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DETERMINATION OF EFFECTS ON INDUCTION MOTOR EFFICIENCY HM Mzungu, P Barendse, MA Khan and M Manyage Department of Electrical Engineering, University of Cape Town, South Africa ABSTRACT This paper will focus on the practical and procedural effects on induction motor efficiency. 1.

INTRODUCTION

The depletion of natural resources and the negative impact of green house gases (GHG) on the environment have put the focus on energy efficiency and conservation. Induction motors, are known as the industrial workhorses and handle the largest percentage loads. In South Africa the industrial and mining sector account for more than two-thirds of the country’s national electricity usage. 64% of the loads in these sectors are motorized systems [1]. With the power situation worsening in South Africa, Eskom Demand Side Management (DSM) is embarking on strategies to reduce electrical energy demand through motor efficiency optimization. One such initiative is to look at the Effects on induction motor efficiency. The project will look to answer questions such as, what effects do different international standards, such as IEC 60034-2 and IEEE 112B, have in determining efficiency and what is the status of motor rewinding techniques used by South African motor repairers and how these techniques affect motor efficiency. This paper will focus on the practical and procedural effects on induction motor efficiency and is organized as follows: Section II will give a brief definition of motor efficiency; Section III will look at the issue of motor repair and rewind in South Africa. These sections look at induction motor damage the effects the rewind and repair process can have. Work in this area is in progress at UCT. Section IV looks at the effects of standards on measuring efficiency; Section V discusses the test setup used for testing; Section VI is the conclusion. 2.

MOTOR EFFICIENCY

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:

η =

Pm e c h = Pe l e c

Pm e c h Pm e c h +

∑

lo s s e s

The difference between the electrical (input) and mechanical (output) power is the sum of the motor losses. This power loss is split into five components: Stator I2R loss, Rotor I2R loss, core loss, windage and friction loss, and Stray load loss (or additional loss). 3.

EFFECT OF REWIND

MOTOR

REPAIR

AND

This section will look at induction motor damages and the effects the rewind and repair process on efficiency. Damage due to faults The increasing pressure of development in South Africa has lead to the maximum operation of all equipment involved [2]. The stress on these equipment, acting especially upon motor windings, rotors, bearings and shafts (figure 1), has resulted in damage and this has caused downtime and production losses.

Figure 1: Motor failure distribution [3] The main stresses that lead to motor damage have been found to affect windings, rotors, bearings and shafts. These are [4]: • Environmental • Thermal • Electrical • Mechanical and • Vibration/Shock Environmental A lack of maintenance and inspection of motors in their working environment can lead to restricted ventilation, high ambient temperature, moisture, foreign material (bearings), corrosion and erosion (or failure modes) on motors.

Thermal Over loading, voltage variation and unbalance, ambient, thermal aging, ventilation, short cycling, friction (lubrication) and hot spots Electrical Phase voltage unbalance, voltage transients, single phasing and power quality issues such as THD. A phase voltage unbalance of 3.5% can cause a 25% increase in thermal stress. Motors should therefore not operate under voltage unbalances of greater than 1% [5]. Mechanical Motor loading (tensional or axial loading on shaft), misalignment (leads to bearing failure), flying objects, coil movement and looses laminations Vibration/shock Vibration (caused poor belt tension and misalignment) is the major cause that leads to many of the listed failures. This stress can be an endless cause of trouble if the motors are not tight and secure. Repair / Rewind procedure The idea that repaired and rewound motors lose efficiency and performance has been around for years now. In some rewind studies done around the world it has been found that repair and rewound motors show loss figures up to 6% [6], show a little impact of rewinding or repeated rewinding on the motor losses and efficiency if “good practice” is followed strictly [7] and it was also noticed that the overall quality of the repair shop ( its adopted maintenance procedures, employed materials, equipment used, and personnel qualifications and capabilities) are of great importance to have a motor successfully repaired [8].It can bee seen that the procedure together with the workmanship of a repair shop play a role in the condition of the repaired motor. A. Standard and procedures There are number of international and local procedures and standards that are followed by South African repair and rewind shops to adhere to the two requirements. • The SABS 0242-1[9] “The rewinding and refurbishing of rotating electrical machines Part 1: Low-voltage three-phase induction motors” and • The Electrical Apparatus Service Association (EASA) “A guide to AC motor repair and replacement’.[3] These standards and procedure are either used separately or as a combination. Table 1 is a list of the different effects that can occur during a rewind and repair process. The most severe is the impact the removal of stator winding from the core [3].

