What's new
What's new

DC bus capacitor new vs. old discharge times are not what I expected.


Mar 5, 2009
California, USA
My vfd dc bus capacitors:


I did a simple charge and monitor open circuit voltage on the capacitors. The newer and more capable capacitor discharges at double the rate
of something like 0.1 volt/sec. The older used capacitors from vfd mfg. date of 2003 :eek: discharge at a slower rate.

At this point I assembled the vfd and powered it up. Couldn't get it to spin my motor because I had to be in manual control
with some program parameter. I don't remember how many times I have gone through that vfd manual. My cheat sheets improve in quality.
This time I'm going to write up notes for any contingency and cover the page with clear plastic. Keep a copy on the vfd cabinet door.
Last edited:
After they sit on the shelf for a while, the capacitors will lose some of their "forming". Modern capacitors are far better than old ones at holding their forming, so the worst you will usually see is a little higher leakage current.

Yes, after a bit of use, they will again be "formed". They would have been completely formed when they left the factory.