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

The usage of synchronous reluctance motors (synchRel) has been gaining importance in many industry applications mainly because of their many advantages over other motors. The performance of a synchRel is greatly dependent on its effective air-gap lengths along the d and q axes. Hence, in this paper, an attempt has been made to obtain the effective air-gap lengths of the machine from the experimental values of the d and q axes reactances and by considering the higher permeance and winding space harmonics. To determine the air gaps, these reactances are compared with their corresponding expressions involving the coefficients of magnetizing and mutual inductances of the stator windings, obtained by using the winding function approach (WFA). Using these air-gap values, a more realistic model of the machine is obtained. A comparative study has been carried out with different permeance and winding space harmonics. The computed q-axis air-gap length seems to be much more sensitive to the operating point and the leakage inductance, compared to the computed d-axis air-gap length. Experimental values near the no-load condition 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 by considering the higher permeance and winding space harmonics, when run as a synchRel (without field excitation). This machine did not have flux barriers, and, hence, the results seem to be more accurate. Finally, the performance of the simulated synchRel has been compared with the experimental results.