Motor bearings in industrial converter-fed threephase motors are mostly equipped with grease-lubricated roller bearings. It is known that under certain operating conditions bearing currents flow that can damage the bearings. When estimating the danger of damaging bearing currents it is important to look at the thickness and the capacitance of the lubricant film as well as the electrical parameters of the drive system. Calculating the thickness of the lubricant film for grease-lubricated bearings is problematical. This paper discusses how this quantity can be determined based on the bearing capacitance. After a brief introduction into the theory of lubricant film formation, the results of extensive tests are presented regarding the dependency of the bearing capacitance on temperature and speed.
Φ Abstract --The basic inter-relationships, which are required to investigate the cross-system subject of "bearing currents in converter operation", are discussed in this paper. In order to obtain an integrated overview of the situation, it is decisive that the main points of investigation that are discussed are combined and viewed holistically. Results from the parameter test area and their link with the overall situation are shown. A higher-level systematic to simulate electric currents flowing through a bearing is outlined, taking into account the various processes that take place in the bearing. The paper is completed by presenting a new type of bearing current sensor system. A review of the steps still required is provided in the conclusion. He has 15 years experience in the development and testing of electrical motors and power electronic converters. He holds several patents in this field. His scientific interests include electric drive systems, power electronics, electrical machines especially the effects of the power electronic converters on electrical machines, e.g. bearing currents, noise emission, insulation stress etc. One of his favour fields of interests is the investigation of the effects of bearing currents in electric drives.
As shown in [6], the issue of current flowing through a motor bearing is a system-related topic. Different aspects have to be taken into account to identify countermeasures to avoid or dampen the current flow. Up until now, it is not possible to define clear limits where the changes in the bearing raceways lead to the so called corrugation and with this to a possible bearing failure. This paper shows the latest results that were obtained with the investigation schematic presented in [6]. New results in the area of field experience with the so called "Bearing Current Sensor" (BCS) will be discussed. Additional results in the area of parameter tests, which take into account the dependency of temperature, speed, voltage as well as the type of lubricant grease, are shown.
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