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The Latest Motor Efficiency Rule Hits June 1, 2016 – Now What?
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 04/18/2016
Learn about what the rulings mean and what motor technologies provide a competitive solution.
If you buy a continuous-duty induction motor after June 1, 2016 don’t be surprised if it looks different than earlier versions. That’s the day the Integral Horsepower Amended Motor Rule of the US Department of Energy (DOE) goes into effect. Amid ongoing concerns about energy independence and greenhouse gas emissions, governments around the globe have been making strategic efforts to increase motor efficiency. Considering that electric motors account for about 38.4% of total electricity consumption in the United States, and motor-driven machinery consumes almost two thirds of the energy used by the industrial sector, an efficiency improvement of even a few percent can have a dramatic impact. As a result, efficiency regulation has been an ongoing process that’s become increasingly strict.
The amended Integral Horsepower Motor rule applies efficiency standards more comprehensively than ever before to continuous-duty AC induction motors manufactured for sale in the United States. Induction motors that meet specification are likely to be larger, longer, heavier, and probably more expensive than their predecessors. On the upside, reduced energy consumption by the improved motors will substantially cut operating costs. The result is faster ROI and significant savings over the motor lifetime, which can last multiple decades. On the downside, they may no longer fit the mounts and housings of pre-existing systems and designs.
Nobody would dispute that AC induction motors are the workhorses of industry. They’re economical, tough, and reliable. Indeed, an audit of more than 4000 motors in Swiss factories revealed that the average age was more than twice the rated lifetime. Still, when it comes to applications like fans, pumps, compressors, and conveyor belts – traditionally the sweet spot for AC induction motors – there’s a more nuanced conversation to be had. Particularly in the context of the form, fit, and function changes introduced by the IHP rule, permanent magnet (PM) motors and other technologies may provide a better all-around solution.
The rule going into force on June 1, 2016 is just the latest advance in an ongoing process. It began in 1992 with the Electric Horsepower Rule, established by the US Energy Policy Act . Per the Electric Horsepower Rule, general-purpose polyphase AC induction motors from 1 to 200 hp had to meet the National Electrical Manufacturers Association (NEMA) Energy Efficient level (standard MG 1, table 12-11). The Energy Independence and Security Act (EISA) of 2007, followed. Effective December 2010, EISA raised the bar for general-purpose 1-to-200 hp motors to the stricter NEMA Premium Efficiency level (MG 1, table 12-12). It also applied the Energy Efficient standard to general-purpose 201-to-500 hp motors and a selection of formerly exempt 1-to-200 hp motors.
In 2010, NEMA and a group of other industry stakeholders formed the Motor Coalition to provide input into a strategy for the DOE as it began to consider the next round of updates. Rather than defining a new efficiency level, the coalition advocated a strategy based on closing EISA loopholes and increasing both the number of motors covered by the existing standards and the efficiency level to which they were held. The result is the Integral Horsepower Motor Amended Rule. Finalized in May 2014, it requires almost all single-speed induction motors from 1 to 500 hp to perform at Premium Efficiency level; in addition, a small number of previously exempt motor types would be held to the Energy Efficient standard. The DOE estimates savings from products sold between 2016 and 2045 to be on the order of 2.05 x 10E12 kWh compared to non-compliant models, or the equivalent of around 395 million metric tons of carbon emissions.
These types of rules are rolled out in stages. Manufacturers had two years between the release of the final rule and the effective date of June 1, 2016 to modify their designs and production facilities. As of the effective date, they can no longer manufacture noncompliant motors for use in the United States. Any units already in inventory, however, can be sold without penalty. It’s important to note that compliance with the standard rests with the motor maker manufacturers and not with the end-user.
Another key DOE efficiency standard is the Small Motor Rule, which holds certain continuous-duty, small-form-factor (NEMA 48, 52 and 56) motors to DOE-defined efficiencies (versus NEMA levels). Effective March 2015, it applies to motors across the range from ¼ to 3 hp. Its scope remains limited at present but expect that to broaden over time.
