CHAPTER 3 EFFICIENCY IMPROVEMENT IN CAGE INDUCTION MOTORS BY USING DCR TECHNOLOGY

Transcription

1 37 CHAPTER 3 EFFICIENCY IMPROVEMENT IN CAGE INDUCTION MOTORS BY USING DCR TECHNOLOGY 3.1 INTRODUCTION This chapter describes, a comparison of the performance characteristics of a 2.2 kw induction motor with conventional Die-cast Aluminium Rotor to that of a 2.2 kw induction motor with proposed Die- cast Copper Rotor. Both the motors are fabricated and its steady state characteristics are compared using MATLAB 7 software, whose inputs are No-load and Blocked rotor tests, obtained from the above motors. These test results are used to determine the equivalent circuit parameters of each motor. The actual load tests were also performed in the motors in accordance with IS 12615: 2004 standard. The possibility of the efficiency improvement for the three phase, low voltage, squirrel-cage induction motor is also experimentally verified between the ratings from 0.37 kw to 11 kw. 3.2 INTERNATIONAL RECOMMENDATIONS ON ENERGY EFFICIENCY In today's power scenario, we are facing a major power crunch. Day by day, the gap between demand and supply of electrical energy is widening at the rate of 3%. Bridging this gap from supply side is very difficult and expensive proposition. Due to the pressure of energy shortages, energy cost and environmental considerations, various Government/Non-

2 38 Government agencies through out the world started looking at the ways of improving the efficiency and creating awareness amongst the industry by enacting mandatory laws/modifying standards for voluntary adoption. Comparative picture of laws and standards as established today in USA, EU and India is given in Table 3.1. Table 3.1 International Recommendations on Energy Efficiency Factors USA (EPAct) EU (CEMEP Agreement) INDIA (Energy Conservation Act) Legislation Mandatory Voluntary Voluntary Type of EE motor 3 Phase 3 Phase 3 Phase Induction Induction Induction Ratings 0.75 kw 150 kw 1.1 kw 90 kw 0.37kW 160 kw Polarities 2,4 & 6 Pole 2 & 4 Pole 2,4,6 & 8 Pole Efficiency Standard Minimum Specified Minimum EFF category to be labelled Minimum Specified as per IS : Testing Specification IEEE 112 B IEC IS Minimum Efficiency Specified 91.0 % 88.4 % 88.4 % for (11kW, 4 Pole, (60 Hz) (50 Hz) (50 Hz) TEFC)

3 FOCUS ON ENERGY EFFICIENCY OF ELECTRICAL MOTORS In simplest terms, energy-efficient electric motors are high-quality versions of standard motor products. They pack more of 'active' electric materials (steel laminations and copper) into essentially the same physical package. By introducing this motor which have different characteristic from a standard motor, less energy is required to produce the same output torque. The usage of energy efficient motor, can reduce financial cost of industrial sector such as the cost of motor maintenance and cost of buying a new motor because it has long life span. The importance of energy saving in induction motor was emphasized about 20 years ago, in the academic area, but the motor manufacturer s interest is focused only on the maximum benefit. As a costumer, it is better to take into account not only the motor price, but also the cost of the used energy during the whole lifetime of the motor. The new requirement to improve the motor efficiency is a serious research subject, which must be about the possibility of loss minimization in the induction motor. For the three phase, low voltage, induction motor, the used material for the squirrel cage is aluminum because of the lower price when compared to copper, which is convenient for the existing technological solutions. As it is known, the copper s resistivity is lower than that of aluminium, and therefore the copper squirrel cage losses decrease with the ratio of resistivity of copper to resistivity of aluminium. Till now, the actual technology has no solutions for low voltage motors and hence new solutions are necessary in the technological area. This chapter analyzes a series of industrial induction machines with squirrel cage, considering the rotor losses decreasing, the material cost increase and the saved energy for the whole machine life.

