The electromagnetic performance of brushless permanent magnet DC motors : with particular reference to noise and vibration
A comprehensive analytical technique is developed for predicting the instantaneous magnetic field distribution in radial-field, surface-mounted permanent magnet brushless DC motors under any load condition and commutation strategy. It is based on a 2-dimensional analysis in polar coordinates and accounts implicitly for the corresponding stator winding current waveforms and the effect of stator slot openings. In addition, a 2-dimensional analytical method for calculating the back-emf waveform is presented, whilst the analytical technique is applied to the prediction of the cogging torque waveform and the calculation of the self- and mutual-winding inductances. Also developed and validated is an analytical model for predicting the steady-state dynamic performance of a 3-phase brushless DC drive, by exploiting the periodicity in the stator winding voltage and current waveforms, with due account of the influence of commutation events in the inverter bridge, the back-emf waveform, current limiting, and commutation timing etc. The model is developed further to couple with the motion equation of the rotor to enable the transient and steady-state dynamic performance of brushless DC drives to be predicted. The effect of end-shields on the vibrational behaviour of stators is investigated by the modem modal analysis technique, and new formulae for the calculation of the acoustic power radiated by a cylindrical stator of finite length, using an analytical method, are presented. A technique which combines the finite element method and Fourier analysis to account for the effects of end-shields on the acoustic radiation is developed, and the spherical acoustic radiation model of motors has been improved by the application of finite elements. Finally, a systematic analytical approach to the estimation and analysis of the acoustic noise from a radial-field, internal rotor, brushless DC motor is presented.