Table 1: Effects of the rewind and repair process on losses Motor loss Affected by Overheating core steel during stripping Damaging core insulation during Stator core losses winding removal Excessive abrasion and grinding during core cleaning Bearings and cooling fan not replaced to original conditions Friction and Incorrect bearing fits Windage losses Incorrect bearing preload Under/over lubrication Reducing conductor crosssectional area Stator Winding Changing number of turns Losses (I2R losses) Using wrong winding configuration Machining rotor Rotor losses (I2R Altering rotor bars and end rings losses) Changing cage design Change in air gap symmetry and air gap unconventional behaviour Bent motor shaft or damaged end Stray load losses shields All the above factors also affect these losses B. Rewind Process The rewind process begins with the removal of the old stator windings. This process is done differently depending on the repair and rewind shop. • •

Oven bake of the core at a Heat cleaning the core with a direct flame or torch

Oven bake Over heating on the core can lead to damage on the interlaminar insulation and this leads to an increase in magnetic and electrical losses. The SABS recommend burn out temperatures not exceed 380 °C [9] and the EASA suggests, applying an oven temperature of 343 °C, and not overheating the stator core above 360 °C are used. Cao et al [7] have found that a temperature of 343 °C was not hot enough and required mechanical force. A temperature ranging from 360-370°C is more adequate. Direct flame (torch) Direct flame has the same damaging effect on the core. The high temperature of torches can lead to the damage on the interlaminar insulation and even create hotspots on the core. Core or loop testing has proven to be very effective in detecting core damage (during and after repair) such as shorted laminations. The test involves creating flux using a single phase loop coil and observing for hotspots.

Replacement of mechanical components, reassembly (right size bearings, lubrication, and paint) and final testing (such as High-voltage test, current balance at noload, input power at no-load, bearing temperature and noise) is the final process. Efficiency tests of the repaired motor are not always done unless requested by the customer. 4.

Table 2: Lower limits of CEMEP classification laws [10] Rated Power

7.5kW

11kW

15kW

(Lower limits)

MEASURING MOTOR EFFICIENCY

When looking at the definition of Motor efficiency, as shown in equation 1, the determination of efficiency is simply calculated by dividing the measured output power by the input power. Because we are dividing between two numbers that are almost equal, a small error in measurement can lead to large error in the efficiency.

4kW

Eff1: “high efficiency” motors

88.3

90.1

91

91.8

Eff2: “energy efficiency” motors

84.3

87

88.4

89,4

Eff3: “standard efficiency” motors

80

83

85

87

The major differences between these standards is in the treatment of stray losses, see Table 3 below. Table 3: Comparison of standards STANDARD

METHOD

STRAY LOAD LOSS

TEMPERATURE CORRECTION

For example, a 55kW rated motor, with an efficiency (true efficiency) of 90%, that is tested with an uncertainty of ±3% can lead to final efficiencies ranging from 84.78 to 95.58% (see calculation example). Other aspects such as temperature have been found to have an impact in the efficiency determination [10].

IEEE 112

Method B

Loss segregation method (direct)

Yes

IEC 60034-2

Direct

Loss segregation method (direct)

Yes

CSA 390

Method 1

Loss segregation method (direct)

Yes

JEC 37

Method 1

Ignores SLL

Yes

Example of error in Pout/Pin method [11]

SANS IEC 60034-2

Indirect Method

Assigns 0.5% of load

No

AS

Direct

Loss segregation method (direct)

Yes

Real Motor Eff = 90% Size = 55kW Measurement Uncertainty = ±3% Motor Input = 61.1kW With Uncertainty the measured Output and Input can range: Output: 59.27 – 62.93kW 56.65kW

Input:

