Influence of Air Gap in Transverse Flux Permanent Magnet Machines for Wind Power Applications

This paper investigates the influence of air gap on the electromagnetic performance of transverse flux permanent magnet (TFPM) machines with reference to surface permanent magnet (SPM) machines at two different power levels, i.e. 3 kW and 3 MW. The performance of a TFPM machine depends significantly on pole number, and therefore, the optimum pole number is selected and compared with an optimized SPM machine for different air gap lengths. It has been emphasized how the air gap length plays an influential role on various aspects of TFPM machines. However, the SPM machine shows much less dependency on the air gap length. For the 3 MW TFPM machine, air gaps below 2.5 mm should be beneficial in terms of mass and cost whereas the threshold for the air gap should be less than 1 mm for the 3 kW TFPM machine.