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Efficiency in induction motors and variable speed drives: not an easy problem

Efficiency in induction motors and variable speed drives: not an easy problem. P. Van Roy, B. Slaets, R. Belmans, Katholieke Universiteit Leuven, Belgium. Overview. 1. Introduction 2. Efficiency standards 3. Measurement set-up 4. Experimental results - Motors 5. Efficiency at partial load

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Efficiency in induction motors and variable speed drives: not an easy problem

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  1. Efficiency in induction motors and variable speed drives: not an easy problem P. Van Roy, B. Slaets, R. Belmans, Katholieke Universiteit Leuven, Belgium

  2. Overview 1. Introduction 2. Efficiency standards 3. Measurement set-up 4. Experimental results - Motors 5. Efficiency at partial load 6. Energy savings 7. Experimental results - Drives 8. Conclusions Induction Motor Efficiency KULeuven Belgium

  3. 1. Introduction • Induction motors use more than 50% of total electricity in industrialised countries • Already high efficiency, can still be improved • Different efficiency standards give different results • Comparison between motors requires reliable standard • With converters: no standards yet Induction Motor Efficiency KULeuven Belgium

  4. 2. Efficiency standards Grid-connected motors • Europe: IEC 60034-2, and the new IEC 61972 • US: IEEE 112 - Method B • Japan: JEC 37 • Difference in efficiency value: up to 3% • Why such a difference? Induction Motor Efficiency KULeuven Belgium

  5. 2. Efficiency standards • The loss consists of five components: • Stator copper losses: Pstator • Iron losses: PFe • Rotor copper losses: Protor • Friction and windage losses: Pfr,w • Stray load losses: Padditional • PFe and Pfr,w: from no-load test • Pstator and Protor: from R, s and Pin • Padditional: can not be measured directly Induction Motor Efficiency KULeuven Belgium

  6. 2. Efficiency standards • Best method of determining Padditional:calculate Padditional for various load levels as • Linearise and correct for measurement errors in function of torque squared as Induction Motor Efficiency KULeuven Belgium

  7. 2. Efficiency standards Induction Motor Efficiency KULeuven Belgium

  8. 2. Efficiency standards • IEEE 112 method B uses this method • Requires measurement of torque and rpm, yielding Pout • Torque measurement was historically difficult, but is now perfectly possible • JEC 37: assumes Padditional = 0 • IEC 60034-2: Padditional = 0.5% . Pin • IEC 61972 : Padditional by measurement or fixed amount depending on motor rating Induction Motor Efficiency KULeuven Belgium

  9. 3. Measurement set-up Induction Motor Efficiency KULeuven Belgium

  10. 3. Measurement set-up Accuracy • Standard deviation, based on : • measurement equipment: 0.9% • Pstray correction factor B: 0.17% • 5 measurements of one motor: 0.12% • Comparison: 4 identical motors with consecutive serial numbers: 0.24% • Careful with small efficiency differences! Induction Motor Efficiency KULeuven Belgium

  11. 4. Experimental results - Motor • 18 motors, 11 kW, 55 kW and 75 kW • Stray load losses at full load: • Average value: 1.7% of Pin Induction Motor Efficiency KULeuven Belgium

  12. 4. Experimental results - Motor • Differences between catalogue efficiency value and measured values: • Conclusions: • Catalogue values are not reliable • Only IEEE standard is meaningful Induction Motor Efficiency KULeuven Belgium

  13. 4. Experimental results - Motor • Comparison IEC - IEEE 6 11 kW 55 kW 75 kW 5 4 D Eff [%] 3 2 1 0 IEC IEEE IEC IEEE IEC IEEE Induction Motor Efficiency KULeuven Belgium

