Switched reluctance motor

The switched reluctance motor (SRM) is a type of stepper motor, an electric motor that runs by reluctance torque. Unlike common DC motor types, power is delivered to windings in the stator (case) rather than the rotor. This greatly simplifies mechanical design as power does not have to be delivered to a moving part, but it complicates the electrical design as some sort of switching system needs to be used to deliver power to the different windings. Electronic devices can precisely time switch, facilitating SRM configurations. Its main drawback is torque ripple. Controller technology that limits torque ripple at low speeds has been demonstrated. The switched reluctance motor (SRM) is a type of stepper motor, an electric motor that runs by reluctance torque. Unlike common DC motor types, power is delivered to windings in the stator (case) rather than the rotor. This greatly simplifies mechanical design as power does not have to be delivered to a moving part, but it complicates the electrical design as some sort of switching system needs to be used to deliver power to the different windings. Electronic devices can precisely time switch, facilitating SRM configurations. Its main drawback is torque ripple. Controller technology that limits torque ripple at low speeds has been demonstrated. An alternate use of the same mechanical design is as a generator. The load is switched to the coils in sequence to synchronize the current flow with the rotation. Such generators can be run at much higher speeds than conventional types as the armature can be made as one piece of magnetisable material, as a slotted cylinder. In this case the abbreviation SRM is extended to mean Switched Reluctance Machine, (along with SRG, Switched Reluctance Generator). A topology that is both motor and generator is useful for starting the prime mover, as it saves a dedicated starter motor. The SRM has wound field coils as in a DC motor for the stator windings. The rotor however has no magnets or coils attached. It is a solid salient-pole rotor (having projecting magnetic poles) made of soft magnetic material (often laminated steel). When power is applied to the stator windings, the rotor's magnetic reluctance creates a force that attempts to align the rotor pole with the nearest stator pole. In order to maintain rotation, an electronic control system switches on the windings of successive stator poles in sequence so that the magnetic field of the stator 'leads' the rotor pole, pulling it forward. Rather than using a mechanical commutator to switch the winding current as in traditional motors, the switched-reluctance motor uses an electronic position sensor to determine the angle of the rotor shaft and solid state electronics to switch the stator windings, which enables dynamic control of pulse timing and shaping. This differs from the apparently similar induction motor that also energizes windings in a rotating phased sequence. In an SRM the rotor magnetization is static (a salient 'North' pole remains so as the motor rotates) while an induction motor has slip, and rotates at slightly less than synchronous speed. SRM's absence of slip makes it possible to know the rotor position exactly, allowing the motor to be stepped arbitrarily slowly.