Rise of the PM machines
Permanent magnet brushless synchronous motors have moved beyond traditional applications with higher efficiency than induction machines.
Frank J. Bartos, P.E. -- Control Engineering, 10/1/2009
No relation to the Terminator 3 saga, this “rise” refers to the expanding applications and growing physical sizes for today’s permanent magnet (PM) machines—motors, generators, and other devices. Times have changed the notion that PM machines comprise mainly small ac (and dc) motors.
PM brushless synchronous motors have moved well beyond machine tool, CNC, and related traditional applications. Even here sizes have grown, for example, up to 220 kW rated power from Fanuc Ltd. Newer PM machine applications include general-purpose industrial processes, electric hybrid vehicles, magnetic (variable-speed) drives, and generators for giant wind turbines. The objective is high efficiency by cutting rotor losses compared to induction machines.
One notable general-purpose product development is Baldor Electric’s RPM AC line of PM synchronous motors. The design combines a laminated steel frame stator and a salient-pole, interior PM (IPM) rotor for power density and efficiency (see Online Ref. 1). Rich Schaefer, marketing manager for variable speed and specialty motors, explains that RPM AC series ratings are now available up to 700 hp (522 kW) in standard air-cooled version. Still higher ratings apply to water-cooled units, such as a gearmotor developed for the drilling industry, rated at 1,020 hp and able to produce 35,000 lb-ft (47,460 N-m) torque in a very compact space, Schaefer notes.
Indicative of further power growth ahead is a development for electrically driven gas compressors from GE Global Research. GEGR described a 6 MW, high-speed drive motor prototype at a March 2009 workshop staged by the U.S. Dept. of Energy and others (see Online Ref. 2). Efficiency and other benefits make PM machine topology the preferred choice above 15,000 rpm speeds, says GEGR.
Material issues
PM machines rely on so-called rare-earth magnet materials—typically neodymium-iron-boron (Nd-Fe-B)—for high torque production. Wider applications mean more demand for magnets whether used in small amounts for high volume products or in large quantities for fewer big machines. The latter translates to hundreds of pounds of magnets per unit, even with IPM rotor design that reduces magnet weight needed for a given torque output. IPM design generates an added reluctance torque component.
Synchronous PM generators for multi-megawatt wind turbines also pose large magnet demands. “In a high-speed generator, magnet weight is in the 150-200 kg (330-440 lb) range,” says Anders Troedson, vice president of The Switch Controls & Converters Inc., a manufacturer of large generators. However, magnet weight skyrockets for slow-speed, direct-drive generators due to the large diameter machine and number of poles needed for torque production (see Online Ref. 3). “The lower the shaft speed the higher the torque required for the same output power,” states Troedson. “Mass of magnets in a large direct-drive generator exceeds two metric tons.” That’s more than 4,400 lb!
Growth of PM machine applications has prompted initial concern that a supply-demand crisis may be coming for rare-earth magnets. Experts see a less critical scenario. “We are not running out of raw materials or production capacity,” notes Troedson. He thinks increased demand will spur manufacturing competition so that magnet prices are not likely to accelerate.
“There is an abundance of rare-earth magnet raw material. The need is to ensure consistent production quality and careful supplier selection,” adds Baldor’s Schaefer.
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