e (V. ) Phase A. Phase B. Phase C. Voltage frequency. 49.75. 49.8. 49.85. 49.9. 49.95. 50. 50.05. 50.1. 50.15. 50.2. 12:00:00 AM 2:24:00 AM 4:48:00 AM 7:12:00 ...
Application of Induction Machine Efficiency Testing Standards in South Africa H.M Mzungu
M.J Manyage
M.A Khan
P. Barendse
T.L Mthombeni
P. Pillay
Electrical Department University of Cape Town Rondebosch, 7701
Electrical Department University of Cape Town Rondebosch, 7701
Electrical Department University of Cape Town Rondebosch, 7701
Electrical Department University of Cape Town Rondebosch, 7701
Anglotechnical Department, Energy Division Marshalltown Johannesburg
ECE Department Concordia University Montreal, Quebec, Canada H3G IM8
Abstract- This paper investigates the application of international induction machine efficiency testing standards in South Africa (SA). Motorized systems in SA account for up to 60% of the total electricity consumption. Recent power crisis in SA has accelerated the need for motor efficiency optimization. The applications of minimum energy performance standards (MEPS) to increase motor efficiency have become the norm around the world. Most electric motors in South Africa are still being rewound, with no consideration for the potential loss of efficiency and the most-likely increase in energy consumption. The status of motor rewinds is not good. Efficiency testing is underway at the University of Cape Town Machine Laboratory (UCTML) to investigate the impact of rewinding on motor efficiency in SA. As one of a very few testing labs, the power quality and instrumentation accuracy limits required in the standards were difficult to achieve. The result of this was errors and poor repeatability in testing. An alternative MG-set was used to create a supply that conforms to the standards. The impact of motor rewinding in SA is presented in this paper. It was found that procedures such as uncontrolled burnout temperatures result in efficiency drops of up to 4% at 25-50% loading points and up to 2% at rated loading points.
I.
INTRODUCTION
In South Africa (SA), the industrial and mining sector is the largest consumer of electricity. Motorized systems in these sectors account for up to 60% of the total electricity consumption and about 57% of SA’s peak power demand [1]. With the recent power crisis in SA, Eskom Demand Side Management (DSM) embarked on strategies to reduce electrical energy demand through motor efficiency optimization. The applications of minimum energy performance standards (MEPS) to increase motor efficiency have become the norm around the world in both developed and developing countries [2]. There are no local manufacturers of low voltage (LV) motors in SA. These motors are imported from different countries, with a total of 1.5 million motors were imported into the country in 2007 (50% of these are AC motors less than 75kW) [2]. There were no energy efficiency classification standards, until the recent promulgation of IEC 60034-30 which is still not considered. Motor efficiency testing was therefore only done by the end-users to prove quoted performance on select large motors. Testing facilities for electric motors in South Africa are in short supply. With the heavy mining industry in South Africa, the motor rewinds industry has grown over the years. While the local bureau of standards has practice guides and standards on motor rewinds, 978-1-4244-4252-2/09/$25.00 ©2009 IEEE
still a majority of the rewind vendors do not have the proper equipment to do quality rewinds. Thus, most electric motor users rewind motors with relatively poor quality, because these offer the lower prices, compared to the bureau certified vendors. Currently, there is no consideration on the impact of rewinds on electric motors when users have to decide whether to rewind or replace motors. This, coupled by the increasing electricity costs presents a significant opportunity for users to lower their costs through controlled rewinds. Industry tends to rewind motors even if the rewind costs are more than 60 % of the new motor price, with no consideration for the potential loss of efficiency after rewinds. Electric motor users do not know how many times a motor should be rewound before it is replaced. Moreover, the “X %” cost