Guide for the Selection and Application of Energy-efficient Motors

Martin Doppelbauer

IEC WG31 SEW Eurodrive GmbH&Co KG, Germany

Abstract

The latest developments and contents of the IEC project 60034-31: Guide for the selection and application of energy-efficient motors including variable-speed applications are presented. Currently the paper is still under development. The final draft will be published in autumn 2009. The official publication of the technical specification is expected for early 2010.

Introduction

Based on a new work item proposal (NWIP) issued by the German national committee DKE K311, working group 31 was established in 2006 by the IEC TC 2 (Technical Committee 2 = Rotating machinery) and assigned the task to define energy efficiency classes for three-phase industrial motors.

The first meeting of WG31 took place in October 2006. Already at the second meeting in May 2007 it became clear that more user guidance for energy efficient operation of electric motors and applications was useful than could be provided in a classification standard.

The idea to create an energy efficiency guide was presented at the general IEC TC2 meeting in May 2007 where the project was confirmed and started.

Until now, two more meetings of WG31 have taken place and the second draft (2CD) of the paper has been released in April 2009. The final draft paper for voting (DTS) is expected for the end of 2009 and the finished guide should be available early in 2010.

The guide provides a guideline on technical aspects of the application of energy-efficient, three-phase, electric motors. It applies to motor-manufacturers, OEMs (original equipment manufacturers), end-users, regulators, legislators and all other interested parties.

The paper is applicable to all electrical machines covered by IEC 60034-30. Most of the information, however, is also relevant for synchronous motors and cage induction machines with output powers exceeding 375 kW. Some sections of the guide were specifically written for variable speed, frequency-converter operated motors.

The guide is partly based on a paper of the US-American Association of Electrical and Medical Imaging Equipment Manufacturers (NEMA MG10).

Saving Energy with electrical drive systems

It is a well known fact that electric motors are energy converters. They can only save a part of their own energy consumption which is only a small part of the total energy they pull from the grid. The green bars in figure 1 are the electrical energy being converted to mechanical energy and the red bars are the own consumption (losses) of the motor. The total energy input of the IE1 motors is set to 100% respectively.

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IE11,1kW

IE21,1kW

IE31,1kW

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IE211kW

IE311kW

IE411kW

IE1110kW

IE2110kW

IE3110kW

IE4110kW

Ploss[%]

Pout[%]

Figure 1. Comparison of energy usage of 4-pole motors of different sizes and energy-efficiency classes; Red bars = own consumption, green bars = transferred energy

Obviously, electrical drive systems can save most energy when the whole picture (the complete application) is taken into account, i.e., when the transferred energy (green bars) is reduced as well. This includes variable speed control, mechanical elements and ultimately the production-processes and machines:

Electricalcomponents

Mechanicalcomponents

FactoryAutomation

EnergyRecouperation

S1Co

ntinuo

usDuty

S2ShortTime

S3...S10

Interm

ittentD

uty

Energyefficiencymotors

Powerfactorcorrectiondevices

Usemost economicalcomponents

Considerrotatinginertia

Variablespeed drivesystems

Properand regular maintenance

Application

Variablespeed drivesystemsSoftstart

with frequencycontrol

Mostefficientpowersupply

Mostefficientpowersupply

Lowenergymodeduringstandstill

Lowenergymodeduringstandstill

Optimizedmassand flow

Energyefficientgearboxes,belts,...

Energyefficientpumps,fans,

compressors,...

Reducing elec.transmission

losses

Regenerativebraking

DClinkcouplingBatteries,ultracaps,

flywheels etc.

Figure 2. Overview of different areas for saving electrical energy with drive systems

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The duty type of the application plays a major part in determining what amount of energy can be saved and which strategy is best.

Applications running in continuous duty (S1) are the best candidates for saving energy. They typically operate between 8 and 24 hours per day and between 250 and 365 days per year.

