Springtime for Switched-Reluctance Motors?

Build a ''better'' electric motor (and its associated controls)—based on simple, rugged design that eliminates costly permanent magnets—and users will flock to apply the technology. Well, this notion has not yet been proven for switched-reluctance (SR) motors, given the present business conditions and a more sober evaluation of SR technology by potential users.

02/01/2003


KEY WORDS

 

  • Motors, drives, & motion control

  • Switched-reluctance technology

  • Induction motors

  • AC/DC drives

  • Electronically controlled motors


Sidebars:
Review the basics on SR motors

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Online Extras

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Build a "better" electric motor (and its associated controls)-based on simple, rugged design that eliminates costly permanent magnets-and users will flock to apply the technology. Well, this notion has not yet been proven for switched-reluctance (SR) motors, given the present business conditions and a more sober evaluation of SR technology by potential users.

After a flurry of excitement and applications of SR motors in commercial products (for example Emerson motors in Maytag Neptune washing machines), a lull has set in for publicized applications during the last couple of years. This is not to say that SR technology is new. In fact an SR motor is probably the oldest of electric motor types. It offered a form of brushless dc design long before that term was coined.

Present activities for SR technology focus on more practical niche areas, such as ''underhood'' automotive and industrial applications that do not grab headlines.

Simple and robust

''Simple, yet inherently reliable construction'' is the main advantage attributed to switched-reluctance motors at Baldor Electric Co. (Fort Smith, AR). For example, only steel laminations assembled on a shaft comprise the rotor, with no windings or cage as in ac induction and brush dc machines.

''This so-called passive rotor design requires that the stator phases be electronically switched to produce a moving magnetic field, which the rotor follows in synchronism,'' says Baldor product manager Bryan Lawson. And the salient-pole stator design essentially eliminates phase-to-phase electrical shorts, especially when using pulse-width modulated adjustable-speed controls, he explains.

Performance advantages of an SR motor and its controls include high-speed capability (100,000 rpm not uncommon), efficiency over wide speed/torque range, excellent heat and vibration tolerance, and high power density.

Ability of SR motors to continue to run with reduced torque output after loss of a phase is another benefit advanced by some manufacturers. Mr. Lawson refers to this characteristic as ''interesting,'' though seldom practical. ''If an SR motor can 'limp' along with loss of torque, then it was likely oversized for the application,'' he states.

Emotron AB (Helsingborg, Sweden; U.S. office: Toledo, OH)-a long-standing manufacturer of switched-reluctance products-refers to SR's main advantage in terms of ''a robust brushless workhorse.'' Once an SR motor drive is placed in operation it virtually ''never'' fails, according to Emotron R&D manager Jürgen Reinert. Since rotor position information is required to operate an SR motor, the available feedback also benefits closed-loop speed control. Particularly when feedback is of the indirect ''sensorless'' type, installation is simplified by eliminating the need for speed-sensor cabling, explains Mr. Reinert.

At Dana Corp., Automotive Systems Group (Ottawa Lake, MI), interest in SR technology centers on automotive applications. John Hickey, director of Advanced Product Development, concurs about previously mentioned advantages of SR and adds to the list easier cooling of the motor, since no heat is produced in the rotor. Also, an SR-based alternator generates more power when an automobile idles than other design choices. Moreover, Mr. Hickey considers SR motors to hold other distinct advantages over the competition-ac synchronous (servo) and brushless dc machines.

Controls remain vital

With their simple construction, SR motors rely heavily on control electronics to obtain the promised performance. Digital signal processors and special algorithms in SR controls are vital to precisely time current pulses fed to the motor windings relative to rotor and stator position, all of which maximizes torque production. This works best when motor position feedback is available. At higher speeds, special methods allow position data to be derived from motor characteristics measured at the drive, without the need for formal feedback, explains Baldor's Mr. Lawson.

SR controls are very similar to those for other types of polyphase motors. One difference is the possibility to cut the number of power-switching devices by one-half, since phase-to-phase currents need not be bipolar (or reversed). However, most suppliers of integrated power modules continue to use the same number of power devices. Instead, connections are modified to allow multiple junctions in silicon to share the output currents. ''The advantage here is that the junction temperature change is less severe than in a single junction per phase solution, increasing reliability and longevity of the power devices,'' says Mr. Lawson.

SR technology has not experienced real breakthroughs recently, according to Emotron. Quite sophisticated controls and motor design refinements are still needed to minimize torque ripple and acoustical noise. From Emotron's perspective, this doesn't make SR the right choice for applications where torque quality or low noise is essential. ''In these applications, both ac induction and brushless dc topologies offer a simpler way,'' states Mr. Reinert.

Specific software and simulation tools have been developed over the years to handle SR motors' nonintuitive design. This trend has changed, placing more emphasis on empirical methods. ''Present simulation tools are sufficient in most cases and are used as a complement to practical measurements and prototyping,'' says Mr. Reinert.

Software tools enhance the controls. ''Improvements in control set-up algorithms that characterize an unknown motor will allow easier commissioning of SR motors and drives that may not have been designed as a matched set or are supplied by different manufacturers,'' adds Baldor's Mr. Lawson.

Limiting the negatives

Ability of SR motors and controls to produce high torque at low speeds-even at zero speed-leads to a downside. Phase-to-phase current pulses that generate torque also excite the motor structures, causing noise akin to hammering and torque ripple greater than in alternative motor types. Says Mr. Lawson, ''If not considered early on during project execution, the inherent noise generation can cause much disappointment in the end product.''

