A consulting engineer discovers an unusual source is turning power boards to toast
By Don Humphrey, Contributing Writer
In the late 90’s I was asked by a drives company to investigate why their drives were failing at an automotive plant in Mexico. Drives were failing at the rate of 2 or 3 a week. The drives company suspected power problems.
When I arrived on site I was met by a rep from a honing machine company. The drives in question were used on one hone per cylinder for two V-8 blocks, for a total of 16 drives .
I asked the plant maintenance, if I could see a bad drive. They pointed me to several drives in the shop that had been changed out. Indeed there were at least ten in boxes. Each one had not only failed, they were toast. The power boards were charred and black.
I first put a line monitor on the incoming three phase 460 ac. From Tuesday through Friday, two drives failed. As is typical, I was not there to witness the failures. There was no recorded power problems in either case.
I stayed around the machine checking, looking and practicing my Spanish, waiting for the next failure.The anxiety level was rising among all concerned. I could only scratch my head, wondering what could it be.
Finally on Sunday afternoon as I was returning from a bathroom break, I was told a drive had just failed with a mighty bang. Rushing back, I took the drive to the shop for disassembly. Examination showed it to be like all the others — the boards blackened beyond recognition.
Then something caught my eye — tiny specks which from a distance looked like pieces of land from the PC board. The lands are how the copper traces on a printed circuit board are referred to. They are not magnetic. Closer examination with a magnifier and a magnet revealed them to be steel filings. These most likely came from the honing process. The question was how?
In looking for clues, I discovered that plant personnel had recently installed an air conditioner in the door of the control cabinet holding the drives. This had warped the door and left a small gap in the top. This kind of honing operation needs good dust filtration.
Upon questioning, I also learned that the honing machine manufacturer had changed from a 5hp to a 10 hp drive as part of a new machine design. The 10 hp drive had a small cooling fan, which was blowing directly across a board with a land carrying 440 ac next to a ground land. The particles of steel were collecting between the lands until they shorted out.
It all seems so simple after the fact! I cleaned all the drive boards, sealed up the cabinet and suggested the plant get better filtering and dust removal for the process.
Contributing Writer Don Humphrey picked up his love of electronics and mechanical tinkering when he got his Ham Radio license at 16. He took math and science courses in college, but he dropped out to see the world with the USAF.
After thirteen years, he entered into the commercial world and has been a field service tech, service manager,and self employed field engineer. He is now retiring to be with his wife and two children.