The paper discusses two finite element open boundary
techniques for the solution of electromagnetic field problems with
translational symmetry. Firstly, the underlying assumptions and ideas
behind the techniques of simple truncation and Kelvin transformation are
presented. The techniques are then analyzed computationally with an
instructive problem. This consists of a pair of two parallel and long
conductive busbars where the force due to currents in opposing
directions is to be evaluated. The value of the force calculated
analytically is used to check the accuracy of the computed forces. The
main features of the numerical models are explained and a comparative
study based on the necessary steps to reach a specified accuracy in
computed forces is presented. Numerical results show that to obtain
accuracy better than 1% using the simple truncation method, very large
outer boundaries must be employed. With the Kelvin method, accuracy
better than 0.1% is readily achieved without using a large number of
mesh elements.
