CAPACITANCES AND LUBRICANT FILM THICKNESSES OF GREASE AND OIL LUBRICATED BEARINGS

摘要

The electrical capacitance of lubricated contacts and bearings is a parameter that has been investigated for many years [1-3] The focus of these investigations was to examine the film-thickness of the elastohydrodynamic contact between the rolling elements and the raceways Nowadays the capacitance is not only of interest for the field of lubrication but also for a drive system behavior and the occurrence of bearing currents and electrical erosive wear. Modern drive systems can be characterized more and more by variable-speed operation that comes with many advantages like an increased efficiency but also some disadvantages One of these is the occurrence of so called parasitic currents which are a result of the common-mode voltage U_(cm) that is inherent in the widely used voltage source inverters. The capacitance of drive system bearings - and other parts like gears [4, 5] - combined with motor inherent capacitances creates a capacitance voltage divider As a result, a voltage U_b that is proportional to the common-mode voltage occurs at the motor bearings. Possible arc discharges in the lubricant gap can melt or vaporize material in the bearing raceways. This leads to a normally grey-frosted raceway with no influence on the bearing lifetime, or to so-called corrugated patterns (see Figure 1 a), which reduce the bearing lifetime. Furthermore, the discharges damage the lubricant due to the high temperature in the discharge arcs (see Figure 1 b) Unscheduled maintenance and therefore higher costs are the result There are three main types of bearing currents. One of them is the so called EDM-current (Electrical Discharge Machining) which depends on the breakdown effects inside the lubrication gap of the rolling element bearing. In contrast to the two other types of bearing currents (circulating and rotor ground currents), the occurrence of EDM currents is strongly influenced by the common-mode voltage and the parasitic capacitance network behind the motor terminals which consists of the stator winding-to-frame capacitance C_(wf), the stator winding-to-rotor capacitance C_(wr), the rotor-to-frame capacitance C_(rf) and the capacitances C_b, of the rolling bearings at the drive end and non drive end of the motor Equation 1 gives the ratio between the common-mode-voltage and the bearing voltage - the so called Bearing Voltage Ratio BVR.- depending on the capacitance network.