In some cases of motor-drive applications, a combination of installation and operational conditions gives rise to bearing currents. Unchecked they might result in bearing failures only a few months after start-up but that doesn’t need to happen. All that is required for peace of mind is a simple check during commissioning. And, if you do have bearing current issues, there are ways to eliminate them.
Bearing currents are not a new phenomenon and there are reports of them causing failures of electric motor bearings as far back as the 1930s. They occur when voltages induced in the motor rotor and shaft are discharged to earth through the bearings. This causes metal to be transferred between the balls and the races – effectively electric discharge machining (EDM) is taking place (Figure 1). The cumulative effect is premature wear, resulting in increased levels of noise and vibration. This ultimately results in early failure – sometimes within a few months after installation – when it is reasonable to expect the bearings in an industrial motor operating 24/7 to last five years or more.
Advances in design and installation procedures have gone a long way to solving bearing currents for large (typical ≥ IEC 280 or NEMA 440 frame size) motors. The issue results from circulating currents due to inductive coupling (inductive bearing currents) and the solution is to break the current path by using one insulated bearing.
However, recently there have been a few reports of bearing currents in motors with smaller frame sizes – up to around 55 kilowatt (kW) rating – in variable speed drive (VSD) applications. At ABB we have received reports relating to some tens of installations at European production facilities. Although this is not a high number (ABB makes over one million motors per year), we are committed to maintaining the highest possible reputation for quality and customer care. Therefore, we have carried out exhaustive testing to identify the cause of bearing currents and to develop solutions.
It appears that recent issues are due to capacitive induced bearing currents, resulting from the common mode voltage in the drive system. The possibility of damage arising from them is closely associated with the specific installation conditions. In fact, data collected from cases where capacitive bearing currents have caused damage suggests that installations with grounded networks have a higher risk for capacitive shaft voltage than those with floating networks. However, the statistical significance of this data is still quite low due to the small number of actual cases reported. There is also no sharp transition based on power or size of motor between inductive and capacitive bearing currents. The information is based on experience and there may in some cases be a transition state where both phenomena may co-exist.
In most cases your first indication of a bearing current issue is an increase in noise and vibration.
With the appropriate training and equipment, it is possible to measure the motor shaft to frame voltage (Figure 2) and we recommend that this should be part of the commissioning procedure. There is no precise rule about what an acceptable shaft voltage is and this will vary according to the motor-drive package and the nature of the installation. If you have any concerns about taking this measurement, ABB can offer guidance and also checking the installation of the complete system.
If you are experiencing bearing current issues in a correctly installed system there are a number of straightforward actions that you can take (Table 1). One solution is to use insulated bearings on both the drive and non-drive ends of the motor. Both bearings must be insulated or the bearing current effect will be focused on the non-insulated bearing and it will fail twice as quickly. We also recommend using hybrid bearings that will offer benefits such as low friction, longer re-lubrication intervals and a longer lifetime compared to a standard bearing. The secondary choice is to use insulated bearings.
However, if both bearings are insulated and conductive coupling between the motor and a grounded application (like a gearbox) occurs, then the bearing current can cause problems in other parts of the system. We suggest two possible solutions in this case: install an insulating coupling between the motor shaft and application; or install a motor shaft grounding brush (Figure 3). The brush can be pre-installed on new motors by specifying a variant code or retro-fitted on site.
In the vast majority of installations this kind of device is not required but it offers a good return on investment when bearing currents are present. Most users of industrial motors should not worry unduly about capacitive bearing current issues as they are very rare and usually limited to motors below the IEC250/NEMA 400 frame size. But, if you think you might have a problem, bearing currents are easy to check for and straightforward to solve.
Table 1: There are three actions that can address bearing current issues.