Motion efficiency is bolstered with newer technologies: Motor Summit 2014
Motion control: Efficiency improvements target electric motors, including developments in different topologies and innovative use of magnet materials, providing highly efficient advanced motor designs. A previous article about Motor Summit (Nov/Dec 2014) focused on trends to raise the efficiency of electrically driven systems that extend beyond the motors.
Three-phase, squirrel-cage asynchronous (or induction) motors have led the way to dramatic energy-efficiency improvements over the past 20 years. The gains were enabled by improved designs—such as use of more electrically active materials and motor circuit advancements—coupled with standardized testing and enactment of mandatory minimum energy performance standards (MEPS). Highly efficient advanced motor designs were among discussions at this 5th Motor Summit, hosted by European and international energy associations and co-hosted by the National Electric Manufacturers Association (NEMA) of the U.S. The event drew 180 participants to Zurich, Switzerland, during Oct. 7-9, 2014 (Figure 1).
It was no accident that ac induction motors were targeted for efficiency improvements as they represent a huge installed base of units worldwide. Rugged, relatively simple construction, and low production cost through large sales volume has made induction machines the industrial "workhorse" of electric motors. Energy requirements of motor-driven systems represent about 46% of total electric power generated globally.
One concise view of electric motor efficiency comes from the IE-class designation, defined in International Electrotechnical Commission (IEC) standard 60034-30 "International Efficiency (IE) Classes." The standard defines increasingly higher efficiency classes IE1 (standard) to IE4 (super premium) along with required nominal efficiency values for 0.12- to 1,000 kW motor sizes. Provision is made for an IE5-class (ultra premium). IE5-class motors are not yet rigorously defined, but proposed as able to reduce motor losses by 20% compared to IE4 class.
So far MEPS have become mandatory only in a select number of countries and only to the IE3 level. The U.S. has led the MEPS effort, with IE3 as the required efficiency level for new installations for some time. Canada and Mexico, among others, have followed, and other nations are heading toward adoption. The European Union has implemented the second of three stages toward IE3 parity as of Jan. 1, 2015, but will not reach full compliance for the all motor sizes until January 2017 (Ref. 1). Motors covered under U.S. regulations currently include virtually all categories of polyphase continuous-duty motors from 1 to 500 hp (0.75
to 375 kW).
As a reference point, an IE3-class motor translates approximately to NEMA Premium efficiency level as applied in the U.S. For example, a 200 hp (150 kW), 4-pole 60 Hz NEMA Premium motor has a required nominal efficiency of 96.2% at full load. A comparable 160 kW, 4-pole 50 Hz IEC motor must provide 95.8% nominal efficiency to be IE3 class.
It should be noted that as IE3 motors become mandatory in more countries, lower efficiency motors cannot legally enter the marketplace. Yet the overall transition has been slow; older, less efficient motors in use still comprise large numbers worldwide (see more at Ref. 1).
Motor manufacturers have kept ahead of mandatory efficiency standards, and today's induction motor designs can meet requirements up to IE4. However, it can be a stretch for some smaller motor sizes and IEC frame sizes. One interesting development for meeting IE4 requirements is the die-cast copper rotor design used in select motor models from SEW Eurodrive and Siemens, among others. The various approaches used to bring induction motors to IE4 efficiency level raise their initial cost as well.
IE5 and beyond
IE4-class induction motors have demonstrated very high efficiencies, especially in larger power sizes. Still, motor developers and manufacturers continue to look ahead to further efficiency gains, even in the face of diminishing returns beyond existing levels. This is where topologies other than the venerable induction motor come into play.
Prime alternative topologies include synchronous reluctance (SynR) and permanent magnet (PM) ac motor types. "Alternative" topology—with the latest design refinements applied—rather than "new" topology is the proper perspective, because most electric motor types have historic origins. Alternative motor designs seek to eliminate the substantial rotor I²R heating losses inherent to the induction motor. Instead, the SynR design uses a rotor with multiple salient poles to optimize magnetic flux direction and produce variable reluctance as the rotor turns. And as the name implies, the PM rotor design incorporates permanent magnets either on its outer surface or internally. Both the SynR and PM motors have a stator structure similar to an induction machine.
A further differentiator of the alternative motors is that they can't run directly from the line. That is, they require electronic control. (Actually, a "line-start" variant of the PM motor exists, but this hybrid design is not covered here.) The need for an electronic drive poses a slight efficiency setback for a SynR or standard PM motor and drive "system," compared to an induction motor that can run directly on line. However, a similar drive efficiency factor applies to the growing number of induction motors controlled by variable-speed drives.
John Petro, consultant at California-based JPEAM LLC, included the above motor topologies in the presentation "Next horizon for efficiency in electric motors" (Ref. 2). He cited examples of commercial and prototype products with IE5 performance available from ABB, Hitachi, and NovaTorque.
ABB's product, named the SynRM², is a synchronous reluctance motor offering energy losses reduced by 20% from the IE4 level. It was introduced in 2014 to target applications in the 1 to 15 kW power and 1,000 to 4,000 rpm speed ranges.
NovaTorque offers ferrite PM motors featuring innovative geometry that maximizes the surface area of the rotor and stator interface. The design enhances magnetic flux development across the interface and is said to obtain efficiency values exceeding the proposed IE5 level-even with the lower cost magnets.
Also in 2014, Hitachi developed an IE5 prototype motor with an amorphous metal core (stator). The 11 kW-rated motor reportedly obtains 96% energy efficiency at full load and reduces motor losses 30% below IE4 class (see Figure 2). Product commercialization is expected in 2015.
Importantly, all three of the above motor examples use ferrite magnets to obtain their high performance. Ferrite magnets are seeing renewed development effort to make the material more competitive with higher strength but much costlier rare-earth magnets.
"In addition, there is significant work being conducted in Japan, the U.S., Europe, and likely China, on making motors that meet or exceed the IE5 standard," Petro said. "Motors beyond IE5 are in development, with cost reduction being the ultimate goal." Moreover, Petro noted the need to define still higher IE6, IE7, and IE8 efficiency classes. "Having these levels gives manufacturers targets to aim for and allows leading manufacturers to differentiate their products in the marketplace," he added.
Figure 3 shows these extended IE levels. Note that an expected 20% loss reduction per each energy class beyond IE5 would require a further 49% cut in motor losses. That's quite an ambitious forecast but a worthy target.