# Charging a drive that has been on shelf



## just the cowboy (Sep 4, 2013)

I have a drive that has been on the shelf for 5 years or more . I was told the caps have too be brought up slowly. Can they be brought up in steps of 24v, 48 v, 120v, 240v, 480volt rectified to dc? Day on each?


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## JRaef (Mar 23, 2009)

just the cowboy said:


> I have a drive that has been on the shelf for 5 years or more . I was told the caps have too be brought up slowly. Can they be brought up in steps of 24v, 48 v, 120v, 240v, 480volt rectified to dc? Day on each?


If nothing else is available, yes, although maybe an hour each is enough. The more steps the better, I use a variac variable transformer and just slowly increase the AC onto two lines, because it gets rectified anyway. But honestly, I generally get impatient and do maybe 4 steps over 6-8 hours.

Do a search on the term "capacitor reforming" with the quotes, you'll get hits on the procedures recommended by most VFD mfrs.


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## LARMGUY (Aug 22, 2010)

I did that once and BOOOM! I guess I did it to fast. Picking up pieces for a week.

It scared me and I'm fearless.


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## tates1882 (Sep 3, 2010)

LARMGUY said:


> I did that once and BOOOM! I guess I did it to fast. Picking up pieces for a week. It scared me and I'm fearless.


 are talking like grenade style explosion, with flying shrapnel? I knew you could fry one but not explode


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## MDShunk (Jan 7, 2007)

tates1882 said:


> are talking like grenade style explosion, with flying shrapnel? I knew you could fry one but not explode


Not grenade. More like shotgun blast with lots of black soot.


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## AK_sparky (Aug 13, 2013)

LARMGUY said:


> I did that once and BOOOM! I guess I did it to fast. Picking up pieces for a week.
> 
> It scared me and I'm fearless.


Sounds more like you mixed up the plus and minus of the caps. Electrolytics don't like reverse polarity at all. They go boom with lots of smoke.


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## JRaef (Mar 23, 2009)

MDShunk said:


> Not grenade. More like shotgun blast with lots of black soot.


Don't forget the goo... Lots of goo on big drive capacitors. It's impossible to clean it thoroughly without it taking more labor time that it would cost to just swap out the drive.

I had the caps blow once on a 40HP drive that froze over night at a remote rock crushing plant. The drive had a NEMA 4X fiberglass enclosure, the force blew the cover off and sent it sailing about 50ft. off the road and down into a ravine.


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## LARMGUY (Aug 22, 2010)

Hard to mix up the polarity using A/C to rejuvenate a cap. And yes, it was a grenade with shrapnel and everything. Darkened underneath my skin on my thumb and finger for about a year finally workeing it's way out. It looked like gun powder.


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## Jlarson (Jun 28, 2009)

No not the goo, anything but the capacitor goo.

Not sure which is worse, cap goo, blown surge protector or melted CT. All seem to end up involving lots of cursing, a carbon scraper and a case of lectra clean. :laughing:


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## AK_sparky (Aug 13, 2013)

LARMGUY said:


> Hard to mix up the polarity using A/C to rejuvenate a cap. And yes, it was a grenade with shrapnel and everything. Darkened underneath my skin on my thumb and finger for about a year finally workeing it's way out. It looked like gun powder.


Every capacitor reforming I've ever seen has used DC directly on the caps. AC directly on the caps would for sure kill them.

But really you should be able to use the rectifier on the drive to get your DC if you somehow limit the current to it.


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## just the cowboy (Sep 4, 2013)

*starting now*

One of the bosses brought in a variac, starting now with low voltage.
12v in first hour 24v second hour, Caps holding charging if power is removed ( bleeding down from internals). Let you know tomorrow when I hit 480V.


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## JRaef (Mar 23, 2009)

So just to be clear, you are not really "charging" anything here. What is happening is that capacitors work by virtue of a thin film of an oxide combined with chemicals in film and the electrolyte that forms on the surface of the layers and layers of film which make up the "plates" of the cap. Those layers are separated by the electrolyte, the goo, but it is conductive, so it is the oxide layers that are actually the insulation layer. When in use, the flow of electrons across the gaps is constantly reforming the oxide layers as they deteriorate. Over time, if left un-powered, the chemicals of the oxide layer dissolve back into the electrolyte. If you re-energize it immediately, the film layers are touching each other and the flow of electricity is immediate across them, burning them (Some are aluminum, many are mylar). If enough of the layers burn through all at once, it creates a gas pressure that explodes. Like a grenade. 

