Maximisation of power density in permanent magnet machines with the aid of optimisation algorithms

This study considers the design of surface-mounted permanent magnet electrical machines for high-speed applications and proposes a methodology to determine the maximum achievable power density. Power density is usually improved by increasing rotational speed. At high speed, a mechanical retaining system for the rotor magnets must be considered. As the speed increases, the thickness of the retaining sleeve becomes larger, reducing torque capability. There will be an optimal speed at which the output power will be maximised. Both structural and electromagnetic design must be considered simultaneously to properly address this design problem. To simplify the design procedure, static finite-element simulations are used for the electromagnetic analysis and analytical formulae are employed for retaining sleeve sizing. The procedure is aided by multi-objective optimisation algorithms. A case study based on the specification of an aeronautical actuator is presented. The performances that can be obtained using different iron cores, high-grade silicon steel, and cobalt iron steel are compared. Finally, results obtained from transient finite-element electromagnetic and structural analysis are presented to validate the feasibility of the proposed procedure.