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

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

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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

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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

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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?

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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

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2. Efficiency standards

• Best method of determining Padditionalcalculate
  Padditional for various load levels as
• Linearise and correct for measurement errors in
  function of torque squared as

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2. Efficiency standards
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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

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3. Measurement set-up
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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!

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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

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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

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4. Experimental results - Motor

• Comparison IEC - IEEE

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11 kW
55 kW
75 kW
5
4
D Eff
3
2
1
0
IEC
IEEE
IEC
IEEE
IEC
IEEE
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4. Experimental results - Motor
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4. Experimental results - Motor
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4. Experimental results - Motor
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4. Experimental results - Motor
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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

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5. Efficiency at partial load
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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

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6. Energy savings
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6. Energy savings
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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)

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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?

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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

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7. Experimental results - Drive
Load torque n2
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7. Experimental results - Drive
Load torque n2
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7. Experimental results - Drive
Load torque n2
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7. Experimental results - Drive
Load fixed freq 50 Hz
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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