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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

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

6. Energy savings

7. Experimental results - Drives

8. Conclusions

Induction Motor Efficiency KULeuven Belgium

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

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

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

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

2. Efficiency standards

Induction Motor Efficiency KULeuven Belgium

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

3. Measurement set-up

Induction Motor Efficiency KULeuven Belgium

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

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

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

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

4. Experimental results - Motor

Induction Motor Efficiency KULeuven Belgium

4. Experimental results - Motor

Induction Motor Efficiency KULeuven Belgium

4. Experimental results - Motor

Induction Motor Efficiency KULeuven Belgium

4. Experimental results - Motor

Induction Motor Efficiency KULeuven Belgium

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

5. Efficiency at partial load

Induction Motor Efficiency KULeuven Belgium

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

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 savingsInduction Motor Efficiency KULeuven Belgium

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 savingsInduction Motor Efficiency KULeuven Belgium

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

- 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

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

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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