Need to Reform Capacitors

[jpanhalt]

I have a TIG welder that has not been used in about 14 years.   It uses a pair of paper oil capacitors (40 uF total, 460 VAC) for power factor correction.  Machine operates on 220 VAC (USA).  I have read that such capacitors can go bad with age, but there are conflicting statements about "reforming" them -- mostly from audionuts.

While PFC is not necessary in my application, the capacitor is a rather large and expensive-looking pair of metal cans.   The cans show no sign of deterioration.   Unit has been stored in a climate controlled and dehumidified environment for 10 of the 14 years.

Do such capacitors benefit from being brought up to operating voltage slowly?

John

[helius]

Electrolytic capacitors are formed during manufacturing. Forming produces a reaction between the anode and the electrolyte (anodization) that grows a thin dielectric layer. The dielectric is what makes it a capacitor, and its thickness determines the capacitance, voltage rating, and leakage. Over years of storage without charge, the anodization can dissolve back into the electrolyte, which reduces the voltage rating and increases leakage.
Reforming is just repeating the procedure that grows the anodization. Since the capacitors are not new, and the electrolyte may have evaporated or degraded, the results are not always as good as new. This only happens with electrolytic capacitors. Reforming film capacitors is a non sequitur.

Note: anodization is a current-controlled process, not a matter of "bringing up the voltage slowly". A variac is not the best way to reform capacitors, although it can prevent inrush surges from damaging vulnerable parts. But some regulator topologies become unstable and can be damaged this way.

[jpanhalt]

Thanks for the prompt reply.   That was my thought too, but I have no experience with oil paper capacitors and thought I would double check in light of what was said on audionut forums. (I know they are not electrolytic and have other failure modes.)

John

[Electro Detective]

FWIW  I've never had an issue with multiple 660 volt ones after switching on ferro resonant transformers after many years of inactivity

and they run at 630 to 660 volts AC constantly   

I do warm up any cold devices that haven't been used for ages to room temperature,
or above with a small room heater to avoid any switch-on  'cold shock' power sucking and avoid tripping a breaker.

Not sure if it's just silly tech voodoo      , 
but I still do it out of habit and it  'works'  for me as a cheap preventative measure   

[jpanhalt]

We are in the midst of a heat spell (for us), temps in the 90's.  I located the only electrolytic in the whole device, and it has 1/4" blade connectors.  So, I will remove it, reform/test it and then fire the unit up.  I so much enjoy TIG welding compared to MIG.  My unit has HF start and foot control. The MIG will be retired to structural steel.

John

[Electro Detective]

Make sure all caps are discharged before removing, if they are in good shape they may still bite 

I would not suggest discharge by shorting with a screwdriver, it doesn't do the caps any favors, millions may disagree