Analysis of armature circuit inductance of DC machines by FEM

A method is described for the calculation of the armature circuit inductance of DC machines by the finite element method (FEM), in which both the flux linkage and the stored energy methods are presented. The inductances calculated by this method are compared with the measured values and those calculated by conventional formulas. The measured inductances decrease rapidly with increase of field current, because of the saturation phenomena of the magnetic circuit. The results obtained by FEM are in good agreement with the measured values. Further, in the conventional formulas, an average gap length is used to calculate the inductance due to the main airgap. In the paper, an improved formula is derived, in which the airgap change in the circumferential direction is taken into account.