# Reforming VFD Capacitors



## micromind (Aug 11, 2007)

I installed 2 VFDs today and they were manufactured in 2013 and had not been energized since then so I reformed the capacitors in both. Actually, both at the same time......lol. 

They both work fine but my question here is this.......

Has anyone seen or heard of capacitors failing (maybe violently........) because they were not reformed after sitting discharged for several years? 

I haven't had any trouble but if in doubt, I will take the time needed to reform them.


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## Cow (Jan 16, 2008)

micromind said:


> I installed 2 VFDs today and they were manufactured in 2013 and had not been energized since then so I reformed the capacitors in both. Actually, both at the same time......lol.
> 
> They both work fine but my question here is this.......
> 
> ...



I've had a couple AB drives that were rebuilt for customers that sat on the shelf for spares, give up the ghost shortly after being energized.


I always assumed it was for the reasons you mentioned. The caps not being brought up to full voltage slowly.


I never investigated. I just replaced the drives and moved on with life. No more issues after that.


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## JRaef (Mar 23, 2009)

I had it happen recently at a large pharma company. They bought spares for all of their drives, but nothing went bad for the first 5 years so they never even look at them other than to count them for inventory. Then they got hit with a spike from the utility and lost a bunch of drives at once (no line reactors). So they started replacing them with the drives off their spares shelf and blew up every one of them, most of them multiple times before calling me. They had never read the instructions about energizing them once per year to avoid having the capacitors deteriorate.


2-3 hours per year of just being _energized _is all it takes to avoid that, no need to connect a motor. But if they sit unused for 2 years or more, you need to do the slow re-forming procedure. Between 1 and 2 years it's a craps shoot.


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## John Valdes (May 17, 2007)

Rob. I had a great customer call me and needed a direct replacement drive. I tried to get him to take a drive off my shelf. Better drive current wise.

Well I had a direct competitor that I knew had this line. I called and he sold me a 3 year old drive for half price, no guarantee.
No time to soak the caps.

Sent my drive guy and once again I show up to job to check how it was going.
This guy turned out to be the best. No drive delivered. 1 Hour later it shows up and my guy jumps right in.
Midnight I asked him to give it a break. He said he had more jobs in the morning.
So here I am with him working like I used to work.

He got it going.


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## stuiec (Sep 25, 2010)

What's the procedure for reforming?


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## JRockey (Apr 9, 2018)

We rebuild drives and often get drives in that have never been energized for 10 plus years. My first technician had worked in the controls department for a large steel plant. He had decades of experience in rebuilding drives and he had very good luck with simply energizing a drive very slowly (over 30 seconds) with a variac. We never lost a unit with this method. I know it varies quite a bit from the manufacturers recommendations of 8+ hours but it worked.


That technician no longer works for us so we looked into doing it the correct way. The biggest problem I have with Rockwell recommendations is that they want you to isolate the bus caps and then use the 8+ hour stepped method. We can do that but it is a lot of extra effort and then safety becomes an issue also. I don't understand why they only place a concern on reforming the bus caps. There are dozens of other electrolytic caps on the circuit boards that according the the cap manufacturers are also in jeopardy of needing reformed yet when you isolate the dc bus caps the other caps will not get reformed.


We have pretty much decided that on larger or more expensive drives, it is worth the time to reform the caps. Smaller drives, we continue to use the variac and have not had an issue yet. We have had one or two techs forget to use the variac and lost a couple drives over the years because they caps were not reformed so it definitely does happen.


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## JRaef (Mar 23, 2009)

The issue regarding isolation is that when you apply low voltage to the AC side of the rectifier (or DC directly to the bus without isolating), the DC/DC converter that is creating the PC board control power off of the DC bus is getting low voltage too and you cannot accurately predict what might happen to the VFD control system. Most of the time it's going to be fine, but you just don't know for absolute sure. I've done it that way myself plenty of times without issue, but if the Mfr makes that recommendation and it DOES screw up the control system, they own the problem. The other issue is that not all VFDs take their internal control power off of the DC bus, some have a separate AC/DC switch mode power supply connected to the line side terminals and because those are often auto-ranging, they can deal with the LV input better. That design philosophy has other negative consequences however, leading to most VFD mfrs moving away from it.


From a chemistry standpoint, you cannot re-form the Aluminum Oxide layers on the Mylar sheets in electrolytic caps in 30 minutes, so if someone was doing that, they got lucky. It physically takes hours for that to chemical process to happen.


As to the other caps in the drives, most modern drives are not using electrolytics on PC boards and such, they are film capacitors now and don't have the Aluminum Oxide depletion issue (which is what the re-forming procedure is about). So the only electrolytic caps left in drives are the DC bus caps.


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## Wiresmith (Feb 9, 2013)

https://www.electriciantalk.com/f28/ppe-vfds-258442/




> we have seen bus caps let go and there is a lot of energy released when this happens.





> Ive seen drives blow and its impressive but in every case the arc force was vented backwards towards the mounting surface. The worst case is when the drive is mounted in a cabinet which redirects the force forwards (door open).


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## drsparky (Nov 13, 2008)

Electrolytic capacitors built between the late 1990s through late 2000s may have the "capacitor plague" I find a lot of them in equipment of that era. The can ruptures or the top looks like it is bulged out. Older and newer caps work fine for years without problems. Most "plague" can be trace back to Taiwanese quality control in the manufacturing process.


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## JRaef (Mar 23, 2009)

drsparky said:


> Electrolytic capacitors built between the late 1990s through late 2000s may have the "capacitor plague" I find a lot of them in equipment of that era. The can ruptures or the top looks like it is bulged out. Older and newer caps work fine for years without problems. Most "plague" can be trace back to Taiwanese quality control in the manufacturing process.


Actually what I heard happened was that someone stole the "secret sauce" recipe for the electrolyte used by a capacitor that allowed caps to be smaller, but more expensive because of the development costs having to be amortized. The developer however hadn't kept all of the info in one place, so when the thieves found buyers for the formula they flooded the market with cheaper capacitors because they had no development or testing costs. Then OEMs snatched them up and incorporated them in lots of different assemblies, the vast majority being PC motherboards and power supplies. The missing elements in the formulation caused them to create gasses, swell up and fail. This cost Dell Computers alone over $3 billion in recalls. There was virtually no company on the planet making consumer electronics that was not affected, but the only place it showed up in VFDs was in small auxiliary power supply boards, not in the big "cans" used for DC bus capacitance. Because of the heat involved, the type of electrolyte used in the bigger DC bus smoothing caps for VFDs is different so it was not affected.


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## Peewee0413 (Oct 18, 2012)

My company just recently tried to cut costs and installed a panel full of used PF VFDs that sat at a warehouse for who knows how long. Two of them let the smoke out on power up.

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