Variable-reluctance (VR) or switched-reluctance (SR) motors also can operate as brushless dc motors. VR motors have salient poles on a soft-iron rotor. Motor action results from interaction between the rotor poles and a rotating field set up by the stator windings. The construction yields high torque-to-inertia ratios.
VR motors cost less than corresponding permanent-magnet brushless types. And because VR types call for unidirectional current, amplifiers for them cost less than those for conventional types. The motors are increasingly used in motion-control systems that require very high torque or high horsepower -- levels where the cost of magnets in conventional PM motors become excessive.
A 150-pole version of a variable-reluctance motor is widely used for powering robots and other machines calling for slow speed and precise positioning. The motors exhibit much wider bandwidths -- typically over 80 Hz -- than conventional motors. A 32-bit microprocessor-based adaptive controller adjusts frequency response in real time.
Another motor recently developed is generally classified as variable reluctance. But the products are sometimes thought of as hybrid permanent-magnet motors because they contain two permanent magnets mounted axially on the rotor. The motors are smaller, less costly, and operate at higher speeds than conventional dc brushless motors. They also develop up to 2,000 oz-in. torque and operate up to 5,000 rpm