Mitigation of Inverter-Induced Noncirculating Bearing Currents by Introducing Grounded Electrodes into Stator Slot Openings

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 paper 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 FEM analysis. The results are verified by laboratory tests. The bearing voltage ratios of the original and modified induction motor are compared. Index Terms-Ball bearings, electrical discharge machining bearing currents, electrical machines, finite element analysis, induction machines, variable speed drives.

On the Application of Extended Grounded Slot Electrodes to Reduce Noncirculating Bearing Currents

“…where C wr is the total winding-to-rotor capacitance, C sr is the stator-to-rotor capacitance, C ws is the total winding-to stator capacitance, and C b is the bearing capacitance. When the BVR is calculated for the machine equipped with slot-opening-embedded electrostatic shields (either short or extended), the following augmented equation should be used [11]:…”

“…At the experimental stage of the work, the proposed machine modernization and effect of electrodes was tested with a fourpole 15-kW machine available in the laboratory. As the basis for the test setup, the bench described in [11] was used. The regular ball bearings were replaced with hybrid ("ceramic") ball bearings to make it possible to measure the shaft voltage.…”

“…There are four types in total. All the types are described in detail both in our papers [9] - [11] and in the publications of other authors [12] - [15]. A summary table of the given types of parasitic currents is given in Table I.…”

“…Circulating currents can also be a result of uneven distribution of the current in the stator winding, rotor eccentricity, or imprecise machine geometry [15]. Inductive bearing current forms a closed loop with a "rotor-bearing-stator-bearing-rotor" path, as shown in [10] or [11]. Modern production methods On the application of extended grounded slot electrodes to reduce non-circulating bearing currents make it possible to manufacture machine parts with sufficiently high accuracy, and thus, the given type does not currently pose the most significant danger to bearings.…”

“…In addition, it is possible to provide parasitic currents with an alternative flow path in order to exclude the bearings from the circuit or use special bearings designed to apply current through them. A detailed review of existing techniques for suppressing bearing currents was recently made in [11].…”