vfd cap bank reform

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Twoskinsoneman

Twoskinsoneman

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I've got some capacitor bank kits for a ACH550 vfd that have been on the shelf for several years.
I'd like to reform the caps on the bench before they're installed. Does this look like it should work?
Basically using a variac to transformer to rectifier to slowly (over several hours) build up the voltage to the cap bank.
I'll be fusing it and monitoring it to ensure I don't go more than 0.1 amp to the caps.
reform.png
 
The circuit arrangement looks sound but what is the voltage rating of the capacitors? Rectified 480V will give you nearly 700Vdc.
Most VFD electrolytics I've come across are up to 450V.
 
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Add a DC voltmeter and don't exceed the rating of the caps.... good to go, then.

(IME, caps that are only a few years old won't need reforming, but it won't do any harm, either, and might reject a marginal cap (sometimes by spectacular failure).)
 
It looks pretty good to me. The 500 ohm resistor will dissipate 5 watts even if you limit the current to 0.1 amps, so make sure the resistor can handle this power level and even higher levels on an intermittent basis.
A shunt bleeder resistor before the series resistor might be wise so that the caps don't retain a lethal voltage (assuming that the cap bank doesn't already have one, which it might).
Use goggles if observing the caps themselves while doing reforming, especially when starting out when the cap status is unknown. Bad electrolytics will sometimes spit out electrolyte through their vents.
 
synchro said:
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Use goggles if observing the caps themselves while doing reforming, especially when starting out when the cap status is unknown. Bad electrolytics will sometimes spit out electrolyte through their vents.
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Good advice.
Been there, done that, got the t-shirt and the hat... It was a MESS!
 
Thanks all. Will take all into account. Good thought on the dc voltage and cap rating. Had planned to monitor output of transformer but will add dc meter on output of rectifier. It doesn't have a bleed resistor. The vfd uses the cooling fan to bleed them after power off so that is also a good thought too. I'll give it a shot. I don't want to take a chance with just installing them without reform. ABB seems to think it should be done if they've been shelved even 1 year.
 
ActionDave said:
Am I the only one who has never heard of reforming caps?
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Well, I have heard of it but never done it. We manufactured variable speed drives up to several MW. Several hundred 2200uF caps. We simply replaced them after a few years and binned the old ones. The data sheets gave an operational life. We kept within that.
 
Electrolytic capacitors depend upon an insulating layer that is electrochemically formed next to one of the electrodes. Sitting unused this insulating layer can break down. If you slowly ramp the applied voltage up you can recreate (reform) this insulating layer.

Radio amateurs often do this to old amplifiers that are being placed back into service.

-Jon
 
Jraef said:
Other than the fusing (which you say you will be adding), you have a good handle on it.

Junkhound, ALL VFDs use electrolytic caps.
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Just so some novice does not take your ALL as gospel:

Probably true IF you add the work COMMERCIAL or 50/60 Hz, but definitely not ALL, as VFDs I have worked on for X-37, F-22, commercial airliners, space station and many aerospace applications use film caps or stacked ceramic caps. Will agree that if you go to Platt, Graybar, Grainger, or ebay for a VFD, they will have only electrolytics in those designs.
 
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