Consumers are increasingly choosing more sophisticated and highly-efficient electronics. To achieve this, these electronics require motors that deliver high efficiency, wide variable speed and compactness. The industry continues to transition from induction motors that are less efficient and more cumbersome from a design perspective, to switched reluctance motors (SRMs). The latter offers efficiency and an ultra-compact size to accommodate the size and weight needs of today's electronics.
The switched reluctance motor is an electric motor in which the torque is produced by the tendency of the rotor to move to a position where the inductance of the excited winding is maximized. During motor operation, each stator phase is excited while its inductance increases, and unexcited as its inductance decreases. The air gap is at a minimum at the aligned position and the magnetic reluctance of the flux flow is at its lowest. An easy way to make the rotor turn is to sequentially switch the current from one phase to the next phase and to synchronize each phase's excitation as a function of the rotor position. The direction of rotation is independent of the direction of the current flowing through the phase. Rather, it only depends on the sequence of the stator winding excitation. This uni-polar principle requires only one switch to be in series with a phase winding. This phase independence and uni-polar principle have encouraged the various converter topologies.
SRMs have several distinct advantages over most motors, including, but not limited to, induction motors. Because a switched reluctance motor has a salient rotor without rotor windings, the material costs are reduced. Furthermore, independent windings make the fault tolerant operation possible and provide a robust structure. This robust structure decreases the actual power consumption as the windings are energized and de-energized only when required. It also has high torque-to-inertia ratio and high starting torque without the problem of in-rush current.
With other motor applications, this in-rush current during start up might cause the line voltage to dip momentarily, which adversely affects the power quality and can pose a problem in meeting government regulations. There can be some drawbacks to using SRM technology which need to be examined. For example, SRM operation requires knowledge of rotor position. Therefore, SRMs usually must include sensors, which can increase the overall cost of the system. Another drawback of SRMs is the need for sophisticated acoustic noise control due to the vibrations inherent with operation and application needs to be unaffected by torque ripple or control.