‘Forward to the Past’ with SR Technology

A rugged, low-cost brushless dc motor technology without magnets, readily available for use! Now there's a bright new idea. Bright yes, new no. At age 150 years plus, and counting, switched-reluctance (SR) motors represent one of the oldest electric motor designs around.Simple construction is a prime feature.

By Frank J. Bartos, Control Engineering November 1, 1999


Motors, drives, and motion control

Switched-reluctance motors

AC/DC drives

Electronically controled motors

Brushless motors

Sidebars: Less ‘reluctance’ to apply SR in industry

A rugged, low-cost brushless dc motor technology without magnets, readily available for use! Now there’s a bright new idea. Bright yes, new no. At age 150 years plus, and counting, switched-reluctance (SR) motors represent one of the oldest electric motor designs around.

Simple construction is a prime feature. SR motors eliminate permanent magnets (PMs), brushes, and commutators. The stator consists of steel laminations forming salient poles. A series of coil windings, independently connected in phase pairs, envelops the stator poles. With no rotor windings, the rotor is basically a piece of steel (and laminations) shaped to form salient poles. It’s 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, more is required to make an SR system go. Motor design is nonintuitive compared to traditional motor types. Software is essential for analyzing magnetic circuits and for overall design simulations. Phase-to-phase switching in the SR drive must be precisely timed with rotor position to obtain smooth rotation and optimal torque output. Rotor position feedback, or a so-called ‘sensorless’ feedback method is needed for proper control.

This is why earlier SR developments have lagged. Today, power/control electronics ad-vancements and better software tools make the difference for the recent surge of developments and interest.

SR technology is drawing the attention of a growing list of manufacturers, as well as intense following by university researchers worldwide.

SR advantages

These motors offer numerous benefits, such as:

Performance-much greater torque output and same (or slightly higher) efficiencies than ‘premium efficiency’ induction motors. Efficiency is flat over a wider speed range;

Small unit size-makes efficient use of materials;

High-speed and acceleration capability-100,000+ rpm, with the proper drive;

Cost-simple to wind stator coils and other low-cost motor parts make for less costly production. Cost versus efficiency favorable relative to other motor types, including brushless dc (bldc);

Cooling-most heat generated in stationary stator which is relatively easy to cool; and

Rugged construction for harsh environments such as high temperature and vibration.

SR controllers add to the benefits. Since they do not need bipolar (reversed) currents, the number of power-switching devices can be cut by 50%, compared to bridge-type inverters of adjustable-speed drives. An SR drive has inherent reliability and fault tolerance. It can run in a ‘limp-home’ mode with diminished performance with one failed transistor in a phase, unlike standard motor drives.

Time is right

In some applications, the time for SR technology has been right for a long while, according to Emotron AB (Helsingborg, Sweden)-a manufacturer of SR products since 1983. Per Zellman, marketing director at Emotron, thinks simultaneous demands of energy- and cost-efficiency in drives are now ‘forcing’ industry to look at SR as a technology alternative.

Other factors have limited use of SR technology until recently, among them a large installed base and well-developed features of ac motors/drives. Marketing of SR products also created customer expectations out of line with technical solutions offered. ‘Many potential users have turned their backs to SR after preliminary tests and comparisons with ac technology. Today, suppliers are more careful about where and when to offer SR solutions,’ says Mr. Zellman.

Virtually all suppliers cite advances in electronics as a catalyst for renewed interest in the switched-reluctance arena. However, Mr. Zellman notes it’s hard to obtain standard components for high-volume SR applications, compared to integrated solutions for ac and bldc motors. ‘This is now slowly changing. The question that remains, ‘Will semiconductor manufacturers see SR as attractive enough to invest in SR-adapted power products?’ ‘ he adds.

Chip producers are entering the scene. Advances in low-cost DSPs, ASICs, and other electronic devices help designers raise performance while holding the low-cost image of the SR motor. See OnLine Extra at www.controleng.com for more about DSP suppliers and some offerings for SR motor control.

