Abstract
Common-mode voltage, caused by a 2-level inverter, is a source of discharge currents in motor bearings. Due to the capacitive coupling, between the stator winding and the rotor, an image of the common-mode voltage is produced on the shaft—which can exceed the dielectric strength of the lubrication film of motor bearings. Accurate determination of the winding-to-rotor capacitance is necessary to predict the shaft voltage. This article proposes a novel analytical determination of the slot and the end-winding portion of the winding-to-rotor capacitance. The calculation rules, which are based on the method of image charges and the charge simulation method, take into account the continuity and the boundary conditions of the field areas. Results are validated by means of finite element method simulations. Notably, deviations are in the single-digit percentage range. Furthermore, the presented methods are simple to implement.
Highlights
A 2-level inverter used for variable-speed operation of an electrical machine impresses a common-mode voltage into the stator winding
The validation of the models is performed with the help of finite element method (FEM) simulations
In contrast to the presented stator slot model, the FEM simulations take into account the radius of the stator and the rotor lamination
Summary
A 2-level inverter used for variable-speed operation of an electrical machine impresses a common-mode voltage into the stator winding. The intensity of the common-mode voltage for a 3-phase electrical machine is equal to the average of the three phase-toground voltages [1]. The common-mode voltage causes a voltage on the rotor shaft via the winding-to-rotor capacitance. The winding-to-rotor capacitance required to predict the shaft voltage is composed of the slot portion and the two end-winding portions [4,5]. The slot portion corresponds to the capacitive coupling in the active part of an electrical machine. For induction machines, the end-winding portion has a large influence on the entire winding-to-rotor capacitance, due to the squirrel-cage ring [6]
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