The efficient evaluation of torque and field gradient in permanenent-magnet machines with small air-gap

The accurate evaluation of the flux density in small air-gaps of electrical machines is important because this is the critical region of the machine as regards energy transfer and losses. Without special care, numerical methods of determining the air-gap flux distribution can give results having a considerable inaccuracy, particularly when the degree of discretisation is limited for reasons of economy in the use of computer time. Reliable methods of solution are used, but particular attention is given to the calculation of torque and flux density. Results are presented for an important class of synchronous permanent-magnet machines.