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How to Push Motor Efficiency

March 12, 2012
Have Induction Motors Reached the End of Their Efficiency Improvement Road?

We have a mixed customer base with regard to preferences for permanent magnet motors or standard induction motors. Have induction motors reached the end of their efficiency improvement road? But we still periodically hear about the potential for magnet material shortages. We'd like to standardize where we can. Anyone else having thoughts about the direction to take?

—From January '12 Control Design


AC Motor Efficiency Improvement
Today's ac premium efficient industrial and commercial motors, which are typically manufactured with premium-grade electrical steel, additional copper in the windings and aluminum cast rotors, have reached a point of diminishing returns. Higher efficiency levels are possible, but incremental cost vs. energy savings makes a reasonable payback period much more difficult to justify.

Copper cast rotors vs. conventional aluminum cast rotors are an excellent next step to higher-efficiency induction motor designs. Copper cast rotors are more expensive to manufacture and currently are produced only in relatively low volume compared with their aluminum rotor counterparts. Higher-efficiency copper cast rotor designs are used today mainly for specialty applications.

Permanent magnet (PM) rotor designs also offer a significant potential for improved efficiency. Most PM rotor motors are designed to run exclusively on adjustable-frequency power. They also run at true synchronous speed without slip and therefore are a synchronous-speed machine. The high-performance neodymium iron boron (NdFeB) magnets found in most PM motors are relatively expensive.

Today China has exclusive control over the NdFeB market, and has driven magnet cost to unprecedented levels in the past year. New global supply markets are emerging to produce these magnets, but this will take time to significantly influence the dominance that China has in the market. However, even with these high magnet costs, many specialty applications can benefit from the PM technology. The increase in efficiency and improvement in power factor can result in smaller, more-compact motor designs. The hybrid electric vehicle is just one example for which ultra-efficient, lightweight and compact PM motor technology can be justified.

One thing seems certain: More efficient motors will be in our future because the demand for electrical energy is going to continue to increase. What technology will evolve as the next generation of ultra-efficient motor? Will they use a copper rotor, PM rotor, a hybrid combination of induction and PM suitable for sine-wave power, conventional induction designs upgraded using higher-grade electrical steels, or some other technology such as synchronous reluctance? 

The answer could lie in how the free market responds to the need for more cost-effective permanent magnets compared with the cost of a high-volume copper rotor design. I highly suspect that not one but a combination of these technologies will evolve and that the customer's application will ultimately dictate the technology solution that gets employed. The next generation of ultra-efficient motors is likely to follow the same progression as today's energy-efficient motors, which started with just a handful of end users concerned with energy saving and evolved into federally mandated premium-efficiency laws. In the end, it all will come down to economics—motor cost for higher efficiency and energy savings based on the cost of energy.

Richard Schaefer,
Senior product manager,
Baldor Electric, www.baldor.com

Rare-Earth Issues Temporary
Permanent magnet motors still hold a significant efficiency and package-size advantage for motors under 3 kW. Both technologies have continued to progress, but for these smaller powered devices, the PM solutions are still superior. The rare-earth magnet issues of late will be only a temporary supply issue. In addition, many manufacturers are using alternative materials to counteract the rare-earth supply issue.

There is a wide variety of rare-earth magnet compounds, and not all experience severe price and availability pressures. For example, samarium used in samarium cobalt (SmCo) alloys does not suffer from as high a price premium or availability issue as neodymium, dysprosium and terbium. SmCo offers performance equal to certain grades of NdFe.

Additionally, ferrite magnet designs have been used to avoid the rare-earth pricing challenge. These motors, however, suffer 25%+ reduction in output from the same size motor. In general, they still would be superior to induction motors, but as the motor's horsepower grows into the 20 hp range, the induction motor closes the gap on performance significantly.

Tom England,
Director of global product planning,
Kollmorgen, www.kollmorgen.com

Ferrite Alternative
Induction motors have been around for nearly 100 years. Over the past two decades, a lot of serious effort has been focused on improving their efficiency. Today, these efforts are reaching the point of rapidly diminishing returns.

Achieving higher efficiency with induction motors will require either using more costly materials (e.g., substituting copper for aluminum in the rotor) or more total material. Either of these costly options must be weighed against the small additional improvement in efficiency that would result.

Although most new permanent magnet motors rely on expensive rare-earth materials, a new design achieves much higher levels with low-cost, readily available ferrite magnets. By using novel, conical air-gap geometry, this motor concentrates the magnetic flux, achieving rare-earth magnet motor performance at a cost competitive with induction motors.

The only significant drawbacks to this design are high rotor inertia, which actually can be an advantage in many applications, including fans and pumps; and that PM motors require a variable-frequency drive to operate, making it most easily justified in applications that already require speed control.

John Petro,
Chief technology officer,
NovaTorque, www.novatorque.com

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