Industrial outlook

‘At one time SR technology had been touted as everything from a general solution for all future drives to a niche-oriented solution for either high-volume/low-cost, high-performance/medium-cost, or extreme-requirements/high-cost,’ says Emotron’s Mr. Zellman. Various application areas look promising, but he sees the future of SR motors and controls in niche sectors not as an ‘across-the-board’ solution. Niche areas include specific production machines, for example, textile machinery where general solutions aren’t available, yet the need for system integration is high. But correctly tailored solutions can span wide industrial sectors. ‘OEM business will be dominant due to a high level of tailoring and know-how required to make good SR products.’

Much better understanding of SR solutions and some commercial successes have demonstrated that customers ‘are more willing to consider SR technology today,’ according to George Holling, president of AMC Technologies (Sun Prairie, Wis.). The company has been a developer in the SR arena since 1989 and offers a full line of SR motors and controllers in the 0.5-15 kW range.

Mr. Holling sees some maturing of the technology, with commercial electronic components becoming more available; for example, control ICs and power modules suited to SR motors.

Earlier SR developments targeted consumer and appliance markets with high-volume potential, able to justify significant spending on R&D and tooling costs.’ Most industrial applications do not offer the volumes to justify development of new products and the supporting infrastructure,’ he continues.

Still, product developments from specific applications trickle down to the industrial marketplace. High-speed capability of SR motors is one drawing card for some industrial users. For example, a major textile machine manufacturer is developing an SR-based spindle drive for 30,000 rpm, notes AMC. However, the most active market for SR developments appears to be traction drives and automotive accessories (see sidebar).

Emerson Electric Co. (St. Louis, Mo.) has been a notable player in SR technology via its local divisions such as U.S. Electrical Motors (USEM, also in St. Louis). Research and development in SR formed a good portion of that work, but ‘without major product implementations” early on, explains Tom Grudkowski, vp of engineering for USEM. With Emerson’s acquisition of SR technology specialist company Switched Reluctance Drives Ltd. (SRDL, Harrogate, U.K.) in 1994, numerous products using SR technology entered the market.

As recent evidence of product success, Mr. Grudkowski notes fractional horsepower Neptune motors in Maytag washing machines, as well as integral hp industrial applications manufactured under license to SRDL (applications sidebar). Emerson Motor Co.’s Appliance division (Paragould, Ark.) makes Neptune motors, while Emerson’s subsidiary, ASTEC in China, produces the electronics.

Working closely with SR Drives Ltd., USEM follows an OEM-specific strategy that translates into SR designs specific to an application. Among many possible requirements, high torque at low-speed could be dominant in one case, while flat efficiency over a wide speed range or high-speed operation might be key in the next case, explains Mr. Grudkowski. And SR must be cost-competitive with alternative solutions-including cost of materials, manufacturing, and long-term reliability. ‘While SR is not the solution for every application, our current OEM product developments in the 1-1,000 hp [0.75-750 kW] range point to an increasing number of applications where it’s the preferred choice,’ he adds.

Under a relatively new company name, Motorsoft Inc. (Lebanon, O.) is a long-term participant in SR technology. Known earlier as Magna Physics, current activities of the company include motor design software, design handbooks, and other educational materials for electric motors. Motorsoft is exclusive U.S. agent for University of Glasgow’s (U.K.) ‘Speed’ motor design simulation software. James Hendershot, Jr., Motorsoft’s president and a pioneering advocate of SR motors in the U.S., participated in the software’s validation. Speed targets SR as well as other motor types and runs under Microsoft Windows 95/98/NT. The software determines general sizing, control requirements, and SR motor performance over a wide range of variables; also, it can be linked to FEA software.

Unique machine

Mr. Hendershot regards the SR motor’s magnetic circuit ‘unique from all electrical machines’ with its single magnetic flux source versus two flux sources (rotor and stator) in all other motors. In his paper, ‘SR Motors for the Year 2000,’ presented at the SMMA Fall Technical Conference (St. Louis, Oct. 1999), he discusses the low market impact of SR technology to date. It’s a case of competing with very successful existing technology such as the ac induction motor. ‘Growth of the SR motor will closely follow the pattern of the PM brushless motor,’ he states. That is, mainly new applications will spur the sales, where existing motors can’t be used and manufacturing infrastructure also needs upgrading. However, Mr. Hendershot thinks the SR drive system has cost advantages compared to a PM brushless approach.

