The effect of repair on motor efficiency

The Effect Of
Repair/Rewinding
On Motor Efficiency
EASA/AEMT Rewind Study
and

Good Practice Guide
To Maintain Motor Efficiency

A EMT

Electrical Apparatus Service Association, Inc.
1331 Baur Boulevard
St. Louis, Missouri 63132 U.S.A.
314-993-2220 • Fax: 314-993-1269
easainfo@easa.com • www.easa.com

Association of Electrical and Mechanical Trades
58 Layerthorpe
York Y031 7YN
England, UK
44-1904-656661 • Fax: 44-1904-670330

Copyright © 2003. St. Louis, Missouri USA
and York, England UK. All rights reserved.

Disclaimer
The information in this report and the Good Practice Guide to Maintain Motor Efficiency (Part 2) was carefully prepared and
is believed to be correct but neither EASA nor the AEMT make any warranties respecting it and disclaim any responsibility or
liability of any kind for any loss or damage as a consequence of anyone’s use of or reliance upon such information.

Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.

(This page intentionally left blank.)

Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.

Foreword

Foreword

Foreword
This publication contains an Executive Summary and a comprehensive report on the results of the EASA/AEMT rewind study
(Part 1); a Good Practice Guide to Maintain Motor Efficiency (Part 2); and Further Reading (Part 3). Its organization assumes
a diverse group of readers whose interest in these subjects will vary widely–e.g., plant managers, purchasing agents,
representatives of government agencies, engineers, and service center technicians. Accordingly, some of the material may
not be of interest to all of them.

Acknowledgments

Principal Contributors • British Nuclear Fuels (BNF)
• John Allen • United States Department of Energy (DOE)
• Energy Efficient Best Practice Program (EEBPP), UK
• Thomas Bishop
• Ministry of Defense Ships Support Agency (MoD
• Austin Bonnett SSA), UK
• Brian Gibbon • UK Water Industry Research, Ltd. (UKWIR)
• Alan Morris
Participating Manufacturers and Institutions
• Cyndi Nyberg
• Ten motor manufacturers provided motors, technical
• David Walters data and assistance for the study: ABB, Baldor, Brook
• Chuck Yung Crompton, GEC, Leeson, Reliance, Siemens,
Toshiba, U.S. Electrical Motors and VEM.
Project Sponsors • The Dowding and Mills facility in Birmingham, UK,
• Electrical Apparatus Service Association, Inc. carried out all motor rewinds and repairs.
(EASA) • The University of Nottingham performed efficiency
• Association of Electrical and Mechanical Trades testing on their dynamometers in Nottingham, UK.
(AEMT)

Acronyms
The acronyms of additional organizations that played an important role in the publication of this document are shown below.

ANSI–American National Standards Institute
CSA–Canadian Standards Association
IEC–International Electrotechnical Commission
IEEE–Institute of Electrical and Electronics Engineers, Inc.
NEMA–National Electrical Manufacturers Association
UL–Underwriters Laboratories, Inc.
BS–British Standards
EN–European Standard

Please direct questions and comments about the rewind study and the Good Practice Guide to EASA, 1331 Baur
Boulevard, St. Louis, Missouri 63132; 314-993-2220; Fax: 314-993-1269; easainfo@easa.com.

Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc. i

Foreword
Foreword


ii Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.

Table of Contents

Table of Contents

Part 1: EASA/AEMT Rewind Study
Executive Summary .................................................................................... 1-3
Test Protocol & Results .............................................................................. 1-9

Table of Contents
Part 2: Good Practice Guide
To Maintain Motor Efficiency
Introduction ................................................................................................. 2-3
Terminology ................................................................................................. 2-3
Energy Losses in Induction Motors .......................................................... 2-4
Motor Repair Processes ........................................................................... 2-10

Part 3: Further Reading
Bibliography ................................................................................................ 3-3
Appendix 1: Chord Factor and Distribution Factor .................................. 3-4
Appendix 2: Analysis of Winding Configurations .................................... 3-6
Appendix 3: Changing To Lap Windings–Examples ............................... 3-7
Appendix 4: Electrical Steels ..................................................................... 3-9
Appendix 5: Repair–Replace Considerations ......................................... 3-17

Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc. iii

Table of Contents
Table of Contents


iv Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.

Part 1 EASA/AEMT Rewind Study

Part 1: EASA/AEMT Rewind Study

Executive Summary .................................................................................... 1-3
Test Protocol & Results .............................................................................. 1-9
Test Protocol–Key Points ......................................................................................... 1-9
Comparison of IEC 60034-2 and IEEE 112-1996 Load Testing Methods .............. 1-11
Loss Segregation Methods Used in EASA/AEMT Rewind Study .......................... 1-13
Core Loss Testing .................................................................................................. 1-15
Test Data ................................................................................................................ 1-16


Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc. 1-1

EASA/AEMT Rewind Study Part 1


1-2 Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.

