A 11 KV, 2.5 MW cage motor has white metal sleeve bearings on both sides where the bearing shells are insulated from the pedestal and the top cover. There are two fiberglass split oil seals for each bearing on either side. The split halves are held in place by an external circular spring.
After two years of running, the motor was opened up for an inspection and the shaft was found to be scored under these seals. The photos of the damages is attached in a PDF file. The scoring is deep to the extent of 4 to 5 mm and they are exactly under the seals. You need to pull the seal axially to see the scoring damages.
The shaft material has formed a hard sludge into the oil seal grooves, which is hard to remove. Strangely, the oil seals themselves do not show any sign of overheating despite the shaft material deposit in them.
The drive end bearing bottom half was found to have a small cable connecting it to the ground. No idea why the chinese OEM would insulate both the bearings and then ground one of them.
A different motor of 3.5 MW capacity from the same chinese OEM at the same plant also has these shaft wear-outs under the non-metal seals.
What could be the reason for this shaft wear-out under the seals, especially when the seal is a non-metal? Did the grounding of one bearing inflict some kind of shaft current damages to these seal areas ? If yes, why under the seals and not anywhere else?
Hoping Gunnar would weigh in. Thanks in advance to all.
PS: The white metal bearings and the shaft journals themselves are clean and do not show any pitting marks due to shaft currents.
Muthu
www.edison.co.in
After two years of running, the motor was opened up for an inspection and the shaft was found to be scored under these seals. The photos of the damages is attached in a PDF file. The scoring is deep to the extent of 4 to 5 mm and they are exactly under the seals. You need to pull the seal axially to see the scoring damages.
The shaft material has formed a hard sludge into the oil seal grooves, which is hard to remove. Strangely, the oil seals themselves do not show any sign of overheating despite the shaft material deposit in them.
The drive end bearing bottom half was found to have a small cable connecting it to the ground. No idea why the chinese OEM would insulate both the bearings and then ground one of them.
A different motor of 3.5 MW capacity from the same chinese OEM at the same plant also has these shaft wear-outs under the non-metal seals.
What could be the reason for this shaft wear-out under the seals, especially when the seal is a non-metal? Did the grounding of one bearing inflict some kind of shaft current damages to these seal areas ? If yes, why under the seals and not anywhere else?
Hoping Gunnar would weigh in. Thanks in advance to all.
PS: The white metal bearings and the shaft journals themselves are clean and do not show any pitting marks due to shaft currents.
Muthu
www.edison.co.in
