RELUCTANCE MOTOR—
This is sort of "Something new under the sun", but not really. The reluctance type motor is similar but different from the well know induction motor and the nearly as well known PMSM, permanent magnet synchronous motor both of which are widely used in sizes ranging from 100 watt up to 500,000 watts.
Let's define reluctance as R=(amp turns)/[magnetic flux (in webers)]The R is usually in a script capital R. The only time I have ever used the reluctance formula was in the redesign of a relay armature. In the case of motors you can think of a reluctance motor as induction wound motor but with the rotor replaced with one of two types of rotors. The first is a smooth iron with strategically placed "flats" and the second is again smooth iron but with strategically placed embedded copper bars.
The upshot of this replacement produces a motor that has induction motor starting characteristic but with synchronous motor characteristics at synchronous speed. Not obvious other advantages that accrue are very high torque output and extreme rpm speed range when driven by VVVF (variable voltage variable frequency) drive. More about that later. Emerson Motor Co. call their offering SR for "switched reluctance" and build in standard NEMA frame sizes. Other companies offering reluctance motors are Swedish Emotron AB and Rocky Mountain
Technologies. SR motors don't do well direct-on-line, and are much better with an associated power converter to complete the package. Further the mechanical tolerances between the rotor and field windings are about 2 to 1 less critical than in either the PMSM or induction types of comparable size. Also the reluctance type motor can be more easily shaped into pancake or long cylinder type as end use requires.