comparison of a 3kw standard and high efficiency induction motor
COMPARISON OF A 3KW STANDARD AND HIGH EFFICIENCY INDUCTION MOTOR MA Khan, D Pati and HM Mzungu University of Cape Town, Cape Town, South Africa ABSTRACT This paper presents a comparison of the operating and performance characteristics of a 3kW standard efficiency (SE) induction motor to that of a 3kW high efficiency (HE) induction motor. Both motors are analysed when driving a centrifugal pump load. The motors are tested according to the IEC 60034-2-1 standard and their operation with the pump load is simulated based on the pump characteristic. 1.
INTRODUCTION
High efficiency (HE) induction motors offer improved efficiency over standard efficiency (SE) induction motors. Over the past few years, very few South African companies opted for HE motors as alternatives to SE motors. This is true for new, replacement or retrofit applications of induction motors. The main reasons for this was the cost differential between HE and SE motors and also the low electricity cost in South Africa. However, the recent constrained energy supply in South Africa has firmly established the need for HE induction motors in South African industry. This is further supported by Eskom DSM’s Energy Efficient Motors program [1]. The main objective of this paper is to compare the operating and performance characteristics of a 3kW standard efficiency (SE) induction motor to that of a 3kW high efficiency (HE) induction motor. The efficiencies of the motors are determined using the IEC 60034-2-1 standard. The no-load and locked rotor tests are then used to determine the equivalent circuit parameters of each motor. The operating characteristics of the motors with the same pump load are then simulated by means of a Matlab model. 2.
IEC 60034-2-1 STANDARD
Several international standards exist for testing the efficiency of induction motors. The standards include: IEEE 112, IEC 60034-2, CSA 390 and JEC 37. The standards differ mainly in their treatment of the stray losses in an induction machine [2],[3],[4]. A detailed comparison of the different efficiency results between the standards was presented in [5]. The IEC standard was used in the research related to this paper. The IEC 60034-2-1 standard uses the segregation of losses method to determine the efficiency of an induction
motor [6]. The efficiency can be expressed in terms of output power (Pout) and the sum of losses (∑Plosses), as:
η=
Pout Pout = Pin Pout + ∑ Plosses
(1)
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: a)
Temperature test: The motor is loaded and allowed to run until its temperature stabilizes. The temperature and winding resistances are recorded.
b)
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.
c)
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.
Temperature correction (to 25ْC) 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 is regarded as the most accurate method for calculating efficiency. This does of course depend on the accuracy of equipment. It also has the advantage of very high repeatability due to the temperature correction of the losses. 3.
LABORATORY SETUP
The IEC 60034-2-1 standard requires the test motor to be coupled to a dynamometer and an accurate method of measuring the shaft torque. The accuracy of torque measurement will determine the accuracy of the efficiency estimate.
No-load Iron losses vs Line voltage 220 200 180
P fe Losses [W]
160 140 120 100 80 60
Std-Motor Cubic-fit H-Eff-Motor Cubic-fit
40
Figure 1: Laboratory setup with the 3kW high efficiency induction motor coupled to a DC generator
20 200
250
300
350 Line voltage [V]
400
450
500
Figure 4: Variation of no-load iron losses with line volatge for the high efficiency and standard efficiency motors
Stator Copper losses vs load 400
A 3kW HE and a 3kW SE induction motor was coupled in-turn to a DC motor that is connected to a four-quadrant DC drive. The DC motor and drive operates in generator mode and therefore applies a load to the shaft of the induction motor. The induction motors were fitted with type K Chromel-Alumel thermocouples to measure the temperature distribution of the motor windings. The laboratory setup is shown in Figure 1 with the HE induction motor being tested.
350
Stator Cu losses [W]
300 250 200 150 100
Std-Motor Cubic-fit H-Eff-Motor Cubic-fit
50 0 20
40
60
80 100 Load [%]
120
140
4. 160
Figure 2: Variation of stator copper losses with load for the high efficiency and standard efficiency motors
The friction and windage losses are determined from the no-load test by varying the line voltage across the motor windings. The friction and windage losses of HE motor is 18.8W and that of the SE motor is 54.3W. The IEC standard was used to determine the stator and rotor copper losses for each motor. The variation of stator copper losses with load for the two motors is shown in Figure 2. The rotor copper losses are shown in Figure 3. The stator and rotor copper losses of the HE motor are lower at all loads than that of the SE motor. This is expected since the stator and rotor winding resistances are smaller by design in the HE motor, in an effort to reduce the copper losses in the motor.