Motors with higher energy efficiency classes (IE2 or IE3) are very beneficial.

An improved power factor (by using frequency-converters and synchronous motors) can help to reduce IR losses in the cables. Also field converters, which are installed close by the motor with short cables, may be helpful to reduce IR losses.

Mechanical optimizations (gearboxes, belts, pumps, fans etc.) may lead to much greater savings than improvements of the electrical motor alone. However, such measures will often require a significant reconstruction of the application.

The application itself must be regarded as well. In many cases variable speed control can be utilized to reduce the energy consumption during light-load periods.

Many industrial plants have a high energy consumption of the low voltage control circuits (typically 24V power supply). Therefore, high-efficiency low-voltage power supply modules should be used. If possible, the factory should be shut down during longer standstill periods (weekends, holidays).

Applications running in short-time duty (S2) are usually not suitable for energy-saving measures. Due to the short running time they do not consume enough energy to justify costly measures. Typical examples are gate or garage door openers, valve controllers, emergency pumps and fans etc.

Intermittent duty applications (S3-S10) have to be regarded on a case-by-case basis. Typical examples are cranes, hoist drives, lifts etc.

Some of these applications are powered directly from the grid and are switched by simple circuit breakers. They may have high switching frequencies (up to several cycles per minute). In these cases, high-efficiency electrical motors may be counterproductive and consume even more energy than standard-efficient motors due to their increased inertia and start-up currents. High-efficient motors will also increase the wear of electromechanical brakes which are frequently used in such applications.

In some extreme cases it might be technically impossible to run the application with high-efficiency motors at all.

Other intermittent duty applications are powered by frequency converters. The converter will reduce the start-up losses considerably and therefore save energy compared to grid operated motors. A higher efficiency of the motor may not improve the overall picture much (or even worsen the situation due to the increased inertia).

Often, just like short-time duty applications, the actual running time of intermittent duty applications is relatively low.

Some intermittent duty applications, where the cycle frequency is rather low and the running time rather high, will benefit from motors of improved efficiency, for example passenger and goods lifts.

Retrofitting existing applications with high- and premium-efficient motors

Due to the fact that cage-induction motors with a high energy-efficiency contain more active material (copper, iron) than their lower efficient counterparts the torque over speed characteristic of these motors is normally stiffer, i.e. the nominal operating speed is higher (see figure 3).

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Torque

[Nm]

speed[1/min]

IE1

IE2

IE3

SquareLoad

Figure 3. Exemplary torque over speed curves for three 4-pole, 11 kW, induction-motors with different energy-efficiency classifications and fan load curve

Note: Due to the scale of the graph and the suppression of speeds below 1000 rpm the black-curve appears to be linear although it is actually a square-load curve.

This can be a potential pitfall when the driven load-machine has a torque characteristic that increases with the square of the speed (typical for all types of fans, compressors and pumps) (see figure 4).

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Torque

[Nm]

Speed[1/min]

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IE3

SquareLoad

Figure 4. Zoomed torque over speed curves for three 4-pole, 11 kW, induction-motors with different energy-efficiency classifications and fan load curve

Note: Due to the scale of the graph and the suppression of speeds below 1450 rpm the black-curve appears to be linear although it is actually a square-load curve.

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The speed increase is depending on the frame-size, number of poles and electromagnetic design of the motor so a general rule cannot be given.

For a typical 4-pole motor series between 0,75 kW and 55 kW a speed increase of 1,1% from IE1 to IE2 and a further 0,5% increase from IE2 to IE3 has been found in average.

The higher speeds translate to an average increase in power demand of square-torque loads compared to the next lower efficiency class of 3,4% and 1,6% for IE2 and IE3 respectively.

This power increase may not seem much but is has to be related to the reduction of losses per efficiency class which is also just a small fraction of the output power.

Figures 5 and 6 give a comparison of the reduction of losses (green bars) in watt compared to the theoretical increase in load demand (red bars) when load is a square of the speed (not taking additional effects like friction or temperature into account).

