Influence of partly closed slots on the differential inductances

A prerequisite for sensorless field oriented control of permanent magnet synchronous machines (PMSMs) at very low speed and standstill operation using signal injection methods is a sufficient saliency ratio of the differential inductances throughout the desired operating area. High frequency signal injection methods are very common in order to perform saliency tracking based sensorless operation. Vanishing saliency for certain operating points or a change of the sign of the difference of the differential inductances, which needs to be known for the sensorless algorithm, causes the loss of stability of the sensorless control method. A closed or partly closed stator slot geometry tends to show a local current dependent loss of the saliency even for low currents. This phenomenon typically shows up as a star-shaped pattern of the inductance surface as discussed further within this paper.