Rotor Copper losses vs load 700
600
500 Rotor Cu losses [W]
PERFORMANCE TESTING
400
300
200 Std-Motor Cubic-fit H-Eff-Motor Cubic-fit
100
0 20
40
60
80 100 Load [%]
120
140
The no-load core (iron) losses of the motors are shown in Figure 4. The IEC standard specifies the core losses under load conditions to be determined as follows: •
An equivalent line voltage is calculated from the load test results, using an appropriate formula given in the standard.
•
The load core losses are then read from the no-load core losses (Figure 4) as the value corresponding to the calculated equivalent line voltage.
160
Figure 3: Variation of rotor copper losses with load for the high efficiency and standard efficiency motors
Indirect Method Efficiency vs Load 0.92
IM Torque vs speed characteristic 60
Std-Motor Cubic-fit H-Eff-Motor Cubic-fit
0.9 0.88
50
40
0.84
Torque [Nm]
Efficiency [pu]
0.86
High Eff Calc Std Calc High Eff Exp Std Exp Centrifugal load
0.82 0.8
30
20
0.78 10
0.76
40
60
80 100 Load [%]
120
140
160
Figure 5: Efficiency vs load characteristics of the 3kW HE and SE induction motors The higher losses associated with the SE induction motor results in a lower overall efficiency of the SE motor compared to the HE motor. A comparison of the efficiency vs load curves of the two motors is shown in Figure 5. It can be seen that the SE motor is designed to operate at maximum efficiency at 60% loading, whereas the HE motor operates at maximum efficiency at approximately 70% loading.
A locked rotor test and a no-load test can be used to determine the equivalent circuit parameters of an induction machine [7]. These tests were performed on each of the motors and the equivalent circuit parameters are listed below: SE Motor HE Motor
1000
1500
Figure 6: Torque vs speed characteristics of the 3kW HE and SE induction motors with pump load IM per-phase stator current vs speed characteristic 30 High Eff Calc Std Calc High Eff Exp Std Exp
25
20
15
10
OPERATING CHARACTERISTICS
The equivalent circuit model of an induction machine can be used to predict its operating characteristics. The variation (with speed) of its torque, current, input power factor, efficiency, etc. determined in this manner can be used to simulate the motor’s operation with an arbitrary load. Comparison of the operating characteristics of the HE and SE induction machines were simulated in this manner. The equivalent circuits of each motor were determined and their operation with the same centrifugal pump load was simulated and analysed by means of a Matlab model.
500 n [rpm]
5
0 0
500
1000
1500
n [rpm]
Figure 7: Current vs speed characteristics of the 3kW HE and SE induction motors IM Efficiency vs speed characteristic 100 90
High Eff Calc. Std. Calc. High Eff. Exp Std. Exp
80 70
Efficiency [%]
5.
0 0
Current [A]
0.74 20
60 50
40 30 20 10 0 1000
1050
1100
1150
1200
1250 n [rpm]
1300
1350
1400
1450
Figure 8: Efficiency vs speed characteristics of the 3kW HE and SE induction motors
1500
The simulated torque vs speed characteristics of the two induction motors are plotted in Figure 6 with a typical centrifugal load torque-speed characteristic. The experimental torque vs speed points determined by means of the IEC standard tests are also included in Figure 6.
7.
REFERENCES
[1]
http://www.eskomdsm.co.za/eem_index.htm
[2]
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.
[3]
B. Slaets, P. Van Roy, R. Belmans, K. Hameyer, “Energy Efficiency of Induction Motors”, Katholieke Universitiet Leuven, E.E. Dept., Div. ESAT/ELEN .
[4]
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.
[5]
H.M. Mzungu, A.B. Sebitosi, M.A. Khan, "Comparison of Standards for Determining Losses and Efficiency of Three-Phase Induction Motors", IEEE PES PowerAfrica 2007 Conference and Exposition, Johannesburg, South Africa, 16 – 20 July 2007.
[6]
IEC 60034-2-1 International Standard, Rotating electrical machines – Part 2-1: “Standard methods for determining losses and efficiency from tests (excluding machines for traction vehicles)”, IEC, Geneva, Switzerland, 2007.
[7]
P.C. Sen, “Principle of Electrical Motors and Power Electronics”, John Wiley & Sons, 1997.
