A position sensor using analogue devices for a SR motor without shaft mounted encoder

In the present work an analogue method to find the unsaturated (air-gap) inductance of the energized phase of a SR motor, on-line, has been developed. The air-gap inductance is an indirect indicator to the rotor position and this fact has been used to decide the turn-on instant of the next phase by comparing it with a reference value of inductance that is equivalent to a reference position. The method eliminates the need for a shaft mounted encoder, has the advantage that it requires no extra inputs other than the voltage and current of the presently energized phase and does not depend on the level of saturation of the magnetic circuit. Further, the self inductance being quite large as compared to mutual inductance between phases, this method is more robust than the methods which depend on the measurement of mutual inductance for position sensing. The effect of saturation on self inductance has been taken care in an interesting and simple way. This is considered to be a significant contribution. Contrary to the methods found largely in literature the present method is implemented with analogue devices thereby eliminating the need for fast digital processors.