So far, we’ve been focusing on the US standards but a similar process is taking place in countries around the globe (see Figure 1). In particular, The International Electrotechnical Commission in 2008 released IEC 60034-30. This defined a set of efficiency levels specified a set of efficiency levels that are essentially equivalent to NEMA levels: IE1, IE2 (Energy Efficient), and IE3 (Premium Efficiency).
Cutting your losses
Let’s take a closer look at the efficiency limitations for AC induction motors. Contributors to loss include resistive losses from windings on the rotor and stator, eddy currents in the laminations that make up the rotor and stator, and hysteresis in the laminations. Winding resistance is a significant loss mechanism for a motor operating at full speed and in part explains why larger motors are inherently more efficient than smaller motors. “The size of the motor really determines the efficiency," says John Petro of Petro & Associates, a consultancy specializing in electric motor design and applications. "If you have a small motor, it’s hard to get 90% efficiency out of it. If you have a large motor it’s hard not to get 90% out of it.” Resistance of a wire goes as the inverse of the cross-sectional area. The larger the space for the windings, the larger the diameter of wire that can be used and the greater the efficiency. “If you’ve got a small motor, it’s hard to get those amp turns in there because you don’t have space for wire,” he adds. “But if you have a large motor, there’s still the same amp turns but now you’ve got space for a large wire. So my resistance is lower, therefore my efficiency is higher.”
Efforts to boost the efficiency of AC induction motors have taken place on multiple fronts. Manufacturers have increased the amount of copper in the windings, swapped out aluminum rotors for copper, experimented with cast rotors and thinner laminations, and used higher quality steel or specialized high resistivity alloys for the laminations. All of this has not only supported the production of Premium Efficiency/IE3 motors but has enabled select manufacturers to build Super Premium/IE4 and IE5 motors.
It’s easy to see how these changes have added weight and size, not to mention cost, to the higher efficiency motors. The changes have also increased motor speed. All induction motors have some degree of slip, which is the amount by which the magnetic field of the rotor lags the magnetic field of the stator. The attraction between the two magnetic fields causes the rotor to “chase” the stator, enabling the motor to develop torque. Increasing motor efficiency decreases the amount of slip. An Energy Efficient two-pole motor might operate at 3425 RPM versus 3580 RPM for a Premium Efficiency version. Depending on the load, swapping a Premium Efficiency motor in place of an Energy Efficient motor may require OEMs and end-users to modify not just the mounting but potentially the horsepower requirements of device. Another option exists – choosing a different motor technology that can deliver the performance needed in a form factor that is as small as or smaller than Energy Efficient models.
PM motors eliminate the need for windings on the rotor, eliminating the bulk of resistive rotor losses. As with all things in engineering, there are trade-offs, however. PM motors require drives. In the past, this has added to the price of the solution compared to fixed-speed induction motors. With the introduction of cheaper drives, however, that is becoming less of a concern.
Another challenge is that drives reduce the efficiency of the overall PM motor system. That said, it is difficult to operate a PM motor without a drive, so any efficiencies measured for a motor system already include losses introduced by the drive. That should be taken into account in a comparison with an induction motor. Particularly when the induction motor is run by a variable-speed drive (VSD), the quality of the drive signal plays a key role in the outcome. Adding a drive cuts the efficiency of an induction motor. Induction motors are designed to run off of a clean sine wave. Drives don’t produce pure sine waves, however, and not all drives are created equal. Any meaningful efficiency comparison between two motor-drive systems needs to take this into account to ensure parity.
“I can put three different drives on a permanent magnet motor and I will get three different efficiencies for those motors,” says Petro. “If I take those same three drives and put them on an induction motor, I’ll get three other motor efficiencies. Again, the motor efficiency is dependent on the kind of signal the drive is putting out.”
One sweet spot for PM motors is for fan applications requiring lower speed operation. Consider a fan operating at 900 RPM. The AC induction motor solutions would include direct drive with an eight-pole motor operating nominally at 900 RPM, adjusting the speed of a four-pole motor (1800 RPM) with a variable-speed drive (VSD), or adjusting the speed of that same motor mechanically by a system of belts.
