Switching in Power Factor Correction Capacitors


I've just purchased a Syncrowave 250, with the power factor capacitors installed. I've noticed from the Miller owner's manual, that the machine draws about 66 amps full-time, whether or not you're welding. However, without the PFCs, the machine draws 92 amps while welding, and about 4.7 while at idle.

Since most of the time when I'm working on a project, the machine is just sitting there idling, while I'm cleaning material, sharpening electrodes, setting-up for the next weld, the machine is drawing a fair amount of power from the wall.

I was considering placing the PFCs across a contactor, which could be triggered from the Gas Valve solenoid, which would connect the PFCs across the primary windings only when the machine was welding.

Are there any flaws to this idea?

Thanks for the continued insight of this group. There is a tremendous amount of knowledge shared here.


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You want the PF correction caps inline when you are not welding. That is what reduces the no load draw from 66 down to 4.7 amps. Most of the no load current is inductive, and there is no real power used. It circulates to and from the power grid and transformer with little loss. The capacitors draw current in the exact opposite phase angle, and cancel it out. There is a little magnetizing loss that can't be canceled out. That is the 4.7 amps you see. That is the true power it takes to keep the transformer powered up.

According to your specs, you could reduce the welding current draw down to

66 from 92 amps by switching out the caps when you start welding.

But... There is a problem.

Your contactor may not last to long.

When the contactor disengages, the cap stays charged at what ever point the ac cycle was at when the cap is disconnected from the line.

If it disconnects at +340 volts. (top peak of the 240Vac cycle) and it tries to reconnect at the bottom peak of the ac cycle. That being -340 volts. Then you have a voltage difference across the contactor of around 680V when it makes contact. The current spikes will be several hundred amps. That will quickly pit the contacts, and/or weld them closed.

That is why it's never a good idea to repetitively switch capacitors in and out of line with little delay between cut in, and cut out.

They only way you can do it is with a sold state active switch that waits until zero crossing of the ac cycle to make a break the connection to the capacitor bank. That device would cost more than your welder did.

Just leave them connected and don't worry about it.

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"N9WOS" > installed. I've noticed from the Miller owner's manual, that the

Hold it... Ignore the last post.

I read your post wrong.

You are saying your unit disconnects the transformer when it is not welding. Instead of disconnecting the transformer from the welding output while leaving the transformer energized?

You could do what you say, but you will still have moderate current spikes when ever your contactor engages. That will reduce the life of the contactor greatly. You will have a flat capacitor bank repetitively connected to full line voltage, and that may cause problems.

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The machine is not drawing nearly as much power when it is idle, as you think. The current is out of phase and carries very little power. So you only pay to your utility for the extra power that is used to heat up your wires inside your walls, which is not much.

I do not personally see a big value in PFC.

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What you're saying makes a great deal of sense. I appreciate the input about switching the capacitors in-and-out of the circuit. I do not have the experience, or depth of knowledge, when it comes to power AC electronics.

It would seem that a bit of time spent with an Amprobe to really determine (read: quantify) how much power is really being used both with and without the PFCs in the circuit.

Most of the welding I will be doing, is going to be 200 amps, or less. I will have to find the averages of power usage per welding session, and either leave in the PFCs or take them out, depending upon how much "real" billable kilowatt*hours used.

Can you explain to me a bit about the theory behind your statement: "Most of the load current is inductive, and there is no real power used." After looking through the google results for "Power Factor Capacitors", this topic of inductance appears, and there appears to be some unclarity as to real power and what the electric company is going to bill for.

Thanks, _kevin

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You need to get this clearly in mind before you start to make measurements. For DC it's really simple. Power = volts x amps. For AC. the equation is P = VA Cos Theta, where Theta is the phase angle between volts and amps. Cos theta is the power factor. For DC, and for non-inductive AC it is 1 (one.) For AC, as the load becomes more inductive or capacitive, the phase angle changes, and Cos theta becomes less than l.

If you make curent measurements only, you will miss the phase angle part, and miss the whole point about the capacitors and their role in power factor correction. Capacitors are used to correct the phase angle, just to eliminate resistive power loss. From your description, it sounds like that is what is happening in your current setup *with* the capacitors, so you probably shouldn't change anything.

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Leo Lichtman

Some things were not making sense with your figures. That is why I got messed up. But I see your figures are right.

I read what you wrote backwards. I though you said that it drew 66 amps at no load withOUT PFC's and that it only drew 4.7 amps with PFC's That would be closer to the way I have normally seen PFC's used. But miller is using them differently. That is why I seen the figures and my mind got screwed up.

A transformer will always draw some current when it's energized. That is called magnetizing current. During idle conditions, the transformer in your welder draws a pretty lousy power factor. Close to a 90 degree phase difference between the current, and the voltage. That causes the power to circulate, instead of going just one way. During part of the cycle, power flows into the transformer and feeds the building magnetic field. During the second part of the cycle, it flows back out as the magnetic field collapses. It generates current on the line, but no power actually goes anywhere. A capacitor has the exact opposite energy flow pattern. When you put them in parallel, they will feed each other, when one is dumping power, the other is absorbing it. And the power flows back and fourth between the two. When the capacitance equals the inductive current, then they cancel each other out. It's called a tuned circuit. There is still current flowing at the terminals of the transformer, but the supply line never sees it, because the capacitor cancels out the current.

