Analytical Optimization of Slotless Surface-Mounted Halbach Magnet Machines

An analytical technique for predicting and optimizing the magnetic field in slotless brushless surface-mounted permanent magnet (PM) machines with mixed grade Halbach magnets is presented. In order to obtain the maximum fundamental air-gap field, the optimal magnet-arc ratio is analytically derived, depending on the pole-pair number, the remanence ratio, the magnet thickness and the magnet relative permeability. The effect of the magnets and the rotor core on the air-gap field is investigated. In addition, the proposed analytical technique can also predict the induced back-electromotive force (EMF), the armature reaction field and the electromagnetic torque. The analytical results show that the electromagnetic torque is improved for the optimized magnet-arc ratio. Predicted results are validated by a finite-element analysis (FEA) for 4-pole slotless PM machines.