Towards 2006 - a new age of motor efficiency
New regulatory requirements for induction motors are in the pipeline and promise to raise the bar on motor minimum energy performance standards (MEPS), according to SEW Eurodrive's Frank Cerra. The three-phase squirrel cage induction motor is the µworkhorse' of modern Australian industry. Simple and robust, the induction motor provides the motive power for some 2 million applications across the country. Not surprisingly, they are responsible for drawing around 30 per cent of Australia's total electricity demand.
Viewed from an environmental perspective, this electric motor energy consumption is a significant source of greenhouse gas emissions. Electricity generation is Australia's most greenhouse gas-intensive energy source. As a result, electric motors account for almost the same annual level of equivalent greenhouse gas emissions as that created by all the motor vehicles on Australian roads.
It is for this reason that the Australian government, via its agency the Australian Greenhouse Office, embarked on a program in the mid-1990s to realise an improvement in the energy efficiency of electric motors used in industrial and commercial applications. The first stage of this program was completed in October 2001 when a range of state and territory legislation was introduced stipulating minimum energy performance standards (MEPS) for 2, 4, 6 and 8 pole electric squirrel cage induction motors ranging in powers from 0.73 to 185 kW.
The MEPS motor efficiency levels are described 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.
MEPS up a notch
The current standard also lists motor efficiency levels for the next level up in motor performance - the so-called µhigh efficiency' motors. While motor manufacturers are not now obliged to meet this level in their µstandard' motor offerings, this situation may change if a recent proposal to increase the stringency of the MEPS is implemented.
In December 2002, the National Appliance and Equipment Energy Efficiency Committee (NAEEEC) put forward a report, proposing that the current high efficiency levels in AS/NZS 1359.5 effectively become the new MEPS levels from 2006 onwards. In effect, the proposal means that all standard motors in the future would rely on the same technology that underlies the high efficiency motors of today.
For a number of quality motor manufacturers, the introduction of the October 2001 first phase regulations represented few challenges. The existing standard motor range was well engineered and readily compliant with the AS/NZS 1359.5 MEPS efficiency demands. If, however, the NAEEEC proposal were implemented, the new 2006 requirements would raise the MEPS bar a deal higher, demanding motor manufacturers closely review and rework their offerings.
To address this challenge, a clear understanding of the various components of the energy loss that occur in electric motors must be had. This is broadly broken down into five categories:
- I2R stator losses: caused by the ohmic resistance in the stator windings.
- I2R rotor losses: caused by the ohmic resistance of the conductor bars of the squirrel cage rotor.
- Friction losses: resulting from the friction within the bearings of the rotor, and the air resistance of the motor's fan.
- Core losses: the magnetic losses resulting from the constantly changing magnetic field in the stator, and to a lesser extent, in the rotor.
- Additional losses: a range of electro-mechanical losses caused by miscellaneous effects mostly related to quality of motor manufacture. Individually they are of low importance, but massed together they are of a significant magnitude.
How this last category is measured is at the heart of an international motor comparison µheadache'. In Australia, two test methods are stipulated for measuring motor losses: Method A (detailed in AS/NZS 1359.102.3), which determines additional load losses from direct measurement of torque and electric power; and Method B (detailed in AS/NZS 1359.102.1, which is equivalent to IEC60034-2), which makes no determination of additional load losses and simply provides for a fixed allowance of 0.5 per cent.
The challenge for Australian motor specifiers and buyers is that the test methods used in other countries vary dramatically. In Japan, the additional losses are ignored completely, whereas in Europe, the specified technique is equivalent to Australian Method B. In the US, the stipulated test method (IEEE 112-B) is similar to Australian test Method A and the new IEC61972. In the long term, IEC countries are expected to move towards this standard. The tolerances applied to the specified losses vary dramatically from country to country, and the impact of the supply frequency used during the test on the final test results complicates things further.
The bottom line is that motor efficiencies - particularly the diverse range of motors seen in Australia - aren't usually readily comparable by just looking at the motor percentage efficiencies. Motor efficiencies should only be compared when you can be absolutely certain that both motors were tested using the same test method, or some adjustment that recognises the percentage differences between the test methods needs to be considered. Most importantly, it should be clear in standard specifications for motors, and OEM-supplied equipment incorporating motors, that Australian Standard MEPS requirements need to be met using Australian Standard test methods.
