Overview of capacitive couplings in windings

Abstract

The use of electrical machines (EMs) with variable-frequency drives (VFDs) results in electromagnetic interference (EMI). At high frequencies (HFs) of conducted EMI, the impedance of an EM insulation system fed from a VFD is small due to the parasitic capacitive couplings. Thus, the conducted EMI currents flow through the insulation into the other conductive parts of the machine or ground. This can cause damage to the insulation, accelerate corrosion and bearings, and influence other electrically and/or mechanically connected equipment in the system. In order to understand the phenomena of EMI and how to minimize it, one must understand parasitic capacitances in a VFD-cable-EM system. Many authors analyze the topic of parasitic capacitances of cables and machine's bearing capacitances. However, not many publications are focused on the detailed analysis of the windings. In most cases, the parasitic capacitances of the winding are represented as a part of a global equivalent RLC circuit. Some authors use simplified models such as solenoids, which are not adequate representatives of the actual winding topologies used in EMs. To date, not much attention is given to the actual distribution of parasitic capacitive couplings in the winding itself, especially for multi-layer coils. Therefore, it appears necessary to provide an overview of the models proposed in the literature and their implementation in the analysis of EMs. This paper presents capacitive couplings from the point of view of winding topology and conductor geometry, including insulation coating between conductors, and conductor and ground, which can be implemented in the winding equivalent RLC circuit, which can further be applied to analyze the machine's behavior at HFs. The discussion section at the end of this paper recommends further steps on how to determine the parasitic capacitive couplings in the future with more accuracy.