4 ENERGY EFFICIENT MOTOR An Energy Efficient motor produces the same shaft output (HP), but absorbs less input power (kw) than a standard motor. Efficiency = Output / Input = (Input Losses) / Input = 1 (Losses* / Input) * Lesser the loss, higher the efficiency. The efficiency can be improved by reducing the various losses in motor. The various losses encountered in an induction motor are given in Table 3.2. Table 3.2 Losses Encountered in Induction Motor S.No. Motor Component Loss Share on Total Loss 1 Stator copper loss 37% 2 Rotor copper (Conductor) loss 18% 3 Iron Loss 20% 4 Friction and Windage loss 9% 5 Stray loss 16% Various attempts are made to reduce the above category of losses in induction motors, primarily to reduce rotor copper loss using DCR technology. Energy - efficient motors, also called premium or high efficiency motors. Motors qualify as energy efficient if they met or exceed the efficiency level listed in the National Electric Manufactures Association s (NEMA s) MG Publication (NEMA Standards Publication MG and 1996).

5 Die-cast Copper Rotor Motor - The Economical way of Energy Conservation 20 HP Induction motor Efficiency Comparison with the Past and Future Nirvana Super Conducting Amorphous Steel Laminations Die Cast Copper Rotor Motor Today s Premium Efficiency Motor Today s Standard Motor Historical Motors Figure 3.1 Technology Vs Efficiency of Induction Motors In conventional design, the cost of motor increases while attempting to reduce the losses. The task of efficiency improvement by various methods is illustrated in Figure 3.1 for a 20 HP rating of motor. DCR has helped to achieve efficiencies to meet EFF1 standards in the previous experiences. It also gave confidence that with optimisation of design. It may be possible to achieve efficiencies above EFF1 level also. The DCR Technology increases the efficiency of motor with a nominal increase in cost. As shown in the Figure 3.2, the DCR technology is the best way in the 21 st century to increase the motor efficiency above the premium level. It is an interesting fact that, Two-third (2/3) of electricity generated globally is used to run motors, which is almost equal to 2 Trillion ( ) kwhr/year. Out of this, about 8.5% of all electricity is consumed to meet the loss in Electrical motors. Of these, motors up to 20HP constitute approximately two third of losses (i.e.) 5.4% of all electricity is wasted as loss for these motors.

6 42 MOTOR LOSSES DCR Motor MOTOR COST PAST 21 ST CENTURY FUTURE Figure 3.2 Induction Motor Efficiency Technology Vs Cost Hence, more than in any other areas, improving efficiency of motors caught the attention of government bodies, producers and consumers of electricity, designers and manufacturers of motors. Improvement in efficiency of motors assumes more significance in India where motors consume over 75% of the electrical energy with very wide coverage of industrial, agriculture and rural sectors. The DCR motor which improves efficiency by 1.5% to 3% would save 30 Billion ( ) kwhr/year, at Rs.4.50 / kwhr, this amounts to Rs. 135 Billion and is equivalent to 78 Million Barrels of oil (Deivasahayam 2005). 3.5 EFFICIENCY IMPROVEMENT IN 2.2 KW (3 HP) INDUCTION MOTOR A lot of research articles have been published in the past 10 years where the results are related with high rating, 4 pole motors and very few low rating (up to 5 HP) 2-pole motors only (Brush et al 2004 and Dale.T.Peters et al., 2007). Therefore this work investigates the performance comparison of DCR and DAR motors of 2.2 kw (3 HP), 2 pole induction motor both

7 43 theoretical calculation using Matlab 7 software and by experimentation in accordance with IS standard IS 12615: 2004 Standard Several international standards exist for testing the efficiency of induction motors. The standards include: IEEE 112, IEC , CSA 390 and JEC 37. The standards differ mainly in their treatment of the stray losses in an induction machine (Renier et al 1999). A detailed comparison of the different efficiency results between the standards was presented. The Indian standard IS 12615: 2004 was used in the research related to this chapter. The IS 12615: 2004 standard uses the segregation of losses method to determine the efficiency of an induction motor as in the same way of reference international standard IEC (Rotating Electrical Machines Testing Standard IEC and IEC ). The efficiency can be expressed in terms of output power (P out ) and the sum of losses ( losses), as: Pout Pout P P P in out losses (3.1) 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: Temperature Test The motor is loaded and allowed to run until its temperature stabilizes. The temperature and winding resistances are recorded.

