Motors for efficiency: Permanent-magnet, reluctance, and induction motors compared

April 10, 2013
• It is possible to find superefficient high-horsepower motors but, at this point, many of these are exotic designs that are practical for limited applications. • Special motor designs may begin replacing induction motors as worldwide efficiency regulations grow ever-more stringent.

Continuing headlines about superefficient electric drives might lead some observers to wonder about the future of three-phase induction motors. The truth is that induction motors aren’t going away any time soon. In fact, they just keep getting better as improved materials continue to make possible more-efficient designs.

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Government legislation supports these efforts: In the U.S., EISA (the Energy Independence and Security Act) went into effect in 2010, mandating higher efficiency standards for general-purpose, three-phase ac industrial motors from 1 to 500 hp manufactured for domestic use. Europe has similar regulations. As of June 2011, the EU only permits motors with efficiency class IE2 (International Efficiency), a boost in efficiency by up to 7% compared to older IEC designs. By 2017, only motors with an IE3 efficiency class will be permitted there. These motors have efficiencies up to 10% higher than older IEC designs and already make economic sense because of their high energy-saving potential.

However, at some point the law of diminishing returns kicks in. With motors operating in the 90%-plus efficiency range for most ratings, there isn’t much more room for improvement.

Laws of physics make it prohibitive to design ac-induction motors with higher efficiencies.

This is where alternative motor designs begin to make sense. As one example, consider permanent-magnet (PM) rotor technology. Unlike induction motors, which induce a secondary magnetic field in the rotor, PM motors use high-performance rotor magnets to create a magnetic field that is always present. This eliminates the secondary circuit rotor I2R (heat) losses found in the induction-motor design, resulting in higher efficiency and a better power factor to boot.

Rating for rating, PM-rotor technology is simply more efficient than induction technology. So, will PM motors completely replace induction designs one day soon? Most industry experts think not.

The deciding factor is cost and payback. In the industrial arena, payback comes down to energy savings. The latest PM-rotor technology uses neodymium-based rareearth magnets, which offer both a high-energy product and high-temperature operating qualities. However, it is no secret that China now controls much of neodymium’s global supply and has been escalating magnet prices and restricting sales. The higher price is making it worthwhile for more countries — including the U.‹S., Australia, South Africa, and Canada — to open mines and processing plants, which will eventually result in more competition for magnets.

Beyond induction designs

The demand for more-efficient motors will continue as the demand for electricity rises worldwide. What the ultraefficient motor of the future will look like is anybody’s guess.

“A motor that is three to four efficiency bands higher than today’s Premium Efficient (induction) motors will likely be a hybrid design,” says Richard Schaefer, senior variable-speed product marketing manager, Baldor Electric Co., Fort Smith, Ark. “It will incorporate both an induction cage for starting and permanent magnets for high-efficiency operation and running at true synchronous speed. This future hybrid may be a squirrel-cage induction or synchronous reluctance design enhanced with permanent-magnet technology. This motor design could ultimately replace today’s induction motors.”

Other motor configurations are also being developed, each with their own special advantages, disadvantages, and application sweet spots. Motors with segmented stators, concentrated windings, copper rotors, salient pole rotors, solid rotors, and hybrids are all either in production or being proposed. The “best” design will be the one most closely matched to application requirements for efficiency, torque, speed, and cost.

It is interesting to explore a few of the more widely discussed designs making their way into use today. One such design uses switched reluctance. Switched-reluctance designs use pulses of current to cause torque and rotation. Their stator contains windings, but the rotor has no magnets or windings. It is made of soft magnetic material (laminated-steel protuberances). When the stator powers up, magnetic poles are induced in the rotor teeth. The rotor’s magnetic reluctance creates a force that attempts to align the rotor with the powered windings. To maintain rotation, adjacent windings get powered up in sequence.

Switched-reluctance motors have fewer rotor poles than stator poles so the poles can’t all align simultaneously, a position which cannot generate torque. In contrast, a related technology called a synchronousreluctance motor has an equal number of stator and rotor poles. The rotor teeth are arranged to introduce internal flux “barriers,” holes which direct the magnetic flux along the so-called direct axis. In either case, the rotor operates at synchronous speed and there are no currentconducting parts in the rotor, so rotor losses are minimal compared to those of an induction motor.

It can be tricky to get an apples-to apples comparison of efficiency for different motor designs. Efficiency depends to some degree on qualities of the drive-current waveform. It would be ideal to do so based on pure sinusoidal waveforms. Unfortunately, this isn’t possible because some designs always are driven from inverters whose outputs are generally characterized by a lot of high-frequency harmonics. With the switched-reluctance design, for example, the waveforms applied to the rotor are nowhere near sinusoidal.

