Theory, performance prediction and indirect sensing of rotor position of aswitched reluctance motor under saturation

A simple and reasonably accurate method of predicting the performance of a SR motor under saturated condition directly from its design data is present ed. First, the L - theta and dL/d theta - theta curves under unsaturated co ndition have been obtained using an approximate analytical method originall y proposed by Pohl for inductor alternators in 1946. The method takes into consideration the slot fringing flux but is easier to apply, if not as exac t, as compared to conformal mapping technique. Assuming the ampere-turns fo r the iron parts of the machine to be dependent only on the total flux, a v ery simple method of calculating the torque developed under saturation cond itions is presented and is used to obtain the static torque against rotor p osition and the torque-speed characteristics of the motor. Finally, a metho d for online measurement of unsaturated phase inductance has been developed and is used to switch different phases of the motor sequentially without t he need for a shaft mounted position sensor. The theory developed has been applied for the prediction of the performance of a small SR motor designed and fabricated locally as well as that of a commercially available motor.