Design Of Low-Cost Pemanent Magnet AC Machines

High performance rare earth permanent magnet (PM) motors have been extensively used for fan and pump applications. Interior permanent magnet (IPM) ac motors have attracted increasing attentions due to their advantages of compact structure, high efficiency, and high torque density. However, the price of the rare earth PM materials NdFeB keeps increasing, therefore more and more motor manufactures have paid attention to the use of inexpensive ferrite permanent magnet motor, especially for cost-sensitive applications.The motor studied in this thesis is a permanent magnet synchronous machine with rated power of 1500W and rated speed of 1000rpm, which is used to drive an industrial fan, a cost-sensitive application. Firstly, the motor structure with 12 slots and 8 poles is studied. Four schemes are proposed, viz, two different kinds of magnet material, NdFeB or ferrite, and two different burying shapes, one magnet bar for each pole or two magnet bars inserted in a V-shaped slot for each pole. Main material cost, mass manufacturing process, and optimization details of these four types of motors are compared comprehensively. Secondly, the model which uses ferrite magnets with two bars inserted in a V-shaped slot for each pole in the rotor, and optimized stator teeth is chosen. The motor is optimized to reduce the cogging torque and back EMF total harmonic distortion (THD). The designed motor is prototyped. Experimental data agree well with the results of finite element analysis (FEA). Finally, torque ripple in the load operation is studied. Analysis proves that the 4th harmonics in self-and mutual inductances cause reluctance torque ripple which takes main part of torque ripple when back EMF THD and cogging torque are small enough to be ignored. It also shows that it is an effective method to keep some cogging torque to counteract the reluctance torque by shifting cogging torque's phase, so as to reduce total torque ripple.