Analytical Prediction of Eddy-Current Loss in Modular Tubular Permanent-Magnet Machines

Abstract

The paper describes an analytical technique for predicting the eddy-current loss in the moving armature of a tubular permanent magnet machine. This loss component is usually neglected in conventional tubular permanent magnet machines since high-order time harmonics in the stator current waveform and space harmonics in the winding magnetomotive force (MMF) distribution are generally considered to be insignificant. However, a relatively new topology of tubular permanent magnet machine, sometimes referred to as "modular", has emerged in which the fundamental component of the stator MMF has fewer poles than that of the permanent-magnet armature, the thrust force being developed by the interaction between a higher order MMF harmonic and the permanent magnet field. Thus, the presence of lower and higher order space harmonics in the winding MMF distribution of a modular machine may gives rise to a significant eddy-current loss in the moving-magnet armature. An analytical model is developed to predict the eddy currents which are induced in the magnets, as well as in any electrically conducting supporting tube which may be employed, and to quantify the effectiveness of axially segmenting the magnets in reducing the eddy-current loss. The validity of the developed model, which is also applicable to conventional designs of tubular permanent-magnet machine, is verified by time-stepped transient finite-element analysis (FEA).