Rise of the PM machines

Permanent magnet brushless synchronous motors have moved beyond traditional applications with higher efficiency than induction machines.


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.

Author Information

Frank J. Bartos, P.E., is a Control Engineering consulting editor. Reach him at braunbart@sbcglobal.net .

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
Strategic outsourcing delivers efficiency; Sleeve bearing clearance; Causes of water hammer; Improve air quality; Maintenance safety; GAMS preview
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role of plant safety and offers advice on best practices.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me