Determination of the Magnetic Stray Field with an Equivalent Source Model

Based on the equivalence of current-carrying circuit-loop and magnetic dipole density the field of a current-carrying air-core coil is approximated by the field of two magnetic equivalent charges which are to be attached to the winding axis at the distance of the coil ends. Thereafter the influence of a highly permeable core on the equivalent charges of the air-core coil are indicated with a known approximation equation, then the final equivalent source model of an El-shaped small transformer is developed. In the final example the maximum stray field of a small transformer, which - as expected - occurs along the winding axis, is estimated and compared to concrete measured values.