Cap Reforming

[bwat]

VFD Gurus: how long would you say a drive had to be powered off before you'd be concerned about reforming the capacitors (or other issues)?

I had one that was powered off for about 6 or 7 months, and upon powering on, it seems to have given up the ghost. Something definitely popped in it. They took cover off to inspect and you can see something definitely failed. Unsure if it was actually the caps or not. Still more investigation underway. 500HP.

 

[hbiss]

6-7 months is not a long time unless the VFD was old to begin with. As for "reforming" caps, I wouldn't try it. Usually a waste of time. Replacement is the right way.

-Hal

 

[synchro]

VFD Gurus: how long would you say a drive had to be powered off before you'd be concerned about reforming the capacitors (or other issues)?

I had one that was powered off for about 6 or 7 months, and upon powering on, it seems to have given up the ghost. Something definitely popped in it. They took cover off to inspect and you can see something definitely failed. Unsure if it was actually the caps or not. Still more investigation underway. 500HP.

I've seen recommendations to reform electrolytic capacitors if the equipment they are in has not been powered up for a year or more.
The thickness of the aluminum oxide dielectric layer starts to degrade after some time and can lower the breakdown voltage enough to cause a permanent failure if full voltage is applied. A reforming process can be used as a preventative measure to restore the dielectric layer if it's not too far gone. But eventually even that will not be sufficient.

 

[zbang]

I've seen recommendation for reforming after a couple of years. Heck, I'll bet there are spares on the shelf at least that old and nobody thinks twice about using them (they should).

VFD Storage and Capacitor Reforming | KEB

This article discusses the importance of periodically reforming VFD capacitors after they have sat for a while. Additonally, time intervals for KEB VFD products is given.

www.kebamerica.com

For example, for a KEB R-housing drive, for storage of under one year, the drive can be started without any special measures. Once an R-housing drive has been in storage for over one year, special reforming procedures are recommended and should be followed. Between 1-2 years of storage, the VFD is to be operated for one hour without any modulation output. Any time after two years of non-powered storage duration will require use of a variable transformer to control the input and slowly increase the voltage to the VFD.

Wouldn't be surprised of other drive makers have similar app notes.