The effect of repair on motor efficiency

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These ebook will absolutely help you to decide when the motor should be replaced - and calculate precisely how much you can spend the money

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The effect of repair on motor efficiency

  1. 1. 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. DisclaimerThe information in this report and the Good Practice Guide to Maintain Motor Efficiency (Part 2) was carefully prepared andis believed to be correct but neither EASA nor the AEMT make any warranties respecting it and disclaim any responsibility orliability 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.
  2. 2. (This page intentionally left blank.)Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.
  3. 3. 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 assumesa 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 maynot 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 BaurBoulevard, 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
  4. 4. ForewordForeword (This page intentionally left blank.) ii Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.
  5. 5. 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 ......................................... 3-17Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc. iii
  6. 6. Table of ContentsTable of Contents (This page intentionally left blank.) iv Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.
  7. 7. 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 EASA/AEMT Rewind StudyEffect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc. 1-1
  8. 8. EASA/AEMT Rewind Study Part 1EASA/AEMT Rewind Study (This page intentionally left blank.) 1-2 Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.
  9. 9. Part 1 Executive Summary Executive Summary The Effect of Repair/Rewinding On Motor EfficiencyIntroduction 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 byelectrical energy used by industrial/commercial applica- Advanced Energy [Ref. 6].tions in the developed world with lifetime energy costsnormally 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 theyenergy 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 Objectivescompany’s short-term profitability; multiple or repeated fail- EASA and AEMT designed this study to find definitiveures can reduce future competitiveness in both the medium answers to efficiency questions, particularly as regardsand 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 ontenance 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 Summarycapital expenditures while assuring reliable operation. Ris- • How different burnout temperatures affect stator coreing energy costs in recent years, however, have led to losses.questions about the energy efficiency of repaired/rewoundmotors. • 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 rewoundand test data. A Good Practice Guide to Maintain Motor motors and prepare a Good Practice Guide to MaintainEfficiency (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 resultsalso 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., thosevert 90% - 95% of input electrical power into mechanical most likely to be rewound), subjecting them to independentwork. Still, given the huge amount of energy they use, even efficiency tests before and after rewinding. Throughout thisminor 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 Universityments in motor efficiency. In the USA, for example, the (UK) for EASA and the AEMT show that good practice repairEnergy Policy Act of 1992 (EPAct) and new premium methods maintain efficiency to within the range of accuracyefficiency designs have boosted efficiency levels to the that it is possible to measure using standard industry testhighest currently available. In Europe, voluntary agree- procedures (± 0.2%), and may sometimes improve it. Thements among leading motor manufacturers and the accompanying report also identifies the good practice repairEuropean 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 Evaluatedlargely on a handful of studies of mostly smaller motors (up The study involved 22 new motors ranging from 50 toEffect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc. 1-3
  10. 10. 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 1Executive Summary 1-4 Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.
  11. 11. Part 1 Executive Summarywere 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 andto accommodate the different test variables. The test results rewound once. Controlled stripping and rewindsummarized below show no significant change in the effi- processes with burnout temperature of 680° Fciency of motors rewound using good practice repair - 700° F (360° C - 370° C).procedures (within the range of accuracy of the IEEE 112Btest method), and that in several cases efficiency actuallyincreased. (For detailed test results, see “EASA/AEMT Test ROUND ROBIN TESTINGProtocol & Results” on Pages 1-7 to 1-19.) AND TEST PROTOCOLGroup 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 forGroup 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 U.S. Electrical Motors 460v/60 Hz 93.1% 86.4% 46.5 42.0° C 1774Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc. 1-5
  12. 12. Executive Summary Part 1 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 efficienciesExecutive 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. 1-6 Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.
  13. 13. Part 1 Executive Summary 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 NameplateEffect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc. 1-7
  14. 14. Executive Summary Part 1 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 1-8 Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.