53.35

-

Efficiency will therefore range from 53.35/62.93 = 84.78% to 56.65/59.27 = 95.58%

A. Motor testing standards International Energy efficiency test standards have been developed to try and rid the errors from the determination of motor efficiency. There exist a number of international standards that are used to test efficiency. The important ones are: • IEEE 112 • IEC 60034-2 • CSA 390 • JEC • SANS IEC 60034-2 • AS 1 359. 1 02 These standards also play a major role in motor classification. In the European Union (CEMEP) and the U.S (EPAC) laws have been set to define the efficiency class of motors [10]. Table 2 shows the lower limits of the CEMEP classification. These limits are determined through testing according to a particular standard (IEC in this case).

The loss segregation method (used in IEEE, IEC, AS and CSA) determines efficiency through the direct method or separation of loss method. This method requires a no-load test, where the core and windage and friction loss are obtained and a load test where the stator and rotor I2R loss are obtained. The Stray load loss (SLL) is determined by subtracting all the major losses from the total measured loss. Because SLL are the smallest loss, they are sensitive to errors in measurement (detailed discussion under instrumentation). This sensitivity allows them to be used to measure the success of tests done on the motors. The correlation of the resultant SLL versus torque squared is used as an indication of the success of the test. The IEEE requires a correlation factor of 0.90 [12][13] while the IEC require 0.95 [14] [15]. The SANS method allocates SLL to be 0.5% [16] of rated load and the JEC ignore SLL completely. The differences in treatment of SLL have been seen to produce different efficiencies on the same tested motor. A 7.5kW and 11kW were tested. Due to the similarity in testing standards, only the IEEE 112B (similar to CSA 390, AS 1359.102), IEC 60034-2 direct method, JEC 37 and SANS IEC 60034-2 loss segregation method were used. B. Test results The results (figure 2-3) of the 15 and 7.5kW motors show that there is a difference in efficiency using different standards. This can be seen on the 15kW motor. The classification according to CEMEP (see table 2) could

•

have this motor as an Eff2 (JEC 37 method) or an Eff3 (Pout/Pin, IEC, IEEE and SANS) efficiency class depending on which standard (or method) is used.

• • • •

92.00 IEEE 112B Catalogue Equivalent Circuit Pout/Pin JEC SANS

90.00

Efficiency (%)

88.00

Wide range of service voltages, with an ability to simulate under-, over- and unbalanced voltages; Mounting of a wide range of machine frame sizes; Ability to test both motors and generators; Ability to test power electronic converters; Ability to make accurate measurements

A. Power Supply As mentioned above, the laboratory has a dedicated 11kV line that is fed into the lab and stepped down to 500V (the most commonly used voltage level in industry [17]) through a three phase transformer. The dedicated line has the advantage of reducing the risks of outages.

86.00

84.00

82.00

80.00 0

20

40

60

80

100

120

140

Loading (%)

Figure 2: Efficiency of 15 kW using different standards

88 IEEE 112B Catalogue

87

Equivalent Circuit Pout/Pin JEC 37 IEC 60034-2

86

The requirement for accurate and repeatable results, according to the standards, is the need for a power supply to have a voltage unbalance of 0.5% and a frequency of ± 0.1% of rated frequency [12][13]. This was not the case with the incoming Eskom line. Figures 4-6 show the fluctuation in voltage magnitude, voltage unbalance and voltage THD. The consequence of the fluctuation has an impact on repeatability and accuracy. 415

SANS IEC 60034-2

Red

85

Yellow

410

Blue

405 83

400

Voltage (V)

82

81

395

390

80

385 79

380 78 0

20

40

60

80

100

120

140

160

375

Loading (%)

Figure 3: Efficiency of 7.5 kW using different standards

370 12:00 AM

2:24 AM

4:48 AM

7:12 AM

9:36 AM

12:00 PM

2:24 PM

4:48 PM

7:12 PM

9:36 PM

12:00 AM

Time

5.