Available and affordable power electronics, plus special design software, make it the right time for SR technology, in the view of Dana Corp., Motor and Electronic Systems business (Ann Arbor, Mich.). Advanced finite element magnetics design software is a must for modeling SR motors’ complex, nonlinear behavior. ‘Absence of brushes provides long life and absence of magnets suits high-temperature automotive environments,’ remarks Ramani Kalpathi, project engineer for SR motors at Dana.

Dana concentrates on the automotive market with its special set of requirements. High-heat and high-vibration are common to these ‘underhood’ applications. Mr. Kalpathi cites long life, lower weight, and favorable cost as benefits of SR motors in this environment. ‘SR products on vehicles must be engineered to optimize their value before a car company will consider the technology. To be successful, SR must compete with different motor technologies on performance and cost,’ he says.

Mavrik Motors Div., Tridelta Industries Inc. (Mentor, O.), specializes in custom design and manufacture of switched-reluctance motor and drive packages. Concentration is on OEM users. The company sees growing applications for SR technology due to improved power electronics and falling prices for electronic components. Software and modeling also help invigorate developments. ‘Use of these advanced methods enable motor system designers to quickly assess requirements, size the motor and determine control profiles needed, estimate cost, and then design the motor package,’ remarks Gary Clark, director of marketing for Mavrik Motors.

Mavrik produces 2- and 3-phase SR motors, as well as higher phase designs. However, focus is on lower cost 2-phase motors, said to offer torque performance and low torque ripple of 3- and 4-phase machines (see next diagram). Mavrik’s patented asymmetrical rotor design combined with proprietary controls also quiets the motor. As a result, these motor/drive systems appeal to blowers for heating appliances and commercial floor-cleaning equipment. Combining soft start with variable-speed SR technology enables one floor-care unit to do the job of separate scrubbing, polishing, and burnishing machines, explains Mr. Clark.

Two-phase SR motors-along with fewer number of poles-also draw attention at Lamb Electric Div. of Ametek (Kent, O.) for reasons of lower cost. Fewer phases and poles mean less electronic parts and smaller unit sizes. Motor construction is likewise simpler with 2-phases and 4/2 stator/rotor poles, but design refinements become necessary.

Ametek Lamb’s first SR product is a motor-blower unit with integral controls and cooling fan in an unusually compact package. Size reduction was a basic design goal versus a prior brush dc motor-based system; brushless motor drives proved not to be cost-effective. Named Infin-A-Tek, the 2-phase SR unit uses only two insulated-gate bipolar transistor switches per phase (four IGBTs in all) and a sensor for position feedback (optical interrupter on the stator and a shutter on the rotor). Fred Dishner, Ametek Lamb senior project engineer, says this approach was preferred for the product, compared to remote sensing requiring a sophisticated microprocessor.

Infin-A-Tek motor-blower units-produced under license from SR Drives Ltd.-likewise find their early application in higher volume floor-care equipment. Three or more times longer motor life compared to brush dc motors is a significant benefit. See the sidebar for an industrial application of this product.

Radio-Energie (Marcoussis, France) manufactures SR motors and digital controllers for low-voltage (24 V dc) forklift trucks, light vehicles, etc. The motor and drive provide 4-quadrant operation with regeneration available during braking. Power range is 0.7-2 kW at 3,000 rpm with up to 80% efficiency. Long motor life is determined by the bearings-the only wear parts. A programmable controller monitors speed, direction, braking, and torque.

Diminish the negative

All technologies have a downside.

Torque ripple and audible noise are inherent drawbacks of SR motors’ simple design. Torque and noise become interrelated effects as the rotating pole pieces ‘switch’ in and out of flux contact with the stator poles. Forces developed during alignment of the salient poles are actually high enough to distort the stator structure. The motor then resonates due to harmonics of these forces to produce audible noise. It can act as a noise amplifier.

Various methods are in place to reduce these effects, with new methods under development. Both the motor and controller offer ways to make improvements, but treating them as a whole system brings the best results.

For motors, Emotron lists methods like over- lapping pole designs, increas-ed structural di-mensions to damp vibration, slots in critical places, and add-ing damping material. Controller refinements include higher switching frequencies and phase-current shaping adapted to motor characteristics (limited to low or medium speeds due to induced emf).

USEM’s Mr. Grudkowski concurs about solving these issues through a systems perspective. At Emerson, solutions ‘take into account motor mechanical design and electromagnetic optimization, system configuration, and control algorithms,’ he says. But, SR motor torque ripple is not viewed as a major issue. It’s regarded as similar to torque ripple in a PM bldc system with trapezoidal excitation.