But even if it doesn't actually explode, damaging the layers will result in a DECREASE in their capacitance. On a VFD, that results in increased DC bus ripple, which can cause the transistors to fire at the wrong time, which leads to failure.

What the "re-forming" procedure is doing is to SLOWLY re-introduce electrolysis, the chemical deposition process of creating (forming) those oxide layers on the film again. The slower the better because by starting off with a low voltage, you are inherently limiting the energy that might be flowing on the direct contact of the film layers, avoiding burning them.


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## LARMGUY (Aug 22, 2010)

AK_sparky said:


> Every capacitor reforming I've ever seen has used DC directly on the caps. AC directly on the caps would for sure kill them.
> 
> But really you should be able to use the rectifier on the drive to get your DC if you somehow limit the current to it.


Ah ha! you caught it!

That was when I learned to do DC on caps. My boss told me to use the Variac to rejuve the cap. I did. The only Variac I had was A/C. It was a learning experience I will never forget. He told me his boss told him to check the voltage on that old O scope. He did so and It shot the end of the CRT across the shop. That's when he learned about the high voltage probe.


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## bill39 (Sep 4, 2009)

This is a very informative post to me. I had never heard of needing to do this with drives.

My takeaway from this post is if this is a concern then I would call the drive manufacturer and get their recommendation. Too much money and safety at stake to try any other way.


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## just the cowboy (Sep 4, 2013)

JRaef said:


> So just to be clear, you are not really "charging" anything here. .


Yea I know it's reforming the cap, was just pointing out that it must be working because it held the charge. 
I did the search on "reforming caps" and allot of interesting info came out about it. The one I found the most was one from 1945 showing this has been a known issue for some time. 
Thanks again for your input, top notch as always.
Harvey


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## dronai (Apr 11, 2011)

Does this just go for the type of Caps used in the VFDs ?


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## just the cowboy (Sep 4, 2013)

*no*



dronai said:


> Does this just go for the type of Caps used in the VFDs ?


They started doing this with radio parts that were on the shelf


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## JRaef (Mar 23, 2009)

just the cowboy said:


> They started doing this with radio parts that were on the shelf


Yep. It applies to all caps, but generally, the little caps on PC boards are at such a low level of energy anyway, it's not as important. 

Happens a lot on motor starting caps with old single phase motors that have not been used for a long time.


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## just the cowboy (Sep 4, 2013)

JRaef said:


> Happens a lot on motor starting caps with old single phase motors that have not been used for a long time.


Never thought of that, but yea I remember pulling caps covered in dust out of spare parts bins and replacing them on motors. Only to have them fail not long after, just assumed it was a defective cap.


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## JRaef (Mar 23, 2009)

just the cowboy said:


> Never thought of that, but yea I remember pulling caps covered in dust out of spare parts bins and replacing them on motors. Only to have them fail not long after, just assumed it was a defective cap.


Well, it was a defective cap, but not by virtue of being manufactured wrong, but rather by being left on the shelf too long.


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## CyberKnight (Sep 3, 2013)

Where I work we have about 100 TB WOODS Etracs. Wfc and afc models. In there manual the say to contact the factory if the drives have been on the shelf for awhile to check and reformat the caps. I did it once but forget how I did it, to long ago LOL. We have had problems with the bus caps going bad. I have found that if I leave them energized all the time they do not act up. We were turning them on and off when not needed and from what i see the caps dont like that. Just something weird I have stumbled upon.


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## JRaef (Mar 23, 2009)

CyberKnight said:


> Where I work we have about 100 TB WOODS Etracs. Wfc and afc models. In there manual the say to contact the factory if the drives have been on the shelf for awhile to check and reformat the caps. I did it once but forget how I did it, to long ago LOL. We have had problems with the bus caps going bad. I have found that if I leave them energized all the time they do not act up. We were turning them on and off when not needed and from what i see the caps dont like that. Just something weird I have stumbled upon.


So regarding turning them on and off a lot:

Capacitors charge up very quickly, in fact so quickly that they draw current into themselves at the level of the available system fault current, which can damage the diode bridge rectifier and the caps themselves. So all drives have what is called a "pre-charge" circuit that is basically a current limiting resistor in the DC bus just ahead of the caps. But to avoid causing a permanent voltage drop, the resistor is really only used for a second when you FIRST apply power, then it is immediately shorted out with a relay contact. By turning the line power on and off a lot, that resistor and relay gets stressed, burns out, and then it either shorts, which takes out the caps next time, or opens, which results in the VFD failing to power up.


