Analytical and FE calculation of eddy-current losses in PM concentrated winding machines for wind turbines

Permanent Magnet (PM) machines with concentrated windings are cost effective, compact and highly efficient. The application of such machines in large Direct Drive (DD) wind turbines can be advantageous. However, these machines have the problem of eddy current losses in solid parts like magnets and rotor back-iron. Apart from cooling problems, these eddy current losses might lead to de-magnetization of PMs because of excessive heating. This paper presents an analytical model for eddy current loss calculation in magnets and solid back-iron of large wind turbines of MW rating. The analytical model has been derived from 2-d magnetic vector potential. The effects of simplifying assumptions have been quantified by comparison with Finite Element (FE) simulations. The analytical model over-estimates the losses in magnets while losses in rotor back-iron are under-estimated. However, the total losses are accurate within 9% of FE calculation. The analytical model is very fast and generic whereby this model can serve as a tool for design optimization where eddy current loss is a criterion. A design with 9 slots per 8 poles has acceptable eddy current losses while in a 3 slots per 4 poles design losses are unacceptable.