The Switch to Switched Reluctance

They can be more efficient than induction motors and work at variable speeds. They are also inherently simple. For a time, they powered Maytag Neptune washing machines.

Switched-reluctance (SR) motors were developed in the 1800s but, apart from a few embedded-drive applications, they have not been widely applied. Their optimum operation depends on relatively sophisticated switching control, something not economical until the advent of compact but powerful solid-state power devices and ICs. Now, with a new emphasis on energy efficiency, switched-reluctance motors may be ready to take a more prominent role in appliances, industrial equipment, and even off-road gear.

A switched-reluctance motor works in a way that is somewhat analogous with a stepper motor. An SR-motor rotor consists of laminated-steel protuberances. It carries no windings, magnets, or other features. The protuberances are strongly magnetically permeable. Areas surrounding them are weakly permeable by virtue of slots cut into them.

SR-torque production resembles that of stepmotors because coils in the stator serve as electromagnets that attract the nearest rotor poles. One important difference between an SR motor and a stepper is that a stepper operates open loop. The rotor follows the magnetic field of the stator, but there is a possibility the two could get out of sync. On the other hand, an SR motor does not operate open loop but rather monitors its rotor position. Stator coils get energized in synchronism with the rotor and only when it is advantageous to do so. Moreover, the motor is configured so the phases overlap. Thus torque transfers smoothly from phase to phase thanks to sophisticated switching.

The torque produced by the SR motor is controlled by adjusting the magnitude of current in the electromagnets. Speed is controlled by modulating the torque (via winding current) in the same way speed is controlled via armature current in traditional brush-dc motors and drives.

And SR motors can be engineered for power rather than for step accuracy as is the case with stepper motors. For example, Emerson Motor Technologies says it has routinely fielded SR motors in the 100 and 200-hp range, sizes not practical with steppers.

SR motors also have advantages in motion-control applications. An SR motor can produce 100% torque at stall indefinitely. This is because there is no heat produced in the rotor at stall. Rotor bearings stay cool as well. Only the stator coils experience a temperature rise, and they can be cooled via fins on the stator housing or other conventional means

SR-stator windings are also simpler than those on induction or permanent-magnet ac motors: Each slot in the stator contains windings for only one phase. A winding that emerges from the stator slots need only loop back around one slot, rather than around multiple slots as on induction motors. This keeps down the volume of end windings and minimizes the risk of a phase-to-phase insulation failure.

Emerson, which has an SR-motor line dubbed SR Drives, points out this construction minimizes the energy lost on coil overhangs at the slot ends, since magnetic fields generated at the end of the slot do not contribute to doing work. A smaller end-winding area also keeps down the length of the motor and the amount of heat to be dissipated. Emerson says the result can sometimes be an SR motor one or two frame sizes smaller than an equivalent induction motor.

A point to note about SR motors concerns their reliance on position feedback from the rotor to operate. The rotor encoder can have a resolution that is relatively coarse compared to a transducer used on a motion-control application. The encoder feedback serves only to switch phases on and off, so it need only have the same number of pulses-per-revolution as the rotor has poles. Simple lowcost, low-resolution Hall-effect devices are sufficient.

Alternatively, it is possible to run the motors in a sensorless scheme. This method uses electronics to detect changes in the phase inductance as the rotor moves past. The inductance changes by factors of 5:1 or more as the rotor moves from a fully aligned to fully misaligned position with respect to the phase windings.