  15. 15. Part 1 Test Protocol & Results EASA/AEMT Test Protocol & ResultsTEST 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, whichrepaired or rewound by a qualified service center reduces indicate that such factors as supply voltage, repeatabilitycapital 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/rewoundmotors. To help answer these questions, the Electrical Standards for Evaluating LossesApparatus Service Association (EASA) and the Association Two principal standards are relevant to this work. IECof Electrical and Mechanical Trades (AEMT) studied the 60034-2 is the current European standard (BS EN 60034-2effects of repair/rewinding on motor efficiency. is the British version), and IEEE 112 is the American standard. The IEEE standard offers several methods ofObjectives 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 strayaccurate assessment possible of the impact of motor repair load loss, rather than assuming a value as the IEC standardon 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 rewoundmotors and prepare a Good Practice Guide to Maintain MethodologyMotor 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. Atests. new 40 hp (30 kW) motor was tested at four different locations (see “Round Robin Testing” on Page 1-11) toProducts 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, whichpower ratings than those in previous studies (i.e., those is the current standard in Europe (see Page 1-14 formost likely to be rewound), subjecting them to independent discussion of IEEE and IEC methods for calculating strayefficiency 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-stateto 225 kW) and 2 smaller motors [7.5 hp (5.5 kW)] were conditions were established and then load tested. Theselected 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.Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc. 1-9
  16. 16. 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 TestsTest Protocol & Results 1-10 Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.
  17. 17. Part 1 Test Protocol & Resultsreclaim 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, aeach rig to ensure maximum accuracy. Power, voltage, 30 kW IEC motor was efficiency tested first by the Universitycurrent, speed and torque were measured with a Norma of Nottingham and then by three other test facilities. TheD6000 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 Oregonaged 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 thenals 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 ofand 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 Associationformed in accordance with IEEE 112B. Test procedures, (NEMA). Initial results of NEMA’s tests varied by 1.7 pointsmeasurement intervals, and thermocouple location on the of efficiency; the variance subsequently was reduced to 0.5winding 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 robinloss 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. Theseentire thermal cycle of the test machine, running at full load results were achieved without standardization and compareuntil the temperature stabilized and the grease in the favorably with the 1.7% variation of the non-standardizedbearings 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 Test Protocol & Results No-load tests were essentially conducted at the tempera- accord with approved industry practice, and that the resultsture of the motor associated with constant, no-load, rated obtained in this study are not skewed by the method ofvoltage 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 ANDmechanical results was taken, and winding temperatures IEEE 112-1996 LOAD TESTING METHODSand resistance were determined. The test motor was thenreturned to full-load operation to restore the full-load tem- The IEEE 112B test procedure was selected over IECperature. Next, part-load readings were taken, starting with method 60034-2 for the EASA/AEMT rewind study becausethe highest load and working down to the lightest load. it measures motor efficiency more accurately. Many of theReadings were taken quickly in each case, after allowing a differences between the two methods are explained belowvery 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 iswithout any modifications was kept especially for this pur- explained more fully on Page 1-14. IEC 60034-2 assumespose. 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 U.S. Electrical Motors 400v/50 Hz 91.9% 86.7% 53.5 59.0° C 1470 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 U.S. Electrical Motors 460v/60 Hz 93.1% 86.4% 46.5 42.0° C 1774Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc. 1-11
  18. 18. Test Protocol & Results Part 1 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° CTest Protocol & Results 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. 1-12 Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.
  19. 19. Part 1 Test Protocol & Results• 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 DifferenceInput - output tests with loss segregation (indirectmeasurement of stray load loss): IEEE 112-1996 vs. 1A 94.1 94.7 0.6IEC 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 0.0Electrical power measurement with loss segregation: 5E 92.3 92.3 0.0IEEE 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 0.1Miscellaneous information: NEMA MG 1-1998, Rev. 3 17H 86.7 87.3 0.6vs. IEC 60034-1-1998 18G 94.2 94.2 0.0• IEC does not use service factors. 19H 93.0 92.7 -0.3 Test Protocol & Results• 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 toNote: Although IEEE does not specify any tolerance, NEMA and procedure. First, the EASA/AEMT study confirmed that theEPACT require that the minimum efficiency of 1 - 500 hp polyphase friction loss does not stabilize until the grease cavity hasmotors not exceed plus 20% increase in loss from the nominal been adequately purged, which may take considerablevalue. 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 arepresent 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 efficiencymethod 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-Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc. 1-13
  20. 20. Test Protocol & Results Part 1 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-Test Protocol & Results 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 1-14 Effect of Repair/Rewinding On Motor Efficiency © 2003, Electrical Apparatus Service Association, Inc.

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