Moving to high efficiency
From a practical perspective, a number of options are open for motor manufacturers to realise the high efficiency performance motor. Reducing the ohmic losses in the stator is most promising. This can be accomplished through increasing the stator conductor cross-section, resulting in a reduced winding resistance (and resultant reduced I2R losses).
It can also be accomplished through reduced motor utilisation (via motor de-rating), which results in less current draw. The second method provides greater loss reduction, but has an obvious disadvantage: the motor cost increases significantly, as does the frame size of the motor. In the case of gear motors, there is a further disadvantage - the motor/gear unit mechanical interface can be changed dramatically with the change in motor frame size.
Another common method for reducing losses is to incorporate low magnetic loss steels in the motor construction. Unfortunately, this can be expensive, and also results in an increase in no-load currents and poorer power factor. This actually results in reduced energy efficiency under partial-load operation.
A technique for realising improved motor efficiency that has long been understood is to improve the conductivity of the rotor itself, thereby reducing the total I2R losses. Using high-conductivity copper in place of aluminium as the material of construction for the rotor cage has long been the obvious choice, but to date, practical limitations of copper casting technologies have prevented this from being economically viable.
Intense research in this area has resulted in the launch of a new series of motors such as the DTE/DVE series, which features cast-copper rotors. These not only exceed current Australian Standard high efficiency motor performance requirements, they provide some of the highest motor power densities available in Australia today. Most importantly, they also achieve this in a motor of the same frame size as the existing MEPS-compliant range (the DT/DV series). As a result, the drive-to-gear interface problems commonly encountered with many high efficiency motors are avoided.
The cast-copper rotor motors also provide improved motor performance, as a result of the rotor's reduced slip characteristic. The motors' torque/speed curves exhibit a stiffer characteristic, ensuring far greater speed stability under varying torque conditions and improved performance in inverter applications.
Total drive system perspective
Regardless of the method used by the motor manufacturer to achieve the new standard, the high efficiency motor is heavier, in many cases larger, and more expensive than conventional MEPS motors. As a result, the high efficiency motor does not provide equal energy savings for all applications.
Optimal energy savings can be achieved in applications involving long periods of continuous operation and load levels of 75 per cent or greater. In high stop-start applications the high efficiency motors are particularly hard to justify on an energy savings basis, due to the extra energy required to accelerate the higher-inertia rotor from a standing start. In such applications, a high efficiency motor might actually use more energy than a standard motor. Here, it is far more important to select a motor that provides optimum dynamic response and requires minimal start-up energy. For example, in the case of brake motors, a lower rotor inertia motor will result in reduced brake wear, a more rapid stop and shorter stopping distances.
High-inertia load applications, such as gantry crane travel drives, are also an interesting case in point. Here, a great deal of torque is needed to get the load up and running, but once running, little power is required and the motor will typically operate at less than 50 per cent load. Once again, very little energy savings can be achieved using a high efficiency motor in such an application.
In reality, a much broader perspective is required to realise optimal energy savings in drive systems. True drive system efficiency can only really be achieved by viewing the entire drive system from end to end: from the driven load itself back to the three-phase supply.
Inefficiencies closest to the load itself can be magnified through the entire chain, so it is important to ensure optimal efficiency at the driven load end as a first priority - most significantly the load itself, and the gear unit driving the load. As an example, there is little to be gained in energy savings by applying a high efficiency motor to an application that is currently using a 50 per cent efficient gear unit. Substantial energy savings can also be realised by effective and judicious application of variable speed drive and soft starter controls.
Preparing for 2006
Clearly, the proposed second phase of the Australian Greenhouse Office's electric motor efficiency strategy could present a number of challenges to Australian motor suppliers and users - challenges beyond those experienced during the first phase. As Australia's motors are sourced from many countries and suppliers, great care would be needed in specifying the required motor efficiency performance and the testing standards used, at the time of purchase.
Most important is the issue of mechanical interface and physical size of the new high efficiency motors. Unfortunately, many motor manufacturers are likely to opt for lesser motor design techniques to realise high efficiency performance. Clearly, this would lead to motors that are physically larger than today's MEPS motors. If care isn't taken at the motor specification stage, this could result in a range of practical drive-to-gear unit interface problems, dynamic performance problems, along with challenges to fitting motors to machines and applications. With careful planning and detailed review of the new standards, such possible post-2006 problems can be avoided.
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