Research on a Tubular Yokeless Linear PM Machine

A tubular yokeless linear permanent-magnet (PM) machine is proposed for free-piston energy converter (FPEC), where high mass and volume power density are required. Topology, operating principle, design, and optimization of the tubular yokeless linear PM machine are studied. With the quasi-sinusoidal speed characteristic of the resonant piston considered, a single-phase yokeless linear PM machine is designed for FPEC by finite-element method. Some main design parameters, which include air-gap diameter, PM thickness, axial lengths of the radially magnetized magnets at center and two ends of the mover, pole pitch, and tooth pitch, are optimized to achieve higher power density. Compared with the conventional linear PM machines, the yokeless linear PM machine has the advantages of higher efficiency, higher mass, and volume power density. The utilization of space and machine material is also improved by adopting the yokeless machine topology.