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.
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.
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.
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
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.
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.
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.
Leo Lichtman wrote:
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
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
If it was me, I would ditch the caps and just run it with a 100 amps
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 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.
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
<I'd like to reserve the right to call the witness in the future> When
I get a better understanding of how to measure the phase angle...
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.
On Sun, 12 Apr 2009 21:21:39 -0700 (PDT), firstname.lastname@example.org
This is what I've been told:
When the power factor is 1 then the voltage and current rise and fall
together perfectly is step. When less than 1 the voltage and current
are out of step. The current causes heating which equals energy loss.
This loss costs the electricity supplier money. Large power users are
charged for lousy power factor because of this and so will want to
correct their power factor to avoid these charges. I guess their power
meters show the power factor. But, a standard residential power meter
measures only the actual watts used and isn't affected by power factor
alterations. Apparently there are all sorts of residential loads that
have differing power factors, from a toaster with a PF of 1 to a
fridge with a much lower PF, which is why the residential meter
ignores power factor. If you are connected to a meter that doesn't
take power factor into account then it seems to me that the best way
to evaluate the PF caps is to watch the meter when the caps are
installed and when they are not.
As others have stated. There is two types of power factor. Leading and
lagging. In an ideal world, an inductor, like a transformer, will have a 0
power factor that is 90 degree lagging. A capacitor will have a 0 power
factor that is 90 degree leading. With 90 degree leading or lagging, there
is no actual power flow. So there is no actual power flow to be billed for.
That is why it is classified as a ZERO power factor. Power factor is the
ratio of apparent power to real power. Apparent power is the value of the
voltage times the value of the current in the circuit. V times A That is
also know as VA or volt amps. Aka "KVA" "MVA"
The zero power factor means that no mater how large the VA value is, there
is no actual power being used.
A purely resistive load has a power factor of 1. The apparent power equals
the real power.
The 66 amps you see during idle with PFC is almost pure capacitive reactive
power. The actual power factor, and billable power used is almost zero.
But you have another problem to worry about which is what I was getting at.
The power lost in the wiring is true power and is billable, not including
the fact that it is hard on wiring and electrical equipment. The 66 amps
isn't full billable power, but it causes wiring losses which do consume
Without PFC, the idling current is very low. You will have low wiring losses
when you have it on and idling. The increased peak current draw during
welding will also push up wiring losses but for only short periods while you
are actually welding.
And all of that is also pretty irrelevant. Considering moderate wiring
length and resistance, you may have an additional 100 to 200 watts of wiring
loss while the welder is powered up and idling with PFC in place. In extreme
cases, you may have 600 to a 1000 watts of wiring losses.
The real cost will be from constant heating of breakers and contacts causing
early electrical failures. Short surges of 80 to 90 amps with breather space
in between is easier on equipment than a constant 60 amp current. Constant
heating causes fatigue of the switching equipment.
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?
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.
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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