Part 1 Executive Summary

Executive Summary
The Effect of Repair/Rewinding On Motor Efficiency

Introduction to 30 hp or 22.5 kW), they often assert that efficiency drops
1 - 5% when a motor is rewound–even more with repeated
Electric motors are key components in most industrial
rewinds [Refs. 1-5]. This perception persists, despite evi-
plants and equipment. They account for two-thirds of all the
dence to the contrary provided by a more recent study by
electrical energy used by industrial/commercial applica-
Advanced Energy [Ref. 6].
tions in the developed world with lifetime energy costs
normally totaling many times the original motor purchase In this context, decision makers today are carefully evalu-
price. In Europe and the USA alone, the annual cost of ating both the reliability and the efficiency of the motors they
energy used by motors is estimated at over $100 billion buy or have repaired. The difficulty they face, however, is
(U.S.). Yet motor failure can cost more in terms of lost how to separate fact from fiction, reality from myth.
production, missed shipping dates and disappointed cus-
tomers. Even a single failure can adversely impact a Objectives
company’s short-term profitability; multiple or repeated fail- EASA and AEMT designed this study to find definitive
ures can reduce future competitiveness in both the medium answers to efficiency questions, particularly as regards
and long term. repaired/rewound motors. The primary objective of the
Clearly, industrial companies need effective motor main- project was to determine the impact of rewinding/repair on
tenance and management strategies to minimize overall induction motor efficiency. This included studying the ef-
motor purchase and running costs while avoiding the pitfalls fects of a number of variables:
caused by unexpected motor failures. • Rewinding motors with no specific controls on stripping
Experienced users long have known that having motors and rewind procedures.
repaired or rewound by a qualified service center reduces • Overgreasing bearings.

Executive Summary
capital expenditures while assuring reliable operation. Ris- • How different burnout temperatures affect stator core
ing energy costs in recent years, however, have led to losses.
questions about the energy efficiency of repaired/rewound
motors. • Repeated rewinds.
To help answer these questions, the Electrical Apparatus • Rewinding low- versus medium-voltage motors.
Service Association (EASA) and the Association of Electri- • Using different winding configurations and slot fills.
cal and Mechanical Trades (AEMT) studied the effects of • Physical (mechanical) damage to stator core.
repair/rewinding on motor efficiency. This Executive Sum-
mary briefly describes the methodology and results of this A second goal was to identify procedures that degrade,
study. The remainder of Part 1 provides additional details help maintain or even improve the efficiency of rewound
and test data. A Good Practice Guide to Maintain Motor motors and prepare a Good Practice Guide to Maintain
Efficiency (Part 2) that identifies procedures for maintaining Motor Efficiency (Part 2).
or even improving the efficiency of motors after rewind is A final objective was to attempt to correlate results
also included. obtained with the running core loss test and static core loss
tests.
Background This research focused on induction motors with higher
Simple, robust and efficient, induction motors often con- power ratings than those in previous studies (i.e., those
vert 90% - 95% of input electrical power into mechanical most likely to be rewound), subjecting them to independent
work. Still, given the huge amount of energy they use, even efficiency tests before and after rewinding. Throughout this
minor changes in efficiency could have a big effect on study EASA and the AEMT have sought a balanced ap-
operating costs. proach that takes account of practical constraints and
Over the past two decades, rising energy costs and overall environmental considerations.
governmental intervention have led to significant improve- The results of tests carried out by Nottingham University
ments in motor efficiency. In the USA, for example, the (UK) for EASA and the AEMT show that good practice repair
Energy Policy Act of 1992 (EPAct) and new premium methods maintain efficiency to within the range of accuracy
efficiency designs have boosted efficiency levels to the that it is possible to measure using standard industry test
highest currently available. In Europe, voluntary agree- procedures (± 0.2%), and may sometimes improve it. The
ments among leading motor manufacturers and the accompanying report also identifies the good practice repair
European Commission (EC) are aiming at the same result processes and provides considerable supporting informa-
with EFF1 category motors. tion.
Meanwhile, claims that repair/rewinding inevitably de-
creases motor efficiency have been commonplace. Based Scope of Products Evaluated
largely on a handful of studies of mostly smaller motors (up The study involved 22 new motors ranging from 50 to


Executive Summary Part 1

300 hp (37.5 to 225 kW) and 2 smaller motors [7.5 hp (5.5 Methodology
kW)]. These included:
All tests were carried out in accordance with IEEE Stan-
• 50 and 60 Hz motors dard 112 Method B using a dynamometer test rig (see
• Low- and medium-voltage motors Figure 1). Instrumentation accuracy exceeded that required
• IEC and NEMA designs by the Standard. A new 40 hp (30 kW) motor was tested at
four different locations (see side-bar “Round Robin Testing
• Open drip-proof (IP 23) and totally enclosed fan-cooled
and Test Protocol”) to verify the accuracy of the test equip-
(IP 54) enclosures
ment and methods used by Nottingham University. For
• 2- and 4-pole motors comparison, efficiencies also were calculated in accor-
• 7.5 hp (5.5 kW) motors (for checking earlier results of dance with BS EN 60034-2, which is the current standard in
multiple burnout cycles) Europe.
• Round robin tests on a new 40 hp (30 kW) motor, which Each new motor was run at full load until steady-state
indicate that such factors as supply voltage, repeatability conditions were established and then load tested, dis-
of the test procedures, and instrumentation, taken to- mantled and the windings burned out in a
gether, can affect test results. controlled-temperature burnout oven. Each motor was then
stripped, rewound, reassembled and retested using the
same test equipment as before. In most cases, core losses