TESTING

Motor Efficiency testing, as part of a Eskom DSM project, are underway at the Electrical Machines Laboratory of the University of Cape Town where specialised test rigs (see figures in Appendix) have been constructed for this purpose. The lab has a flexible distribution system and can provide both AC and DC supplies. It has an 11kV ring mains that is stepped down to 500V through a three-phase transformer. The lab contains two 250kW 4-quadrant dc machine drive, two 250kW dc motors, a 6.6kV, 520kW alternator, a 75kW induction machine and 75kW AC variable speed drive. The 75kW induction motor is coupled to the alternator which in turn is coupled to a 250kW dc motor. Standard efficiency squirrel cage induction motors with capacities ranging from 3 kW to 55 kW (potential to test up to 250kW) has been lined up to undergo tests. According to Wallace et al [17], the elements for a highly functional and efficient testing laboratory are: • Industrial power supply rating capability;

Figure 4: Three phase mains supply voltage over 24hrs

4.5

4

3.5

3

THD (%)

Efficiency (%)

84

2.5

2

1.5

1

0.5

0 12:00 AM

2:24 AM

4:48 AM

7:12 AM

9:36 AM

12:00 PM

2:24 PM

4:48 PM

Time

Figure 5: Supply voltage THD over 24hrs

7:12 PM

9:36 PM

12:00 AM

300

0.6

250 Stray Load Loss (W)

0.7

Voltage Unbalance (%)

0.5

0.4

0.3

Afternoon Morning

200 150 100 50

0.2

0 0

0.1

20

40

60

80

100

120

Loading (%) 0 12:00 AM

2:24 AM

4:48 AM

7:12 AM

9:36 AM

12:00 PM

2:24 PM

4:48 PM

7:12 PM

9:36 PM

12:00 AM

Time

Figure 6: Supply Unbalance over 24hrs (above 0.5%) Figure 7 shows the impact on efficiency using the Eskom supply. A difference of 1% efficiency can be seen between the two supplies. This difference can be attributed to the increase in core and stray load loss (figure 8 and 9) caused by the voltage unbalance and THD in the mains supply. Morning Afternoon

90

89.5

Efficiency (%)

89

88.5

88

87.5

87

86.5

86 0

20

40

60

80

100

120

140

Loading (%)

Figure 7: Efficiency of 15kW at two different times of the day 350 300

Core loss (W)

250 Afternoon

200

Morning

150 100 50 0 0

20

40

60

80

100

120

A generator set, made up of 520kVA, 6.6kV alternator driven by a 250kW DC motor, was used to solve the power quality issues. Apart from a perfect supply being produced, the generator gives the testing centre a wide range of service voltages, with ability to simulate under-, over- and unbalanced voltages. B. Test Platform Motors come in a wide range of frame sizes, masses and aspect ratios. A test platform must be able to accommodate and be adjustable to all the experimental motors. The UCT machines laboratory has a base plate that is rigid, flat and forms a single plane. This feature has the advantage keeping the overall test rig securely positioned and provides vibration damping [18].

91

90.5

Figure 9: SLL of 15kW motor at two different times of the day

140

Loading

Figure 8: Core loss of 15kW motor at two different times of the day

Two test rigs have been constructed in the UCT machines laboratory. The large test rig (foot-mounted motor sizes up to 250kW) can be seen in figure 10. The rig has been constructed to only accommodate foot-mounted motors. A 250kW DC acts as the dynamometer. This was mounted via four base plates to the base plates and tightly fastened down. The coupling of the dynamometer and the tested motor is done through a cambelt pulley arrangement. The pulley arrangement, with a gear ratio of 2:3, was chosen to accommodate the motor speeds. Torque is accurately measured through a 2kNm inline transducer. The smaller test rig (foot-mounted motor sizes up to 15kW) can be seen in figure 11. This test rig utilizes the reaction torque though a force transducer to measure torque.

140

Figure 12: SLL sensitivity hierarchy[10] The higher the accuracy of the instrumentation used to measure the listed data, the more accurate the efficiency calculation.

