Determination of effective air-gap length of reluctance synchronous motors from experimental data

Because of many advantages of reluctance synchronous motors (RSM) over other motors, their usage in AC drives have been gaining importance for many industry applications. The performance of RSM is greatly dependent on the effective air-gap lengths of the machine along its d and q axes. In this paper, an attempt has been made to obtain the effective air-gap lengths of the machine from the experimental values of direct and quadrature axis reactances by considering the higher permeance and winding space harmonics of a RSM. These reactances are then compared with the coefficients of the magnetizing inductances of the stator windings, obtained by using winding function approach (WFA), to determine the air-gaps. A comparative study has been carried out with different permeance and winding space harmonics. The q-axis air-gap length seems to be much more sensitive to the operating point and the leakage inductance compared to the d-axis air-gap length. Experimental values near no-load have been considered to minimize flux barrier effects, as WFA cannot simulate these effects without changing the length of the pole arc. The effective air-gaps of another salient pole synchronous machine with damper bars have also been determined and compared with various permeance and winding space harmonics, when run as a RSM (without field excitation). This machine did not have flux barriers and hence the results seem to be more accurate.