Figure 1
Executive Summary



were measured before burnout and after stripping using a Results: Average efficiency change of -0.1%
loop (ring) test and/or two commercial core loss testers. (range +0.7 to -0.6%) after 3 rewinds (3 ma-
chines) and 2 rewinds (2 machines).
Results of Efficiency Tests on Rewound Motors Group C2 Two low-voltage motors [7.5 hp (5.5 kW)] pro-
The 22 new motors studied were divided into four groups cessed in burnout oven three times and
to accommodate the different test variables. The test results rewound once. Controlled stripping and rewind
summarized below show no significant change in the effi- processes with burnout temperature of 680° F
ciency of motors rewound using good practice repair - 700° F (360° C - 370° C).
procedures (within the range of accuracy of the IEEE 112B
test method), and that in several cases efficiency actually
increased. (For detailed test results, see “EASA/AEMT Test ROUND ROBIN TESTING
Protocol & Results” on Pages 1-7 to 1-19.) AND TEST PROTOCOL
Group A Six low-voltage motors [100 - 150 hp (75 - 112 To ensure accurate tests results, a 30 kW IEC motor
kW) rewound once. No specific controls on was efficiency tested first by the University of Not-
stripping and rewind processes with burnout tingham and then by three other test facilities. The
temperature of 660° F (350° C). other facilities were: U.S. Electrical Motors, St. Louis,
Missouri; Baldor Electric Co., Fort Smith, Arkansas;
Results: Initially showed average efficiency and Oregon State University, Corvallis, Oregon.
change of -0.6% after 1 rewind (range -0.3 to
-1.0%). Each facility tested the motor at 50 and 60 Hz using
the IEEE 112 Method B (IEEE 112B) test procedure.
However, two motors that showed the greatest All testing used the loss-segregation method (at no
efficiency reduction had been relubricated dur- load and full load), which allowed for detailed analy-
ing assembly, which increased the friction loss. sis.
After this was corrected the average efficiency As a benchmark, the results were compared with
change was -0.4% (range -0.3 to -0.5%). those of round robin test programs previously con-
Group B Ten low-voltage motors [60 - 200 hp (45 - 150 ducted by members of the National Electrical

Executive Summary
kW)] rewound once. Controlled stripping and Manufacturers Association (NEMA). Initial results of
rewind processes with burnout temperature NEMA’s tests varied by 1.7 points of efficiency; the
of 680° F - 700° F (360° C - 370° C). variance subsequently was reduced to 0.5 points of
Results: Average efficiency change of efficiency by standardizing test procedures.
-0.1% (range +0.2 to -0.7%). As Table 1 shows, the range of results for round robin
One motor was subsequently found to have tests of the 30 kW motor in this study did not exceed
faulty interlaminar insulation as supplied. Omit- 0.9 points of efficiency at 60 Hz, and 0.5 points at 50
ting the result from this motor, the average Hz. These results were achieved without standard-
efficiency change was -0.03% (range +0.2 to ization and compare favorably with the 1.7% variation
-0.2%). of the NEMA tests.
These results also verify that the test protocol for
Group C1 Five low-voltage motors [100 - 200 hp (75 - 150
determining the impact of rewinding on motor effi-
kW)] rewound two or three times. Controlled
ciency is in accord with approved industry practice,
stripping and rewind processes with burnout
and that the results obtained in this study are not
temperature of 680° F - 700° F (360° C -
skewed by the method of evaluation.
370° C).

TABLE 1
ROUND ROBIN TEST RESULTS OF 30 KW, 4-POLE MOTOR
Full load Full load Full load Temperature
Test location Test efficiency power factor amps rise rpm

Baldor 400v/50 Hz 91.8% 86.8% 54.0 69.4° C 1469

Nottingham 400v/50 Hz 92.3% 87.0% 54.2 68.0° C 1469

U.S. Electrical Motors 400v/50 Hz 91.9% 86.7% 53.5 59.0° C 1470

Nottingham 460/60 Hz 93.5% 85.9% 47.0 53.9° C 1776

Oregon State 460v/60 Hz 92.6% 85.9% 47.0 50.0° C 1774




Results: Average efficiency change of Group A) included control of core cleaning methods and
+0.5% (range +0.2 to +0.8%). rewind details such as turns/coil, mean length of turn, and
Group D One medium-voltage motor [300 hp (225 kW)] conductor cross sectional area.
with formed stator coils rewound once. Con- The benefits of these controls, which form the basis of the
trolled stripping and rewind processes with Good Practice Guide to Maintain Motor Efficiency (Part 2),
burnout temperature of 680° F - 700° F (360° C are clearly shown in Figure 2, which compares the results
- 370° C). for motors in Groups A and B.
Results: Efficiency change of -0.2%. The
behavior of this motor was similar to the low-
Conclusion
voltage machines rewound with specific This report is the work of a team of leading international
controls. personnel from industry and academia. The results clearly
demonstrate that motor efficiency can be maintained pro-
Significance of Tests Results vided repairers use the methods outlined in the Good
Practice Guide to Maintain Motor Efficiency (Part 2).
The test results for all groups fall within the range of the
deviation of the round robin tests, indicating that test proce- Partial List of Supporting Information Provided
dures were in accordance with approved industry practice
Elsewhere in This Publication
(see side-bar on “Round Robin Testing”).
The average efficiency change for each group also falls • EASA/AEMT Test Protocol & Results (Part 1). This
within the range of accuracy for the test method (± 0.2%), includes a full account of the details of the study, as well
showing that motors repaired/rewound following good prac- as actual test data. In addition, this section explains in
tices maintained their original efficiency, and that in several simple terms how motor losses were calculated for this
instances efficiency actually improved. (See side-bar “Ex- study in accordance with IEEE 112 Method B, widely
planation of Nameplate Efficiency.”) recognized as one of the most accurate test standards
currently in use. It also summarizes the main differences
All motors were burned out at controlled temperatures. between the IEC test standard (BS EN 60034-2) and
Other specific controls applied to motors (except those in IEEE 112-1996 and compares the motor efficiencies
Executive Summary

measured for this project but calculated by the two
different methods. Finally, this section demonstrates that
93.8 tests commonly used by service centers are effective in
93.75

93.7 determining if repair processes (particularly winding
93.6 burnout and removal) have affected motor efficiency.
93.5 94.16 • Good Practice Guide to Maintain Motor Efficiency
94.16

