Switched-reluctance motors are not new technology, but commercial versions have emerged only in the last few years. Manufacturers of these motors claim they offer better performance and reliability, higher efficiency, and lower price than standard motors. Should you consider switched reluctance motors for your applications? Here’s a look at what these motors can offer.
Switched-reluctance (SR) motor concept has been known since 1838. But the lack of semiconductors to rapidly switch current on and off and a limited supply of engineers with the technical knowledge to design with this technology inhibited availability. Today, though, these limitations are gone.
Benefits attributed to switched reluctance (also called variable-reluctance) motors include high motor-drive efficiency and reliability, low overall system cost, and increased performance. According to several manufacturers of switched reluctance systems (among them Switched Reluctance Drives Ltd. of England, acquired by Emerson Electric Co., and Sichmemotori of Italy) their motors perform better than standard induction or other adjustable speed motors.
These motors have control characteristics similar to electronically commutated dc motors and can operate in a variety of applications, specifically where horsepower to weight ratios and sizing are critical. SR motors also meet requirements for unusual ruggedness and reliability.
One application of a SR motor was a 50-ton coal-shearing machine that needed a motor that could provide a high torque in a harsh environment. A 200-hp switched reluctance motor replaced the induction motor. In a friction welding application, two metal surfaces are rubbed together until they melt and fuse. Other motors used in this application tended to get too hot to touch after 20 minutes of operation. Switched reluctance motors, however, tend to run cool. The 22-kW motor now used in this application gets only warm to the touch after 3,500 cycles in 6 hr. In a textile-spinning application, a 55 W switched reluctance motor spins at 26,200 rpm with constant torque and peak efficiency of 80%. If a thread breaks, the motor can stop in less than one revolution. Other markets include generators, household appliances, and automotive auxiliary such as windshield wipers.
This motor looks simple, Figure 1. The rotor has no magnets or windings. It is made of salient, laminated iron that spins in exact synchrony with the drive-controlled rotating stator field. The stator, with its salient poles, has a simple construction. The number of stator and rotor poles differ, such as six and four respectively, Figure 2. No matter where the rotor comes to rest, it will always be misaligned with any applied stator field enabling a reliable re-start. The 6/4 pole configuration is common. Other possible configurations include 4/2, 8/6, 12/8, 16/12, or 32/24.
Unlike other types of motors, these motors will not run without their electronics. The number of wires connecting the motor to the control depends on the number of phases of the particular switched reluctance motor. There will be 2 wires per phase.
Switched reluctance motors exploit the fact that the forces from a magnetic field on the iron in the rotor can be up to ten times greater than the magnetic forces on the current carrying conductors.
The drive rotates the magnetic field by continuously and sequentially switching the current on and off (electronic commutation) through successive stator windings. The rotor spins, chasing the current, to try to stay aligned with the magnetic field to minimize the reluctance in the magnetic flux path. A rotor position transducer, an RPT, determines when and which of the stator poles require energizing.
Induction motors, by contrast, turn when the rotating stator field induces a voltage into the rotor windings, causing a current, which produces another magnetic field that reacts with the stator field and thus, creates a rotating torque.
The speed of the switched reluctance rotor is determined by the sequential switching speed of the stator poles, which is controlled by the semiconductor switches. The output torque is determined by the amount of current passing through the stator windings and is proportional to the square of the current. Both the speed and the torque are controllable in negative and positive directions, enabling these motors to offer four-quadrant operation.
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Some vendors claim that switched reluctance motors offer the same or better energy efficiencies than high-efficiency PWM induction motors, 93% vs. 90% without drives, for a 3.5 hp heat-pump application. In addition, switched reluctance motor efficiency can be maintained through a wide torque and speed range, Figure 3. In general, BLDC motors have higher efficiency at low speeds, SR motors have higher efficiency at high speeds.
High-efficiency motors are standard motors with specific design changes, such as larger gauge wire and high-grade silicone steels, to reduce energy losses. (See PTD, “Getting the most from high-efficiency motors,” 5/95).
Switched reluctance motors are made of standard gage copper magnet wire and low carbon steel laminations. They get their efficiencies from their construction. For example, the squirrel cage has been eliminated from the rotor. Therefore rotor losses are reduced and problems associated with over-heating of the rotor are almost eliminated. The stator winding is simplified and the end turns are reduced for less energy loss, increased winding reliability, and to simplify the manufacturing process.
Because of their construction, switched reluctance motors require fewer production processes and are therefore less expensive to manufacture than standard motors. When buying an SR motor, the drive electronics are included in the total price. Some estimates say total purchase price of one of these motor systems is at least 11% less expensive than other types of motors. (The costs of installation and wiring are not included in these estimates).
Manufacturers of switched reluctance motors claim that they can outperform standard induction motors in speed, torque, reliability, and robustness.
Switched reluctance motors can operate at speeds down to 0 rpm (with a resolver if the speed will be maintained below 50 rpm) to speeds of more than 100,000 rpm. For example, a 5 MW motor can run at a speed of 50 rpm and a 10 kW motor on a machine spindle can run at a speed of 100,000 rpm. For a given motor, 100:1 speed range is common and 1,000:1 is available. Their speed is limited primarily by the type of bearing used.
According to vendors, these motors deliver 2 to 4 times the starting and accelerating torque of a same size induction motor. This lets switched reluctance motors be one to two frame sizes smaller than an equivalent inertial-load induction motor. At rated speed, the switched reluctance motor will operate at close to its rated load, which differs from some induction motors that may run at a smaller fraction of their rated load if they were oversized to provide the necessary starting torque.
Switched reluctance motors can run forward and backward, as a motor or generator, with torque and speed independently controllable over a wide range. The responsiveness of these motors is due to their torque-inertia ratio, which is a result of the light-weight rotor and torque density. A NEMA 215T frame SR motor, unloaded, has a reversal time from 2,500 rpm to 22,500 rpm of 0.9 sec.
The control of a switched reluctance motor can program the speed-torque relationship to match load characteristics in real time, within thermal and peak torque limits. For example, one motor was reprogrammed through software to change its identity from a constanttorque industrial motor to a high-starting torque motor. Because of this wide operating speed and torque range, these motors can often be directly coupled to loads, eliminating belts, gears, and transmissions. In many applications, there’s no need to balance the motor.
Torque ripples used to be a problem in early designs. The improved design of the waveform reduces torque ripples to levels equal to or below those of other motor designs.
Because the rotor and stator are of simple design and construction, there are fewer fault modes. The motor circuit design, Figure 4, has no short-circuit paths across a dc power supply in case of failure. Each stator winding is independent, enabling switched reluctance motors to continue running with one or more disabled or shorted poles, albeit with reduced torque and smoothness. As long as at least one pole-pair is intact, the motor can run and even start with a failed condition provided sufficient torque remains.
Information for this article was supplied by the U.S. Motors Div. of Emerson Electric Co., St. Louis, Mo.