Print this page 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:
- Understand the effect of motor efficiency at different load factors;
- Learn the methods of determining motor load based on various measured parameters;
- Describe the conditions, procedure and equations used for slip, voltage compensate slip and current ratio methods;
- Compare the basic field efficiency evaluation measurement techniques;
- Learn the accuracy limitations of various techniques;
- Calculate motor efficiency with measured load data;
- Learn the importance of power factor measurement and improvement;
- Point out different measurement instruments, techniques and a few common measurement errors; and
- Learn few energy
conservation measures on induction motors.
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.
Quiz
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.
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.
