Bearing currents in modern drive systems (2)

Spotting bearing damage

The time it takes for bearing damage to occur varies with the drive set-up, but is typically detected after a few months of use. Sectional drives are more prone to bearing current damage than stand-alone ones, as several parallel inverter-motor circuits result in a lower impedance for the common mode current.
Bearing damage can be detected by a loud running noise, or by using continuous vibration monitoring.The damaged bearing should be replaced before its total destruction causes loss of production. Current damage can be verified if the rolling tracks of the bearing are fluted or frosted, and the rolling elements have a grey, dull finish.
How can damage be prevented?
There are three approaches used to affect the high frequency bearing currents: a proper cabling and earthing system; breaking the bearing current loops; and damping the high frequency common mode current.
Use symmetrical multicore motor cables. The protective earth (PE) conductor must be symmetrical to avoid bearing currents at fundamental frequency.The symmetry is achieved by a conductor surrounding all the phase leads or a cable that contains symmetrical three phase leads and three earth conductors.
Define a short, low impedance path for common mode current to return to the inverter.The best and easiest way to do this is to use shielded motor cables.The shield must be continuous and of good conducting material, i.e. copper or aluminium and the connections at both ends need to be made with 360° termination.
Add high frequency bonding connections between the installation and known earth reference points to equalise the potential of affected items, using braided straps of copper 50 -100mm wide; flat conductors will provide a lower inductance path than round wires.
Break the bearing current loop by insulating the non-drive end bearing, by insulating the shaft from the frame, or by using an insulated coupling between the motor and the driven machinery.
Dampen the high frequency common mode current by using dedicated filters, which add impedance in the common mode loop.
Finally, in many inverters the carrier frequency can be adjusted between 1 kHz and 10 kHz. In cases where bearing damage has occurred, some short term relief can be achieved by reducing the carrier to the 1-3 kHz range.

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