This article reviews variable-switched reluctance (VSR) motors that are now entering mainstream use from jet fighters to washing machines. A VSR motor is generally used as a stepper motor and, if properly controlled, can be made to behave like a servomotor. Basically, the motor is a rotor and stator with a coil winding in the stator. VSR motors also provide other benefits. They can be programmed to precisely match the loads they serve, and their simple, rugged construction has no expensive magnets or squirrel cages like the ac induction motor. It can be difficult to give VSR motors a smooth torque profile, so they are used more often in place of variable speed motors than as servomotors. There are ways to control torque ripple, such as adding encoders and electronics to compensate, but these added controls could cost at least as much as what the motor itself would save. VSR motors work with relatively small air gaps. If the shaft is off-center, unbalanced tangential forces come into play, so shafts and bearing systems generally need to be of a higher quality than with other motors.
Locomotives in the early 19th century were the first machines to use variable-switched-reluctance (VSR) motors. They did not perform satisfactorily," said George Holling, president of Advanced Motion Controls (AMC) in Prince ton, Wis., "and they soon faded into obscurity." Recently, however, organizations as diverse as the Department of Defense, automakers, and consumer-appliance manufacturers have realized the benefits of VSR motors.
A VSR motor is generally used as a stepper motor and, if properly controlled, can be made to behave like a servomotor. Basically, the motor is a rotor and stator with a coil winding in the stator. The rotor, which consists of a laminated permeable material with teeth, ' is a passive device with no coil winding or permanent magnets. The stator typically consists of slots containing a series of coil windings, the energization of which is electronically switched to generate a moving field. For the most part, only a single coil set is activated at anyone time.
When one stator coil set is on, a magnetic flux path is generated around the coil and the rotor. The rotor experiences a torque and moves the rotor in line with the energized coils, minimizing the flux path. With the appropriate switching and energization of the stator coils, the rotor can be encouraged to rotate at any desired speed and torque.
This setup offers better performance than many other types of motors. A VSR motor does not require sinusoidal exciting waveforms for efficient operation, so it can maintain higher torque and efficiency over broader speed ranges than is possible with other advanced variable- speed systems. The optimal waveforms needed to excite a VSR motor have a high natural harmonic content, and are typically the result of a fixed voltage applied to the motor coils at predetermined rotor angles. Such waveforms can be achieved at virtually any speed.
In addition, as long as the commutation can be accurately controlled with respect to the rotor angle, the motor will operate at its predicted high efficiency. "With VSR technology," Hoiling said, "it is possible to design a low-cost motor with over 90- percent system. efficiency and variable speed at a good price."
VSR motors also provide other benefits. They can be programmed to precisely match the loads they serve, and their simple, rugged construction has no expensive magnets or squirrel cages like the ac induction motor. With no internal excitation or permanent magnet, the motor is inherently resistant to overload and immune to single-point failure. Finally, once in high-volume production, they are likely to be less expensive than competing systems.
Out of Obscurity
These benefits remained mostly hidden following the initial use of VSR motors in the opening decades of the 1800s. The motors reappeared in the early 1980s, with the development of electronic controllers for brush less motors; these same controllers could also be used for the highly nonlinear VSR motors. As a result, researchers in both the public and private sectors began to look at VSR motors seriously and identify potential applications.
Even with the electronic controllers, VSR motors failed to gain widespread acceptance or go into commercial production. Huge investments in permanent magnet technology that were new at the time had not yet been amortized , so many players in the motor industry were hesitant to dive into another unproven technology right away. Nevertheless, many organizations conducted a significant amount of research into this technology.
A barrier to commercializing VSR motors has been that few engineers are trained to design the technology.
The Department of Defense has pursued VSR-motor applications aggressively. The motors are now used in such military applications as generators for turbine engines and pump motors for jet fighters. Military planners and researchers liked the technology mainly because of their high reliability. Since the volume of motors purchased was relatively low, the cost was high; reliability was the top priority, not cost. For the same reason, the aerospace industry-primarily in Great Britain-also has begun to use VSR motors for applications including actuator controls for aircraft flaps. Nevertheless, VSR motors had yet to permeate the high-volume applications that would lower the cost.
The past few years have brought an increase in the use of VSR motors for high-volume consumer products. Among the first products to use the technology is a power-assisted steering system developed by Dana Corp. in Pittsburgh for the Ford Motor Co. in Dearborn, Mich. VSR motors are well suited for this application because they can deliver a high power density in a small volume, and they are less expensive. Various companies are looking to adopt VSR motors for other automotive applications, such as windshield-wiper motors and alternators.
Emerson Electric Co. in Danville, Va., is also using VSR motors. Although the company is planning on incorporating them into many of its appliances and tools, its biggest success so far has been with one of Emerson's new washing-machine models. In these washers, the VSR motors eliminate all the mechanical gearing for the spin cycle, so no taps are needed to get multispeed performance, resulting in a significant cost saving. Emerson, which uses an estimated 2,000 motors per day for this application, plans on introducing its first industrial VSR motor this year.
AMC, which has partnered with NEC/Densai in Japan, is making VSR traction drive products for various companies, such as for Pittsburgh-based Westinghouse Electric Corp. It also offers a traction drive suitable for amusement-park rides.
A barrier to the rapid commercialization of VSR motors has been that few engineers are trained to perform the exacting and specialized design the technology requires. This hurdle is gradually shrinking, with more than two dozen firms now designing or manufacturing VSR drives, and several are moving into mass-production applications. As these and other firms gain experience with the technology, new opportunities will arise for utilities, energy users, and original equipment manufacturers to capture the benefits of VSR-motor systems.
Making the motor truly practical in many applications requires sensorless control. A sensor attached to the motor represents a major cost item in any volume application. Hall sensors are the most common motor sensors; because the VSR motor has no internal magnets, which can be used for sensing, external magnets must be provided. Sensing devices add at least $1 to system cost, which can prove too great for high-volume systems. Furthermore, these sensors need additional wires, which can be prohibitive. AMC has developed a sensorless method that enables the commutation of a switched-reluctance motor from stall through very high speeds of up to 100,000 rpm. The sensorless control scheme delivers excellent starting torque even at zero speed.
VSR motors are not without their drawbacks, however. The most significant downside is the acoustic noise and large vibrations often caused by the motor's high pulsating magnetic flux. This noise can be reduced by adding components to the electronics , designing special magnetic circuit , and tweaking the mechanical design, but taking some or all of these steps could compromise the motor's benefits. Designers generally select the right combination of noise reduction and performance to suit the particular application.
Another limitation is torque ripple. It can be difficult to give VSR motors a smooth torque profile, so they are used more often in place of variable speed motors than as servomotors. There are ways to control torque ripple, such as adding encoders and electronics to compensate, but these added controls could cost at least as much as what the motor itself would save. If torque ripple is of primary concern, the best alternative might be a permanent magnet motor instead.
VSR motors work with relatively small air gaps. If the shaft is off-center, unbalanced tangential forces come into play, so shafts and bearing systems generally need to be of a higher quality than with other motors. Various motor designers are working on designs to widen the air gap.
"The adoption and proliferation of VSR motors is about 15 years behind brushless motors,' said Dan Jones, a Thousand Oaks, Calif., consultant to the motion-control industry. "However, it appears that they are experiencing the same acceleration curve as permanent magnet motor. Although they will never be ideally suited for all applications, they are emerging as a viable competitor to ac induction motors and permanent magnet motors."