Resurgence for SR Motors, Drives?
Advances in controls and simulation software help switched reluctance (SR) motors-one of the oldest electric motor types-remain competitive. SR systems deliver efficient performance in a simple, rugged package without permanent magnets.
Sheer numbers of induction and brushless permanent magnet (PM) motors at work in industrial and commercial applications testify to their well-established manufacturing infrastructure and user acceptance. This has limited wide use of switched reluctance (SR) motors—a technology that offers a practical alternative for various demanding applications. SR motors have been around for a long time but the sophistication of controls needed to exploit their benefits has had a shorter history. SR motors have experienced waves of interest from time to time and perhaps a new resurgence is at hand.
SR technology promises an impressive set of benefits over its competition. Among these are high efficiency over a wide speed range (and partial loads), high-speed capability (>100 krpm, with the proper drive), easy cooling with heat source only in the stator, ruggedness for high-temperature or vibration environments, and relatively simple mechanical construction (see “SR motor anatomy” online).
Why consider SR?
Growing demand for energy efficiency is driving OEMs and end users to consider alternatives to induction motors, according to Emerson Motor Co. “These customers are increasingly looking to SR technology to provide a competitive and highly efficient alternative,” says Rob Boteler, director of marketing at Emerson Motor.
SR motors can’t run direct-on-line, thus require an associated power converter (drive) to complete an SR system. “As more applications become variable speed, the SR option, whose cost is competitive with an equivalent inverter-fed induction motor, becomes viable across a growing range of applications,” he states.
Compared to conventional variable-speed solutions, SR systems deliver high efficiency across the entire load range, Boteler continues. “Substantial energy savings result in applications where a significant part of the operation occurs at part load or above or below rated speed.”
Emerson Electric Co. has long recognized the importance of SR and, through its SR Drives division (formerly, SR Drives Ltd. of the U.K), has developed and applied this technology for more than 20 years, Boteler explains. “Emerson is meeting the current resurgence in demand for SR technology with a variety of platforms intended for industry, aimed at two speed regimes: super-synchronous applications [well above 3,600 rpm]—such as screw compressors, blowers, and high-speed pumps—and low-speed, high-torque areas (extruders, conveyors, and feeders), where SR has more traditionally been considered.”
Rocky Mountain Technologies (RMT)—a designer and manufacturer of SR motors and drives since 1994—notes significant industrial R&D expenditure and well-funded product development going into specific SR applications. “Unlike past developments that mainly focused on small units (fractional to low integral hp) we now see activity in the 100 kW to 1 MW range,” says George Holling, RMT’s technical director. “This big shift involving larger SR machines is real.” RMT is currently designing SR machines rated over 1 MW. Users’ motivation to look at SR as an alternative comes from concern about magnet material cost in PM synchronous motors and a desire to move away from induction motors for overall efficiency and system cost, he notes.
However, SR systems must focus on specific applications to successfully compete with the huge installed base of induction and brushless PM technology. “There is too much infrastructure to fight; recouping your investment from going into the general market is just a tremendous challenge,” states Holling. He mentions one new application in smaller wind turbines, where minimal drag and non-cogging performance of a 7.5 kW SR generator (with proper current control) becomes advantageous, enabling operation in low winds. Reportedly, the SR generator’s efficiency reaches 95.5%, or higher, at low speeds.
Intended for traction applications and rated for 2.5 kW, Rocky Mountain Technologies' 4-phase SR motor and traction controller (for 3- and 4-phase motors) constitute an SR system. The company has built and tested SR machines up to 300 kW and speeds to 120,000 rpm.
For severe duty
Swedish company Emotron AB, a long-standing provider of SR technology, notes little increase in demand from the industrial arena. Rather, Emotron sees SR’s role in OEM products for applications requiring higher volumes and robustness. “We find SR motors and drives hard to beat in more unique applications,” says Per Zellman, vice president of product marketing at Emotron. Mining machinery is a prime application along with other severe environments where extreme motor speeds are needed or acoustic noise of uncompensated SR motors is tolerable. “Motor noise is mitigated through damping and savvy electromechanical design, but it comes at increased product cost,” Zellman says.
Cool-running SR motors also allow temporary high-current operation for peak torque output for severe applications. Zellman mentions versatile motor geometry as another SR technology benefit—citing motor versions that range from long cylindrical units to pancake models.