SR drives remain attractive to numerous applications requiring high torque, yet can tolerate some torque ripple and acoustical noise.

Dana Corp.'s Mr. Hickey believes that SR motors' interdependent problems of acoustic noise and torque ripple can be ''properly managed'' through careful attention to design details, such as selection of the number of phases, number of teeth on rotor and stator (see sidebar diagram), and proper shaping of teeth. ''Software methods that shape the current pulses in the windings also can be used to further reduce acoustic noise and torque ripple on a limited basis,'' he says. Overall, better design analysis tools exist today to assist SR motor and controls developers.

Within limits, SR drives can also be used in closed-loop servo motion control. Baldor Electric mentions start-stop applications, such as cut-to-length, where moving a load from point to point is the main objective, with little attention paid to torque ripple (or resulting velocity ripple) during transit. However, applications with higher precision, such as CNC machining, would be off limits.

Diminishing the inherent negative effects of SR motors is the subject of intense R&D work in academia and industry, especially in Europe. Current smoothing to reduce the ''hammer'' effect of pulsed currents is one approach used in newer SR controls, according to Baldor. However, the improvement comes at the expense of torque production. Other approaches combine current control with dampening of vibration sources and other mechanical suppression in the motor design. ''It is not likely, however, that noise will be reduced [drastically]..., without reducing the performance to unjustifiable levels,'' says Mr. Lawson.

Right time for SR?

Some say recent interest in applying SR motors and controls has peaked, others disagree. In part, improper application choice has led to less-than-complete success of some SR solutions.

Emotron notes reduced interest in SR technology in the last two years. ''As long as nothing drastically new is presented, ac induction drives and brushless dc will be the main alternatives to most users,'' says Mr. Reinert. Also, the company believes some questionable marketing practices have hurt SR's image. ''Intense marketing with big promises by some SR players has left many potential customers disappointed and with a disbelief in SR. This will be difficult to overcome,'' he adds. Even so, Emotron sees SR technology and products as important parts of its drives business, and continues to develop existing and new OEM applications.

During the late 1990s, SR technology generated a lot of interest and resulted in commercial and industrial products using SR motors, explains James Hendershot Jr., president of Motorsoft Inc. (Lebanon, OH) and a pioneer developer of SR motors in the U.S. ''Unfortunately, initial publicity about SR technology was overly enthusiastic, with its virtues and capabilities sometimes overestimated,'' he says. Possibly the best-known example of a well-accepted, successful product incorporating SR motors was the Maytag Neptune washing machine, which forced competitors to rethink their inverter-driven, induction-motor-based designs, explains Mr. Hendershot.

Yet, even this seemingly successful application has faded with marketplace caprice. As of mid-2002, latest model Maytag washing machines reverted back to a more conventional induction motor/drive design. Reportedly, other design changes were needed as well, because SR motor performance could not quite be matched. Pricing became an issue versus mass-produced induction motors, given the lower sales volume of SR motors.

Motorsoft Inc. sees some indications of waning interest in SR technology. The firm also is the U.S. marketing agent for an extensive database on SR machines known as Brivit. The trademarked site ( www.brivit.com ) contains nearly 10,000 documents.

Some companies in CE 's November 1999 article on SR technology are absent here. One has gone out of business, one was acquired, and others chose not to participate in this article, despite stating that new SR projects are under development. (See box for more SR sources.)

Baldor Electric suggests that successful integration of SR drive technology is extremely application dependent. ''The design, manufacturing, and performance challenges associated with the similarities (among many diverse installations still being evaluated) will drive SR technology to more highly developed levels and wider use in varied industries over time,'' says Mr. Lawson.

''Application of SR technology in the automotive industry is just in its infancy,'' is the enthusiastic view ahead offered by Dana Corp.'s Mr. Hickey. Among unique applications, he cites an SR starter/alternator (see article's first photo) that's 40% more efficient than present alternator systems. ''Other applications in electric power steering and turbocharger boost could be ideally suited to SR machines,'' he concludes.

As we now pass through the depths of a cold, calendar winter, new and more focused niche applications could propel switched-reluctance technology into another springtime.

-Comments? E-mail fbartos@reedbusiness.com




Review the basics on SR motors

Simple mechanical construction is the most obvious characteristic of switched-reluctance (SR) motors. SR motors eliminate permanent magnets, besides brushes and commutators. The stator consists of steel laminations forming salient poles on which are wound a series of coils, independently connected in phase pairs. With no windings required, the rotor is basically a piece of steel (and laminations) shaped to form salient poles. An SR motor is the only motor type with salient poles on both rotor and stator.

To operate an SR motor, current is ''switched'' among the phase coil windings in a sequential pattern to develop a rotating magnetic field. ''Reluctance'' refers to the resistive property of a magnetic circuit, also termed magnetic resistance. When a phase circuit-or pair of stator poles-is energized, the rotor moves to align with the stator poles (see diagram). Magnetic path reluctance of each phase circuit varies with rotor position, but becomes a minimum as pairs of rotor and stator poles align.

However, much more is required to make these motors thrive. Phase-to-phase switching in the SR controller must be precisely timed with rotor position to obtain smooth rotation and optimal torque output. Some form of rotor position feedback is required for proper control. Motor design is nonintuitive compared to traditional motor types. Software tools are essential to design and analyze not-so-simple magnetic circuits of SR motors.





Online Extras

More information on SR developments and applications: soon to come.

Also see '' CE , Nov. 1999.

Other SR technology providers include:

Ametek Lamb Electric

Emerson Motors

Radio-Energie

SR Drives Ltd.



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