Alternative electrical machine solutions for a free piston generator

In this paper dimensioning details for some linear electrical machines are provided for application in a free piston generator (FPG). The free piston generator integrates the functions of a combustion engine, the crankshaft, the connecting rod and the rotating electrical generator used in conventional hybrid schemes into a single unit. It has promising applications in series hybrid vehicles and in distributed generation units. Previous work by the authors has investigated the surface mounted moving magnet machines, the synchronous reluctance moving iron machine and the moving coil linear induction machines. As a continuation of that work, the buried-magnets moving magnet machines, the switched-reluctance and the hybrid-reluctance moving iron machines, and the moving coil-machines having field excitation from magnets in the stator are investigated. It is found that only the transverse-flux machines (TFM) of both the surface-mounted and the buried-magnet designs are promising as far as the fulfilment of system specifications are concerned. As for the transverse-flux machines a finite-element verification of analytical results is always required, a single-pole of a surface-mounted TFM has been simulated in 3D-FEM. It is found that the FEM results closely follow the analytical models, verifying the promising behaviour of the TFM. However, since these machines have low power factor, it is recommended to find measures to counter this drawback.