  14. 4. Experimental results - Motor Induction Motor Efficiency KULeuven Belgium

  15. 4. Experimental results - Motor Induction Motor Efficiency KULeuven Belgium

  16. 4. Experimental results - Motor Induction Motor Efficiency KULeuven Belgium

  17. 4. Experimental results - Motor Induction Motor Efficiency KULeuven Belgium

  18. 5. Efficiency at partial load • Motors are usually overdimensioned • Efficiency at 50 and 75 % load should also be mentionned • Definition ‘average weighted efficiency’: • (1 x Eff100 + 0.75 x Eff75 + 0.5 x Eff50)/2.25 or • (0.75 x Eff100 + 1 x Eff75 + 0.5 x Eff50)/2.25 Induction Motor Efficiency KULeuven Belgium

  19. 5. Efficiency at partial load Induction Motor Efficiency KULeuven Belgium

  20. 6. Energy savings • Assume 11 kW motor 1 and 2 • Typical purchase cost: 37.5 EUR/kW • Energy cost: 0.075 EUR/kWh • Annual time of use e.g. 5000 h at • Partial load: 75 % • Efficiency: motor 1: 88 % motor 2: 86 % • Energy saving: +/- 1100 kWh/year • Cost saving: +/- 80 EUR/year +/- 20 % of purchase cost Induction Motor Efficiency KULeuven Belgium

  21. 60 48 Annual 250 36 cost Annual 200 saving 24 cost [%] saving 12 150 [EUR] 40 100 50 50 60 70 0 Partial load [%] 80 8000 90 6000 100 Annual time 4000 Motors 11 A – 11 D of usage [h] 6. Energy savings Induction Motor Efficiency KULeuven Belgium

  22. 48 36 Annual 1000 cost 24 Annual saving cost 750 [%] 12 saving [EUR] 40 500 50 60 250 70 Partial load [%] 80 0 8000 90 6000 Annual time Motors 55 C – 55 F 100 4000 of usage [h] 6. Energy savings Induction Motor Efficiency KULeuven Belgium

  23. 6. Energy savings Conclusions • Annual cost savings can be as high as 50% of the typical purchase cost • More efficient motor can be more expensive, but pays itself back • Efficiency at partial load is very important • Overdimensioned motor choice • Use at partial load • Need for reliable standard (IEEE, new IEC) Induction Motor Efficiency KULeuven Belgium

  24. 7. Experimental results - Drive • Variable speed drive, using induction motor and frequency converter • Energy saving potential up to 50% in • pump drives • ventilator drives • compressor drives • … when compared with fixed speed on/off, throttle or bypass system • What with efficiency between drives? Induction Motor Efficiency KULeuven Belgium

  25. 7. Experimental results - Drive • Converter efficiency: 95 to 98%, even at low load • Motor efficiency: comparable with grid-connected efficiency, even higher at low load when using flux-optimisation • Overall drive efficiency at 50 Hz: 2% lower compared with grid-connected motor • Difference in drive efficiency: 3 to 4% • Advice: efficient motor with user-friendly converter, with flux-optimisation Induction Motor Efficiency KULeuven Belgium

  26. 7. Experimental results - Drive Load: torque ~ n2 Induction Motor Efficiency KULeuven Belgium

  27. 7. Experimental results - Drive Load: torque ~ n2 Induction Motor Efficiency KULeuven Belgium

  28. 7. Experimental results - Drive Load: torque ~ n2 Induction Motor Efficiency KULeuven Belgium

  29. 7. Experimental results - Drive Load: fixed freq: 50 Hz Induction Motor Efficiency KULeuven Belgium

  30. 8. Conclusions • IEC 34.2 is not reliable • Catalogue value usually too high, 3 to 4% • Partial load efficiency is important as well • Fixed allowance for additional load losses can not be defended • More efficient motor: energy and cost savings, as high as 60% of purchase cost • Variable speed drive: energy saving: 50% NEED FOR RELIABLE STANDARD AND RELIABLE MANUFACTURER INFORMATION Induction Motor Efficiency KULeuven Belgium

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