8.
AUTHORS
The current vs speed and efficiency vs speed characteristics of the motors are plotted in Figure 8. Good correlation can be observed between the experimental and simulated characteristics of both motors in Figure 6, Figure 7 and Figure 8. The steady-state operating points of the two motors were determined for same pump load shown in Figure 6. The operating points of the motors are summarised in the Table 1: Table 1: Operating points of the motor SE motor HE motor Slip 6.1 % 4.85 % Speed 1408.5 rpm 1427.3 rpm Line current 6.28 A 6.1 A Input power 0.78 0.8 lagging factor lagging Efficiency 83.1 % 87.8 % Input power 3.39 kW 3.38 kW A comparison of the operating points of the motors shows that the HE motor operates with a 4.7% increase in efficiency over the SE motor under the same load. The HE motor drives the centrifugal load at a higher speed and delivers motor torque to its shaft. More output power and hence increased mass flow is therefore delivered to the mechanical process that the HE motor-pump drive is connected to. This is achieved with approximately the same input power for the two motors (0.3% increase for the HE motor). If the increased mass flow is perceived by the process operator as enhanced productivity, a temptation would exist to run the HE motor-pump drive for the same duration as the SE motor-pump drive, in order to increase revenue. The benefit of the higher efficiency of the HE motor in reducing electrical power consumption is therefore almost entirely lost.
6.
CONCLUSION
It was shown that by replacing a pump drive with a high efficiency motor, the net reduction in electrical power consumption is only 0,3% even though the motor itself was almost 5% more efficient. The reason being that the high efficiency motor operates with a lower slip. The resulting speed difference results in the pump delivering more, and also drawing more power so that the benefit of the higher efficiency (in terms of reducing electrical power consumption) is almost entirely lost.
Principal Author: Azeem Khan holds a PhD degree in Electrical Engineering from the University of Cape Town (UCT). He is currently a Senior Lecturer in the Department of Electrical Engineering at UCT. His research interests include: electrical machines & drives, renewable energy and energy efficiency. Co-author: Dumisani Pati holds a BSc degree in Electrical Engineering from the University of Cape Town. He is currently employed by Eskom. Co-author: Heskin Mzungu holds a BSc degree in Electrical Engineering from the University of Cape Town. He is currently working towards an MSc degree in the Department of Electrical Engineering at UCT. His research is on the impact of repairs on the efficiency of induction motors. Presenter: The paper will be presented by Azeem Khan.
INTRODUCTION
High efficiency (HE) induction motors offer improved efficiency over standard efficiency (SE) induction motors. Over the past few years, very few South African companies opted for HE motors as alternatives to SE motors. This is true for new, replacement or retrofit applications of induction motors. The main reasons for this was the cost differential between HE and SE motors and also the low electricity cost in South Africa. However, the recent constrained energy supply in South Africa has firmly established the need for HE induction motors in South African industry. This is further supported by Eskom DSM’s Energy Efficient Motors program [1]. The main objective of this paper is to compare the operating and performance characteristics of a 3kW standard efficiency (SE) induction motor to that of a 3kW high efficiency (HE) induction motor. The efficiencies of the motors are determined using the IEC 60034-2-1 standard. The no-load and locked rotor tests are then used to determine the equivalent circuit parameters of each motor. The operating characteristics of the motors with the same pump load are then simulated by means of a Matlab model. 2.
IEC 60034-2-1 STANDARD
Several international standards exist for testing the efficiency of induction motors. The standards include: IEEE 112, IEC 60034-2, CSA 390 and JEC 37. The standards differ mainly in their treatment of the stray losses in an induction machine [2],[3],[4]. A detailed comparison of the different efficiency results between the standards was presented in [5]. The IEC standard was used in the research related to this paper. The IEC 60034-2-1 standard uses the segregation of losses method to determine the efficiency of an induction
motor [6]. The efficiency can be expressed in terms of output power (Pout) and the sum of losses (∑Plosses), as:
η=
Pout Pout = Pin Pout + ∑ Plosses
(1)
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: a)
Temperature test: The motor is loaded and allowed to run until its temperature stabilizes. The temperature and winding resistances are recorded.
b)
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.
c)
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.
Temperature correction (to 25ْC) 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 is regarded as the most accurate method for calculating efficiency. This does of course depend on the accuracy of equipment. It also has the advantage of very high repeatability due to the temperature correction of the losses. 3.
