Although induction motor drives still are the workhorse of industry, the switched reluctance motor drive has been actively researched over the past decade with very promising results. The switched reluctance machine has a simple and rugged construction as well as very good overall performance over a wide torque-speed range. Recently, doubly-salient switched reluctance motors have been found to be an attractive alternative to more conventional synchronous and induction machines in low horsepower converter fed variable speed drive applications.
Because the current waveform of the switched reluctance motor must be carefully programmed to extract the maximum torque per ampere, this machine is more accurately termed a current regulated stepping motor (CRSM or CRS motor). The fundamental feature of this type of motor drive is that the CRS motor requires only a unidirectional current and thus the drive circuit topology and corresponding switching algorithm is greatly simplified. A detailed comparison of the CRS motor drive with a high efficiency induction motor drive has indicated that performance parameters such as torque per unit stator volume, torque per unit inertia and torque per unit copper weight, can be made equal to that of an induction machine, or in some cases, even exceed the induction machine. M. R. Harris et al., "A review of the Integral Horsepower Switched Reluctance Drive," IEEE Trans. Ind. Appl. Soc., Vol. IA-22, July/Aug. 1986, pp. 716-721.
While the recent work on CRS machines has yielded encouraging results, in several respects the machine is less than optimum. For example, taking an eight stator/six rotor pole CRS motor, with typical excitation of one of the four phases, only one quarter of the stator inner circumference is utilized to make a contribution to torque development at any instant. Secondly, the inductance variation of the occupying coils over each one fourth of the machine is limited by the so called double salient design. To have a comparable power rating to that of the conventional induction machine of the same size with such a limited air gap inner surface area, the CRS machine is designed to operate in a deeply saturated condition. The corresponding active material is thus under severe electromagnetic stress. It is apparent that any method of ensuring that the other three quarters of the inner circumference of the stator remain "active" would be a very significant step toward improving torque production and therefore, towards relaxing the severe electromagnetic stress and consequent iron losses in the active materials of the machine.
Electromechancial energy conversion in a CRS motor is accomplished by means of a time varying inductance due to the temporal variation of the rotor position. The switched reluctance motor typically has phase winding coils located on opposite sides of the stator and a multi-pole rotor. The poles of the rotor are brought into and out of alignment with the poles of the stator as the rotor rotates, periodically increasing and decreasing the inductance of the coils for each phase. In an idealized machine, the inductance varies with rotor angle as a form of triangular wave, with current being supplied by the converter to the phase coil during the time that inductance is increasing to obtain motoring operation. Generating torque is produced if the machine is excited during the interval in which the inductance of the winding is decreasing. Torque production is proportional to the square of the current and therefore independent of current polarity if mutual inductance is not involved. Hence, the windings can be excited with unidirectional currents.
In general, to maximize the torque for a given current in a CRS machine, the change in inductance as a function of rotor position should be maximized. However, in a conventional CRS motor, the ability to maximize the change of inductance with rotor position is limited. To maximize inductance, the stator pole should be completely aligned with the rotor pole but to minimize inductance the stator pole should be totally nonaligned with the rotor pole. In general, this requirement, and limitations imposed by leakage flux and the permeance of the main flux, result in a maximum achievable ratio of maximum inductance to minimum inductance of ten to one, when saturation is neglected. Another limitation, set by the small working area of the double salient structure, is that the permeability of cross section is strongly affected by the excitation level, which is detrimental to maximizing the ratio of maximum to minimum inductance. Thus, a significant improvement in the change in inductance with respect to rotor position in a CRS machine is not practical without increasing the number of poles, which unfortunately reduces the power output or speed of the machine or, conversely, requires very high switching frequencies in the associated power converter.