approach is often very user specific and should typically not be applied universally, as is often the case. Electric motor users need to understand and apply the concept of total cost of ownership, in which decisions are not made based on the capital spent today, but look at the motor “from cradle to grave”. The main objectives of this paper are to present the factors that make motor efficiency testing in SA difficult and to examine the effect of rewinding on the efficiency of induction motors in SA by profiling the efficiency loss or gain after rewinding process. The electric motor efficiency loss due to poor quality rewinds is a country specific one; as it depends on the local skills and workmanship; more of a problem in developing nations. However, it is hoped that this paper will not only assist motor users in developing nations, but also serve to benchmark other nations in developed nations. II. TEST-BEDS DEVELOPMENT The University of Cape Town Machine Laboratory (UCTML) is one of very few testing laboratories in SA. The laboratory has a flexible distribution system which can supply AC and DC power at flexible voltage and current levels. The UCTML supply is rated at 500kVA and is fed from a dedicated 11kV utility supply. The lab contains two 500V 250kW 4quadrant DC drives, two 250kW, DC motors; a 6.6kV, 520kW, 57A alternator and a 75kW AC variable speed drive. Three test-beds were constructed to test motors from 3kW up to 250kW. Fig. 1 shows a state-of-the-art 250kW test-bed. Figure 2 shows the schematic of all three test-beds. The test
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facilities consisted of a base plate that is rigid, perfectly flat and forms a single plane; a generator set, made up of 520kVA, 6.6kV alternator driven by a 250kW DC motor, three dc-loads (or dynamometers), 3.5kW, 15kW and 250kW, coupled to the tested motors through torque transducers, a high precision power analyzer and two 500V DC drives. The ac supply to the test motors was either supplied from the mains (which was found to be inadequate [3]) or from the generator set. The generated supply provided precise and constant voltage and frequency.
Standard efficiency squirrel cage induction motors ranging from 3 kW to 55 kW have undergone tests as part of an investigation into the ‘Impact of armature rewinding on motor efficiency in South Africa’. A number of aspects were found to affect the determination of efficiency. III. EFFECT OF POWER QUALITY AND INSTRUMENTATION A. Power Quality As mentioned, the UCTML has a dedicated 11kV line that feeds into the laboratory, which is stepped down to 500V (the most commonly used voltage level in industry [3]). The requirement for accurate and repeatable results is good supply. Testing standards limits are, voltage unbalance of 0.5% and frequency of ± 0.1% of rated frequency [4][5]. These limits were not met with the incoming utility supply. Figs. 3 and 4 show the fluctuation in voltage magnitude, frequency, voltage unbalance and total harmonic distortion (THD) over a period of 24hrs. Voltage magnitude 415
Phase A Phase B Phase C
410
405
Figure 1 250kW test-bed coupled to a DC Drive
Voltage (V)
400
395
390
385
380
375
370 12:00:00 AM
2:24:00 AM
4:48:00 AM
7:12:00 AM
9:36:00 AM
12:00:00 PM
2:24:00 PM
4:48:00 PM
7:12:00 PM
9:36:00 PM
12:00:00 AM
Time
Voltage frequency 50.2
50.15
50.1
Frequency (Hz)
50.05
50
49.95
49.9
49.85
49.8
49.75 12:00:00 AM
2:24:00 AM
4:48:00 AM
7:12:00 AM
9:36:00 AM
12:00:00 PM
2:24:00 PM
4:48:00 PM
7:12:00 PM
9:36:00 PM
12:00:00 AM
Time
Figure 3: Three phase mains supply voltage magnitude and frequency over 24hrs
Power
Data
Figure 2: Schematic of test-beds
The utility supply variations highlight the difficulty in motor efficiency testing in SA. The need for a separate generator, which is not always available in many testing or rewind centers, is required to meet the international standards. Fig. 5 shows the impact of supply variations on efficiency using the utility supply. A difference of 1% efficiency can be seen between the two tests done on the same day at different times. This difference can be attributed to the increase in motor losses (e.g. core and stray load loss) caused by the voltage unbalance and THD in the mains supply.
