questions on Large can electrolytic Capacitors

admin

Hello AllSomewhere online I read and I can't remember where, that several smaller value multiple capacitors are used in our mig welders as opposed to fewer larger value caps to accomplish better heat dissipation and to distribute this heat over a larger number of caps. This would seem to make sense.For example I have three different migs one is a 300 amp Airco Dip Cor which uses (8) 10,000 UF caps for a total combined capacitance of 80,000 UF opposed to say 4 caps @ 20,000UF's each. for the same total capacitance.Also I have a 225 amp L-Tec which uses 5 caps @ 9,800UF's (which L-Tec later upped to 6 caps in later models) for a total of 49,000UF's as opposed to say 4 @ 12,250UF's and if you were to look inside a MillerMatic 200 you would find 8 caps rated @ 15,000Uf's each for a combined total of 120,000UF'sSo do think heat dissipation was one of the main design factors in all of this?Also would there be any thing wrong with using the same number of caps with a considerably higher capacitance Value? Since it isn't always possible to find the exact replacements in the same physical size and specs?Would these higher values result in better performance or is more  actually less as they sayLast edited by jzr75601; 02-12-2021 at 12:24 AM.
Reply:Someone like Duane would be more qualified to fully answer this but I will give it a go.  The number of caps is not really that important but the caps must be equal in value. The total capacity of the group is important.  With that said they will work fine if the replacement caps are close to the original capacity. But again they must all be the same. I would never go with less total capacity than the manufacturer used and would try to stay as close as I could to the original value. The manufacturer has lee way to choose what caps to use and I would say their decision is based more on cost than how many are used.
Reply:Anytime you are working with capacitors the first most important thing is the voltage rating. Appling to high of voltage will damage or explode the capacitor.As far as the capacitance rating and do I use one or more in parallel. I am sure there is an engineering book with all the reasons. From what I have seen, the small 90 to 130 amp welders only use one cap with no problems. 150 amp and higher welders always use cap banks of 4 or more. The total value of the cap bank varies by the size and type of welder. the cap bank and stabilizer ( inductor) values have to work together to produce a stable arc. The value of the stabilizer seams to be stable but the cap bank value is variable to a point. two small and arc pops out. to large cost more money. So from a cost design point you want just enough for a good arc and to hopefully last the design life of the welder. From a repair point we can't get the right caps sometimes because of manufacturing changes during the life of the welder. so the original size caps are not made anymore. we have to find some that are close in size and just a little larger in value. To large just costs more. In one of my electronics classes where we were working with formulas. Used in electronic design. we learned that the prefect  values are sometimes not made.so you have to find a close value that will work. so in all designs there is a little room for changes of values.
Reply:One way or another I'd bet it always comes down to cost.  Smaller caps may have higher voltage ratings at a lesser cost so the price per uF at a specific voltage is less with multiple small caps.It could also be due to the ESR (equivalent series resistance) which determines how much power the capacitor will dissipate.  With caps in parallel, the capacitances add together.  With resistors (or ESR) the resistance goes down (with 4 in parallel, the equivalent ESR would be 1/4 the individual ESR).I've also seen part changes go to bigger/higher voltage components that would seem to be more expensive, but rather come from a much more commonly used part with higher manufacturing volume and thus lower per unit cost.  The other end of that stick is asking a manufacturer to make a custom part for you, if they will do it at all they will charge a ton.MillerMatic 252, HTP 221 w/cooler, Hypertherm PM45, Lincoln IdealArc 250 AC/DC"I'd like to believe as many true things and as few false things as possible"