Magnetic material optimization for hybrid vehicle permanent magnet motor drives

Authors:

Sigrid Jacobs, Daniel Ruiz Romera, Patrick Goes, Dietrich Hectors, Emmanuel Attrazic, Sebastiao Paolinelli: ArcelorMittal

François Henrotte, Martin Hafner, Kay Hameyer: Institute of Electrical machines, RWTH Aachen University, Germany

ArcelorMittal’s product range of Electrical Steels includes specific Fully Processed grades which have been developed for enhancing the performance of electrical machines.  We felt the need to quantify better the impact of the steel choice on the performance of the machine, in particular for the permanent magnet synchronous machine used in electrified automotive traction.

The steel efficiency is determined as the ratio of the mechanical power to the sum of the mechanical power and the iron losses and is introduced the first time in this paper.

Steel efficiency reaches its maximum in an operation range (low frequency, high currents) where the overall efficiency of the machine is very bad. The two objectives (maximizing total efficiency and maximizing steel efficiency) are thus contradictory; they have nearly opposite gradients in the operation space and the question arises which trade off will yield the optimum motor design.

Clearly the overall efficiency of the machine must remain the deciding factor, but still there is a potential for energy saving and efficiency improvement through an optimum and appropriate choice of magnetic material, in particular via thin low loss Fully Processed grades .

Our study has shown that the question can actually not be solved on basis of material considerations only. The optimum material is the one that offers minimum losses under typical operation of the machine. It is therefore a constrained optimisation problem, where the constraint is application dependent.  In the case of PMSM drives for hybrid vehicles, one can rely on standard driving cycles. We have developed a method allowing to combine efficiently the material information (loss and saturation curves) with statistical drive cycles and a finite element model of the PMSM.

The method involves an accurate computation of all losses, a breakdown into its different components (Joule, eddy current, hysteresis), as well as estimation of the influence of higher harmonics and rotational hysteresis. The coupling of the finite element model with the statistical drive cycle analysis is rendered tractable in terms of computation time by the extraction of histograms of peak induction in the machine.