The figures were derived from motor data of an actual 4-pole motor series of one manufacturer and are not universal for all types of motors or all manufacturers.

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Losses

[W]

Nominalmotoroutputpower[kW]

IE2comparedtoIE1

Lossreductionduetohigherefficiencyclass

Powerdemandincreaseduetospeedincrease

Figure 5. Exemplary reduction of losses of 4-pole IE2 motors compared to IE1 motors and increase of power demand in square torque applications (fans, pumps, compressors)

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Losses

[W]

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IE3comparedtoIE2

Lossreductionduetohigherefficiencyclass

Powerdemandincreaseduetospeedincrease

Figure 6. Exemplary reduction of losses of 4-pole IE3 motors compared to IE2 motors and increase of power demand in square torque applications (fans, pumps, compressors)

In reality, the saved energy is just the difference between the red bars and the green bars. Whenever the red bars exceed the green bars the energy-efficient motor actually consumes more energy than the lower-efficient motor. Of course it also runs faster and produces more flow. But that might not be the desired goal.

In general, the speed and output-power increase associated with the selection of motors of a higher energy efficiency class may reduce or, in some cases, actually reverse the benefit of the efficiency improvement in fan, compressor and pump applications.

A variable frequency converter can be used to reduce the speed to a lower level. But a converter is a costly device and will also add further losses to the system. For these reasons, frequency converters should not be used in full-speed, full-load applications just for the purpose of constant speed reduction alone.

Only in applications where a variable speed is beneficial anyhow and/or where longer periods of part-load operation are common, a frequency converter will be a viable alternative.

Otherwise the only solution to avoid the increased losses of higher speed motors may be to mechanically reconstruct either the fan, compressor or pump (smaller blades or impellers etc.) or to adapt the conduit system to utilize the higher speed for the same mass flow (larger cross-sections, increased turning radiuses etc.).

Interpolation of part-load losses

Most manufacturers give the efficiency for full load and three-quarter load in their catalogues.

In order to calculate the savings of an application a detailed knowledge of part-load efficiencies may be beneficial. For these cases the guide gives a simple set of formulas which can be used with good accuracy to interpolate any part-load efficiency (chapter 5.5).

The formulas are based on universal physical properties of three-phase rotary-field motors and can be used for induction- and synchronous-machines alike:

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pp L

p

L

L

++=

=

=

0

1000

75100

1

1

114375.0

1175.011

with:

100 = Efficiency at rated load (from 0...1 with 1 equals 100%) 75 = Efficiency at 3/4 load (from 0...1 with 1 equals 100%) L, 0 = Intermediate results p = Desired power (relative to rated load, i.e. from 0...1...overload) p = Resulting efficiency (from 0...1 with 1 equals 100%) Super-Premium Efficiency IE4

Already very early on in the project working group 31 decided to add a further energy efficiency class for industrial motors to the established Standard- (IE1), High- (IE2) and Premium-Efficiency (IE3) scheme.

The new class was intended to unify the historical differences of 50 Hz and 60 Hz countries and also to broaden the application range to all kinds of industrial motors including frequency converter driven motors like permanent-magnet synchronous-types.

For that reason it was decided to base the new levels on speed classes and output torque rather than on number of poles and output power.

It became clear that further harmonization would be needed and so the specifications for IE4 were moved from the classification standard IEC 60034-30 into the guide IEC 60034-31 as an informative annex. At the next revision of IEC 60034-30, when more experience is available, an updated definition of IE4 shall be pulled back into the standard.

Figure 7 gives an overview of the new limiting curves as published in the second CD of IEC 60034-30.

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Efficiency[%

]

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Figure 7. IE4-efficiency limits

The curves are a compromise of different goals.

In average, the reduction of losses of IE4 should be some 15% compared to IE3.

