Published in IET Electric Power Applications Received on 2nd January 2007 Revised on 13th September 2008 doi: 10.1049/iet-epa.2007.0391 ISSN 1751-8660 Performance improvement of permanent magnet machines by modular poles A.H. Isfahani 1 S. Vaez-Zadeh 1 M.A. Rahman 2 1 School of Electrical and Computer Engineering, University of Tehran, North Kargar Avenue, P.O. Box 14395/515, Tehran, Iran 2 Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X5 E-mail: ahassanpour@ut.ac.ir Abstract: Modular permanent magnet poles have recently been proposed to enhance the performance of permanent magnet machines and improve the exploitation of used magnet materials. An optimisation method for these poles is proposed for use in linear permanent magnet synchronous machines. The main objective of the optimisation is to select proper dimensions and material properties of modular permanent magnet poles to enhance the machine developed thrust. The optimisation is carried out based on a mathematical model of the machine obtained analytically. In particular, the developed thrust of linear permanent synchronous machines with modular poles is given by the model. Genetic algorithm is then employed to optimise pole parameters where the ratio of thrust ripples to average thrust is chosen as an objective function. Extensive investigations carried out by analytical and finite element methods confirm that substantial lower thrust ripples are produced with almost the same average thrust. 1 Introduction Different types of rotary and linear permanent magnet (PM) electric machines including permanent magnet direct current (PMDC), brushless direct current (BLDC), permanent magnet synchronous (PMS) and interior permanent magnet (IPM) machines are widely used in industrial applications [1]. In all these machines, PM poles provide the magnetic flux required for torque or thrust production. Therefore the flux density distribution (FDD) of a PM pole has a crucial role in a PM machine performance through the development of torque or thrust [2]. Also the FDD greatly influences pulsations of torque and thrust. In a BLDC machine for instance, the best performance is achieved by a rectangular or trapezoidal FDD, whereas a PMS machine usually needs a sinusoidal FDD. Therefore the shaping of FDD of a PM pole is critical in any machine design. The shaping of the FDD can mainly be done via designing appropriate shape and dimensions of PM poles, which in turn depends on many technical and economical factors. Many design methods for PM poles in synchronous machines have been proposed so far. The simplest shape for the cross section of a PM pole is a rectangle. Although the manufacturing of such a pole is inexpensive, its FDD is far from a sinusoidal shape distribution as it is flat in the middle of the PM pole. It means that the FDD contains harmonics of higher order, in addition to its fundamental component. It is well known that these harmonics cause torque or thrust ripples in a rotary or linear PMS machine, respectively. Different methods have been proposed to make the shape of such a FDD as close to a sinusoidal shape as possible to reduce ripples. A simple method is the elimination of one or more harmonics by adjusting the width of the PM pole [3]. However, this method may reduce the force density of the PM pole and the machine torque or thrust, since they are influenced by the pole width too. Therefore the PM material of the PM pole may not be used wisely. Another method is the use of a PM pole consisting of PM pieces of different heights as shown in Fig. 1 together with its magnetic field intensity. However, this PM pole, when used in a PM machine, causes a non-uniform machine air gap and results in audible noise and air resistance leading to a deterioration of the machine performance and efficiency. IET Electr. Power Appl., 2009, Vol. 3, Iss. 4, pp. 343–351 343 doi: 10.1049/iet-epa.2007.0391 & The Institution of Engineering and Technology 2009 www.ietdl.org