Mavrik attacks torque ripple and noise reduction via a staggered-pole (asymmetrical) rotor geometry. Rotor pole ‘teeth’ have unequal widths so that two pole sets can be active in the magnetic circuit and produce more evenly distributed torque as the phases are energized in sequence. Control of the power-switching devices’ turn-on/turn-off angles sets the start and end points of electrical phase commutation.

‘This design allows two magnetic strokes within a single electrical stroke during the revolution [see diagram],’ explains Mavrik’s SR motor technical specialist Wayne Pengov. ‘Staggered rotor teeth minimize effects of stator distortion as phase-to-phase switching occurs and help diminish constant frequency harmonics,’ he says. Drive electronics and control algorithms also help limit motor noise.

Ametek Lamb’s Infin-A-Tek, 2-phase SR motor design also uses a version of rotor pole shaping to ncrease efficiency and minimize harmonic noise. The rotor pole face is much wider than the stator face and has a stepped radius, explains Mr. Dishner. ‘Width of the rotor pole face permits energizing two phases with some overlap,’ he says. ‘The second phase is fired be-fore the first switches completely off, smoothing the transition between phases.’

Rotor pole shaping also eases a startup problem with 2-phase SR motors. It helps eliminate a positioning magnet usually needed for motor starting.

Stator design can also optimize performance. Infin-A-Tek’s asymmetrical stator geometry moves stator poles away from a 0-90 degree layout, creating thicker lamination sections in some vital areas. This stiffens the structure to minimize magnetically induced mechanical stresses that cause noise and harmonics.

Because SR systems are mostly applied in speed-control rather than servo control mode, AMC Technologies agrees that torque disturbances are ‘often less of an issue.’ Mr. Holling says, ‘Where needed, electronic control methods coupled with lamination design reduce or virtually eliminate torque ripple in almost all cases. Reducing torque ripple also lowers audible noise in the SR motor.’

Less publicized, but an important issue, is air gap size for SR motors. Past designs called for small air gap (& 0.25 mm) to achieve efficient motor operation. ‘We now have the technology to make good, efficient SR motors with air gaps of up to 1 mm,’ states Mr. Holling. Larger air gaps bring lower costs and simplify design. Some examples include less expensive bearings, wider part tolerances, and less chance of trapping contaminants. ‘Larger air gap is also an effective means to control the motor’s audible noise,’ he adds.

Minimizing torque ripple becomes particularly important when SR motors are used in servo positioning as in Dana’s automotive applications (sidebar). One ap-proach is to produce stator and rotor laminations in a twin-stack arrangement. The patented design changes pole width abruptly along the pole piece’s axial dimension. ‘This helps produce a flat torque profile,’ explains Mr. Kalpathi. Improved FEA tools also contribute, making torque ripple (and noise) control an easier task for the designer.

‘Sensorless’ operation

For smooth SR motor rotation, current switching between phases must occur at precise angular points. Rotor position feedback is one way to obtain such control, but this raises cost and reliability concerns. As a result most SR motor suppliers offer or have under development sensorless (more correctly, ‘remote sensing’) methods for feedback. Sensorless operation involves measuring motor characteristics at the drive and then using sophisticated calculations to derive the required position data.

Emotron uses motor current and voltage measurements to calculate rotor position with its IntraSens technique. This patented second-generation sensorless SR system launched in 1996 follows the company’s first sensorless approach introduced a decade ago.

Mavrik Motors also reports considerable development of sensorless commutation, resulting in the elimination of the encoder or resolver used for rotor position sensing. ‘A novel inductance measurement scheme as a function of rotor position has been developed, opening up some new applications,’ according to the company.

Texas Instruments Europe (Nice, France) mentions inductance sensing and measurement of motor flux/current characteristics as two methods for ‘sensorless’ operation of SR motors. See more at OnLine Extra at www.controleng.com . Several factors-sensorless feedback, control of torque ripple and noise, and favorable performance-to-cost ratio-now strive to bring a renaissance to switched-reluctance technology. This twist on ‘back to the future’ we choose to call ‘forward to the past.’

