What is a switched reluctance motor?
Working principle
A switched reluctance motor (SRM) has salient poles on both the stator and the rotor. The stator carries simple concentrated coils; the rotor is only stacked steel, with no magnets, no cage and no windings. When a stator phase is energised, the nearest rotor poles are pulled into line with it to shorten the magnetic path, and that pull is the torque. The phases are then energised one after another, so the rotor keeps chasing the energised pole pair around.
Torque and inductance
The torque of one phase is T = ½·i²·dL/dθ. It is positive while the phase inductance rises, as a rotor pole approaches the stator pole, and negative while it falls. Because the current is squared, the direction of the torque does not depend on the direction of the current, so each phase needs only a unipolar current. That is why the converter can be an asymmetric half-bridge with two switches and two diodes per phase, which can never short the DC link.
Turn-on and turn-off angles
The converter applies +V across a phase from the turn-on angle to the turn-off angle, then −V until the current has fallen back to zero. At low speed the current rises fast and is chopped to hold a reference. At high speed there is little time, the current never reaches the reference and a single pulse remains, so torque falls roughly with the square of the speed. A turn-off that is too late leaves current in the falling-inductance region and brakes the rotor.
Torque ripple and applications
Torque arrives in pulses, one per stroke, so the ripple is large and so are vibration and acoustic noise. An 8/6 machine with four phases overlaps its strokes and runs smoother than a 6/4 with three. In return the rotor is cheap, robust and can run very hot or very fast, and a failed phase does not stop the machine. SRMs are used in vacuum cleaners, textile machines, aircraft starter-generators and traction drives.
Frequently asked questions
- Why does a switched reluctance motor not need magnets?
- The rotor is pulled towards any energised stator pole because magnetic flux takes the path of least reluctance, so no rotor field is needed. The torque follows ½·i²·dL/dθ and does not depend on the sign of the current. In the simulator, the cycle chart shows torque appearing only while the inductance rises.
- Why is the torque ripple of an SRM so large?
- Each phase gives a pulse of torque during only part of the rotor pole pitch, and the pulse follows the current as it builds and decays. Where the pulses of neighbouring phases overlap poorly, the total torque dips between them. Switch from 6/4 to 8/6 and compare the torque curve: the ripple falls because more phases share each pitch.
- What happens if the phase is switched off too late?
- The current cannot drop instantly: it decays under −V while the poles approach alignment. If it is still flowing after alignment, the inductance is falling, so the same current now produces negative torque that brakes the rotor and wastes energy. Move the turn-off angle towards 0° and beyond, and watch the negative torque share and the efficiency.