Influence of flux penetration on inductance and rotor position estimation accuracy of switched reluctance machines

In one class of position-sensorless control strategies for switched reluctance motors, the position estimation relies on the measured inductance of an active or idle motor phase. The inductance is calculated from the response of the current to a voltage excitation, e.g. pulse-width modulation in an active phase or injection of high-frequency voltage pulses in an idle phase. With the calculated inductance or flux linkage, the rotor position is retrieved by means of a look-up table or an analytical formula. It is shown in this paper that the measured inductance can significantly deviate from the static inductance when flux penetration in the magnetic core becomes important. This is especially the case when the switched reluctance motor is in a position near alignment and when high-frequency voltage pulses are applied. An analytical wide-frequency model for the motor phase impedance in the aligned position is proposed and calculated results are compared to measured data. The influence of excitation frequency and duty ratio, magnetic core properties and motor geometry on the accuracy of phase inductance and rotor position estimation is discussed.