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.

Construction

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.

Operation

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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