Center 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 architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.