8 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 No-load Test The motor is run at no-load with a varying supply voltage between 125% to 20% of rated voltage. The friction and windage and core losses are calculated from this. Temperature correction (to 25C ) 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 of course depends on the accuracy of the equipment. It also has the advantage of very high repeatability, due to the temperature correction of the losses. 3.6 PERFORMANCE CHARACTERISTICS In order to analyze the performance of an induction motor, first, the conventional aluminium die cast material is placed in its rotor. After that, DCR is substituted in place of former one. A locked rotor test and a no-load test can be used to determine the equivalent circuit parameters of an induction machine. (Nasar et al 1979; Rajput 1993 and Engelmann et al 1995). So, these tests were performed on each of the motor and the test results are shown in Table 3.3 and 3.4 respectively. The equivalent circuit parameters for the DCR motor and the comparison with the DAR motor are shown in Table 3.5.

9 45 Subsequently, the calculated equivalent circuit parameters are fed in to Matlab 7 software to determine the steady-state characteristic of the induction motor. From Table 3.5 it is observed that the value of no load resistance (R 0 ) in the DCR motor is more than that of DAR motor. This is due to slot geometry which is designed for DAR motor. i.e., the slot design had not been optimized for copper. Table 3.3 No-Load test results for 2.2 kw (3 HP), 415 V, 50 Hz, 2-pole, 3-phase induction motor Parameters DAR Motor DCR Motor No load voltage/ phase 240V 240V No load current / phase 2A 1.90A No load power 220W 180W Table 3.4 Locked rotor test results for 2.2 kw (3 HP), 415 V, 50 Hz, 2- pole, 3-phase induction motor Parameters DAR Motor DCR Motor L.R. voltage / phase 48.5V 47.3V L.R. current / phase 4.6A 4.47A L.R. power 410W 320W Stator resistance per phase

10 46 Table 3.5 Comparison of equivalent circuit parameters of 2.2 kw (3 HP) DCR motor with DAR motor Parameters DAR Motor DCR Motor Iw (A) Iu (A) I 0 (A) i i R 0 ( ) X 0 ( ) Z 01 ( ) R 01 ( ) X 01 ( ) R 1 2 ( ) R 1 ( ) Figure 3.3 Equivalent circuit of an induction motor

11 47 (a) (b) (c) (d) (e) Figure 3.4 Performance characteristics of a 2.2 kw (3 HP) induction motor

12 48 The equivalent circuit model of an induction machine can be used to predict its performance characteristics (Sen 1999 and Wildi 2000). One such circuit is shown in Figure 3.3. The variation (with output power) of its torque, current, input power factor, efficiency, and slip are determined using Matlab 7 which is shown in Figure 3.4 (a-e) (Ong 1998 and Okoro 2004). in Figure 3.5. The experimental set up for conducting the full load test is shown Figure 3.5 Experimental setup of 3-HP DCR Motor The results from the tests obtained on one of the 2-pole motors of 3-phase, 2.2 kw (3 HP), 415V, 50 Hz are shown in Table 3.6. The important parameters responsible for the induction motor characteristics like Efficiency, Locked rotor Torque, Slip etc., are calculated from Matlab 7 and compared to the tested results as shown in Table 3.7. Both the results are found equivalent with each other.

13 49 Table 3.6 Comparison of test results of 2.2 kw (3 HP) DAR and DCR motors DAR DCR Parameters 75% 100% 110% 75% 100% 110% Voltage (Volts) Current (Amps) Input Power (Watts) Speed (RPM) Output Power (Watts) Efficiency (%) Power factor Pull out torque (Break down) % of rated value LR Torque (% of full load Torque) Table 3.7 Comparison of tested and calculated values of 3 HP DAR and DCR motors under rated conditions Parameters Results DAR motor DCR motor Efficiency (%) Speed (RPM) Power factor Input Power (Watts) Output power (Watts) Calculated Tested Calculated Tested Calculated Tested Calculated Tested Calculated Tested

