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Shaft Currents

Destructive Implications Of Shaft Currents On Motor Bearings

Bearing current problems are nothing new to engineers, but it is the alarming rate of increase in this problem that is becoming a real concern to factories and mines. SGS, a company in the USA, undertook a survey on 1000 AC motors on Variable Speed Drives. 250 Of the motors monitored had bearing faults appearing by month 18, and of motors averaging 24 months produce on time, 65% had electrical bearing faults appearing.

Older VSD’s operated at a lower switching frequency and did not create many bearing problems, with the advent of the newer IGBT VSDs, the higher switching frequencies have created more unbalance and thereby creating higher shaft currents that increase motor failure.

“Will electrical bearing damage always occur in a VSD application?"
The answer is "Yes, in 9 out of 10 motors - with no alternate path to discharge shaft current, pitting of the motor bearing will take place during VSD operation.  These discharges will continue and will always seek the path of least resistance - usually through the motor bearings".

Yes, many motors still last for some time on a VSD application but bearing life is drastically reduced by up to 50 percent, even when the problem is not critical.

Bearing failure

Destructive Implications Of Shaft Currents On Bearing Grease

Electrical Arcing Destroys Grease in VFD driven Electric motors.

One can observe strong oxidation and hardening of the grease that occurs following high-temperature stress, which is produced through electrical grounding (arcing). Loss of lubricant health produces mixed friction and wear in the roller contact area.

The fact that a bearing cannot be easily relubricated from the outside plays a crucial role in eventual element failure. The newly added grease cannot displace the hardened and oxidized lubricant already present, and it makes an exchange of grease impossible. With normal relubrication intervals, bearing failure is inevitable.

Relubrication is not possible because the bearing has already failed due to damage from poor lubrication.

Bearing failure