Optimal Design Of Modulation Ring In Concentric Magnetic Gear Considering Magnetic Performance And Mechanical Strength

In a concentric magnetic gear,a space harmonic field that is produced by the inner/outer magnets at the outer/inner air gap has equal number of poles and rotates synchronously with a space harmonic field that is produced by the outer/inner magnets,thanks to the flux modulating ring.A concentric magnetic gear operates based on the fields that are produced by the inner and outer magnets and coupled due to the modulating effect.Conventionally,the flux modulating ring is assembled by stacking small separated pieces of steel laminations.Interconnected flux modulators made from full laminations have been implemented in recent years.A modulating ring made from complete laminations with bridges between the steel pieces needs less manufacturing effort and lower cost than a conventional modulating ring.Meanwhile,this design improves the mechanical strength and stability of the modulation ring.However,the introduction of bridges apparently increases the magnetic leakage flux and hence downgrades the torque capability of the magnetic gear.In addition,the design of the bridged flux modulators should determine the number of clamping rods and the geometry of modulating ring considering the trade-off between the magnetic performance and the mechanical strength.This thesis investigates the influences of the design factors,such as modulating ring geometry and clamping rod count,on the torque capability of the magnetic gear and the mechanical stress of the bridged modulating ring.The geometry of modulating ring is characterized by thickness of bridge,ratio of pole arc to pole pitch and fillet radius at junctions between modulaors and bridges.The transient magnetic and stress fields of concentric magnetic gear are both modeled by three-dimensional(3-D)time-stepping finite element method(T-S FEM).The distributed magnetic force on the modulating ring is calculated by the magnetic FEM,and the mechanical finite element model of the modulating ring is loaded with the distributed magnetic force for the stress calculation.The following results are proved by the finite element calculations.The thickness of bridge is a very influential factor for the torque capability of the magnetic gear and the maximum stress in the modulating ring.The increasing number of clamping rods within a certain range reduces the maximum stress in the modulating ring significantly.However,if the number of clamping rods continues to be increased,the maximum stress will not keep decreasing significantly.With a relatively small number of clamping rods that does not saturate the effect of stress reduction,the adjustment of pole-arc-to-pole-pitch ratio within a certain range can significantly reduce the stress in the modulating ring at very small cost of torque capability.The adjustment of fillet radius can effectively reduce the stress in the modulating ring only at certain values of pole-arc-to-pole-pitch ratio.A design procedure of bridged modulating ring is proposed considering magnetic performances,mechanical strength and manufacturing cost.The thickness of bridge is determined as the first step of the design process,according to the required torque capability;the number of clamping rods is increased as the second step until the acceptable levels of manufacturing cost and mechanical strength are reached;the other geometric parameters of modulating ring including the ratio of pole arc to pole pitch and the fillet radius are adjusted for further potential reduction of stress and high reliability at no noticeable cost of torque capability.