advantages for motor starting, speed control and smooth operation (low torque ripple). The inductance of each phase winding in the motor varies with position, because the reluctance also varies with position. This presents a control systems challenge. Types Synchronous reluctance Synchronous reluctance motors have an equal number of stator and rotor poles. The projections on the rotor are arranged to introduce internal flux “barriers“, holes that direct the magnetic flux along the so-called direct axis. The number of poles must be even, typically 4 or 6. The rotor operates at synchronous speeds without current-conducting parts. Rotor losses are minimal compared to those of an induction motor, however it doesn't normally have a lot of torque. Once started at synchronous speed, the motor can operate with sinusoidal voltage. Speed control requires a variable-frequency drive. Switched reluctance or variable reluctance The switched reluctance motor (SRM) is a form of stepper motor that uses fewer poles. The most rudimentary form of a SRM has the lowest construction cost of any electric motor because of its simple structure, and even industrial motors may have some cost reduction due to the lack of rotor windings or permanent magnets. Common uses include applications where the rotor must be held stationary for long periods, and in potentially explosive environments such as mining because it operates without a mechanical commutator. The phase windings in an SRM are electrically isolated from each other, resulting in higher fault tolerance than inverter-driven AC induction motors. The optimal drive waveform is not a pure sinusoid, due to the non-linear torque relative to rotor displacement, and the highly position-dependent inductance of the stator phase windings. Applications Analog electric meters Analog electric clocks Some washing machine designs Control rod drive mechanisms of nuclear reactors Hard disk drive motor Electric vehicles? Power tools such as drill presses, lathes, and bandsaws See also Electric vehicle motor Flux switching alternator, a similar machine arrangement, used as a generator. Switched reluctance motor Stepping motor References External links Real-Time Simulation of Switched Reluctance Motor Drives Technical Paper Electric motors