93.4 94.15 (Part 2). Intended primarily for service center personnel,
93.3 94.14 this outlines the good practice repair methods used to
93.32

93.2 94.13 achieve the results given in this study. It can be used as
94.13

93.1 94.12 a stand-alone document. It also contains repair tips,
93.0 94.10 relevant motor terminology, and information about
sources of losses in induction motors that affect effi-
ciency. Included, too, is a useful analysis of stray load
100
loss, which is currently treated differently in IEC and
90 IEEE motor test standards.
• Appendix 4: Electrical Steels. The type of electrical
80
steel and interlaminar insulation chosen for the stator
70 and rotor laminations are very important in determining
Percent efficiency

motor performance and efficiency. Improper repair pro-
60 cesses, however, can alter the qualities of the steel core
Before rewind

Before rewind
After rewind

After rewind

and its interlaminar insulation. This appendix reviews the
50 various types of electrical steel used throughout the
40 world and explains in greater detail the reasons for some
of the good practices suggested in Part 2.
30 • Appendix 5: Repair or Replace?. This often difficult
question is covered comprehensively here. Replacing a
20
motor with a new one of higher efficiency is often the best
10 financial option. At other times, repairing the existing
motor will yield better results. Key factors include annual
0 running hours, the availability of a suitable high efficiency
Group A Group B
replacement motor, downtime, and reliability. This chap-
Figure 2. Average efficiency. ter also contains charts that can help both users and
repairers make the best choice.



EXPLANATION OF Table 2 NEMA/EPACT Efficiency Levels
NAMEPLATE EFFICIENCY
Minimum Efficiency
Nameplate efficiency is the benchmark for compar-
Nominal Based on 20%
ing efficiencies before and after a motor rewind. It is Efficiency Loss Difference
important to understand the basis for and limitation of
nameplate values. 94.1 93.0
The nameplate may state the nominal efficiency, the 93.6 92.4
minimum (also called “guaranteed”) efficiency, or 93.0 91.7
both. If only one is listed, it usually is the nominal 92.4 91.0
value, which always has an associated minimum
value (to allow for higher losses). If no efficiency is 91.7 90.2
shown on the nameplate, contact the motor manu- 91.0 89.5
facturer or consult catalogs or technical literature. 90.2 88.5
Nominal and minimum efficiencies are best under- 89.5 87.5
stood as averages for particular motor designs–not
as actual tested efficiencies for a particular motor. 88.5 86.5
They are derived by testing sample motors of a single 87.5 85.5
design. Reference: NEMA MG 1-1998 (Rev. 3), Table 12-10.
As Tables 2 and 3 show, the efficiencies for NEMA
and IEC motors cover a range of values (between the
Table 3. IEC 60034-1, 1998 Efficiency Levels
minimum and nominal efficiencies). They are not
discrete values. Consequently, it can be misleading Minimum Efficiency Minimum Efficiency
to compare the tested efficiency of a new or rewound Nominal <50 kW (15% >50 kW (20%
motor with its nameplate efficiency. Efficiency Loss Difference) Loss Difference)
The minimum efficiency is based on a “loss differ- 94.1 93.3 93.5

Executive Summary
ence” of 20% for NEMA motors and 10 or 15% for IEC 93.6 92.7 93.0
motors. This allows for variations in material, manu-
93.0 92.0 92.3
facturing processes, and test results in motor-to-motor
efficiency for a given motor in a large population of 92.4 91.3 91.6
motors of a single design. 91.7 90.5 90.9
Nominal and minimum efficiency values are only 91.0 89.7 90.1
accurate at full load, with rated and balanced sinusoi-
90.2 88.7 89.2
dal voltage and frequency applied at sea level and at
an ambient of 25° C. Therefore, it usually is imprac- 89.5 87.9 88.5
tical to measure efficiency in situ to the levels of 88.5 86.2 87.4
accuracy implied by the three significant figures that
87.5 85.9 86.3
may be shown on the nameplate. The fact that the
tested efficiency does not match the nominal name- Reference: IEC 60034-1, Table 18. Nominal and minimum
plate efficiency does not imply that the motor was efficiencies for IEC motors measured by summation of loss
made or repaired improperly. method.
Figures 2 and 3 show typical nameplates for IEC and
NEMA motors. CATALOG # MODEL #
SHAFT END BRG OPP END BRG
Reference: NEMA MG 1-1998 (Rev. 3).
FR TYPE ENCL
PH MAX AMB °C ID#
INSUL CLASS DUTY WT BAL
AC MOTOR IEC 60034 EFF1
HP RPM SF HZ
TYP SER. NO. YEAR
KW r/min V A HZ
VOLTS MAX KVAR NEMA NOM EFF

KW r/min V A HZ AMPS CODE DES

DUTY INSUL AMB °C RISE K DESIGN 3 PHASE SF AMPS PF GUARANTEED EFF

COS Ø CODE IP IC SERVICE FACTOR
GREASE DE BRG NDE BRG
DIAG IA/IN MA/MN kg MOTOR WT

Figure 2. Typical IEC Motor Nameplate Figure 3. Typical NEMA Motor Nameplate



References
[1] William U. McGovern, “High Efficiency Motors for Up-
grading Plant Performance,” Electric Forum 10, No. 2
(1984), pp. 14-18.
[2] Roy S. Colby and Denise L. Flora, Measured Efficiency
of High Efficiency and Standard Induction Motors
(North Carolina State University, Department of Electri-
cal and Computer Engineering (IEL), 1990).
[3] D. H. Dederer, “Rewound Motor Efficiency,” Ontario
Hydro Technology Profile (Ontario Hydro, November
1991).
[4] Zeller, “Rewound High-Efficiency Motor Performance,”
Guides to Energy Management (BC Hydro, 1992).
[5] Rewound Motor Efficiency, TP-91-125 (Ontario Hydro,
1991).
[6] Advanced Energy, “The Effect of Rewinding on Induc-
tion Motor Losses and Efficiency” (EEMODS 02, 2002).
Executive Summary


