At the point shown in Figure (b) the rotor is aligned with the stator, and the current is switched off. Its momentum then carries the rotor on round over one-quarter of a turn, to the position of (c). Here the magnetic field is re-applied, in the same direction as before. Again, the field exerts a torque to reduce the air gap and make the field symmetrical, which pulls the rotor on round. When the rotor lines up with the stator again, the current would be switched off. In the switched reluctance motor, the rotor is simply a piece of magnetically soft iron.

Also, the current in the coil does not need to alternate. Essentially then, this is a very simple and potentially low cost motor. The speed can be controlled by altering the length of time that the current is on for in each power pulse. Also, since the rotor is not a permanent magnet, there is no back EMF generated in the way it is with the BLDC motor, which means that higher speeds are possible. In the fuel cell context, this makes the SR motor particularly suitable for radial compressors and blowers.

The main difficulty with SR motor is that the timing of the turning on and off of the stator currents must be much more carefully controlled. For example, if the rotor is 90? out of line, as in Figure (a), and the coil is magnetised, no torque will be produced, as the field would be symmetrical. So, the torque is much more variable, and as a result early SR motors had a reputation for being noisy.

The torque can be made much smoother by adding more coils to the stator. The rotor is again laminated iron, but has ‘salient poles’, i.e. protruding lumps. The number of salient poles will often be two less than the number of coils. Figure (d) shows the principle. In figure (a) coil A is magnetised, exerting a clockwise force on the rotor. When the salient poles are coming into line with coil A, the current in A is switched off. Two other salient poles are now nearly in line with coil C, which is energised, keeping the rotor smoothly turning. Correct turning on and off of the currents in each coil clearly needs good information about the position of the rotor. This is usually provided by sensors, but modern control systems can do without these. The position of the rotor is inferred from the voltage and current patterns in the coils. This clearly requires some very rapid and complex analysis of the voltage and current waveforms, and is achieved using a special type of microprocessor called a digital signal processor.

The greater control precision needed for the currents in the coils makes these motors somewhat harder to apply on a ‘few-of’ basis, with the result that they are most widely used in cost-sensitive mass-produced goods such as washing machines and food processors.

Figura (d)

El torque se puede hacer mucho más suave agregando más bobinas al estator. El rotor es nuevamente de hierro laminado, pero tiene 'polos salientes', es decir, protuberancias sobresalientes. El número de polos salientes a menudo será dos menos que el número de bobinas. La figura (d) muestra el principio. En la figura (a) la bobina A está magnetizada, ejerciendo una fuerza en el sentido de las agujas del reloj sobre el rotor. Cuando los polos salientes se alinean con la bobina A, la corriente en A se desconecta. Los otros dos polos salientes ahora están casi en línea con la bobina C, que está energizada, manteniendo el rotor girando suavemente. El encendido y apagado correcto de las corrientes en cada bobina claramente necesita una buena información sobre la posición del rotor. Esto es proporcionado generalmente por sensores, pero los sistemas de control modernos pueden prescindir de los mismos. La posición del rotor se deduce de los patrones de voltaje y corriente en las bobinas. Esto claramente requiere un análisis muy rápido y complejo de las formas de onda de voltaje y corriente, y se logra utilizando un tipo especial de microprocesador llamado procesador de señal digital.

La mayor precisión de control necesaria para las corrientes en las bobinas hace que estos motores sean un poco más difíciles de ser utilizados a baja escala de producción, con el resultado de que se usan más ampliamente en bienes producidos en masa, sensibles al costo, como lavadoras y procesadoras de alimentos.

Para este propósito, la técnica más utilizada para fabricar diodos de conmutación es añadir al semiconductor un metal que sea eficaz para disminuir los potadores. En el caso de los diodos de silicio, se suele dopar con oro.