Abstract
Modern converter-supplied ac motors are exposed to bearing currents. Despite extensive research and development, the industry still does not have a final solution with acceptable cost and high efficiency. This article focuses on capacitive bearing currents. After a brief explanation of the phenomenon, an unconventional approach for effective mitigation of the capacitive bearing currents is proposed. The approach suggests using grounded electrodes in the slot openings to reduce the stator-winding-to-rotor-core capacitance and thereby the bearing currents. Different electrode diameters are considered and evaluated by a finite element method (FEM) analysis. The results are verified by laboratory tests. The bearing voltage ratios of the original and modified induction motor are compared.
Highlights
B EARING currents are an adverse phenomenon in modern electrical drives
Low-frequency circulating currents have been known since the infancy of electrical machines and are mainly caused by an unbalanced magnetic flux, which results from an imperfect motor geometry, or stator segmentation
Preliminary results were obtained by a finite element analysis (FEA), performed in the FEMM v.4.2 software
Summary
B EARING currents are an adverse phenomenon in modern electrical drives. The problem was recognized already in the infancy of electrical motors. As there are conductive parts insulated from each other, an electrical machine can be represented as a set of parasitic capacitances. These capacitances are shown in Fig. 1; they are the stator-winding-to-rotor-core capacitance Cwr, the stator-winding-to-stator-core capacitance Cws, the stator-core-to-rotor-core capacitance Csr, and the capacitance of the bearing Cb. Different types of bearing currents are recognized. The dominating type of bearing current typically depends on the frame size. Among the abovementioned bearing currents, there are other less significant types of parasitic currents in electrical drive systems: shaft grounding current and stator grounding current, which are reported e.g., in [1]−[3]
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