Part 1 Test Protocol & Results

EASA/AEMT Test Protocol & Results

TEST PROTOCOL–KEY POINTS • 7.5 hp (5.5 kW) motors (for checking earlier results of
multiple burnout cycles)
Experienced users long have known that having motors • Round robin tests on a new 40 hp (30 kW) motor, which
repaired or rewound by a qualified service center reduces indicate that such factors as supply voltage, repeatability
capital expenditures while assuring reliable operation. Ris- of the test procedures, and instrumentation, taken to-
ing energy costs in recent years, however, have led to gether, can affect test results.
questions about the energy efficiency of repaired/rewound
motors. To help answer these questions, the Electrical Standards for Evaluating Losses
Apparatus Service Association (EASA) and the Association
Two principal standards are relevant to this work. IEC
of Electrical and Mechanical Trades (AEMT) studied the
60034-2 is the current European standard (BS EN 60034-2
effects of repair/rewinding on motor efficiency.
is the British version), and IEEE 112 is the American
standard. The IEEE standard offers several methods of
Objectives of the Study translating test results into a specification of motor effi-
ciency. IEEE 112 Method B (IEEE 112B) was used for this
The primary objective of the study was to provide the most
study because it provides an indirect measurement of stray
accurate assessment possible of the impact of motor repair
load loss, rather than assuming a value as the IEC standard
on rewinding. This included studying the effects of a number
does. IEEE 112B therefore measures efficiency more accu-
of variables:
rately than the IEC method.
• Rewinding motors with no specific controls on stripping
Both IEC 60034-2 and IEEE 112B efficiency test proce-
and rewind procedures.
dures require no-load, full-load and part-load tests. The
• Overgreasing bearings. IEEE approach requires no-load tests over a range of

Test Protocol & Results
voltages and a wider range of loads for the part-load
• Different burnout temperatures on stator core losses. conditions. The IEEE 112B also requires precise torque
• Repeated rewinds. measurement, whereas the IEC test does not.
Although the study was conducted in accordance with
• Rewinding low- versus medium-voltage motors. IEEE 112B test procedures, the results are quoted to both
• Using different winding configurations and slot fills. IEC and IEEE standards. Interestingly, the most significant
difference between them is in the area of stray load loss.
• Physical (mechanical) damage to stator core. (For an in-depth comparison of IEEE 112B and IEC 60034-2,
see Page 1-12; and for an explanation of loss segregation
A second goal was to identify procedures that degrade, according to IEEE 112-1996, see Page 1-14. )
help maintain or even improve the efficiency of rewound
motors and prepare a Good Practice Guide to Maintain Methodology
Motor Efficiency (Part 2).
All tests were carried out in accordance with IEEE 112B
A final objective was to attempt to correlate results using a dynamometer test rig (see Figure 1). Instrumenta-
obtained with the running core loss test and static core loss tion accuracy exceeded that required by the standard. A
tests. new 40 hp (30 kW) motor was tested at four different
locations (see “Round Robin Testing” on Page 1-11) to
Products Evaluated verify the accuracy of the test equipment and methods used
by Nottingham University. For comparison, efficiencies also
This research focused on induction motors with higher
were calculated in accordance with BS EN 60034-2, which
power ratings than those in previous studies (i.e., those
is the current standard in Europe (see Page 1-14 for
most likely to be rewound), subjecting them to independent
discussion of IEEE and IEC methods for calculating stray
efficiency tests before and after rewinding [Refs. 1 - 6].
load losses).
Twenty-two new motors ranging from 50 to 300 hp (37.5
Each motor was initially run at full load until steady-state
to 225 kW) and 2 smaller motors [7.5 hp (5.5 kW)] were
conditions were established and then load tested. The
selected for the study. These included:
motors were then dismantled, the stators were processed in
• 50 and 60 Hz motors a controlled-temperature oven, and the windings were
• Low- and medium-voltage motors removed. Next, each motor was rewound, reassembled and
retested using the same test equipment as before. In most
• IEC and NEMA designs cases, core losses were measured before burnout and after
• Open dripproof (IP 23) and totally enclosed fan-cooled (IP coil removal using a loop (ring) test and/or two commercial
54) enclosures core loss testers. To minimize performance changes due to
factors other than normal rewind procedures, rotor assem-
• 2- and 4-pole motors blies were not changed.


Test Protocol & Results Part 1

Potential Sources of Error Repeatability of Results
Ideally, the electrical supply to a machine under test Although accuracy of the highest order obviously was
should be a perfectly sinusoidal and balanced set of three- required, repeatability was even more important. Therefore,
phase voltages. Unbalance in the phase voltages (line-to-line the test rig for this project (Figure 1) was designed to control
as only three wire supplies are used) or imperfection in the three of four basic factors that contribute to repeatability: the
120 electrical degree phase difference between adjacent power supply system, the mechanical loading system, and
phases will increase machine losses. Although losses the instrumentation. The fourth variable, test procedures, is
change with the changing unbalance during the day in the discussed separately below.
normal supply system, phase voltage regulation can miti- Test rig and equipment. The test equipment used by the
gate this. University of Nottingham consisted of a DC load machine
The presence of voltage harmonics or distortion in the that was coupled to the test motor by a torque transducer
supply also will increase the power loss in a machine. The mounted in a universal joint. The AC supply to the test
considerable distortion present on normal mains supplies motors was provided by an AC generator that was driven by
changes constantly throughout the day and from day to day. an inverter-fed synchronous motor. This setup provided a
Such potential sources of error were avoided in this constant sinusoidal voltage of almost perfect balance and
project by rigorously adhering to the IEEE 112B test proce- waveform purity. A second DC machine was coupled to the
dures and using a well-designed test rig. same shaft as the generator and synchronous motor to