14 EFFICIENCY IMPROVEMENTS IN SMALL/MEDIUM RATED MOTORS Experimental Results During the first test process, seven rotors were cast for 15-HP (11 kw), 4 Pole, 50 Hz motor and were 150 mm in diameter with 165 mm core length containing 7 kg of copper in the conductor bars and end rings. It is important to note that, the laminations used here were designed for aluminium, i.e., the slot design had not been optimized for copper. Rotor watt loss averaged 135 watts with a range of 131 to 145 W losses. Table 3.8 shows the CSA390 test results for the seven rotors tested. Similarly, the CSA390 test results for the 10-HP (7.5 kw), 4 Pole, 50 Hz motor and the segregation of losses is given in Table 3.9. Relative costs are summed up for each rating as per present research. Table 3.8 Test readings of 7.5 kw (10 HP), 4 Pole, 50 Hz motor Types of Losses and performance parameters Constant loss (Iron loss + Friction and Windage loss) Std. Eff. motor (DAR) Energy Eff. motor (DAR) Energy Eff. motor (DCR) 505 W 342 W 202 W Stator copper loss 452 W 424 W 422 W Rotor copper loss 357 W 243 W 166 W Stray loss 37 W 37 W 38 W Speed (Rpm) Efficiency (%) Cost (%)

15 51 Table 3.9 Test readings of 11 kw (15 HP), 4 Pole, 50Hz motor Types of Losses Std. Eff. Energy Eff. Energy Eff. and performance motor motor motor parameters (DAR) (DAR) (DCR) Constant loss (Iron loss + Friction and Windage loss) 555 W 422 W 222 W Stator copper loss 761 W 714 W 666 W Rotor copper loss 287 W 266 W 135 W Stray loss 56 W 56 W 55 W Speed (Rpm) Efficiency (%) Cost (%) Table 3.10 Performance comparison of motors with DAR and DCR Motor Efficiency (Al) Efficiency Incremental Rating (%) / RPM (Cu) Difference (%) / RPM Eff. / RPM 0.37kW 2 Pole 72.8 / / / kw 2 Pole / / / kw 2 Pole 84.0 / / / kw 4 Pole 73.1 / / / kw 4 Pole 82.0 / / / kw 4 Pole 83.5 / / / kw 4 Pole 84.0 / / / 40 Average Efficiency improvement 2.5%

16 52 Improved efficiencies obtained by using DCR in the existing designs are given in Table 3.10 for certain popular ratings. This goes to prove that the DCR motor is the only economical way of obtaining energy efficiency above levels acceptable. Table 3.11 Test readings of 10 HP (7.5 kw) 4 pole Induction motor under Full load condition Parameter DAR DCR Voltage (Volts) Current (Amps) Input Power (Watts) Speed (RPM) Output Power (Watts) Efficiency (%) Power factor Slip (%) Pull out torque (Break down) % of rated value LR Torque (in kg-m) LR Current (Amps) As expected, in the higher conductivity (copper) rotor material, the speed of DCR motor is increased slightly, the slip and input currents are reduced and the efficiency is increased. For the same output power, by merely replacing the DAR with DCR (without changing the other parameters), the efficiency of the motor is increased by nearly 2.6% points in the 2.2 kw, 2 pole motor. The efficiencies of small/medium rated motors between the

17 53 ratings (0.37 kw- 3.7 kw) were increased by about 1.5% and nearly 3% points, a rather remarkable improvement. The test methods for the above motors are loss separation method of CSA 390 and the test conditions are 3 Phase, 415 V, 50 Hz. Results from tests on one of the 4 pole, 7.5 kw motor and one of the 2 pole, 1.5 kw motor is shown in Tables 3.11 and 3.12 respectively. It may be noted that the breakdown torque values of the DCR motor has 30% points over those for similar motors with aluminium rotor. Table 3.12 Test readings of 2 HP (1.5 kw) 2 pole 50 Hz induction motor under Full load condition Parameter DAR DCR Voltage (Volts) Current (Amps) Input Power (Watts) Speed (RPM) Output Power (Watts) Efficiency (%) Power factor Slip (%) 4 3 Pull out torque (Break down) % of rated value LR Torque (in kg-m) LR Current (Amps)