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B. Instrumentation Table I shows the accuracy of the instrumentation used in tests at the UCTML. According to Cao [11], the estimated worst case error (WCE) if the minimum instrument accuracy is met is 0.29% (for IEC 34-2 and IEEE 112). Using the same calculation procedure in [6], the WCE in the UCTML tests is 0.33% and 0.71% for the IEEE/IEC and SANS 34-2 (SA efficiency test standard) respectively. This shows the difficulty in meeting the standards instrument accuracy requirements.
instrument on the same item and under the same conditions. Three variables were found to be very important in order to achieve high repeatability: The supply, the procedures and data collection [3]. The three controlled variables provided very high repeatability between tests and test rigs. This can be seen in Fig. 6 where the efficiencies are almost exact after repeated tests over a two week period. To ensure maximum accuracy, all instrumentation were calibrated, with particular attention paid to the digital power meter; the torque transducers and speed pick up (tachometer). Details can be found in [3]. This therefore confirms the reliability of rewind results tested.
Voltage THD 4.5
Phase A Phase B Phase C
TABLE I
UCTML INSTRUMENT ACCURACY
4
Instruments
UCTML
Power (W) Current (A) Voltage (V) Frequency (Hz) Speed (RPM) Torque (Nm) Resistance (Ohms) Temperature (ºC)
±0,2% ±0,2% ±0,05% ±0,1% ±2RPM ±0,3% ±0,5% ±1 ºC
IEEE 112 (2004) ±0,2% ±0,2% ±0,2% ±0,1% ±1 RPM ±0,2% ±0,2% ±1 ºC
THD (%)
3.5
3
2.5
2 12:00:00 AM
2:24:00 AM
4:48:00 AM
7:12:00 AM
9:36:00 AM
12:00:00 PM
2:24:00 PM
4:48:00 PM
7:12:00 PM
9:36:00 PM
12:00:00 AM
Time
IEC (2007) ±0,2% ±0,2% ±0,2% ±0,1% ±1 RPM ±0,2% ±0,2% ±1 ºC
Volatge Unbalance 0.7
Effiency for 55kW motor - IEC
0.65
0.96
0.94
0.55
0.5
0.92 Efficiency (p.u)
Unbalance (%)
0.6
0.45
0.4
0.35
0.9
0.88
0.3 12:00:00 AM
2:24:00 AM
4:48:00 AM
7:12:00 AM
9:36:00 AM
12:00:00 PM
2:24:00 PM
4:48:00 PM
7:12:00 PM
9:36:00 PM
12:00:00 AM
0.86
Time
Figure 4: Supply THD and Unbalance over 24hrs 0.84 E ffec ts of P ower Q aulity
0
0.91
50
100
150
Loading (%)
Figure 6: Test showing repeatability of a 55kW motor
0.9
IV. EFFICIENCY TEST PROCEDURES AND STANDARDS
Efficiency (%)
0.89
The efficiency of an induction machine represents the success of the machine in converting electrical power to mechanical energy. This can be expressed by the following equation:
0.88
0.87
0.86
0.85 20
η = 40
60
80 100 Loading (% )
120
140
160
Figure 5: Efficiency of 15kW at two different times of the day
Repeatability and Accuracy Although accuracy is very important, repeatability is even more crucial in the rewind project. Repeatability is the variation in measurements taken by a single person or
Pm e c h Pm e c h +
∑
(1)
lo s s e s
The five power loss components are; Stator I2R loss, Rotor I R loss, core loss, windage and friction loss, and Stray load loss (or additional loss). The determination of efficiency is simply calculated by dividing the measured output power by the input power. However, this method is subject to large 2
C.
