Drive Failures, Main Breaker Tripping


Senior Member
Allentown, PA
I was called out to a customer site to troubleshoot a drive failure. When I arrived I found they were fixing the AC Unit that cooled the drives, and was told the cabinet was very hot. I found a drive had burnt up components. I meggered the motor, and everything looked fine. Swapped out the drive, and they ran for 6 hours with no issue.

Got called back by the customer that the machine was back down. They said that the Main Breaker tripped and when they reset it and then enabled the drives they heard popping. When I arrived I found that several capacitors in the power section of the same servo drive rack had failed. I also attributed this failure to the cabinet overheating. But I was not sure why the Main 200A SD KA breaker would trip before 100A Siemens 3NA6 Fuse (These do appear to coordinate well). I changed out the power supply and they ran for another 2 days.

While I was onsite looking at a another issue, they had the Main 200A breaker trip again. And again the capacitors in the new power section of the same servo rack had failed. At the time the machine was not running. One of the first things that came to mind is if the system was grounded. I opened up the isolation transformer that fed the machine and it is a 480 Delta to 480 Wye secondary with the X0 not grounded. I checked several times through the week and the voltage always measure 480 L-L and 277 L-G, so I never detected a fault that would create the first ground reference.

I read the manual on the power section Indramat HVE and it is an issue to be installed on an ungrounded system. It has been installed this way for 10 years. I am going to ground the X0 on Monday.

I am thinking sometime else is going on. But I don't know where to look. I am having a hard time grasping what would cause the main breaker to trip and cause the capacitors to fail at the same time, while not blowing the 100A fuse. There are 5 drive sections that are mostly identical, and it is just the one section that has been lossing drive/power supplies.

The drives and power supplys are very expensive so I fear installing another one. If anyone has suggestions as to things to check I am all ears. I attached a singleline drawing.




Suggestion - breaker trip

Suggestion - breaker trip

Coming from a electrical utility background involving customer complaints -- I suggest that you check ALL connections feeding the motor drive. We had customers with motor drive tripping off line and some times found problem with thermal imager of our lines (bad phase connection of conductors -- bad connection was slightly arcing and heating up enough to cause the problem --visual was OK but thermal imager showed problem quick) -- replacement of bad conductor connector quickly solved problems --sometimes it was the customer's system. Oh by the way, lightning often is the major cause of capacitor failure --with the lightning storms this year --could have caused this problem which will not be fixed until all lightning induced weaknesses are found. --got access to thermal imager?