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Motor efficiency highly rated

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WHETHER in pumping, grinding, mixing, materials conveying, lifting, air conditioning, ventilation or refrigeration, most industrial applications incorporate an electric motor to convert electrical energy into motive power. In actuality, electric motor-driven applications account for one-third of Australia’s total electricity consumption and more than 10 percent of greenhouse gas emissions.

Because electric motors are used so extensively, a small increase in their operating efficiencies can lead to significant reductions in national energy usage.

In recognition of this and in aid of a national greenhouse response strategy, the Australian Greenhouse Office (AGO) created the National Appliance and Equipment Energy Efficiency Program (NAEEEP) in 1992. The NAEEEP is a set of legislated efficiency schemes used to promote the use of energy efficient products.

In October 2001, as part of the NAEEEP, the energy efficiencies of 2, 4, 6 and 8 pole electric squirrel cage induction motors were targeted through the introduction of Minimum Energy Performance Standards (MEPS).

At this time, it was legislated that three-phase electric motors ranging from 0.73 to 185 kW, manufactured in or imported into Australia, had to comply with efficiency requirements set out in the Australian standard AS/NZS 1359.5-2000 Rotating electrical machines - General requirements Part 5: Three-phase cage induction motors - High efficiency and minimum energy performance standards requirements.

The standard also listed motor efficiency obligations for the next level up in motor performance - the so-called “high-efficiency” (HE) motors.

The National Appliance and Equipment Energy Efficiency Committee (NAEEEC) proposed a revision to this standard - one that saw that the HE levels specified in AS/NZS 1359.5:2000 become the new MEPS levels from 2006 onwards (as described in AS/NZS1359.5:2004).

This more stringent proposal means that, from April 2006, all standard motors must comply with MEPS, while all HE motors must comply with a new raised HE level.

Energy is money

ALTHOUGH MEPS was introduced to promote national eco-efficiency, the improvement in energy efficiency can lead to substantial operating cost savings for the end-user.

In most cases, the upfront cost of a motor represents a fraction of the operating cost over an average 15-year life. An increase in motor efficiency of just one percent can reduce an end-user’s annual electricity costs by several hundred dollars - depending, of course, on the rated motor output, duty cycle and running time.

In order to meet or exceed the minimum efficiency levels set out in AS/NZS 1359.5:2004, most motor manufacturers are targeting a two to five percent improvement in energy efficiency. This efficiency depends on a number of factors, with larger motors being more efficient.

The typical overall conversion efficiencies of electric motors are in the realms of 80 to 96 percent, making them intrinsically effective ways of converting electrical energy.

In simple terms, energy losses that occur in electric motors can be broadly broken down into five categories: stator losses, rotor losses, friction losses, iron losses and any additional losses (Figure 1).

Stator and rotor losses are caused by the ohmic resistance in the stator windings and conductor bars of the squirrel-cage rotor. The friction losses result from the friction within the bearings of the rotor, and the air resistance of the motor’s fan. The iron losses are magnetic losses resulting from the constantly alternating magnetic field in the stator and rotor core laminations and other minor eddy current losses.

Additional losses are generally electro-mechanical losses caused by miscellaneous effects; individually they are of low importance, but massed together they can be of a noteworthy magnitude.

Evaluation and validation

IN AS/NZS1359.5:2004, motor efficiencies are defined by two test methods.

The first is Method A to Australian Standard 1359.102.3 (equivalent to the International Electrotechnical Commission (IEC) test method IEC61972), which determines additional load losses from direct measurement of torque and electric power.

The second is Method B to the Australian Standard 1359.102.1 (equivalent to IEC60034-2). This test method assumes that additional load losses (also called “stray load losses”) are equal to 0.5 percent of the rated input power, whereas it is generally known that such losses are far higher than this percentage figure.

Using each test method reveals a different value of efficiency.

For example, a 4 kW motor that demonstrates an efficiency of 87 percent when tested under Method A records an 88.3 percent efficiency when tested using Method B. Again, this is due to the two methods’ different treatment of stray load losses.

This relative merits of Test Method A and Test Method B were discussed at the NAEEC 2005 Motor Forum in Adelaide late last year, with the forum agreeing that Test Method A is the more accurate approach.

