Inherent Difference in Saliency for Generators with Different PM Materials

The inherent differences between salient and nonsalient electrical machines are evaluated for two permanent magnet generators with different configurations. The neodymium based (NdFeB) permanent magnets (PMs) in a generator are substituted with ferrite magnets and the characteristics of the NdFeB generator and the ferrite generator are compared through FEM simulations. The NdFeB generator is a nonsalient generator, whereas the ferrite machine is a salient-pole generator, with small saliency. The two generators have almost identical properties at rated load operation. However, at overload the behaviour differs between the two generators. The salient-pole, ferrite generator has lower maximum torque than the NdFeB generator and a larger voltage drop at high current. It is concluded that, for applications where overload capability is important, saliency must be considered and the generator design adapted according to the behaviour at overload operation. Furthermore, if the maximum torque is the design criteria, additional PM mass will be required for the salient-pole machine. 1. Introduction The material most commonly used as permanent magnets (PMs) in electrical machines is neodymium-iron-boron, Nd2Fe14B (shortened as NdFeB). During the last five years the price for NdFeB has increased and fluctuated extremely. In addition, 97% of all mining currently occurs in China and the export quotas for rare-earth metals from China are politically controlled [1]. Furthermore, the environmental aspects of rare-earth metal are an issue. Therefore, the idea to substitute NdFeB with ferrites has been suggested [2] and tested [3]. Several theoretical comparisons of generators with NdFeB and ferrites for wind power generators have previously been performed [2, 4–6]. In most of these studies [4–6] the better magnetic performance of NdFeB is used as an argument for its advantage. However, when the availability and unstable price development for NdFeB are considered, ferrites are considered superior, especially for applications where the weight increase is of less importance [2]. NdFeB generators have been extensively studied [7, 8]. In wind turbines, variable speed operation is increasingly popular. Therefore, the behaviour of a permanent magnet synchronous generator (PMSG) with NdFeB at variable speed and load has been especially evaluated [9]. When substituting NdFeB with ferrites, very similar characteristics can be found at the nominal operating point [2]. However, the intrinsic difference between these two generator types was not considered in [2]; that is, the