A New Perspective on the Operating Principle of Direct Current Machine Based on Airgap Field Modulation Theory

In this paper, the operating principle and electromagnetic performances of the direct current machine (DCM) are reiterated from a new perspective, viz., the airgap field modulation perspective. The equivalent airgap model containing three cascaded elements is applied, based on the general airgap field modulation theory (GAFMT), to expound the operating principle and common features of the four most conventional topologies in a unified way. The general analysis of DCM can be achieved existing in the only difference of modulation behavior, that is modulator like that in induction machine (IM) and synchronous machine (SM), whose results are consistent with the classical theory on electrical machines. Theoretical expressions for electromagnetic parameters are developed and fairly compared between different topologies to verify the validity of the proposed analytical model. In addition, torque density comparison under different constraints are revealed and iron saturation has been treated as a constant discount on the airgap magnetomotive force (MMF) due to the synchronous modulation mechanism. The theoretical predictions are demonstrated by 2D finite element analysis (FEA) and achieve good agreement.