Less ‘reluctance’ to apply SR in industry

Switched-reluctance (SR) technology is moving beyond consumer and appliance markets. Automotive accessories and electric vehicle drives see considerable play, while some industrial users emerge.

Air compressors represent a prime industrial application for SR drives. CompAir Ltd., Broomwade (Bucks, U.K.) manufactures reciprocating- and screw-type air compressors in the 1-300 kW power range. Its Cyclon 475SR and 345SR oil-injected screw air compressors apply SR dc drives (produced under license to SRDL) to precisely match power consumption to air demand (cutaway photo).

Field trials show average energy efficiency gain and operational cost savings of over 25% compared to conventional air compressors of the same rating using an ac drive and inverter, according to Tom Grudkowski, vp of engineering at USEM Div. of Emerson Electric Co. (St. Louis, Mo.). The SR drive includes soft starting to eliminate current peaks during start and acceleration. ‘With speed regulation of the compression element, Cyclon SR maintains a steady delivery pressure enabling further energy savings by optimizing air system pressure levels,’ he adds.

Ametek Lamb’s (Kent, O.) Infin-A-Tek SR motor-blower units are seeing industrial usage in pneumatic systems, transporting plastic pellets from a storage area to the molding machines at Conair Inc. (Franklin, Pa). In a cooperative program between Conair and Lamb, Infin-A-Tek replaces existing series universal motor-driven units. ‘Extended service life and lack of carbon emissions are prime motivations,’ comments Peter Pavlick, director of marketing at Ametek Lamb.

The existing centrifugal fan system is a low cost design small enough to fit directly onto a chamber above the molding machine. However, it’s limited by carbon brush life of the dc motor and brush emissions. Lamb’s SR system provides much improved service life (estimated at 4X) and control capabilities without dc brush problems, according to the company. Furthermore, the SR solution is implemented at about the same cost and within the same space configuration as the existing product. ‘This allows Conair to fit the unit into existing equipment without extensive retooling,’ adds Mr. Pavlick.

Dosing pumps for industrial processes require high starting torque and accurate speed control. ‘SR motors have proved toχbe a good alternative to frequency inverter controlled induction motors for dosing pumps,’ says Per Zellman, marketing director at Emotron AB (Helsingborg, Sweden). Higher efficiency compared to induction motors and a wide speed range are other benefits.

‘Plunger and diaphragm pumps are especially difficult to control due to their pulsating torque,’ he states.

Sensorless SR, motors supplied by Emotron, drive plunger pumps for chemical injection at the Ringsjöverket water treatment plant near Helsingborg. High static speed accuracy of the motors results in nearly pressure-independent flow rates (excluding pump leakage). Operated by Sydvatten AB , the plant supplies drinking water to a large part of the southern Swedish district of western Skåne.

Automotive interests

Recently, AMC Technologies (Sun Prairie, Wis.). has successfully tested an SR motor/drive as a potential replacement for the original brushless dc (bldc) drive on the Neighborhood Electric Vehicle made by Corbin Motors (Hollister, Calif.).

Dubbed the ‘Sparrow,’ this three-wheel, 614 kg, one-passenger vehicle boasts top speed of 104 km/hr and nearly 100 km max. range on 12 V battery power supply (first photo). ‘The SR motor generates more torque and draws 20% less current than the competing specific PM bldc motor with rare-earth Nd-Fe-B magnets,’ says AMC president George Holling. ‘Noise was substantially equal to that of the bldc motor.’ AMC’s role in the alternative SR design included prototype development of the motor and controls. Densei Motors of Japan, now part of Invensys, built the motor. AMC has a strategic alliance with Densei for marketing SR and other motor types in the Americas and Western Europe.

Dana Corp., Motor & Electronic Systems business (Ann Arbor, Mich.) supplies an SR electronic throttle control motor for the Jaguar S V-6 and V-8 automobile engines. The motor positions a throttle plate to provide the requested vehicle speed. This switched-reluctance motor meets positioning accuracy requirements specified by Ford Motor Co. (Dearborn, Mich.). The SR approach further satisfies severe temperature and vibration demands found in an automotive environment.

A novel encoder-actually a miniature SR motor-handles closed-loop positioning for the servo type control. This approach proved to be a low-cost way to sense and interpolate the main SR motor’s coil induction variations for servo control, says Dana.