For buyers of plant and machinery who appreciate that the cost of ownership is more significant than the purchase price, energy efficiency is a very important factor. Alistair Rae reports on the European Minimum Energy Performance Standard scheme for electric motors, and highlights some of the most efficient motors currently available.
According to the IEC (International Electrotechnical Commission), industry accounts for approximately 42 per cent of the world's consumption of electricity, with around two-thirds of this used by electric motors. Given the need to reduce energy consumption, designers are starting to consider motors that are even more efficient than the units built to comply with current legislation.
The efficiency of an electric motor is defined as the ratio of usable shaft power to electric input power. International standard IEC 60034-30, Rotating electrical machines - Part 30: Efficiency classes of single-speed, three-phase, cage-induction motors (IE-code), lists three motor efficiency classes: IE1 is the base standard, IE2 stands for high efficiency and IE3 for premium efficiency. The standard also mentions a future level above IE3 to be called IE4 super premium efficiency. Even though the standard does not yet provide the full technical specifications, a few manufacturers are introducing IE4 motors.
As of 16 June 2011, only motors that meet or exceed the IE2 level are permitted to be sold and installed in the EU. From January 2015 all motors will need to reach the IE3 level (or IE2 motors can be used if they are controlled by variable-speed drives). The EU Meps (Minimum Energy Performance Standard) scheme, which mandates compliance with the IEC 60034-30 energy-efficiency classes, covers most two-, four- and six-pole motors rated from 0.75 to 375kW for power supplies at 50 and 60Hz.
It is predicted that some 30 million existing industrial motors in Europe alone will gradually be replaced under the Meps scheme, resulting in energy savings in the order of 5.5 billion kilowatt-hours of electricity each year and a corresponding reduction in carbon dioxide emissions of 3.4 million tonnes.
Before looking at IE4 motors, it is worth reviewing how IE2 and IE3 motors differ from IE1 motors. Dr Sebastiao Lauro Nau is responsible for motor research and development at WEG. He explains that IE2 and IE3 motors use low-loss grades of steel for the laminations and more active material - such as steel laminations, copper and aluminium - to reduce the total losses. These lower losses increase energy efficiency and reduce the temperature rise in the motor, which also extends its operating life. Others performance characteristics remain the same as for IE1 motors.
Nau comments: "The efficiency performance of IE3 motors combined with variable-speed drives is better than that of IE2 motors with the same drive. The type and form of windings for IE2 and IE3 motors are the same; the difference is the amount of active material including steel lamination, copper and aluminium. What distinguishes IE3 motors from IE2 is the higher quality and quantity of the materials that compound the core of the motor.
"IE4 motors available today have a new technology. They are Permanent Magnet Synchronous Motors (PMSM). The high-energy rare earth permanent magnets are quite expensive; that makes the motor more expensive, despite having less active material - in this case only copper and steel lamination - than IE3 motors. PMSM requires a drive to start and control the speed. They are not able to start directly from the grid. Another IE4 motor type is the line-start permanent magnet (PM) motor. In this case it is possible to start it directly from the grid but its size and weight is comparable to IE2.
IE3 motors are normally heavier and physically bigger than IE1 motors, but Nau says that IE4 permanent magnet motors (except line-start types) will be smaller and lighter than IE3 motors. He adds: "If we intend to reach the IE4 efficiency with induction motors, probably most of them will be little heavier than IE3. We are working towards having an IE4 induction motor line in the near future. This is possible for motors of 15kW upwards using low-loss materials by redesigning the core lamination."
Energy losses are mainly attributable to heat caused by electrical resistance in the coil winding, losses in the rotor bars and slip rings, losses due to magnetising of the iron core, and losses from friction in the bearings. WEG claims it has found a way to limit motor temperature rise at low speeds by using its inverter-based Optimal Flux Technology.
WEG's WQuattro super premium efficiency motors employ a hybrid design to exceed the requirements of the IE4 classification. These motors integrate a conventional three-phase distributed winding with a rotor featuring an aluminium cage and internal high-energy magnets. This combination makes WQuattro motors suitable for direct-on-line starting and acceleration up to synchronous speed. If required, WQuattro motors can be used with inverters to give an extended speed range with constant torque.
Customers upgrading motors on existing designs or equipment already in service benefit from the fact that WQuattro motors employ the same frame size as standard induction motors with the same output. WQuattro motors are available in four- and six-pole versions, with frame sizes from 80 to 132S and outputs of 0.37kW to 7.5kW. The motors feature class F insulation and are suitable for operation on 230/400, 400/690 or 525V supplies.
Configured for maximum efficiency, ABB's new motor and drive packages are claimed to have energy losses that are 40per cent lower than for conventional motors. Åke Andersson, the technical manager of ABB Low Voltage Motors, states: "Variable-speed drives save customers a lot of money since they reduce the energy used by the motor. Running a pump or a fan at 80 per cent speed can reduce the energy consumption to one half compared to running at full speed. The low bearing operating temperature extends bearing lifetime while reducing greasing intervals. This brings down maintenance costs and improves reliability."
SEW-Eurodrive has created the IE4-rated DRU motor as part of its modular DR family that can be configured with a choice of integral brakes and encoders. DRU motors will initially be offered only as four-pole units in frame sizes from 71 to 100 and power ratings of 0.18 to 2.2kW, with operation from 50Hz supplies. Line-start versions feature permanent magnets installed within the cage.
Siemens displayed a motor concept for the future at the 2011 Hanover Fair. Michael Müller of Siemens motor development, states: "Our version of the IE4 asynchronous motor is a further refinement of the IE3 motor. We improved the manufacturing technology for the motor by using more energy-efficient and low-loss materials. This corresponds to an efficiency rate of 94 per cent."
According to Geoff Spear of Lenze UK, the jump from IE1 to IE2 was not big; running currents fell a little, starting currents tended to rise and a small number of customers had to review their contactors and fusing. In most cases frame sizes remained unchanged, though motor lengths sometimes increased slightly. He comments: "The step up to IE3, however, will not be so easy, as we anticipate problems with increasing dimensions. We see the take up of IE3 motors as being slow, with little demand at this time - only a few big cases like airports. Yes there will be a minority of customers who want the highest levels of efficiency but also need variable speed."
IE4 motors will, like many other energy-saving measures, require a higher initial investment. Whether or not the extra cost is worthwhile will depend on the predicted service life and usage pattern.
Given that IE3 motors are larger than similarly-rated IE1 and IE2 motors, while some IE4 motors may be no larger than IE2 motors, some designers may find it preferable to opt for IE4 rather than IE3 motors. It is too soon to tell how the market will react to IE4 motors, but those manufacturers that are already offering compliant products have perhaps made a wise decision