Theory, performance prediction and indirect sensing of rotor position of a switched 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 presented. First, the L-θ and dL/dθ-θ curves under unsaturated conditions have been obtained using an approximate analytical method originally 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 exact, as compared to the conformal mapping technique. Assuming the ampere-turns for the iron parts of the machine to be dependent only on the total flux, a very simple method of calculating the torque developed under saturation conditions is presented and is used to obtain the static torque against rotor position and the torque-speed characteristics of the motor. Finally, a method for online measurement of unsaturated phase inductance has been developed and is used to switch different phases of the motor sequentially without the 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