Small and high-temperature electrical machine for vehicle applications

Abstract

Interior permanent magnet (IPM) motors are a very promising design alternative in comparison with other types of electrical motors. Even though the price of rare-earth magnets has become a severe concern, IPM motors are gaining increasing attention due to their high torque density and excellent field weakening performance. Therefore, researchers have attempted to reduce the use of magnet materials but, at the same time, maintain the output performance of IPM motors. One solution is to reduce the size of the machine, which will also reduce the amounts of all materials used. Generally, a small scale is a profound advantage. Still, it may constitute a deficiency from the thermal point of view by contributing to higher loss density and problems operating at a higher temperature. An IPM motor may fail due to winding failure or the demagnetization of permanent magnets. It is crucial to make sure that these motors can run safely. The purpose of this study is to develop a new electrical machine for automotive applications that is smaller in size with minimised use of magnets and which meets all requirements. The focus is on design alterations to reduce the size of the motor. Furthermore, high-temperature materials are used to ensure that the motor can work safely, even in hotter conditions. A comparison is conducted on the performance of different sizes of the motor using finite element analysis in attempting to reduce the usage of the magnet material. Then, the temperature and heat transfer exposure of IPM motors are predicted by applying thermal modelling. Research is also conducted to find the most suitable material for smaller IPM motors to run at higher temperatures. Besides using neodymium-iron-boron as magnet material for an existing IPM motor, this study also analyses an IPM motor with samarium-cobalt, which has advantages in terms of higher temperature operation. The characteristics of IPM motors equipped with distributed and concentrated winding for automotive applications are also considered, and a proper motor winding is proposed. Two prototype IPM motors with different sizes and magnet materials are built and tested to validate the finite element analysis results. The first machine is developed using the same materials as in the existing Nissan Leaf machine, while the second is designed using high-temperature materials. The most suitable size of a smaller IPM motor is ultimately determined, which can reduce the usage of permanent magnet and other material, but which maintains the output performance of existing IPM motors. The design allows significant weight and size reductions in comparison with existing PM motors due to the use of high-temperature materials, which makes this electrical motor the right candidate for traction drive applications. The motor also satisfies all safety requirements.