Switched-Reluctance Motors and Controls Offer an Alternative Solution
Online Extra to Feb. 2003 Control Engineering article,‘Springtime for Switched-Reluctance Motors?’ Click here to return to main article
Switched-reluctance (SR) technology offers users a simple, rugged type of electric motor, able to deliver high torque and speed. However, control electronics and a sophisticated drive are required to obtain such high-performance from these machines. As well, some form of feedback sensor is needed to properly commutate the phase windings of SR motors. To cut feedback device costs, sensorless (or encoderless) feedback methods are being investigated and implemented.
Just what do we mean when we say that SR motors are simple? Here’s a perspective from John Hickey, director of Advanced Product Development at Dana Corp., Automotive Systems Group (Ottawa Lake, MI). “The switched reluctance machine is a synchronous motor in which torque is produced by the rotor as it moves to a position where the inductance of the excited phase winding is maximized,” he says. In an SR motor, both rotor and stator consist only of steel laminations shaped to form salient poles (or teeth) and “welded into stacked assemblies.” The stator winding contains a set of coils on each stator pole, connected in either series or parallel, depending on the number of phases and stator poles of a particular motor design. “Motor excitation is simply a sequence of current pulses applied to each phase in a precise fashion to achieve smooth and efficient operation,” adds Mr. Hickey.
Pros and cons
Attesting to SR motors’ construction simplicity, Brian Lawson, product manager at Baldor Electric Co. (Fort Smith, AR), says, “The rotor, void of aluminum or copper conductors, and consisting of only a stack of passive laminations, could not be more reliable. Save for the shaft-to-stack assembly, there is no other component on the rotor to cause lifetime failures. Bearing integrity truly becomes the lifetime limit on this‘brushless motor.’ ”
At the same time, Mr. Lawson mentions that the SR motor has detrimental torque ripple, especially at low speeds. “It acts like a huge step motor, [but] performance is quite acceptable for certain niche applications.”
Advantages and downside issues of SR motors—for example, torque ripple and acoustic noise—are discussed in the main article . Improvements are being made at substantial developmental effort and cost, which cuts into potential savings of the lower-cost motor. SR motors also face stiff competition from alternative technologies. A gigantic installed base of ac induction motors and permanent magnet synchronous motors exists, against which SR-motor-based systems must compete head to head. Because of their relatively smaller production numbers, manufacturing costs for SR technology tend to be higher.
According to Baldor product manager John Mazurkiewicz, SR motors and drives represent custom applications, where every motor and control has to be optimized. “This can be potentially less expensive if the volume of business is there,” he says.
Baldor Electric is currently tooled to produce SR motors and controls for a “commercial mixing” application, requiring 2.7 hp at 1,200 rpm. This motor is an open lamination, fan-cooled design, and approximates a 180 NEMA frame. The SR drive has an open-chassis design that fits inside a panel on the mixing machine. The drive accepts commands over an RS-485 serial bus from an operator interface that holds timer recipes for different mixes.
On the controls side
SR controls have essentially the same design as other types of polyphase motor controls.
Typical SR controls consist of a converter section for changing ac input power into dc power using rectification and filtering, followed by a power control section that uses digital electronics to provide low-level control signals, then an inverter section, where drivers and power output devices supply dc bus powered, pulse-width modulated (PWM) currents to the motor phases.
Some manufacturers say they need more suppliers of integrated power modules (IPMs) that can meet power requirements of industrial SR controls. Suitable IPMs could lower the cost of SR power controls and add reliability.
Switched-reluctance technology is going after niche industrial markets. This follows a spurt of commercial applications in the past few years, the most publicized of which was the use of SR motors in Maytag washing machines. Automotive applications—especially in hybrid-electric vehicles—are now drawing the attention of SR motors for such uses as brake-by-wire and steering assist. Centrifuges for laboratories, textile machinery spindle axes, and screw-type air compressors are among present industrial applications. Ability of SR motors to withstand extreme temperatures (up to 1,000 °F) is also being investigated.
Emotron AB (Helsingborg, Sweden; U.S. office: Toledo, OH)—a long-standing manufacturer of switched-reluctance products—mentions that its latest SR motor and drive product, made in large quantities, is at work in commercial building ventilation systems. EMS-VVX low-speed motors and drives provide efficient control of rotary heat exchangers in the ventilation system, without the need for a gear reducer. According to Emotron marketing director Per Zellman, these rotating heat recovery wheels can be as large as 3.5 m (11.5 ft) in diameter (see photo). “The drive system is based on sensorless SR motor control and incorporates low-speed operation as well as braking capacity,” says Mr. Zellman.
Emotron’s EMS-VVX switched-reluctance drive system is available in three motor sizes with up to 6 Nm (53.1 lb-in.) torque output and with matching sizes of controllers. For the ventilation-system application shown, the three sizes of SR drives can handle heat exchanger wheels of 1.5, 2.5, and 3.5 m maximum diameter, respectively, at 10-12 rpm.
Baldor Electric cites the following “application opportunities” for which it is currently developing SR motors: Screw-type air compressors; centrifugal liquid pumps; and commercial washing, extracting, and drying equipment.
Additional SR technology suppliers
A number of manufacturers are active in the field of switched-reluctance motors and drives, in addition to those covered in the main article. Here is a sampling.
Ametek Lamb Electric (Kent, OH) produces a line of vacuum fan systems driven by switched-reluctance motors. The company is a major name in vacuum/air moving motors for original equipment manufacturers (OEMs) in floor care, outdoor power equipment, and other markets.
Radio-Energie (Marcoussis, France) manufactures a line of brushless SR motors and drives that operate on 24 V dc power supply. Power range of the motors is 0.7-2 kW at 3,000 rpm. These magnet-free products are said to provide a competitive solution for forklift trucks, light electric vehicles, and related traction applications. For more information, visit www.radio-energie.fr
SR Drives Ltd. (Harrogate, U.K.), a division of Emerson Electric Co. of the U.S., is a well-known designer and developer of switched-reluctance technology. The company provides all major SR system elements—motor, drive, and associated power and control electronics, circuits, etc. For more information, click www.srdrives.com