Study and geometry optimization of hybrid excited synchronous alternators for automotive applications

Due to the increasing demand for power a continuous development of automotive alternators concerning power, efficiency and power density is required. In this paper an alternative arrangement to conventional Lundell automotive generators is examined. This geometry is characterized by its hybrid excitation combining the high energy density of permanent magnets and the controllability of commonly used electrical excitation - thus a simple and cost-effective control of output voltage for application in on board supply systems is possible. A simulation of the alternator using the finite-element method (FEM) is performed to optimize the rotor geometry and to develop a design, ready for prototyping. The finite-element calculation of the alternator behavior and the unknown stator current, including their reaction on the entire magnetic field, requires a transient solving process taking the geometry rotation into account and which is coupled to an electric circuit. The formulation for the transient solver is given and the coupling with external circuit exemplified. The accomplished studies, simulations and geometry optimizations are presented. In a final step the calculations are compared to prototype measurements and confronted with conventional alternator geometries. The developed geometry absorbed about five percent more power but was fifteen percent lighter than compared Lundell generator.