Power factor caps and how to calculate them?

Calculating isn't easy. Much easier to add some capacitance and check current, add some more, keep testing until you find a minimum idle current.

I believe that for optimal performance you would want SQRT(2*PI/LC) = 60 Hz.

You know everything in that equation except the C. That assumes you know the effective inductance "looking into" the primary of the transformer.

I think that an LC network looks resistive at the resonant frequency, and that the resonant frequency is SQRT(2*PI/LC). That's why if you add just the right amount of capacitance for the inductance of your welder, then the resonant frequency is the same as that of the power grid and you get zero imaginary current; i.e. all of your current will be real current.

Real current and imaginary current are electrical engineering terms. Imaginary current can burn up a wire just like real current, it just doesn't show up on your power meter.

I used to know all this stuff ..

Grant

Er, F = 1 / (2 * pi * sqrt(L*C))

I suppose you could measure unloaded current draw. But some of that is actual power too...

Yep, the C and L cancel. But if you're blowing breakers, you've probably already got trouble simply turning it on. That power-on spike is none too fun for the circuit with *just* the inductance, let alone the caps (which look like a short to such transients)!

And I'm still learning it. :o

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

Add it where, and how much do I start with? My power bill from Pacific Greed and Extortion (PG&E) was a $100 higher this month from what I suspect is the welders being used a fair amount..most of which was simply idling.

I never knew any of that stuff...shrug.

Gunnner

"To be civilized is to restrain the ability to commit mayhem. To be incapable of committing mayhem is not the mark of the civilized, merely the domesticated." - Trefor Thomas

Would it be better to switch in the caps AFTER turn-on?

Add capacitance right across the power lines, from L1 to L2; i.e. connect each capacitor in parallel with the whole welder.

I'd scrounge some 10uF motor run caps and add one and turn on your welder and measure the current draw (with a loop-type current meter) and then add another cap and measure it again. If it goes down then just keep adding capacitance. You might want to also add some 30 or 35 uF run caps just so you can build up quicker.

If you need help scrounging motor run caps drop me an email. I have an unimpeachable source.

Grant

Do you have one of the new "speed read" meters, or the old manual type? When there is a sudden jump in the bill it's always a good idea to check that the meter reading actually matches what the bill says.

Twice before they upgraded to the speed read meters I had to call in corrected readings when the meter reader misread one of the upper digits causing a 1,000 kwh jump in the bill. I probably didn't notice other errors in lower digits and fortunately the next correct reading will put things back on track anyway.

$100 jump would seem to be an awful lot of KWH, somewhere between 400 and 800 kwh depending on how bad your electric rates are. Even at 400 kwh that works out to on the order of 13 kwh / day more consumption

Pete C.

Gunner wrote:

Gunner; Have you checked to see if Miller sold Power factor Caps for your machine? Lincoln supplied the ones in my Squarewave 275. It was about $100 cdn at the time. Miller might give specs for theirs (if they sell them)

Pete

Gunner wrote:

If so, then balancing isnt going to lower the operating costs anyways.....

This is true. Power delivered is power sold. IMO, it is highly unlikely Gunner used $100 worth of juice in one month just heating up the wiring during welder idle time. The extra charge is probably due to an error in meter reading - or - Gunner just did a lot more welding in that month.

Bob Swinney

You want to measure the current on the meter side of the caps; not on the welder side.

More like: Resonant Frequency = [1 / (2*PI*SQRT(L*C) ] and solving for C = SQRT[1 / (*60*60*4*PI*PI*L)] But as stated you'd have to know the value of L . . . . .

Bob Swinney

anyways.....

Yeah, I know......I just that I wanted third party verification for Gunner's sake.....

( Pretty sure he has me branded as being 'Liberal'--and so anything *I* might say is *definately* suspect )

Gunner, you might want to check your rate structure. PG&E may have something like a demand charge on its residential (I assume you're a residential customer) rates that kicks in if you have a big surge in demand over something like a 5-minute interval.

That sort of thing is not common in residential rates, but PG&E is notorious for its creative rate plans.

--RC

Projects expand to fill the clamps available -- plus 20 percent

"Political stuff" is always suspect, regardless of who it was uttered it.

True for now Don, talking residential that is. But probably not forever here in the USA. New Zealand already charges residential customers for KVARs, and I recall seeing some outfits already selling "whole house" power factor correction boxes here, which IMO are a sham because they don't automatically track the requirement and correct for it. Capacitors of the right size at each load are the easiest solution and also reduce the I^2*R losses in the house's wiring, giving a tiny additional savings there.

BTW, there was some work done by some Navy guy a while back showing how much power could be saved by adding a running capacitor to exixting dumb old split phase induction motors. IIRC it increased the efficiency of the average motor by about 10%.

Just my .02,

Jeff

Yes, I know how to calculate the required capacitor value, if you can give me the idling inductance of the transformer primary. But you don't know that, and neither to I.

I do know that Miller uses 250 uF of capacitance in their power factor correction kit for the Synchrowave 250. That makes it tempting to say 1 uF per amp of output, but that would be wrong. It depends on the particular transformer. Something in the neighborhood of 250 uF for your machines should be about right though. You don't have to hit it right on the nose to get a useful amount of PF correction.

Gary

Hey Gunner,

For what it's worth, I brought home some oil-filled caps from Apex Surplus out in the Sun Valley area of LA. For my comments here, I'm making San Fernando as east-west, and Lankershim as north-south. Anyway, just in case you are NOT familiar with the place, it's on San Fernando just east of Lankershim. Finding anything in there on your own is a bit of a bitch though, but a real adventure. Ask at the counter if you're short on time. The larger ones I got were at the west end of the first or second row about waist high, and the smaller ones were on the north wall due north of the large ones (but over two rows). I haven't even unpacked mine yet though, so I don't know that they are "good" yet. The guy said I could bring them back if they didn't work, and he knew right away they were for a Rotary Phase Convertor.

Take care.

Brian Laws>Any idea of how to calculate the amount of capacitance needed to

Thanks Brian. Im well familiar with Apex. Someday I aspire to be 1% of them

Sorry we didnt meet up this trip. I got your voice mail today. I was up on the roof in the rain..sigh.

Next trip, try to make a little time to come visit the homestead. Im sure we can find Stuff to fit in your suitcase

Gunner

"The French are a smallish, monkey-looking bunch and not dressed any better, on average, than the citizens of Baltimore. True, you can sit outside in Paris and drink little cups of coffee, but why this is more stylish than sitting inside and drinking large glasses of whiskey I don't know." -- P.J O'Rourke (1989)

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