Figure 10: Large test rig

Figure 11: Smaller Test rig Vibration Vibration or "soft foot" distortion can also be eliminated by tight fastening and good alignment (to within 0.1mm parallel) between the tested motor and dynamometer. Motor vibration is one of the biggest headaches in motor testing. Vibration can lead to an increase in heating and therefore lower the determined efficiency. The life span of the test equipment is also affected by vibration. During testing, it was found that the bearings (see figure in Appendix) of the gimbaled arrangement were the first to fail due to vibrations caused by misalignment. C. Instrumentation Instrumentation plays a large role in determining efficiency accurately and with a high repeatability. Accuracy As described earlier, SLL are the most sensitive to measurement errors and therefore are used to judge the accuracy of tests. A sensitivity analysis done by Boglietti [10], shows that the most sensitive measurement to SLL is the measurement of the input power. This is followed by the measurement of torque, stator current, speed and then the winding temperature.

Calibration Calibration was done on three of the main instruments used in the study. This included: a digital power meter, the torque transducers and a tachometer. The Yokogawa WT1600 digital power meter, which measures voltage, current and power was calibrated in Johannesburg by the distributors for Yokogawa in South Africa. The torque sensors were calibrated by means of torque arms and weights. The weights were weighed on a calibrated scale in the University’s Materials Engineering Department. The torque arms used in the calibration were measured precisely to the nearest millimetre by a large venire. The optical tachometer was calibrated by measuring the speed of a synchronous motor and comparing it to the measured frequency from the calibrated digital power meter (Yokogawa). Calibration of equipment gives the confidence in the efficiency testing. Repeatability The credibility of results from any test is dependant on the repeatability of results from similar tests. The repeatability of results for this study is therefore an important consideration. A measure of repeatability of results is given by means the calculated standard deviation (STDEV) between results of similar tests. The lower the STDEV the better the repeatability. A 15kW motor was used to measure the repeatability by running full tests on the motor on two different days. The repeatability values can be shown in table 4. Table 4: Repeatability of test rig using a 15kW motor Loading 15kW 0.28 134.1 0.3 117.3 0.29 100.2 0.19 75.3 0.13 50.3 0.16 27.8 D. Results Four motors (3, 7.5, 11 and 15kW) have been tested to produce preliminary results. These are shown in figure 13-16. The results show good repeatability (table 4) and accuracy.

85.0

To support the analyses and to get more information on the motors, free and locked rotor and temperature rise test were done on the motors.

Test 1

84.0

Test 2

83.0

Test 3

Efficiency (%)

82.0 81.0 80.0

6.

79.0

77.0 76.0 75.0 0.0

20.0

40.0

60.0 80.0 100.0 Loading (% of full load)

120.0

140.0

160.0

1

Figure 13: Efficiency of 3kW motor

86

Test 1

85

Test 3

The determination of induction motors efficiency for either classification or verification can be affected a many aspects. This paper is an attempt to highlight some these factors that were found during testing. The issue of rewind and repair is in progress. Motors will be tested before and after rewind. 7.

87

Efficiency (%)

CONCLUSION

78.0

Test 2

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support of Dr Lotten Mthombeni from Eskom, DSM

84 83

Chris Wozniak from the University of Cape Town Machine Lab for all the technical support

82 81 80

Eskom, DSM for the financial support

79 78 0

20

40

60

80

100

120

140

160

Loading (%)

8.

APPENDIX

Figure 14: Efficiency of 7.5kW motor

90.0 Test 1

88.0

Test 2

Efficiency (%)

Test 3

86.0 84.0 82.0 80.0 78.0 0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

160.0

Loading (%)

Figure 15: Efficiency of 11kW motor 92.0 Test 1

91.0

Test 2

90.0 L o a d i n g (% )

Appendix Figure 1: Damaged bearings with and without grease

Test 3

9.