18 54 It is observed that this increment in efficiency in DCR is due to the better conductivity of DCR. Starting (locked rotor) Torque is reduced somewhat for aluminium laminations with slots designed for aluminium. In applications where the lower starting torque is a problem, a modified design of the rotor slots offers the solution. The pull out (Breakdown) torque values are also reported in Table 3.6 for a 2.2 kw motor. It is observed that the pull out torque in DCR is high. i.e., 365 % of the rated torque compared to 352 % of the rated torque for the same motor with DAR. From the mechanical characteristics shown in Figure 3.4 (c), the DCR motor has the advantage of a higher torque at running speed. This characteristic of a DCR motor often finds very useful in applications such as centrifugal pumps, which need high torque at high speed. The p.f of the motor under various load conditions of both the DAR and DCR motors are nearly equal. The pictorial views of 11 kw and 1.5 kw DCR s are shown in Figure 3.6. A photomicrograph of a 2.2 kw DCR is shown in Figure Efficiency Comparison The graphical representations of efficiency comparison of two 4 pole motors are shown in Figure 3.8 and 3.9. Both these motors are 415V, 50 Hz, 3 phase. Figure 3.9 shows the DCR of 11kW and 1.5 kw respectively. A core plate of a 5 HP DCR is shown in Figure 3.10.

19 55 Figure 3.6 DCR of 11 kw and 1.5 kw ratings Figure 3.7 A photomicrograph of a 2.2 kw (3 HP) DCR

20 56 2 HP (1.5 kw) % LOAD 75 % LOAD 100 % LOAD LOAD (%) ALUMINIUM COPPER Figure 3.8 Efficiency Comparison of 2 HP Motor 0.5 HP (0.375 kw) % LOAD 75 % LOAD 100 % LOAD LOAD (%) ALUMINIUM COPPER Figure 3.9 Efficiency Comparison of 0.5 HP Motor

21 57 Figure 3.10 Core plate of 5HP DCR 3.8 SAMPLE WORKING OF ENERGY SAVINGS AND PAYBACK The slightly higher (15%) initial cost of DCR motors is often misunderstood as a demerit. It is not at all true. The increase in initial cost is offset by the energy saved and the following equation outlines a method for calculating the cost savings from improved motor efficiency. The difference in losses for the two motors is: 1 1 L kw x F x (3.2) E E FL SM EM where, kw FL = Full load rating (kw) (taken from motor nameplate) E SM E EM F = Full load efficiency (%) of Standard Motor = Full load efficiency (%) of EE Motor = Running load as a % of full load

22 58 Pay Back Period Months Capital Cost Differential Savings per Month (3.3) In the above equation for SM at part loads falls steeply to a lower value unlike E EM which remains almost constant. Hence the payback period reduces more, than what is calculated using the above equation. Consider an example, assume we have a standard-efficiency 11 kw motor. Assume the motor operates 600 hours, and that power costs Rs.4.50/kWh. Such motors have an average efficiency rating of 87% at full load and the capital cost is $350. This motor may be replaced with a DCR motor having an efficiency of 91% and the capital cost is $380. The pay back period is calculated as follows: Increased cost of motor with 91% efficiency is $30 Let, X - Input power per hour for 87% efficiency motor = 11/0.87 = 12.6 units Y - Input power per hour for 91% efficiency motor = 11/0.91 = units Reduction in electricity cost for 91% efficiency motor per month = (X-Y)*600*0.1= $31.2 Hence payback of incremental cost is less than 2 months.

23 CONCLUSION This chapter is a comparative analysis of performance characteristics of 2.2 kw, 3-phase 2 pole TEFC induction motor and practical verification using conventional Die-cast Aluminium Rotor with proposed Diecast Copper Rotor. The equivalent circuit parameters of an induction motor is obtained from No load and Blocked rotor tests are fed to Matlab 7 software to draw the performance characteristics of both DAR and DCR motors. The experimental results are compared with the calculated results. Both the results are almost tallying with each other. It was found that by using DCR technique the efficiency of 2.2 kw, 3-phase 2 pole TEFC induction motor is improved by 2.6%. Similarly, performance comparison with DAR and DCR were carried out between the ranges from 0.37 kw to 11 kw. By merely replacing the DAR with DCR (without change in the other parameters), the efficiency of the motor were proven to improve by 1.5% to 3% and the average efficiency improvement of 2.5% is achieved.