An improved method for virtual air gap length computation

An approach to the analysis of a single-phase transformer with a virtual air gap is presented. The transformer geometry in the paper shows four auxiliary windings embedded in its core and is connected to a dc supply. The purpose of these windings is to locally saturate the core. By changing the magnetic permeability of the ferromagnetic core material in the region of the auxiliary windings, a virtual air gap is created. An equivalent mechanical air gap length can be associated with the virtual air gap. It results from an increase in core reluctance with local saturation and is obtained by an analytical approach based on finding the distribution of the magnetic field in the core produced by the auxiliary windings. The accurate calculation of the magnetic field is necessary to obtain both the operating point of the core and the change in core reluctance. The proposed method consists of 5 steps. The result obtained from the proposed method is compared with experimental and analytical results.