Friday, July 8, 2011

Unusual VFD problem

So here is one that doesn't happen very often.  Back in March, we had a customer that lost a 900 horsepower drive that was over 10 years old.  Due to the age of the drive, the customer decided that they wanted to strip out the guts of the old unit and retrofit the cabinet with a new 2 module unit. 

Zach and Brandan when to the site and did this work in a couple of days.  Following them getting done, Madhu went up and did a start-up on the unit.  All seemed to be well and we all felt pretty good about the work that was accomplished in the short amount of time.

About 4 to 5 weeks after the installation, the customer had a power bump and the drive would not start back up.  They were on the phone with Madhu describing the problems when Madhu just told them to reset the unit and try it started and we forgot the entire episode.

Last week, the drive was shut down due to a problem with a stack fan.  When the customer went to start the drive back up it started again to fault out on M2 overcurrent. 

They had me on the phone for the better part of the day and I had them run various tests on the drive.  The drive passed every test we put it through but still continued to fault out on the M2 OC fault.  After consulting with the factory and doing my own thinking about this, we all reach the conclusion that the motor must have a problem. 

We shipped up the spare we had in our warehouse.  They installed it but this did not fix the problem.  The motor that was removed was not meggering correctly so the focus then turn to the motor leads.  The customer went through the process of changing out the leads but this did not clear the problem either.

Thursday night, 10:20 pm:  The customer calls me and wants me at the site to get the beast to run.  I left early the next morning and during the course of traveling the 500+ miles to the site, I had parts shipped in to the local airport.  After arriving at the site, we did several tests on the unit.  In checking the unit I noticed that the dip switches on both paralleling boards we set the same (to module 2).  This is not correct as the dip switches are what set the address for each module.  As soon as I changed the settings the fault moved to module one.

After going to the local airport and getting the parts, I went back to the site and started the process of elimination to see what could be wrong.  After a quick check of several items and eliminating them as the problem, I decided to change something that I have only seen go bad 2 other time in the past 28 years, the Hall CT's.  That did the trick. 

I still don't understand what happened or whether they were bad from the beginning but that did it.  Odd thing about this, I did numerous voltage checks to make sure the Hall CT's were operating correctly and it seemed that they were.  Obviously they were not.


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