A Dirty Little Secret

In 2008, one of my bridges underwent a rehabilitation project and I got my first experience with variable frequency drives (VFD). Since that time, all of my bridges have been outfitted with VFD’s. So after more than a decade’s worth of experience, I was shocked when I learned just last year that there was a vital piece of maintenance information that I had not been told.

I had experienced a failure with a drive and was just starting the process of planning to remove and replace the drive with my spare unit. Fortunately, I happened to bring this up in conversation with a control vendor who asked if I had reformed the drive capacitors.

I had not.

I had never even heard of capacitor reforming and set about immediately to learn everything I could about this process. I’ll not go in depth here, but to summarize the issue:

The insulating oxide layer (or dielectric) that is created and maintained when a capacitor is charged up deteriorates over time with the lack of a rejuvenating voltage. Without this layer, the capacitors short circuit drawing large amounts of current. This boils the electrolyte solution and turns it into a gas. This happens very quickly and the resulting failure can be extremely dramatic. Basically…boom!

It was sheer dumb luck that I had mentioned this to this vendor at all as we were discussing an entirely different issue. But that lucky conversation prevented me from destroying a $25K drive and a $50 pair of trousers.

If you have VFD’s on the shelf as spare parts, please read about this issue on your drive manufacturer’s website. Each manufacturer can have different procedures for reforming but basically the idea is to subject capacitors to low voltage and slowly step this up until the dielectric is restored.

If you would like additional information or would be interested in learning about the reforming rig that I built from my manufacturer’s recommendations, please feel free to contact me.

I would be interested in hearing any stories you may have about this issue. Please feel free to share your comments below.