Torsional Shear Stress Minimization Techniques and Implications on Electromagnetic Performance of Flux-Modulated Double Rotors

Magnetic-geared machines are based on the flux-modulation principle and typically constructed with a flux-modulated double rotor (FMDR). The modulator experiences high torsional stresses when designed for high torque transmission. As a result, the FMDR has high probability of failure if not designed appropriately. In this paper, the FMDR design options are investigated with mechanical and electromagnetic finite element (FE) modeling. The addition of filler material in between the poles of the modulator pole-pieces is found to reduce the shear stresses by 61.7% to 87.2% at the cost of 2.6% to 3.2% lower pull-out torque. Possible shear stress reduction with the design of bridged modulators is investigated. Increase in pole number is shown to increase localized stress levels in bridged modulators. Electromagnetic simulations show that for a given FMDR specification, the highest pull-out torque is at an optimal pole number. The tradeoff between FMDR weight, losses, and shear stress are discussed. A scaled-size prototype FMDR with a bridged modulator is constructed. The FMDR is tested and is shown to achieve comparable results with the FE simulations. At field intensity levels above 600 kA/m in the periphery, the FMDR achieves reduced pull-out torque 28.7% lower in comparison with three-dimensional simulations. The losses at different load levels of the FMDR are also compared with experimental and FE simulation result and are shown to confirm the FE simulation results.