While high-speed capability is more known, Zellman notes a very successful low-speed SR motor application. Emotron is the reputed market leader in Europe for rotary heat exchangers, for which SR technology provides a solution at 400 rpm nominal speed without the need for a gearbox. Lower speed operation also reduces noise, which is of concern in this application.
SR drive is key
SR drive (power converter) topology differs from that of conventional ac drives in the arrangement of power switch and fly-back diode circuits. For smaller drives, use of power modules is a cost-effective design route, but off-the-shelf modules are not available for SR as for other motor technologies. Custom modules or other more costly designs are often used. “Having low-cost power modules available for the drive would be a key step to make SR systems more competitive. “Drives in the 1-50 kW range are most affected,” says Holling. The situation is different for larger drives. “For really big drives we work with individual power devices, not modules, and device costs are similar to other drive types of similar size,” he says.
SR drives operate at switching frequencies typically 10 times lower than comparable ac drives because SR motors run efficiently without the need for sinusoidal voltage or current. Benefits derived include reduced motor harmonic losses, less electronics heating, lower RFI, and better system efficiency, according to Emerson. Holling at Rocky Mountain Technologies also points to this SR drive advantage, citing “very high efficiency obtained with SR drives that output non-sinusoidal waveforms.”
To obtain smooth rotation, stator coils are switched on/off in strict synchronism with rotor angular position (see diagram), meaning that the phase currents’ electrical frequency follows rotor speed, not vice versa, Boteler explains. SR drives require some type of rotor position feedback to precisely time switching among motor phases. This can come from a simple low-resolution encoder or by “sensorless” methods that estimate rotor position from phase current and voltage. Phase-switching becomes critical at high motor speeds because significant rotor motion occurs during current build up or decay time. “It’s necessary to compensate for this by advancing switching angles as speed increases—analogous to 'vacuum advance’ in an IC engine,” Boteler adds.
SR motor isn’t a stepper
Emerson calls SR motors “self-synchronous” machines intended to provide smooth, continuous rotation—“in contrast to superficially similar stepper motors.” (Some other sources seem to put both motors in the same category.) Emotron concurs that today’s SR motor is not a stepping motor since current is continuously monitored and controlled relative to rotor angular position. Emotron’s expertise includes sensorless control methods for SR motors. Zellman cites more than 20,000 SR system installed with its patented IntraSens solution to monitor rotor position.
With the SR drive’s optimal control of switching angles and current compensation, interrelated cogging and audible noise of an uncompensated SR motor can be minimized, enabling smooth rotation and best torque output. “Motor mechanical and electromagnetic construction must also be designed to reduce audible noise, which may be a big issue when competing in general-purpose applications,” Holling adds. “Combining the right motor geometry and proper controls will make a noticeably quieter motor. It’s a system issue.”
In addition, design tools are essential to solve non-linear current and torque relationships associated with SR motors. Holling cites SPEED software (developed at Univ. of Glasgow, U.K.--Scottish Power Electronics and Electric Drives is a consortium) as a valuable design tool for building SR motors at RMT. With large air gap motors (see below), RMT uses finite element analysis in conjunction with The Mathworks’ Matlab-Simulink for overall system optimization—for example, when designing motors for above 50,000 rpm.
Air gap economy
Rocky Mountain Technologies stresses that key to make economical SR motors is ability to design with large air gaps, which simplifies mechanical construction and assembly. “We routinely use 20 to 60 mil (0.5-1.5 mm) air gaps for integral hp motors and preferably 40-60 mil,” Holling states. “Small air gaps can lead to problems, such as bearing wear, torque imbalance, and acoustic noise.” This contrasts with many motors designed with tighter air gaps, typically in the 0.010-0.020 in. range.
Holling illustrates the point by comparing two gap sizes when the rotor often is subjected to high side forces. For a 10 mil air gap and
With a 40 mil air gap and the same
With numerous developments and initiatives in the pipeline—such as Emerson Motor Co.’s new SR platform introduction to the North American market (see “Product Exclusive” section on page 10) —switched reluctance systems are likely to see wider adoption ahead.
- SR motor anatomy: See inside switched reluctance motors ;
- Springtime for Switched Reluctance Motors? (Feb. 2003);
- ' Forward to the Past' with SR Technology (Nov. 1999); and
- Product Exclusive: Emerson switched reluctance motor system .
Frank J. Bartos, P.E., is Control Engineering consulting editor. Reach him at firstname.lastname@example.org .