89.0

REFERENCES

88.0 87.0 86.0 85.0 84.0 0.0

20.0

40.0

60.0

80.0

Efficiency (%)

Figure 16: Efficiency of 15kW motor

100.0

120.0

140.0

[1] Dr Lotten Mthombeni, “DSM presentation”, [Online] Available: [2] Martin Creamer, Repair stalwart, Creamer’s Media’s Engineering News, Published 2/12/05, [Online] Available: [3] EASA , ‘A guide to motor repair and replacement’. [Online] Available: http://www.easa.com/indus/ac_gd199.pdf

[4] Austin H. Bonnett, Fellow, IEEE, ‘Root Cause AC Motor Failure Analysis with a Focus on Shaft Failures’ IEEE Transactions On Industry Applications, Vol. 36, No. 5, September/October 2000 [5] The power factor Magazine, volume 4 issue 2 1990 [6] J. C. Hirzel, “Impact of rewinding on motor efficiency,” in Conf. Rec.Annu. Pulp and Paper Ind. Tech. Conf., Jun. 20–24, 1994, pp. 104–107. [7] Assessing the Impacts of Rewind and Repeated Rewinds on Induction Motors: Is an Opportunity for Re-Designing the Machine Being Wasted? Wenping Cao, Member, IEEE, and Keith J. Bradley, Associate Member, IEEE [8] Analysis of Repairs on Three-Phase Squirrel-Cage Induction Motors Performance Edson da Costa Bortoni, Senior Member, IEEE, Jamil Haddad, Afonso Henriques Moreira Santos, Erick Menezes de Azevedo, and Roberto Akira Yamachita [9] SABS 0242-1 The rewinding and refurbishing of rotating electrical machines Part 1: Low-voltage three-phase induction motors [10] Boglietti, Member, IEEE, Andrea Cavagnino, Member, IEEE, Mario Lazzari, and Michele Pastorelli, “International Standards for the Induction Motor Efficiency Evaluation: A Critical Analysis of the Stray-Load Loss Determination Aldo IEEE Standard Test Procedure for Polyphase Induction Motors and Generators, IEEE std 112-B, 1996.” IEEE transactions on industry applications, vol. 40, no. 5, september/october 2004 [11] EASA, “Understanding Energy Efficient Motors”, [Online] Available: https://smtp.cpuaid.com/easa/resources/cgis/displayit em.cgi?category=2&name=Promotional+Materials&i tem=5 [12] Standard Test Procedure for Polyphase Induction Motors and Generators, IEEE Std 112-1996, 1997. [13] Energy Efficiency Test Methods for Three-Phase Induction Motors, CSA C390-1993, 1998 [14] Method for Determining Losses and Efficiency of Three Phase, Cage Induction Motors., IEC Std 61 972 ,2000. [15] Methods for Determining Losses and Efficiency of Rotating Electrical Machinery From Tests, IEC 60034-2, 2007. [16] Methods for determining losses and efficiency of rotating electrical machinery from tests (excluding machines for traction vehicles), SANS 34-2 1972 [17] A. K Wallace, T. E Rollman “High Efficiency Testing Laboratory for motors, drives and generators”, [18] Nailen, Richard L ‘Designing an a-c motor test center (Part 2)’, Electrical Apparatus, Oct 1999 [19] Aníbal T. de Almeida, Member, IEEE, Fernando J. T. E. Ferreira, John F. Busch, and Pierre Angers, ‘Comparative Analysis of IEEE 112-B and IEC 342Efficiency Testing Standards Using Stray Load Losses in Low-Voltage Three-Phase, Cage Induction Motors, IEEE transactions on industry applications, vol. 38, no. 2, march/april 2002

[20] Electrical Apparatus Service Association (EASA) “A guide to AC motor repair and replacement’ [Online] Available: [21] EASA, ‘The Effect of repair /rewinding on motor efficiency’. [Online] Available: http://www.easa.com/indus/rwstdy1203.pdf Principal Author: H. Mzungu holds a BSc degree in Electrical Engineering from the University of Cape Town. At present he is pursuing a MSc at the University of Cape Town.

Co-author: P. Barendse holds a PhD degree in Engineering from the University of Cape Town. He is presently a lecturer specializing in Machines at the University of Cape Town. Co-author: A. Khan holds a PhD degree in Engineering from the University of Cape Town. He is presently a lecturer specializing in Power Engineering and machine design at the University of Cape Town. Co-author: M. Manyage holds a Bsc degree in Engineering from the University of Cape Town. He is presently awaiting his PhD in Machine Design at the University of Cape Town. Presenter: The paper is presented by Heskin Mzungu.