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## GrayHair (Jan 14, 2013)

Worked part time in a TV/Radio repair shop and I had to reform all the electrolytics every 6 months. Owner had a pretty old one pop after he installed it and started reforming after that.


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## Electric_Light (Apr 6, 2010)

I would use a 250 to 500VA size 120<->240/480 isolated transformer, then limit the current on the PRIMARY side by plugging into a wall outlet through a 100 or 150W light bulb in series. Connect the 480v side to two of the input terminals so you're feeding 480v single phase. If it won't allow you to charge it up like that, use a 1,000v rated bridge to feed directly into the bridge. 

Hook everything up first, THEN plug in. The light bulb on 120v side will light up bright, then fade off unless the drive is shorted or you reversed the polarity into DC Link. Confirm that voltage builds up to about 680v on DC bus. Unplug. Let it bleed down naturally. Repeat it one more time. Using this setup lets you control everything from the 120v and fault current is limited as you won't have to handle live 480v service. If the bulb doesn't fade down, don't hook the drive up to regular power.

Energize the VFD normally, but leave it in standby for number of hours in years that it was unused (5hrs if it was shelved for 5 yrs)


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## don_resqcapt19 (Jul 18, 2010)

JRaef said:


> ...
> I had the caps blow once on a 40HP drive that froze over night at a remote rock crushing plant. ..


So is that why some drives have a temperature sensor wired in to prevent the drive from running if it is too cold?


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## JRaef (Mar 23, 2009)

don_resqcapt19 said:


> So is that why some drives have a temperature sensor wired in to prevent the drive from running if it is too cold?


Haven't seen that. To do that, they would need to have a way to keep power off of the DC bus, not just prevent the transistors from firing. It's just APPLYING power to the line terminals that is the problem if the caps are frozen. 

Where this comes up most often is on portable equipment that is being used in extreme cold, where panel heaters are not going to work because the generator is not running. So I just put a line isolation contactor ahead of the drive and control it with a thermostat set to close on temperature rise above 40F (often called a "freeze-stat". You start the generator, it powers the heaters, but no power gets applied to the line side of the drive until the air in the box is 40F. In the case of the one that exploded, I had warned the owner when I was building the panel but he didn't want to pay extra, claiming that if it was that cold they wounldn't be running the rock crusher anyway. He forgot to pass that concept on to his workers, they fired up the generator, flipped the main breaker and Boom!

There may be other issues of too rapid of a thermal change in the transistors or something by _running _a drive if it's too cold, but I've never run across that one before.


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## oliquir (Jan 13, 2011)

don_resqcapt19 said:


> So is that why some drives have a temperature sensor wired in to prevent the drive from running if it is too cold?


yes it will block the vfd from running, but the capacitor will still charge, but it will prevent any current draw from them


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## MDShunk (Jan 7, 2007)

With the drives integrated inside the MCC bucket in recent years, a very popular "reset feature" has begun to prevail among non-electrical types. Pull the sidearm down to cycle power to the whole bucket, count to 10 or 15, and turn the power back on. That's gonna start to bite people as drives age. It's actually quicker to reset the fault through the HIM (or equivalent) on the front of the bucket door than it is to cycle power.


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## JRaef (Mar 23, 2009)

Yes, but if there is a shortcut, people will find it.

There are several solid state overload relays on the market that when initially released, had a major flaw. If the OL relay tripped, you could not reset it until the motor had time to cool down, which is a basic requirement. But on some SSOLs, they failed to think of having "retentive thermal memory" and if you cycled control power, you could reset it immediately, right into a hot motor, and the OL relay would think it was cold. Oops... And of course, operators figure that one out almost immediately.

I'm pretty sure those have all been recalled and fixed by now, that was a decade ago or so.


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## KennyW (Aug 31, 2013)

All that aside though, first step should be output filter, IMO. Relatively cheap and easy, and will have a greater affect than a cable change.


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## just the cowboy (Sep 4, 2013)

*Took my time*

Reformed caps no problem. Went to 625vdc on caps with variac and stepup xformer hooked to two input phases.
Thanks for input
Harvey


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