Scaling of AC copper loss in thermal modeling of electrical machines

The accurate prediction of the ac copper loss together with its thermal behaviour is crucial in the design of electrical machines, transformers and inductors. The commonly used figure of merit, when describing the ac copper loss, is a ratio of the equivalent ac and dc resistance (R_ac/R_dc). This ratio defines the average ac copper loss as a function of frequency. However, as the ac and dc components of the copper loss scale differently with temperature a single value of R_ac/R_dc derived for one temperature might be inadequate when used in thermal modelling. In this paper, the authors discuss limitations and applicability of the existing scaling techniques. The emphasis is placed on the method proposed earlier by the authors. The proposed scaling approach is illustrated on example test coils. Both theoretical and experimental data is provided and discussed.