On the Use of Pulse Width Modulation Method for the Elimination of Flux Density Harmonics in the Air-Gap of Surface PM Motors

Abstract

This paper presents a novel design for the elimination of radial air-gap flux density harmonics of surface mounted permanent magnet (PM) motors by using the pulse width modulation (PWM) method. Instead of conventional full arc-shape magnets, the block magnets are configured to have a shape similar to PWM signals used in power electronics in order to achieve a quasi sinusoidal radial magnetic flux density in the air-gap. Then the obtained electromotive force does not contain the low order harmonics and is almost sinusoidal. The computation of the block magnet sizes is performed by solving a constrained optimization problem. All important variable parameters and constraints are carefully considered in the design process. The solutions are achieved with an optimal design method based on finite element analysis and Matlab optimization toolbox. The effect of different PWM segmented configurations on the flux density waveforms illustrates a high improvement in the quality of the back-EMF as well as the torque.