The normal use for a PFC that I am familiar with is to cancel out the idling current of the transformer. Like the welder I have. It draws close to 15 amps at idle without the PFC. With the PFC it draws around 2 amps.

But...There is another inductance that popes up when you load a welding transformer. A welding transformer is designed as a current limiting transformer. So, when it is directly shorted, then the line current is almost purely inductive. It is a result of the inductive nature of the current limiting. That welding inductance almost always produces a far greater line current than the magnetizing current. When you are actually welding, and there is some voltage across the output, and actual energy is being consumed, then the power factor of the input will shift closer to the resistive side. The current will be more in phase with the voltage. That is because there is actual power being consumed.

(There is some current limiting systems that will actually produce a capacitive power factor when you have shorted output. That is what I had assumed when I saw the figures you posted.)

But in your case, Millar is trying to use the PFC to cancel out the welding inductance. Not the idle inductance. So that means they have to have big PFC capacitors. That way overshoots the normal idle inductance. To the point that the input is heavily capacitive during no load conditions. That is the massive 66 amps you see during idle. You basically tack on 60 amps of capacitive current at idle to cancel out 26 amps of welding inductance current at full load.

If you have a high duty cycle application, like a production line. Then it makes sense. But in the home environment with low duty cycle, then no, it don't.

And all the above makes the problem of capacitive switching that I covered in my first post all the more important. A capacitor bank of that size will create big spikes during hot switching. The contactor won't last long at all.

If it was me, I would ditch the caps and just run it with a 100 amps circuit.

As for as the power company. They only charge you for the power you use. The power company only charges for power that goes into your home and stays. Power that circulates in and back out from and inductive load, is not charged for. That is because the power company still has it at the end of the day.

The capacitors on the line will cause real losses. That is the resistance losses from the current flowing through the house wiring. Those losses will be consuming real power that you will be charged for all the time your welder is on. But without them, you only have added resistance losses when you are welding at full power. So you don't come out ahead.

The only time they don't negatively affect you is if you are a large company that already has a lot of inductive loads, that capacitive current from the welder will help cancel out a lot of inductive current from your other equipment. But for most houses, you have no where close to 60 amps of inductive loads for it to cancel out. More like 4 or 6 amps in reality. So, for a normal home workshop, it is just a waste of time to worry about PFC caps of that size.

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Good one Leo well said.

Remember the little terms.

ICE current leads voltage in capacitance loads ELI voltage leads current in inductance loads

The phase angle between the Current and Voltage is the Theta.

I remembered them by saying Eli the Ice man.

Mart> You need to get this clearly in mind before you start to make measurements.

Reply to
Martin H. Eastburn

Gentlemen, I really appreciate all of your help.

Excellent explanations. I'm going to have to learn more about the phase angle relationship.

From what has been discussed, without the PFCs, the welder is an inductive load. That makes total sense to me. Additionally, as the magnetic field builds in the transformer, there is power being consumed (billable by the electric company). Am I to understand that when that magnetic field collapses, a voltage is induced into the windings, and the voltage feeds the electric company, essentially reversing the power previously consumed? Of course there will be some heat losses. So the net result is a small charge for the electricity be the utility. Is that correct? Is the 4.7amps consumed, are those mostly heat losses?

With the PFCs in place, the load is now more capacitive, and from NW9OS's explanation, results in "real" energy charges from ComEd. Did I understand that properly?

I'm still trying to get my head around the real power consumption with the PFCs in place. I'm going to go to some of my AC training books, and get some required knowledge to better understand the concept of a capacitive load.

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Don't know much about the theory. But we have a Synchrowave 250 that someone determined needed the PFCs. The PFCs were installed and the single circuit breaker/disconnect that worked fine beore, got so hot it was smoking. The PFCs were removed and the breaker was replaced and it worked fine. The duty cycle is low for this machine although I've welded some fairly heavy aluminum projects that gave the machine a real work out. I don't know what made them think it needed the PFCs. It is in an industrial plant and It actually could have hooked up to

480V if they would have desired.


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Would it be possible or desirable to add PFC capacitors to a DialArc HF without them? I have a bucket of 440VAC oil-filled capacitors surplus from Seattle City Light. Would it make sense to add them one at a time between L1 and L2 until the idling power measured lowest using a clamp-on AC current meter?


Reply to
Grant Erwin

Yes. As you increase the size/number of capacitors on the circuit, you should notice the idling current going down, until you get to a transition point where it starts to go back up with increasing capacitance. If you carefully select the size, you should be able to get it down to 2 to 3 amps for almost any transformer. A transformer in the 15 to 30KVA range will normally have around a 50 to 200 watts of real power loss while idling on line. That is a result of core losses.

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