Re: Switching in Power Factor Correction Capacitors


"N9WOS" <Don't@xxxxxx> wrote in message
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<karchiba@xxxxxxxxxxxxxxx> wrote in message
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All:

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?


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.


.

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