Pm e c h = Pe l e c
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errors caused by small measuring errors in the two numbers [3]. The loss segregation method in International Energy efficiency test standards such as IEC 60034-2-1 (2007) have been developed to reduce errors from the determination of motor efficiency. Three tests are performed in order to determine the losses in an induction machine accurately. The tests and results associated with each are as follows:
the largest efficiency drop at rated load of 1.647 and 1.859% respectively, and up to 4% drop at 25% loading, The 45kW and 55kW motors show very small efficiency drops (0.04%) when repeatability factor is included. The larger efficiency drops are seen at lower loading points of the motors (50, 25%). 92.00 New
91.20 90.96
90.62 90.37
89.98
Rew
89.88
90.00
89.28 88.57 87.49
88.00 86.98 86.14 Efficiency (%)
Temperature test: The motor is loaded and allowed to run until its temperature stabilizes. The temperature and winding resistances are recorded. Load test The motor is loaded at six different loading points ranging from 25%-150% of rated load. The stator and rotor copper losses are calculated from this.
85.94
86.00
85.42
84.10 84.00
82.00
80.00
80.02 79.05
78.00
No-load test: The motor is run at no-load with a varying supply voltage between 125% to 20% of rated voltage. The friction & windage and core losses are calculated from this.
7.5
11
15
22
37
45
55
Motor Rating (kW)
Figure 7: Efficiency change before and after rewind 11kW New vs Rewind Efficiency 90.00
88.00
87.40
87.34
85.94
86.00
85.11 84.15
84.10
83.71
84.00 82.74 Efficiency (% )
Temperature correction (to 25 ْC) is done on the Stator and Rotor losses using the winding temperature and resistance from the temperature test. The stray load losses (SLL) are then found by subtracting all the calculated losses from the measured loss. The loss segregation method used in IEC and IEEE112B is regarded as the most accurate method for calculating efficiency [4] [5] [6]. This also depends on the accuracy of equipment. It also has the advantage of very high repeatability due to the temperature correction of the losses.
3
82.11 82.00
80.94
80.00
78.00
New Rew 79.21
77.60
76.00
74.00
V. EFFECT OF MOTOR REWINDING ON MOTOR EFFICIENCY
72.00
Eight new squirrel-cage induction motors ranging from 3kW to 55kW were purchased for testing. The motors tested are 3, 7.5, 11, 15, 22, 37, 45 and 55kW. The IEC and IEEE 112 standards were used to test the motors. Several tests were performed on each motor to assess the accuracy and repeatability of test results. Statistical analyses were performed on the test results for each motor to obtain a representative average efficiency performance curve for each motor. The motors were then sent for complete stator rewinds and retested.
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50
75
100
125
150
Loading (%)
Figure 8: 11kW efficiency before and after rewind 15kW New vs Rewind Efficiency 90.00 89.10
88.91
88.00
87.49 86.29
86.00
Efficiency (%)
A. Efficiency Measurements The results in Fig. 7 clearly show that the impact of rewind practices on motor efficiency results in a drop in efficiency. This is evident in all eight motors. Figs. 7-12 show efficiency drops at other loading points ranging from 25 - 150%. The smaller motors (3-15kW), which generally have lower efficiencies, show very large efficiency drops compared to larger motors (45-55kW). The 37kW motor efficiency drop is insignificant, 0.046% at rated load, as it falls within the repeatability of the test rig. The 11kW and 15kW motors show
25
86.68
85.43
85.43
85.25
84.00
83.18
82.00 80.98 80.00
78.00
76.00 25
50
75
100
125
Loading (%)
Figure 9: 15kW efficiency before and after rewind
New Rew
B. Motor losses 37kW New vs Rewind Efficiency 92.00
91.00
90.83
90.62
90.58
90.17
89.98 89.88
90.00
89.00 88.94
Efficiency (% )
89.00
87.82 87.75
88.00
New Rew 87.00
86.74 86.16
Stator copper losses The major contributor to the drop in efficiency in the majority of the motors is the increase in stator losses. Statistical analyses were performed on the loss results for each motor to obtain a representative average loss curve for each motor. Six out of the eight motors show an increase of 3 - 15 % after rewind. This is due to the increase in stator resistance caused by the increase in total length of the winding. Controlled, tighter hand wound coils has been seen to lead to a decrease in winding resistance (up to 5.3%) [7].
