Capacitances and lubricant film thicknesses of motor bearings under different operating conditions

Abstract

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.

Experimental Rig for Measuring Lubricant Film Thickness in Rolling Bearings

“…Wittek et al investigated the film thickness in thrust bearings [16]. They were particularly interested by grease -lubricated bearings used in electrical machines and on how the electrical current might discharge through the film.…”

“…At over 200 nm the ratio of outside region to Hertzian area capacitance is about 3 and increasing. Analysis of data from reference [16] by Wittek and co-workers shows that most of their film thickness exceed 200 nm, so the value of 3.5 for the ratio of outside region to Hertzian area capacitance may be justified, however to be noted that this is only valid at relatively thick films and it is not constant.…”

Evaluation of Oil-Film Thickness Along the Path of Contact in a Gear Mesh by Capacitance Measurement

“…For this measurement the insulation of the gear is bypassed. While the general setup is similar to existing ones [3], an additional high speed measurement system in combination with a detection of the shaft position is necessary to divide the path of contact into small areas. With each turn of the shaft another section of the gear mesh is evaluated.…”

“…Different to the measurements shown in [3] -where a pair of bearings is always measured together -a single machine element can be evaluated, as each element is insulated from the gearbox housing (see Fig. 8).…”

“…With the knowledge of the lubricants' permittivity and the contact area it is possible to calculate the film thickness. This measurement method has been successfully used to investigate the film thickness in ball and roller bearings [3]. In the gear mesh the measurement has to be conducted at a high frequency as the time of constant contact conditions is only a few milliseconds depending on speed and number of teeth.…”