LABORATORY SETUP
The IEC 60034-2-1 standard requires the test motor to be coupled to a dynamometer and an accurate method of measuring the shaft torque. The accuracy of torque measurement will determine the accuracy of the efficiency estimate.
No-load Iron losses vs Line voltage 220 200 180
P fe Losses [W]
160 140 120 100 80 60
Std-Motor Cubic-fit H-Eff-Motor Cubic-fit
40
Figure 1: Laboratory setup with the 3kW high efficiency induction motor coupled to a DC generator
20 200
250
300
350 Line voltage [V]
400
450
500
Figure 4: Variation of no-load iron losses with line volatge for the high efficiency and standard efficiency motors
Stator Copper losses vs load 400
A 3kW HE and a 3kW SE induction motor was coupled in-turn to a DC motor that is connected to a four-quadrant DC drive. The DC motor and drive operates in generator mode and therefore applies a load to the shaft of the induction motor. The induction motors were fitted with type K Chromel-Alumel thermocouples to measure the temperature distribution of the motor windings. The laboratory setup is shown in Figure 1 with the HE induction motor being tested.
350
Stator Cu losses [W]
300 250 200 150 100
Std-Motor Cubic-fit H-Eff-Motor Cubic-fit
50 0 20
40
60
80 100 Load [%]
120
140
4. 160
Figure 2: Variation of stator copper losses with load for the high efficiency and standard efficiency motors
The friction and windage losses are determined from the no-load test by varying the line voltage across the motor windings. The friction and windage losses of HE motor is 18.8W and that of the SE motor is 54.3W. The IEC standard was used to determine the stator and rotor copper losses for each motor. The variation of stator copper losses with load for the two motors is shown in Figure 2. The rotor copper losses are shown in Figure 3. The stator and rotor copper losses of the HE motor are lower at all loads than that of the SE motor. This is expected since the stator and rotor winding resistances are smaller by design in the HE motor, in an effort to reduce the copper losses in the motor.
Rotor Copper losses vs load 700
600
500 Rotor Cu losses [W]
PERFORMANCE TESTING
400
300
200 Std-Motor Cubic-fit H-Eff-Motor Cubic-fit
100
0 20
40
60
80 100 Load [%]
120
140
The no-load core (iron) losses of the motors are shown in Figure 4. The IEC standard specifies the core losses under load conditions to be determined as follows: •
An equivalent line voltage is calculated from the load test results, using an appropriate formula given in the standard.
•
The load core losses are then read from the no-load core losses (Figure 4) as the value corresponding to the calculated equivalent line voltage.
160
Figure 3: Variation of rotor copper losses with load for the high efficiency and standard efficiency motors
Indirect Method Efficiency vs Load 0.92
IM Torque vs speed characteristic 60
Std-Motor Cubic-fit H-Eff-Motor Cubic-fit
0.9 0.88
50
40
0.84
Torque [Nm]
Efficiency [pu]
0.86
High Eff Calc Std Calc High Eff Exp Std Exp Centrifugal load
0.82 0.8
30
20
0.78 10
0.76
40
60
80 100 Load [%]
120
140
160
Figure 5: Efficiency vs load characteristics of the 3kW HE and SE induction motors The higher losses associated with the SE induction motor results in a lower overall efficiency of the SE motor compared to the HE motor. A comparison of the efficiency vs load curves of the two motors is shown in Figure 5. It can be seen that the SE motor is designed to operate at maximum efficiency at 60% loading, whereas the HE motor operates at maximum efficiency at approximately 70% loading.
A locked rotor test and a no-load test can be used to determine the equivalent circuit parameters of an induction machine [7]. These tests were performed on each of the motors and the equivalent circuit parameters are listed below: SE Motor HE Motor
1000
1500
Figure 6: Torque vs speed characteristics of the 3kW HE and SE induction motors with pump load IM per-phase stator current vs speed characteristic 30 High Eff Calc Std Calc High Eff Exp Std Exp
25
20
15
10
OPERATING CHARACTERISTICS
The equivalent circuit model of an induction machine can be used to predict its operating characteristics. The variation (with speed) of its torque, current, input power factor, efficiency, etc. determined in this manner can be used to simulate the motor’s operation with an arbitrary load. Comparison of the operating characteristics of the HE and SE induction machines were simulated in this manner. The equivalent circuits of each motor were determined and their operation with the same centrifugal pump load was simulated and analysed by means of a Matlab model.