Figure 1. Schematic of Test Rig Used for IEEE 112 Method B Tests



reclaim energy from the DC load machine. Round Robin Testing of 30 kW IEC Motor
A range of in-line torque transducers was employed in As an additional check to ensure accurate test results, a
each rig to ensure maximum accuracy. Power, voltage, 30 kW IEC motor was efficiency tested first by the University
current, speed and torque were measured with a Norma of Nottingham and then by three other test facilities. The
D6000 wattmeter with motor option. All torque, speed and other facilities were: U.S. Electrical Motors, St. Louis, Mis-
power readings were taken at the same instant and aver- souri; Baldor Electric Co., Fort Smith, Arkansas; and Oregon
aged over several slip cycles to minimize reading fluctuations. State University, Corvallis, Oregon.
The winding resistance was measured at the motor termi-
Each facility tested the motor at 50 and 60 Hz using the
nals with a four-wire Valhalla electronic bridge with a basic
IEEE 112B test procedure. All testing used the loss-segre-
accuracy of 0.02%.
gation method (at no load and full load), which allowed for
The test setup therefore controled three of the four detailed analysis.
potential sources of error–power supply, loading system
As a benchmark, the results were compared with those of
and test equipment. That leaves just one–test procedures.
round robin test programs previously conducted by mem-
Test procedures. The tests for this study were per- bers of the National Electrical Manufacturers Association
formed in accordance with IEEE 112B. Test procedures, (NEMA). Initial results of NEMA’s tests varied by 1.7 points
measurement intervals, and thermocouple location on the of efficiency; the variance subsequently was reduced to 0.5
winding were optimized by comparing results for a 30 kW points of efficiency by standardizing test procedures.
test motor with those obtained using direct measurement of
As Table 1 shows, the range of results for round robin
loss by calorimeter.
tests of the 30 kW motor in this study did not exceed 0.9
As a precursor to the load test, each motor completed an points of efficiency at 60 Hz, and 0.5 points at 50 Hz. These
entire thermal cycle of the test machine, running at full load results were achieved without standardization and compare
until the temperature stabilized and the grease in the favorably with the 1.7% variation of the non-standardized
bearings settled. Typically, this took a minimum of four NEMA tests.
hours at load. The machine was then allowed to cool to room
These results also verify that the test protocol for deter-
temperature.
mining the impact of rewinding on motor efficiency is in

No-load tests were essentially conducted at the tempera- accord with approved industry practice, and that the results
ture of the motor associated with constant, no-load, rated obtained in this study are not skewed by the method of
voltage operation. Winding temperatures were measured evaluation.
by thermocouples embedded in the coil extensions.
Once temperatures stabilized, a set of electrical and COMPARISON OF IEC 60034-2 AND
mechanical results was taken, and winding temperatures
IEEE 112-1996 LOAD TESTING METHODS
and resistance were determined. The test motor was then
returned to full-load operation to restore the full-load tem- The IEEE 112B test procedure was selected over IEC
perature. Next, part-load readings were taken, starting with method 60034-2 for the EASA/AEMT rewind study because
the highest load and working down to the lightest load. it measures motor efficiency more accurately. Many of the
Readings were taken quickly in each case, after allowing a differences between the two methods are explained below
very brief interval for the machine to settle to its new load. and illustrated in Tables 2 - 7.
The techniques and equipment described above ensured The most significant difference between the two meth-
repeatability to within 0.1% for tests conducted on a stock ods, however, is how they determine stray load loss (SLL).
motor at intervals of several months. A 100 hp (75 kW) motor IEEE 112B uses the segregated loss method, which is
without any modifications was kept especially for this pur- explained more fully on Page 1-14. IEC 60034-2 assumes
pose. a loss of 0.5% of the input power at rated load, which is

TABLE 1
ROUND ROBIN TEST RESULTS OF 30 KW, 4-POLE MOTOR
Full load Full load Full load Temperature
Test location Test efficiency power factor amps rise rpm

Baldor 400v/50 Hz 91.8% 86.8% 54.0 69.4° C 1469

Nottingham 400v/50 Hz 92.3% 87.0% 54.2 68.0° C 1469


Nottingham 460v/60 Hz 93.5% 85.9% 47.0 53.9° C 1776

Oregon State 460v/60 Hz 92.6% 85.9% 47.0 50.0° C 1774




assumed to vary as the square of the stator current at other ings of 3% or less in half-hour intervals.
load points. The effect can be to overstate the level of • For load testing, IEC uses tested temperature for I2R loss
efficiency by up to 1.5 points, depending on what percent of of the stator. IEEE uses tested temperature rise plus
the total loss is represented by the stray load loss. The 25° C.
differences in EASA/AEMT rewind study were less (see
Table 7). • For load testing, IEC does not specify any temperature
correction for slip (rotor I2R loss). IEEE corrects to speci-
TABLE 2. METHODS fied stator temperature.
IEC 60034-2 • For temperature correction of copper windings, IEC uses
IEEE 112B
234.5. degrees C. IEEE proposes to use 235° C.
Input - output Braking test
TABLE 4. REFERENCE TEMPERATURE
Input - output with loss
segregation IEEE 112-1996 IEC 60034-2
——-
——
Indirect measurement of stray
load loss Ambient 25° C 20° C
Duplicate machines Mechanical back-to-back Specified
Electrical power measurements 1) Test Preferred Used only for
under load with segregation of load test
losses Summation of losses
1) Direct measurement of stray (Calibrated driving machine) 2) Other
load loss Class A/E 75° C 75° C
2) Assumed value of stray load
loss–load point calibrated Class B 95° C 95° C
Equivalent circuit Class F 115° C 115° C
1) Direct measurement of stray
load loss–loads point calibrated ——
——
Class H 130° C 130° C