86.00
Core losses Increase in core loss was evident on the 11kW and 15kW motors where the core losses have almost doubled. Damage to the core is the result of this. Higher W/kg core losses were measured prior to rewinding. The 11kW motor failed the core test. This result is critical as it shows that core damage may exist in new machines. The 11kW motor was rewound with the failed core. The benefit of annealing is negated by improper handling of the core during burnout, winding removal, and rewinding process. Fig. 13 shows an example of a damage core during winding removal.
85.00
84.00
83.00 25
51
75
101
120
140
Loading (%)
Figure 3: 37kW efficiency before and after rewind 45kW New vs Rewind Efficiency 92.00 91.33 91.07 91.00
91.03 90.64
90.63
90.38
90.00 89.43
89.20
Efficiency (%)
89.00
New Rew
88.00 87.36 86.92
87.00
86.00
85.00
84.00 26
50
75
100
126
Loading (%)
Stray load losses (SLL) Three motors showed an increase in SLL and three showed drop after rewinding. In documented research a high correlation was found between temperature rise, rotor losses and SLL. According to Cao [9], the drop in SLL after rewind was seen to correlate with a drop in temperature rise and rotor losses. The impact of the lower rotor losses, and therefore rotor resistance, will result in the leakage current in harmonics equivalent circuits to be diverted into the magnetizing branch. The result of this would be less drag torque and therefore less stray load loss. One of the eight of the motors (37kW) showed the trend with temperature rise and rotor loss.
Figure 11: 45kW efficiency before and after rewind 55kW New vs Rewind Efficiency 94.00
93.00
92.49
92.66 92.20 92.18
92.00 91.16 90.75
Efficiency (% )
91.00
90.00
90.96 90.05
90.01
89.62
89.00
New Rew 88.67 87.94
88.00
87.00
Figure 13: Damage on slot teeth from winding extraction
86.00
Rotor losses The rotor losses in theory should not change since the rotor was not changed in any way during the rewind process. One of the eight motors shows a drop of 4% in rotor losses while the rest show an increase of 2-14%. An increase in temperature rise due to a drop in speed causes a higher slip and therefore a
85.00 25
50
75
101
120
139
Loading (%)
Figure 12: 55kW efficiency before and after rewind
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lower power transfer to the shaft. The losses are also in the range of the repeatability and therefore the change is insignificant. Friction & Windage losses A drop in the friction and windage loss in all the motors was observed. This drop in loss is a result of greasing during the refitting of bearings and fan changing. Changes in these losses are generally due to the quality of workmanship. C. Efficiency profile The maximum efficiency point in induction motor occurs when a motors load-dependant losses equals its loadindependent losses. Changes in any of the losses results in a change in the maximum efficiency point and efficiency profile. The efficiency profile of the 55kW motor in Fig. 14 shows a shift in maximum peak consequent after motor rewind.
VI. MOTOR REWIND VS REPLACE With the ongoing power crisis, in South Africa, and the increase in electricity prices, electricity costs have become a large contributor to the cost of any business. The decisions to rewind or replace motors are based on economic models. The expenditure related to the rewind or replacement of failed motors can be evaluated based on, annual energy costs, pay back period or life cycle cost [10] [11]. Life Cycle Cost (LCC) is regarded as a good comparison tool to evaluate alternatives. The life cycle cost of motors (or any equipment) is the total cost incurred during its lifetime. This is made up of the purchase price, installation and commission costs, operating costs, rewind and maintenance costs, energy costs, operation and downtime costs, environmental and disposal costs. Equation (2) is the LCC equation with all the cost elements.