500 n [rpm]
5
0 0
500
1000
1500
n [rpm]
Figure 7: Current vs speed characteristics of the 3kW HE and SE induction motors IM Efficiency vs speed characteristic 100 90
High Eff Calc. Std. Calc. High Eff. Exp Std. Exp
80 70
Efficiency [%]
5.
0 0
Current [A]
0.74 20
60 50
40 30 20 10 0 1000
1050
1100
1150
1200
1250 n [rpm]
1300
1350
1400
1450
Figure 8: Efficiency vs speed characteristics of the 3kW HE and SE induction motors
1500
The simulated torque vs speed characteristics of the two induction motors are plotted in Figure 6 with a typical centrifugal load torque-speed characteristic. The experimental torque vs speed points determined by means of the IEC standard tests are also included in Figure 6.
7.
REFERENCES
[1]
http://www.eskomdsm.co.za/eem_index.htm
[2]
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.
[3]
B. Slaets, P. Van Roy, R. Belmans, K. Hameyer, “Energy Efficiency of Induction Motors”, Katholieke Universitiet Leuven, E.E. Dept., Div. ESAT/ELEN .
[4]
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.
[5]
H.M. Mzungu, A.B. Sebitosi, M.A. Khan, "Comparison of Standards for Determining Losses and Efficiency of Three-Phase Induction Motors", IEEE PES PowerAfrica 2007 Conference and Exposition, Johannesburg, South Africa, 16 – 20 July 2007.
[6]
IEC 60034-2-1 International Standard, Rotating electrical machines – Part 2-1: “Standard methods for determining losses and efficiency from tests (excluding machines for traction vehicles)”, IEC, Geneva, Switzerland, 2007.
[7]
P.C. Sen, “Principle of Electrical Motors and Power Electronics”, John Wiley & Sons, 1997.
8.
AUTHORS
The current vs speed and efficiency vs speed characteristics of the motors are plotted in Figure 8. Good correlation can be observed between the experimental and simulated characteristics of both motors in Figure 6, Figure 7 and Figure 8. The steady-state operating points of the two motors were determined for same pump load shown in Figure 6. The operating points of the motors are summarised in the Table 1: Table 1: Operating points of the motor SE motor HE motor Slip 6.1 % 4.85 % Speed 1408.5 rpm 1427.3 rpm Line current 6.28 A 6.1 A Input power 0.78 0.8 lagging factor lagging Efficiency 83.1 % 87.8 % Input power 3.39 kW 3.38 kW A comparison of the operating points of the motors shows that the HE motor operates with a 4.7% increase in efficiency over the SE motor under the same load. The HE motor drives the centrifugal load at a higher speed and delivers motor torque to its shaft. More output power and hence increased mass flow is therefore delivered to the mechanical process that the HE motor-pump drive is connected to. This is achieved with approximately the same input power for the two motors (0.3% increase for the HE motor). If the increased mass flow is perceived by the process operator as enhanced productivity, a temptation would exist to run the HE motor-pump drive for the same duration as the SE motor-pump drive, in order to increase revenue. The benefit of the higher efficiency of the HE motor in reducing electrical power consumption is therefore almost entirely lost.
6.
CONCLUSION
It was shown that by replacing a pump drive with a high efficiency motor, the net reduction in electrical power consumption is only 0,3% even though the motor itself was almost 5% more efficient. The reason being that the high efficiency motor operates with a lower slip. The resulting speed difference results in the pump delivering more, and also drawing more power so that the benefit of the higher efficiency (in terms of reducing electrical power consumption) is almost entirely lost.
Principal Author: Azeem Khan holds a PhD degree in Electrical Engineering from the University of Cape Town (UCT). He is currently a Senior Lecturer in the Department of Electrical Engineering at UCT. His research interests include: electrical machines & drives, renewable energy and energy efficiency. Co-author: Dumisani Pati holds a BSc degree in Electrical Engineering from the University of Cape Town. He is currently employed by Eskom. Co-author: Heskin Mzungu holds a BSc degree in Electrical Engineering from the University of Cape Town. He is currently working towards an MSc degree in the Department of Electrical Engineering at UCT. His research is on the impact of repairs on the efficiency of induction motors. Presenter: The paper will be presented by Azeem Khan.