High-pole-count induction motors are expensive, and depending on the frame size, they may not operate well. Adding a VSD to the system cuts the efficiency by a percent or two. Operating the motor at lower speeds also magnifies the effect of slip. For a four-pole induction motor operating at top speed, 50 RPM of slip represents about 2.7% of loss, but at 900 RPM, that number doubles. At 500 RPM, the slip represents a 10% loss. “You still need that 50 RPM of slip to generate the torque," says Alan Crapo, chief technology officer at NovaTorque (Freemont, California). "The efficiency just plummets when you go to lower and lower speeds, whereas permanent magnet motors in general do not have that issue.” Even at their best, belts might operate at around 90% efficiency; worn or improperly adjusted belts can drop efficiency down to 85% or even 75%. A brushless PM motor can offer a more efficient alternative.
PM motors have higher price tags than induction motors as a result of the cost of the magnets and the drive. In motion control applications, which are sensitive to performance and form factor, the benefits of rare-earth magnets justify the price premium. For continuous-duty applications like pumps and fans, non-rare-earth PM motors provide a more practical alternative (see Figure 2).
Skyrocketing prices during the rare-earth bubble prompted the search for ways to economically achieve that performance at a lower cost. That led to the development of three-dimensional flux-focusing designs that deliver efficiencies as high as 95% from ferrite magnets and achieve ROI in 12 to 18 months. Unlike conventional axial flux motors, which typically have a rotor on the inside and a stator on the outside (or vice versa), the focused flux design consists of a pair of conical rotors, one at each end of the motor. The flux return path passes through the two rotors. There is no flux return path in the stator, which helps reduce stator losses. The conical air gap concentrates the magnetic field to give a flux density comparable to that of a rare-earth PM motor.
Because they are based on a PM design, the motors can deliver constant torque over a very wide speed range. Their 210-frame motor functions as a 10 hp motor at 1800 RPM but operates as a 20 hp motor at 3600 RPM, for example. Meanwhile, the conical air gap enables it to achieve very high efficiency, not just for the motor drive but for the system as a whole. “I think that really is an important thing to look at, and you can be misled by looking at just motor only numbers or drive numbers because it’s really the application,” says Crapo. “If you are already using a variable-speed drive, it’s simple to try it out with one of our motors.”
The synchronous reluctance motor is another alternative to continuous-duty induction motors. Although the stator is designed and constructed similarly to that of an induction motor, that is where the comparison ends. Unlike the rotor of a conventional induction motor, which generates magnetic poles by running current through the windings, the rotor of a synchronous reluctance motor does not include a conductor. Instead, it consists of a stack of laminations structured to create a specific magnetic flux pattern. The interaction between that flux and the magnetic field distribution of the stator causes the rotor to turn, generating torque.
On the downside, it requires a drive, adding cost and complexity. On the upside, the rotor design cuts losses dramatically, allowing synchronous reluctance motors to reach efficiency levels of IE4 and beyond.
The focus on energy independence and climate change will continue, as will efforts to increase efficiency. Permanent magnet motors and alternative technologies like synchronous reluctance motors may ultimately provide a far more effective solution to minimize electricity consumption. For those reasons and most especially because every virtually every manufacturing organization needs continuous-duty motors, the motion industry needs to pay attention to the combination of opportunity and responsibility offered by this application area.
Thanks go to John Malinowski, senior manager of industry affairs, Baldor Electric Co. and past chairman NEMA Motor & Generator Section for background information.
- Premium Efficiency Motor Selection And Application Guide, US Department of Energy, DOE/GO-102014-4107, February 2014. A detailed, quantitative overview of the category.
- The Impact of the Integral Horsepower Amended Rule, NEMA, April 2014. A summary of what the rule means for users, which motors qualify and which do not.
- Swiss motor efficiency program EASY: results 2010 – 2014, Rita Werle, Conrad U. Brunner, Rolf Tieben, Impact Energy Inc., Switzerland ABSTRACT, ©2015 ACEEE Summer Study on Energy Efficiency in Industry
- Checking in on the Rare-Earth Magnet Market