LCC= CIC + CIN + CE + CO + CM + CS + CENV + CD
(2)
Efficiency for 55kW motor - New vs Rew 0.93
Where, C IC = Initial costs (purchase price of entire motor system)
New Rew New Rew
0.92
Efficiency (p.u)
0.91
C IN
0.9
= Installation and commissioning costs
0.88
C E = Energy costs (entire motor system) CO = Operating costs (labour cost)
0.87
CM
0.89
= Maintenance and Rewind costs C S = Down Time costs C ENV = Environmental costs (contamination cost)
0.86 0.85
0
50
100
150
C D = Decommissioning and Disposal costs
Loading (% )
Figure 14: Shift in peak due to changes in losses
The rewound motor has an improved efficiency below 60% and lower efficiency above 60% loading. The maximum efficiency point shift is due to the increase in the stator and rotor losses (dependant losses) and decrease in core and W&F loss (independent losses). Fig. 15 shows a plot of the dependant and independent losses of the new and rewound 55kW motor. Maximum efficiency occurs at a point where the two types of losses are equal. 55kW Losses 7000
New Dep loss New Indep loss Rew Dep loss Rew Indep loss
6000
5000
Loss (W)
4000
Maximum Efficiency Peak has moved
3000
2000
Table II shows the LCC analysis of a scenario where the rewound motor and new motor options are analyzed over a 20 year lifetime (new motor’s life span). Motor temperature rise has a negative influence in a motor’s life. Motor insulation lifetime halves for every 10 degree temperature rise according to [12]. The operating temperature of the 15kW motor in this rewind project increased by more than 10 degrees consequent of rewinding. If this temperature increase was sustained then the insulation life would be reduced by 50%. The motor would therefore potentially fail within ten years. This result therefore supports the assumption made in this scenario. Accumulated damage from minor repairs and/or ageing insulation can also result in small efficiency drops in the 20 year life cycle of the motors. A value if 1% after 5 years has been given for this. The LCC analysis of the scenario shows that the option to rewind a failed motor will cost about 24% more than replacing the motor in the 20 year lifetime.
1000
0 20
40
60
80
100
120
140
Loading (%)
Figure 15: Shift in peak due to changes in losses
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TABLE II LCC ANALYSIS OF A MOTOR PUMP SYSTEM WITH MOTOR FAILURE
Motor LCC Elements
Rewind
Replace
Initial Investment (Purchase and Rewind costs) (R)
R 4,250.0
R 6,167.0
Installation (R)
R 200.0
R 200.0
Power Rating (kW)
15.00
15.00
Efficiency (%)
87
89
Efficiency change (after 5 years) (%)
-1.00
-1.00
Annual cost of energy (R)
R 15,513.68
R 15,168.54
Average Operating hrs/yr
4000.00
4000.00
Energy Cost (c/kWh)
R 0.30
R 0.30
Operation cost (routine supervision)
R 3,000.00
R 3,000.00
Maintenance cost/yr
R 3,500.00
R 3,500.00
Down time cost/yr
R 9,000.00
R 5,000.00
Environmental cost ( R)
0.0
0.0
Decomm/Dispo cost ( R)
0.0
R2500.00
Life Time (yrs)
20.0
20.0
Second rewind (after 10 yrs)
R 4,250.0
0.0
Return in investment (%)
35.0
35.0
Interest Rate (%)
12.0
12.0
Inflation rate (%)
8.80
8.80
Present Worth Evaluation Factor (PWEF)
4.10
4.10
Annual Energy cost Increase (20%)
1.44
1.44
LCC
R 39,713.68
R 35,535.54
Present LCC
R 160,898.55
R 145,595.49
VII. CONCLUSIONS A rewind study conducted at the University of Cape Town has been presented. It is hoped that three induction motor testbeds constructed at the university will be a resource to local industry. New squirrel-cage induction motors ranging from 3kW to 55kW were purchased for testing. Several efficiency tests were performed on each motor to assess the accuracy and establish the repeatability of the tests. The motors were then sent for complete armature rewinding and retested after. Efficiency tests after the motors were rewound show efficiency drops ranging from 0.1 to 1.9%. The changes in the core and stator copper losses have the biggest contribution to efficiency changes. The increase in core losses is a result of damage on the core during the winding removal. Better insertion of the stator winding coils, while keeping the same number of effective turns produces lower stator conductor losses. A
change in the efficiency profile is also evident as a result of the changes in the constant and load dependant losses. This paper presented efficiency values before and after rewinds, at various loading points, this is important to the user as motors are usually run below the 100 % load point. The paper also informs the motor user in part, on how many times a particular size motor can be rewound, without significant loss of efficiency. These kinds of results also inform the user on which kW size range not to rewind, instead replace, where suitable with high efficiency motors. Future work would include repeated rewinds study and a national roll-out study to assess the status of electric motor repairs across the country, this would then allow for a larger sample of motors to be tested and results more representative.
