Investigation of the usability of 2D- and 3D-FEM for a hybrid stepper motor

The optimization of hybrid stepper motors and similar special machine geometries can not be performed effectively due to a large number of elements which follow from the fact, that the dimension of the air gap is tiny against the whole geometry and that it is necessary to model a relative big part of the machine. The presented paper shows how to model hybrid stepper motors with finite element method more effectively. Based on a certain machine, the limits and problems using full 3D models are identified. Further attempts with 2D models follow. It becomes clear that, depending on the features of special software, these 2D models can only be used to analyze certain aspects like detent torque or holding torque around small stationary working points. The biggest problem is to rotate the rotor part against a stator with impressed current. Therefore a possibility has been searched to model a full working machine without the disadvantages of a full 3D model. A solution was found with a reduced 3D model. This is the quintessence of the paper. We also investigated which modeling depth has to be performed. It will become clear which boundary conditions have to be taken into account to reduce the model. Moreover the theoretical background which is necessary to do the reduction is given. It can be seen, that the reduced model can be used to get reliable results for an optimization. To conclude a comparison between the different models and results shows how useful the reduced model will be, if optimization or investigation of such a machine shall be done.