Spare Drive Forming Capacitor

[W@ttson]

Hello all,

We have a spare 480V drive that has been sitting around in a box for 3 - 4 years. I want to get a variable transformer so that I can apply a stepped up voltage scheme to the drive to reform the capacitors. I will start at 200V and wait for about 10min, then move on to 250V and wait about 30min, then go up to 350V and wait about 20min. Then I will bring it up to 480V and wait 3 hrs.

Does anyone have a recommended variable transformer for this purpose?

Do you think it would be sufficient to just connect to 208V panel for an hr then connect to 480V for 3 hrs, so that we don't have to go and buy some extra device?

 

[__dan]

A small variable 120 V transformer might be easily available, probably an Ebay item. Then you could run that through a small step up 480 V transformer, probably a common control sized transformer, usually 480 to 120, again maybe Ebay. The load would be tiny or near zero just to float the caps, DC float at the caps. Probably make sure it is rectified and not AC on the DC bus but I would read the manufacturer's procedure just to be sure.

Pretty ambitious to do that. I have suggested it on occasion but never found a taker.

 

[ptonsparky]

The VFD I have on the shelf recommends power to it with no load connected for several hours if the time offline is not known. You milage will vary of course.

 

[W@ttson]

A small variable 120 V transformer might be easily available, probably an Ebay item. Then you could run that through a small step up 480 V transformer, probably a common control sized transformer, usually 480 to 120, again maybe Ebay. The load would be tiny or near zero just to float the caps, DC float at the caps. Probably make sure it is rectified and not AC on the DC bus but I would read the manufacturer's procedure just to be sure.

Pretty ambitious to do that. I have suggested it on occasion but never found a taker.

mine would need to be a 3ph setup.

 

[W@ttson]

The VFD I have on the shelf recommends power to it with no load connected for several hours if the time offline is not known. You milage will vary of course.

Wouldn't it tell you to ramp the voltage up in steps and not just dump the rated voltage if the shelf time was unknown?

On different drives I have seen a marker of 2 years where you dont need to do anything but just power it up and let it sit for an hr for every year it has been off.

If you are over 2 years, then you got to do steps and each step is a soak time of 15 to 30 min.

 

[Jraef]

mine would need to be a 3ph setup.

No, it doesn't. It gets rectified to DC, which is where the caps are, so it doesn't matter where that comes from on the AC side, so single phase is fine. You also don't need a lot of power, a few amps is fine. I bought a Variac on Fleabay a few years ago for $10 (before everyone figured out they were fun for electric guitars) that plugs into 120V. Then I take that variable output and run it through a little control power transformer I had laying around to step it up to 480 or 240, then feed that into drives (everything fused of course). Works great. I do 50V increments for 15 minutes each.

If you want to AVOID having to do this, put a sticky note on the box of the VFD on the shelf reminding yourself (or someone) to take it out and apply power to it for an hour once every year, like on Tax Day. It doesn't need to be hooked to a motor or anything, just 2 poles of line power at the rated voltage.

PS: It's best to see if the original VFD mfr provides their own instructions for "capacitor reforming" of their product. Some don't like having you apply variable low voltage to the AC line terminals, because it affects the control boards if they get low voltage for too long. So they will tell you that you need to disconnect the rectifier and apply variable DC to the cap bank terminals directly. Those are a PITA...

 

[W@ttson]

No, it doesn't. It gets rectified to DC, which is where the caps are, so it doesn't matter where that comes from on the AC side, so single phase is fine. You also don't need a lot of power, a few amps is fine. I bought a Variac on Fleabay a few years ago for $10 (before everyone figured out they were fun for electric guitars) that plugs into 120V. Then I take that variable output and run it through a little control power transformer I had laying around to step it up to 480 or 240, then feed that into drives (everything fused of course). Works great. I do 50V increments for 15 minutes each.

If you want to AVOID having to do this, put a sticky note on the box of the VFD on the shelf reminding yourself (or someone) to take it out and apply power to it for an hour once every year, like on Tax Day. It doesn't need to be hooked to a motor or anything, just 2 poles of line power at the rated voltage.

PS: It's best to see if the original VFD mfr provides their own instructions for "capacitor reforming" of their product. Some don't like having you apply variable low voltage to the AC line terminals, because it affects the control boards if they get low voltage for too long. So they will tell you that you need to disconnect the rectifier and apply variable DC to the cap bank terminals directly. Those are a PITA...

Oh wow! very important and eye opening. thank you

If I don't have that variac and I am not too worried since it hasn't been too long (relatively), would my 208V panel hook up work? That would be 43% of rated voltage. I am thinking that would be a good start to not stress things too much. Then after it soaking there for a bit (lets say 30 min to 1 hr), connect it to 480V. Then keep it there for 2 hrs or so.

Looking at this KEB forming video, it looks like it might be good:

 

[garbo]

The VFD I have on the shelf recommends power to it with no load connected for several hours if the time offline is not known. You milage will vary of course.

We had 5 or 6 VFD'S ( 60 to 75_HP ) sitting around due to part of a job was cancelled. Danfoss drive tech told me that you should apply power for a few hours once a year. If the drive was 3 phase you would be better off supplying three phase power. Only had to replace capacitors a couple of times on a 50 & 125 HP drive. They usually are the first things bolted into drive so to try and rearrange cap wires to use single phase could be a project If you can get to inputs if IGBT''S you might be able to connect power to cap bank. If you have a small say 480 to 120/208 bolt three phase transformer you or a 120/208 panel you could take off 208 volts and use that for a few hours. Even a 100 or 200 HP VFD would draw more then hopefully a few amps just to energise the drive. Last capacitor bank that I replaced in a VFD had 9 capacitors. Read that on a normal VFD drive the capacitors should last 75,000 hours. At large place that I retired from they had over 500 drives and some were over 15 years old and over 125,000 hours run time . If you do use single phase remember that all drives 250 HP & smaller ( at least the ones that I worked on ) only uses single phase to supply power to control board that in turn supplies low voltage to LCP ( WHAT Danfoss calls local control panel = touch screen ) so you will only get screen to illuminate when power is applied to say A & B phase. No screen if power is only applied to A& C or B & C phase. Touch panel does not have any control to three phase power to cap bank. The three phase drive input goes right to the three phase rectifier bank that capacitors are connected to . From there this DC Buss voltage ( usually around 660 volts DC For 480 volt input ) supplies power to the IGBT'S. At a large newspaper that I worked at we had spare 17.5 HP A& B and some Finn Cor 60 HP drives that sat on shelves for several years before being used. We never applied yearly power even after I asked to do it.Guess they were just lucky that they worked.