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ACKNOWLEDGEMENTS The authors wish to thank Eskom Demand Side Management for the financial support to carry out this study and the motor rewind vendors who contributed, sharing their knowledge of the industry. DISCLAIMER
[12] Energy Management Guide For Selection and Use of Fixed Frequency Medium AC Squirrel-Cage Polyphase Induction Motors, NEMA Standards Publication, MG 102001. [13] P. Pillay, and M. Manyage, ‘Loss of Life in Induction Machines Operating With Unbalanced Supplies’, December 14, 2004. Paper no. TEC-00058-2002.
The views reflected here are not those of Angloamerican and all of it subsidiaries. REFERENCES [1] Department of Minerals and Energy, ‘Energy Efficiency Strategy of the Republic of South Africa’, South Africa Department of Minerals, March 2005 [2] T. L. Mthombeni and A. B. Sebitosi, ‘Impact of introducing minimum energy performance standards for electric motors in South Africa’, ICUE Conference, South Africa, Cape Town, May 2008. [3] IEC 60034-2 International Standard, Rotating electrical machines – Part 2: “Standard methods for determining losses and efficiency from tests (excluding machines for traction vehicles)”, IEC, Geneva, Switzerland, 2007. [4] H. M. Mzungu, P. Barendse, M. A. Khan and M. Manyage, “Determination of Effects on Induction Motor Efficiency”, ICUE Conference, South Africa, Cape Town, May 2008. [5] A Boglietti, A. Cavagnino, M. Lazzari, M. Pasterolli, “International Standards for the Induction Motor Efficiency Evaluation: A Critical Analysis of Stray-Load Loss Determination”, IEEE Transaction on Industry applications, Vol. 40, No. 5, September/October 2004. [6] B. Renier, K. Hameyer, R. Belmans, “Comparison of standards for determining efficiency of three phase induction motors”, IEEE Transactions on Energy Conversion, Vol. 14, No. 3, Septermber 1999. [7] H.M. Mzungu, A.B. Sebitosi, M.A. Khan, "Comparison of Standards for Determining Losses and Efficiency of ThreePhase Induction Motors", IEEE PES PowerAfrica 2007 Conference and Exposition, Johannesburg, South Africa, 16 – 20 July 2007. [8] W. Cao, and K. J. Bradley, ‘Assessing the Impacts of Rewind and Repeated Rewinds on Induction Motors: Is an Opportunity for Re-Designing the Machine Being Wasted?’, IEEE Transactions On Industry Applications, Vol. 42, No. 4, July/August 2006. [9] H. du Preez, ‘Efficiency and the rewind of motors’, LHM Motor Conference, September 2007. [10] W. Cao, K.J. Bradley and J. Allen ‘Evaluation of additional loss in induction motors consequent on rewind and rewinding’, IEEE Proceedings Power Application, Vol. 153, No. 1, January 2006. [11] N. M. Kaufman, ‘A 100 Motor Study: Investigating PreEpact Motors As A Subset Of The Industrial Motor Population With Regards To The Economics Of Motor Rewind/Replace Decisions’, MSc Thesis, North Carolina State University, 2005.
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