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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

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

- 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

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

- 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

- 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

Induction Motor Efficiency KULeuven Belgium

- 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

Induction Motor Efficiency KULeuven Belgium

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

- 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

- Differences between catalogue efficiency value and measured values:
- Conclusions:
- Catalogue values are not reliable
- Only IEEE standard is meaningful

Induction Motor Efficiency KULeuven Belgium

- 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

Induction Motor Efficiency KULeuven Belgium

Induction Motor Efficiency KULeuven Belgium

Induction Motor Efficiency KULeuven Belgium

Induction Motor Efficiency KULeuven Belgium

- 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

Induction Motor Efficiency KULeuven Belgium

- 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

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]

Induction Motor Efficiency KULeuven Belgium

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]

Induction Motor Efficiency KULeuven Belgium

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

- 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

- 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

Load: torque ~ n2

Induction Motor Efficiency KULeuven Belgium

Load: torque ~ n2

Induction Motor Efficiency KULeuven Belgium

Load: torque ~ n2

Induction Motor Efficiency KULeuven Belgium

Load: fixed freq: 50 Hz

Induction Motor Efficiency KULeuven Belgium

- 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