Re: Switching in Power Factor Correction Capacitors
- From: "N9WOS" <Don't@xxxxxx>
- Date: Thu, 9 Apr 2009 13:52:57 -0500
<karchiba@xxxxxxxxxxxxxxx> wrote in message
news:e0bf20ae-e26f-4bcf-a8eb-3768926b2582@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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.
.
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