Nevertheless, it is standard practice for efficiency comparisons to assume ideal current and voltage waveforms for each technology. For example, in line-fed cases, this means perfect sinusoids. In converter-fed instances, most analysis assumes waveforms that produce the lowest motor losses.

Induction motors develop torque because of a slip between the speed of the rotor and the speed of the magnetic flux rotating around the stator winding. (For this reason they are considered an asynchronous-motor technology.) Thus, there is always an energy loss associated with slip that reduces induction-motor efficiency. In contrast to asynchronous motors, synchronous motors have no such slip loss but still can have a rotor loss, though it is usually small. This type of loss typically arises from eddy currents in magnets and laminations. Switched-reluctance and synchronous-reluctance motors are both synchronous designs. Two other motor designs known for high-energy efficiency are synchronous designs as well. They are the salient pole PM motors and nonsalient PM motors.

In the switched-reluctance design, however, the magnetic field in the rotor is time varying. This time-varying field typically causes rotor hysteresis and eddy-current losses. Depending on the particular design and operating conditions, these rotor- core losses can be fairly large and at a rather high frequency.

Salient-pole PM motors carry magnets buried in the rotor iron or placed in slots on the rotor surface. The space above the magnets may hold a cage to permit starting. Nonsalient PM motors typically have the magnets on the rotor surface. Nonsalience means the inductance of the motor measured at the terminals is constant, regardless of rotor position.

From most efficient to least, here’s how the five motor technologies stack up: salient-pole PM, nonsalient PM, synchronous-reluctance, switched-reluctance and, last but not least, induction motors. Regarding power and torque density, as may be expected, the rankings mirror those for efficiency, with the PM designs having the highest torque density and induction designs the least. This makes sense because the need to dissipate losses and keep within a certain temperature limit is common to all the motor technologies.

Several motor technologies today can operate across a range of speeds when used with adjustable-frequency supplies. Operation at fixed speed without an adjustablefrequency supply requires a starting cage. This type of cage is inherent to the induction-motor design, and in some cases can be integrated into both synchronousreluctance and salient-pole PM motors. It’s important to note that nonsalient PM, cage-free salient-pole PM, cage-free synchronous reluctance, and switchedreluctance motors are always run with an inverter and, therefore, can be considered part of a variable-speed system.

Where they’re used

Induction motors are widely available from many manufacturers and are, therefore, relatively inexpensive. These industrial workhorses are often considered general-purpose motors, running everything from fans and pumps to compressors and conveyors. Salient- pole PM motors come in a limited range of ratings (up to 1,200 hp) from fewer manufacturers. These motors have served in a wide range of applications that demand high power density, high efficiency, and operation across a wide range of speeds and loads.

Both salient-pole and nonsalient PM motors are relatively more expensive than the other technologies because they incorporate rare-earth magnets. Nonsalient PM motors are available over a limited range of ratings from just a few manufacturers and typically find use as torque-rated servomotors.

Synchronous-reluctance motors are available from only a few manufacturers over a limited range of ratings — from 2 to 450 hp. Although these motors were once relegated to low-power applications such as web processing, they are beginning to emerge in general-purpose variable-speed applications such as fans and pumps. Recall that in this design, the rotor is free of both magnets and conductors. And its stator shares the lamination and winding configuration of widely available induction motors, making it a relatively affordable technology.

Switched-reluctance designs are available from just a handful of manufacturers and mostly as OEM-specific designs rather than general-purpose motors. The simple structure of both the rotor and stator help keep costs down. These motors have been applied in a range of niche applications where high speed is a factor, such as motion control in printers, traction applications in mining, and air compressors.

Pros and cons

There is no such thing as a universally perfect motor, only a motor that is the right fit for the intended application. Each technology has different strengths and weaknesses. Induction motors are inexpensive, widely available, and do not incorporate expensive permanent magnets; their inherent weakness is their rotor-slip losses. Nonsalient PM designs offer high efficiency, high torque density, and high speed, but only limited operation above their base speed. They also require permanent excitation and use expensive magnets. Salient-pole PM motors also offer high efficiency and high torque density, but feature permanent excitation and expensive magnets like their nonsalient counterparts.

Synchronous-reluctance designs work at high efficiency and high torque density without the need for permanent excitation or permanent magnets. However, they only offer a low power factor and limited high-speeds. Finally, switchedreluctance designs offer high-speeds and high-torque density, along with no need for permanent excitation or permanent magnets. Their drawbacks include acoustic noise, torque ripple, rotor-core loss, high fundamental frequency, and the need for a six-lead connection.

Baldor Electric Co.• Comparing efficient motors • Copper rotor induction motors
IPM versus three-phase induction motors • Switched reluctance motors

© 2013 Penton Media, Inc.

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