2) Assumed value of stray load
loss–load point calibrated Stray load loss (SLL). Except for load tests (braking,
——
—— Electrical back-to-back
back-to-back, and calibrated machine), IEC uses a speci-
fied percentage for SLL. The specified value is 0.5% of input
at rated load, which is assumed to vary as the square of the
stator current at other loads.
TABLE 3. INSTRUMENT ACCURACY
For all load tests except input-output, IEEE requires
IEEE 112-1996 IEC 60034-2 determination of the SLL by indirect measurement with data
smoothing–i.e., raw SLL is the total loss minus remaining
General ±0.2% ±0.5%
segregated (and measurable) losses.
Three-phase ±0.2% ±1.0%
wattmeter For non-load tests, IEEE requires direct measurement of
SLL unless otherwise agreed upon. Table 5 shows the
Transformers ±0.2% Included
assumed value at rated load. Values of SLL at other loads
Stroboscope/ Stroboscope/ are assumed to vary as the square of the rotor current.
Speed/slip
digital digital
Torque TABLE 5. IEEE 112 ASSUMED STRAY
a) Rating 15% —-
— LOAD LOSS VS. HP/KW

b) Sensitivity ±0.25% —-
— Stray load loss %
Machine rating of rated output
EPACT (IEEE
112-1996) 1 - 125 hp / 0.75 - 93 kW 1.8%
—-
—
General ±0.2% 126 - 500 hp / 94 - 373 kW 1.5%
Transformers ±0.2% —-
— 501 - 2499 hp / 374 - 1864 kW 1.2%
Combined ±0.2% —-
— 2500 hp / 1865 kW and larger 0.9%

Speed ±1 rpm —-
—
Input - output tests: IEEE 112-1996 vs. IEC 60034-2
Torque ±0.2% —-
—
• IEC does not specify any limitations on dynamometer
size or sensitivity.
General differences between IEEE 112B
and IEC 60034-2 • IEC does not specify dynamometer correction for friction
and windage.
• IEC does not require bearing temperature stabilization for
determining core loss and friction and windage (F&W) • IEC uses tested temperature rise without correction.
loss from no-load test. IEEE requires successive read- IEEE uses tested temperature rise plus 25° C.



• IEEE specifies 6 load points. IEC does not specify any TABLE 7. IEEE AND IEC EFFICIENCY
load points. COMPARISON FOR EASA/AEMT STUDY
Motor IEEE Efficiency IEC Efficiency Difference
Input - output tests with loss segregation (indirect
measurement of stray load loss): IEEE 112-1996 vs. 1A 94.1 94.7 0.6
IEC 60034-2 2B 92.9 93.5 0.6
IEC has no equivalent test method. 3C 94.5 95.3 0.8
4D 95.0 95.0 0.0
Electrical power measurement with loss segregation:
5E 92.3 92.3 0.0
IEEE 112-1996 vs. IEC 60034-2
6F 94.4 94.4 0.0
• IEEE requires actual measurement of SLL by reverse
rotation test. Specified value accepted only by agree- 7B 93.7 94.0 0.3
ment. IEC uses a conservative specified value. 8C 96.2 96.3 0.1
• IEEE requires actual loading of the machine at 6 load 9E 90.1 90.3 0.2
points. IEC does not specify the number of load points 10D 95.4 95.3 -0.1
and allows the use of reduced voltage loading at constant 11F 96.4 95.9 -0.5
slip and with vector correction of the stator current to 12F 95.9 95.5 -0.4
determine load losses.
13G 94.8 95.3 0.5
• Both IEEE and IEC correct stator I2R losses to the same
14H 89.9 91.2 1.3
specified temperature. However, IEC makes no tempera-
ture correction for rotor I2R losses. 15J 93.0 94.2 1.2
16H 95.4 95.5 0.1
Miscellaneous information: NEMA MG 1-1998, Rev. 3 17H 86.7 87.3 0.6
vs. IEC 60034-1-1998 18G 94.2 94.2 0.0
• IEC does not use service factors. 19H 93.0 92.7 -0.3