This forum included a visit and tour of the new independent CalTest Testing Laboratory. Located 80 kilometres outside of Adelaide, the CalTest Testing Laboratory is one of Australia’s two NATA-accredited electric motor testing facilities. In order for a motor to be registered for MEPS, the manufacturer or importer must be able to demonstrate that it meets the either the MEPS or High Efficiency levels set out in the Australian standard.

Check testing of the motors is undertaken by the AGO at such facilities as the CalTest. Compliance is policed by the AGO and State Electricity regulators. A full list of motors regulated and registered for MEPS can be found on the government website: www.energyrating.gov.au.

A copper solution

SEW-Eurodrive’s answer to the higher-efficiency demands of the updated AS/NZS1359.5:2004 and NAEEEP is its DTE/DVE series of motors.

Launched in 2001 to comply with the High Efficiency levels of the first phase of MEPS, the DTE/DVE motor is still compliant with the requirements of MEPS in this second phase. Its design is continually being revised for further development.

The series addresses the new efficiency requirements by featuring die-cast electrical-grade copper rotors. Because the electrical resistance of copper is about 37 percent lower than that of the same volume of aluminium, the rotors enable all energy losses to be significantly reduced without any appreciable increase in overall size.

However, because the melting point of copper is much higher than that of aluminium, higher casting temperatures, technologically advanced materials and handling are required.

Through extensive research and development, SEW-Eurodrive has addressed this copper die-casting challenge and is now able to produce the more conductive copper rotor cost-effectively. This is combined with improvements in the rotor conductor bar shape, stator laminations and winding designs. The stator iron losses have also been trimmed. By using stator lamination materials with improved magnetic properties, iron losses have been reduced.

All these modifications have succeeded in raising the efficiency over the entire load spectrum, while maintaining the overall motor frame size, starting torque and other critical points on the torque-load curve.

Rotor reduction

WHEREAS SEW-Eurodrive has chosen to revisit the entire design and material selection for its motors in order to improve energy efficiency, the conventional approach within industry has been to implement minor adjustments and modifications.

The easiest method to improve a motor’s energy efficiency is to increase the active material; this entails increasing the size of the motor. To produce a 0.8 percent improvement in energy efficiency, the frame size would typically need to be augmented by10 percent.

In contrast to this, the use of copper in the rotor cage allows reductions in rotor diameter, in iron required for laminations and in stator copper windings.

As a result, SEW-Eurodrive is one of the few manufacturers to offer a fully MEPS-compliant range of motors with frame sizes identical to legacy (non-MEPS) motors of the same rated output. Not only does this prevent the drive-to-gear interface problems commonly encountered with many high-efficiency motors, but it results in significant cost savings when retrofitting a new motor into an existing system.

However, this is only true if a philosophy of energy efficiency is applied to the design of the complete drive train, and not just that of the motor.

There can be very significant energy improvements and energy cost reductions made by looking, not only at the motor, but also at the driven application and the mechanical transmission equipment in the drive train.

For example, a 90.8 percent efficient motor coupled to a 95 percent efficient helical or bevel-gear unit will yield an overall drive efficiency of 86.3 percent. The same 90.8 percent efficient motor cascaded with a 60 percent efficient worm-gear unit will result in an overall drive efficiency of just 54 percent - an efficiency reduction of 32.3 percent! Such an efficiency improvement would result in a significant reduction in energy operating costs.

Movers and shakers

IN effect, it is not the motor that consumes energy but what it is driving. In order to maximise efficiency gains and energy cost reductions, the motor’s application needs to be accurately specified in terms of the rated output, duty cycle and function.

Cyclic applications with continual start/stops - such as those encountered in materials handling - consume less energy when controlled by low-inertia motors, rather than high-efficiency motors. This also results in reduced brake wear.

Air-conditioning or heating applications, by contrast, demonstrate a constant demand, ensuring that high-efficiency motors running at or above 75 percent load can yield significant energy cost savings.

As such, the motor needs to be viewed in the context of the entire facility and its environmental targets.

The energy-efficiency of motors is an area where Australian legislation is leading the technology and driving progress forward. What’s more, as Australia is heavily reliant on internationally manufactured motors, the country is essentially setting the pace for motor technology on a global level.

* Commentary by Frank Cerra, engineering manager, SEW-Eurodrive

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