A new topology of CMG for high torque and low loss

Abstract

Aiming at the problems that the coaxial magnetic gear (CMG) has lower torque density and higher eddy current loss, a relation model between the effective components of the modulation magnetic field and the original field is deduced based on the calculation method of the air gap flux density of the permanent magnetic motor and its unilateral grooved air gap permeance. According to this model, the modulation effect of the modulating ring is the best when the original magnetic air gap field of the permanent magnetic ring in CMG is close to sinusoid. Hereby a new topology is proposed where the traditional yoke magnetic conductive structure is replaced by the structure of the permanent magnet itself, thus the permanent magnet of the permanent magnetic ring in CMG would be of a higher utility rate of space and form a more ideal air gap magnetic field. Using control variable method (CVM), 2D finite element method (2D FEM) and fast Fourier transform (FFT), the simulated analysis between the old and the new structures proves that when without a modulating ring, the original air gap magnetic field formed by the traditional yoke magnetic conductive structure is square wave, whose magnetic field modulated by the modulating ring is mainly composed of harmful fundamental wave. Meanwhile, however, the new topology presented is of an original air gap magnetic field closer to the ideal sinusoid, and the modulated magnetic field of which is mainly composed of harmonic wave coupling with the permanent magnet on the permanent magnetic ring of the other side. Taking advantage of CVM and 2D FEM, the performance differences of the old and the new structures are analyzed on the aspects such as torque density and eddy current loss, and the results show that with the same inner and outer rotor volume, as more permanent magnets could be placed, the new topology is of a higher torque density. Moreover, the increase of the torque density lessens the volume of the outer magnetic ring, thus lessening its change of flux. As a result, with identical speed and maximum torque, the eddy current loss of the new structure is also lower.