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Induction vs Permanent Magnet Motors

Release time:2017-05-08 14:17     Author:Nature

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Keywords: Permanent Magnet Motors

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
 
Answers
 
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.
 
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.


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