Stability Limits of Saturated Interior Permanent Magnet Motors

In the modern designs of interior permanent magnet (IPM) motors, cage windings may be eliminated. Motor starting is achieved using different well-established drive techniques. However, the absence of cage windings results in less inherent stability of the motor. This paper presents a detailed investigation for the steady state stability limits of IPM motors in the absence of cage windings. The main attention has been given to the effect of magnetic saturation on both direct and quadrature axis inductances as well as on the induced emf due to the permanent magnet. Three saturation factors have been defined. The effect of these factors on the motor steady state stability limits is discussed for different conditions. The results have been checked in the time domain using the motor dynamic model. The attained conclusions are expected to be a useful guide for IPM motor designers. Being a component of the drive system, enhancing motor stability is reflected on the whole system performance. Moreover, understanding saturation effects may facilitate its implementation to obtain better performance and to extend operation range of the drive system