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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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Efficiency in induction motors and variable speed drives not an easy problem l.jpg

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

P. Van Roy, B. Slaets, R. Belmans, Katholieke Universiteit Leuven, Belgium


Overview l.jpg

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


1 introduction l.jpg

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


2 efficiency standards l.jpg

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


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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


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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


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2. Efficiency standards

Induction Motor Efficiency KULeuven Belgium


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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


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3. Measurement set-up

Induction Motor Efficiency KULeuven Belgium


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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


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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


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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


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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


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4. Experimental results - Motor

Induction Motor Efficiency KULeuven Belgium


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4. Experimental results - Motor

Induction Motor Efficiency KULeuven Belgium


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4. Experimental results - Motor

Induction Motor Efficiency KULeuven Belgium


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4. Experimental results - Motor

Induction Motor Efficiency KULeuven Belgium


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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


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5. Efficiency at partial load

Induction Motor Efficiency KULeuven Belgium


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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


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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


6 energy savings22 l.jpg

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


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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


7 experimental results drive l.jpg

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


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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


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7. Experimental results - Drive

Load: torque ~ n2

Induction Motor Efficiency KULeuven Belgium


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7. Experimental results - Drive

Load: torque ~ n2

Induction Motor Efficiency KULeuven Belgium


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7. Experimental results - Drive

Load: torque ~ n2

Induction Motor Efficiency KULeuven Belgium


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7. Experimental results - Drive

Load: fixed freq: 50 Hz

Induction Motor Efficiency KULeuven Belgium


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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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