Influence of PM Coating on PM Magnetization State Estimation Methods Based on Magnetoresistive Effect

Variable flux permanent magnet synchronous machine (PMSMs) (VF-PMSMs) are gaining importance, especially in automotive applications. The use of such machines will require the development of methods to estimate the permanent-magnet (PM) magnetization state. PM magnetization state estimation methods reported in literature include back-electromagnetic (EMF) compatibility and high-frequency signal injection. One advantage of high-frequency signal injection methods is that they can operate over the whole speed range, with practically no interference with the regular operation of the machine. Magnetization estimation using high-frequency injection relies on the variation of the d-axis high-frequency inductance with the saturation produced by the magnets. Alternatively, the changes of the magnet electrical resistance due to the magnetoresistive effect can also be used for this purpose. These methods have been tested with different materials, including NdFeB, AlNiCo, and SmCo magnets. NdFeB magnets are usually protected from oxidation using NiCuNi coating. However, NiCuNi also shows magnetoresistance effect and can therefore affect the performance of the method when used with magnets having NiCuNi coating. This paper studies the effects of PM coating on the performance of methods, which estimate the magnetization state of the magnet using the magnetoresistance effect by means of high-frequency signal injection. The analysis will include NdFeB, AlNiCo, and SmCo magnets.