• IEC allows less power supply variations. 20H 93.9 94.1 0.2
• Temperature rise limits are generally the same. 21J 93.7 94.6 0.9
• Torque characteristics are very similar. 22H 83.2 84.0 0.8
23K 95.7 95.7 0.0
• IEC inrush current requirements are not as tight as
NEMA’s and generally allow 20% or greater on 5 hp (3.5 24E 95.1 95.1 0.0
kW) and larger.
cess. The actual test procedures for determining these
• IEC does not assign a specific output rating to a frame, but
losses are described in the standard. Discussion of how
does specify preferred outputs.
instrumentation, dynamometer calibration, methods of tem-
perature correction and numerous other procedural items
TABLE 6. TOLERANCES
can affect the accuracy of the acquired data is beyond the
IEEE 112-1996 IEC60034-1-1998 scope of this section.
Summation Similar relevant testing standards include Canadian Stan-
See note. 50 kW -15% of (1 - eff.) dard C390, Australian/New Zealand Standard AS/NZS
of losses
1359.5, Japanese Standard JEC 2137-2000, and the re-
See note. >5 0 kW -10% of (1 - eff.)
cently adopted IEC 61972. As explained on Page 1-12, the
Input/output See note. -15% of (1 - eff.) test standard currently used in Europe (IEC 60034-2) differs
Total losses See note. >50 kW +10% of (1 - eff.) from these standards.
Several key issues need to be emphasized in regard to
Note: Although IEEE does not specify any tolerance, NEMA and procedure. First, the EASA/AEMT study confirmed that the
EPACT require that the minimum efficiency of 1 - 500 hp polyphase friction loss does not stabilize until the grease cavity has
motors not exceed plus 20% increase in loss from the nominal
been adequately purged, which may take considerable
value.
time. The study also suggests that in some cases a break-
Table 7 compares the results of IEEE and IEC efficiency in heat run may affect other losses.
testing of the motors in the EASA/AEMT study. The figures The test protocol employed for this project included a
represent the efficiency of each motor before rewind. break-in heat run for each unit. Once this was done, care
was taken not to alter the grease fill during disassembly,
LOSS SEGREGATION METHOD USED except on motors 1A and 3C, where grease was added.
IN EASA/AEMT REWIND STUDY
The EASA/AEMT rewind study used the IEEE 112-1996 Determination of efficiency
method to segregate losses. Applicable sections of the Efficiency is the ratio of output power to total input power.
standard are summarized below to help explain the pro- Output power equals input power minus the losses. There-



fore, if two of the three variables (output, input, or losses) are voltage axis is the friction and windage loss. The intercept
known, the efficiency can be determined by one of the may be determined more accurately if the input minus stator
following equations: I2R loss is plotted against the voltage squared for values in
the lower voltage range.
efficiency = output power
input power Core loss. The core loss at no load and rated voltage is
obtained by subtracting the value of friction and windage
efficiency = input power - losses loss from the sum of the friction, windage, and core loss.
input power Stray-load loss. The stray load loss is that portion of the
total loss in a machine not accounted for by the sum of
Test method 112 B: input - output with friction and windage, stator I2R loss, rotor I2R loss, and core
loss segregation loss.
This method consists of several steps. All data is taken Indirect measurement of stray load loss. The stray
with the machine operating either as a motor or as a load loss is determined by measuring the total losses, and
generator, depending upon the region of operation for which subtracting from these losses the sum of the friction and
the efficiency data is required. The apparent total loss (input windage, core loss, stator I2R loss, and rotor I2R loss.
minus output) is segregated into its various components,
Stray load loss cannot be measured directly since it has
with stray load loss defined as the difference between the
many sources and their relative contribution will change
apparent total loss and the sum of the conventional losses
between machines of different design and manufacture. In
(stator and rotor I2R loss, core loss, and friction and windage
IEEE 112B, residual loss is evaluated by subtracting the
loss). The calculated value of stray load loss is plotted vs.
measured output power of the motor from the input power
torque squared, and a linear regression is used to reduce
less all of the other losses.
the effect of random errors in the test measurements. The
smoothed stray load loss data is used to calculate the final Residual loss will equal stray load loss if there is no
value of total loss and the efficiency. measurement error. Since two large quantities of almost
equal value are being subtracted to yield a very small
Types of losses quantity, a high degree of measurement accuracy is re-

quired. The biggest error, however, can come from the need
Stator I2R loss. The stator I2R loss (in watts) equals 1.5
for an accurate measurement of torque (of the order of 0.1%
x I2R for three-phase machines, where:
error or better) to evaluate output power precisely.
I = the measured or calculated rms current per line
The determination of true zero torque is always a prob-
terminal at the specified load
lem. The IEEE standard suggests comparing input and
R = the DC resistance between any two line terminals output powers at very light load, where most of the motor
corrected to the specified temperature losses are due to windage and friction, the stator winding,
Rotor I2R loss. The rotor I2R loss should be determined and the machine core. Here stray load loss can be assumed
from the per unit slip, whenever the slip can be determined to be insignificant. The torque reading can be adjusted
accurately, using the following equation: under this condition so that input power less known losses
Rotor I2R loss = (measured stator input power - stator equals output power.
I2R loss - core loss) • slip
Impact of too much bearing grease
Core loss and friction and windage loss (no-load
test). The test is made by running the machine as a motor, A number of studies have found that over-greasing the
at rated voltage and frequency without connected load. To bearings can increase friction losses (see Part 2: Good
ensure that the correct value of friction loss is obtained, the Practice Guide To Maintain Motor Efficiency for more infor-
machine should be operated until the input has stabilized. mation). For the EASA/AEMT rewind study, grease was
added to the bearings of two rewound test units in Group A.
No-load current. The current in each line is read. The
average of the line currents is the no-load current.
No-load losses. The reading of input power is the total of 1000
the losses in the motor at no-load. Subtracting the stator I2R
loss (at the temperature of this test) from the input gives the
WF loss (w)

900
sum of the friction (including brush-friction loss on wound-
rotor motors), windage, and core losses.
Separation of core loss from friction and windage 800
loss. Separation of the core loss from the friction and
windage loss may be made by reading voltage, current, and 700
power input at rated frequency and at voltages ranging from 0 2 4 6 8 10
125% of rated voltage down to the point where further Time (hours)
voltage reduction increases the current.
Figure 2. Reduction in F & W losses during the break-
Friction and windage. Power input minus the stator I2R in run for a 60 hp (45 kW) motor with proper grease fill
loss is plotted vs. voltage, and the curve so obtained is tested in the EASA/AEMT rewind study.
extended to zero voltage. The intercept with the zero


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