Use of transverse-flux machines in a free-piston generator

This paper investigates the use of transverse-flux machines (TEM) in a free-piston generator. A free-piston generator integrates a combustion engine and a linear electrical machine to provide benefits in e.g., efficiency, emissions and reliability. This work is the continuation of the authors' earlier work. It was concluded then that a transverse-flux machine is the only promising candidate, if any, to meet the demands upon the electrical machine in such an application. Conventional TFM designs are investigated first. It is found that these cannot fulfil the system demands. Several improved surface-mounted TFM designs were thus investigated. Though these designs seemed promising during the analytical design phase, they faded during the 3D-FEM analysis, mainly due to the predominant armature-flux leakage in the axial direction. Improved designs for the buried-magnets TFM variant were thus sought instead. It was found that one such design allowed the fulfilment of system demands. A 5 kW proof-of-concept prototype was dimensioned. Analytical results were found to approximately hold during the 3D-FEM analysis as well. One-phase of the machine was built. Some measurements were performed, which identified some of the manufacturing defects due to small pole dimensions. Nevertheless, the electromagnetic soundness of the design was approximately verified. Problems surrounding the mechanical stability and the manufacturing process are still to be solved.