Practically, this could only be reached in relation to the 60 Hz IE3-curve with 16.6%, 16.5% and 16.2% difference in average losses for 2-, 4- and 6-pole machines respectively.

Due to the generally smaller size of 50 Hz IEC motors compared to 60 Hz NEMA motors and the energetic disadvantages of 50 Hz versus 60 Hz, the IE3 limits for 50 Hz motors are lower than for 60 Hz motors and so the difference to the frequency independent IE4 class must be bigger.

In the current draft the differences in losses of 50 Hz 2-, 4- and 6-pole machines of efficiency class IE4 compared to IE3 are in average 22.6%, 26.9% and 19.5% respectively.

For small motors it is typical to reach the highest efficiency at high output speeds around 3000 to 5000/min. For larger motors, the peak efficiency is reached at lower speeds. The IE4 curves model this physical characteristic in principle. They can of course never be exact as this phenomenon is depending on many individual motor design parameters, most importantly on the type of ventilation.

In order to maintain compatibility with the existing IE1, IE2 and IE3 curves and to simplify the application of the new class to conventional three-phase, cage-induction motors, additional tables are given in the standard with references to output power, grid frequency and pole-number (see figures 8 and 9).

These tables however are derived from the original torque/speed/efficiency table which remains the normative basis.

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[%]

[kW]

4pole

IE4 SuperPremiumEfficiency60HzIE4 SuperPremiumEfficiency50HzIE3 PremiumEfficiency60HzIE3 PremiumEfficiency50HzIE2 HighEfficiency60HzIE2 HighEfficiency50HzIE1 StandardEfficiency60HzIE1 StandardEfficiency50Hz

Figure 8. Efficiency limits for 4-pole cage-induction motors related to output power

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Figure 9. Efficiency limits for IE4 motors in relation to nominal motor torque and nominal speed

It must be noted that IE4 only gives efficiency limits for the nominal motor speed and nominal motor torque. If motors are rated for a speed range (and possibly a nominal torque range) then the limits of IE4 must be reached for all ratings.

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On the other hand, when a motor is operated at partial load it does not need to reach the efficiency defined in IE4 for that particular torque.

The efficiency is always tested between the motor input terminals (winding connection wires) and the mechanical output (shaft). Losses in cables and converters as well as losses in external devices like electromechanical brakes, clutches, speed encoders, external fans etc. are not taken into account for the classification according to IEC 60034-30 and -31.

The energy-efficiency classes IE1, IE2 and IE3 as defined in the current edition of IEC 60034-30 may only be applied to three-phase, cage-induction motors within the specified voltage and power range. These classes are not applicable to any other types of motors.

Only the efficiency class IE4 as defined in the current draft of IEC 60034-31 may be applied to all kinds of low-voltage electric motors within the specified nominal output torque and speed range.

Conclusion

The drafted new IEC 60034-31 application guide gives background information for the application of energy efficient motors and drive systems. It is useful for regulators, manufacturers, end-users and OEMs. While some of the material is very technical and aimed at manufacturers and OEMs (like converter losses, power factor improvements etc.) other information is also relevant for regulators and end-users.

The important topics covered include applications running under partial load and/or in part time, replacement of standard motors by energy-efficient motors in square-load type applications, additional losses introduced by electronic frequency converters, effects on power-factor, comparison of 50 Hz and 60 Hz grid frequency, starting performance, voltage unbalance and the introduction of a new efficiency class (IE4) for all types of electrical machines and not just cage-induction motors.

For a large number of different types of applications the guide gives specific tips and tricks regarding important features to look after.

References

[1] IEC 60034-30 (2008-10): Rotating electrical machines Part 30: Efficiency classes of single-speed, three-phase, cage-induction motors (IE-code)

[2] IEC 2/1554/CD (2009-04): IEC 60034-31: Rotating electrical machines Part 31: Guide for the selection and application of energy-efficient motors including variable-speed applications