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.

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