Electromagnetic Characteristics of a Flux-Modulated Permanent Magnet Linear Machine With Sandwich Armature for Free-Piston Energy Converter System

The linear machine is the key part of free-piston energy converter system, and its electromagnetic characteristics directly determine system performance. This paper proposes a novel flux-modulated permanent magnet (PM) linear machine with sandwich armature for free-piston energy converter system. The sandwich armature is composed of modulated layer, air-core winding layer and iron core. It integrates electromagnetic advantages of a flux-modulated and slotless machine, so it can achieve good comprehensive performance by appropriate design of sandwich armature. Firstly, an accurate subdomain model is established to analyze magnetic field characteristics of proposed machine, and then the suitable pole/slot combinations are determined. Subsequently, machine performance such as thrust force and efficiency are discussed based on design dimensional constraints. Finally, structural optimization of cylinder armature is developed for eddy-current loss suppression, and then experiments on prototype are carried out. The performance comparisons with flux-modulated and slotless machines show that the proposed prototype has high peak thrust, small electromechanical time constant, and strong design flexibility. Hence it is a preferred solution for free-piston energy converter system.