Analytical Analysis Of Magnetic Field And Optimization Of Surface-Mounted Permanent Magnet Brushless Machines

Brushless permanent magnet(PM)machines have the characteristics of simple structure,high efficiency,high torque/power density,low maintenance rate and so on.These advantages make brushless PM machines used in various fields,such as electric vehicles,wind tubines and ocean energy converters,etc.Therefore,it is of great theoretical significance and practical engineering value to study the structure and electromagnetic characteristics of PM machines.An analytical technique for predicting the magnetic field distribution in slotless brushless surface-mounted PM machines having sector-ring-shaped auxiliary interpole magnets with tangential magnetization has been presented.For the maximum fundamental amplitude of the air-gap flux density,the optimal magnet pole arc ratio has been analytically derived,depending on the pole-pair number,magnet thickness and magnet relative permeability.In addition,the proposed analytical technique can also predict the induced back-electromotive force(EMF),armature reaction field and electromagnetic torque.On this basis,the effect of the interpole magnets and the rotor core on the fundamental air-gap flux density for the machine has been investigated.Predicted results have been validated by finite-element analysis for a 4-pole slotless PM machine.For comparison,the sector-ring-shaped magnets are changed into the arc-trapezoidal magnets.Using a layer superposition method,the analytical expression of air-gap flux density and the optimal trapezoid angle and pole arc ratio have been obtained.The back-EMF and electromagnetic torque have also been discussed.Compared with the sector-ring-shaped magnetic poles,the electromagnetic performance of the PM machine with arc-trapezoidal magnets is improved.Using improved Carter's coefficients,the analytical expressions of the air-gap flux density,back-EMF and electromagnetic torque have been obtained for the slotted PM machines with four kinds of slots.The validity of the presented method is verified by the finite element method.Therefore,the presented methods and results are helpful for the optimization design of the PM machines.