Electromagnetic Field Analysis Of Multiphase Permanent Magnet Generator

Multiphase permanent magnet generator combines the merits of multiphase machines and permanent magnet machines together, whose research and application are still of frontier science nowadays. Compared with traditional electric machines, multiphase permanent magnet generators are more suitable in high power low voltage DC system for a high reliability. Based on the elementary principles of electromagnetic field, the performance and parameters of nine phase semi-symmetrical permanent magnet generators are analyzed and calculated with finite element method.The background, tendency as well as significance of this subject are briefly introduced at the first part of this paper. The concept and types of multiphase windings are illustrated with fundamental rotating magnetomotive force(MMF) of this nine phase semi-symmetrical windings analyzed. The advantages of such windings over traditional three phase ones are listed afterwards.The theoretical basis of electromagnetic field in multiphase permanent magnet generators and corresponding analytical softwares in finite element calculation like Ansys and Ansoft are illustrated. The multiphase permanent magnet generators are analyzed in static field for calculation of some important parameters such as no-load leakage flux coefficient, waveform coefficient, Carter’s coefficient and the effective pole-arc coefficient. The magnetic flux density in air gap of no-load operation is then calculated, from which the fundamental component is extracted to derive the induced voltage. And both the waveshape and value of output voltage for rated load are derived with current iteration method. The direct and quadrature armature reaction reactances are also calculated and can be mutually verified with above two softwares. Energy method is used to determine the stator slot leakage reactance and compared with the traditional magnetic circuit method.As a particular phenomenon in permanent magnet machines, cogging torque is analyzed detailedly in theory with the most effective parameters proposed. Based on the quantitive finite element analysis, pole-arc coefficient, stator slot opening width and the magnet thickness all affect the cogging torque to an extent, which is significant for the elimination of vibration and noise in practice.The transient field model is established at last for a field-circuit coupling simulation of multiphase permanent magnet generator in time-stepping finite element method with end winding effect taken into consideration. The sudden short circuit fault in DC side of the multiphase permanent magnet generator rectifier power system in no-load operation is studied with the peak value of short circuit and the time when it appears recorded, which is meaningful for the setting of relay protection devices.This paper is of reference value for research and design of multiphase permanent magnet generators.