Re: Strange control problem -- SOLVED!!! -- 3 phase "firing system"

On Tue, 16 May 2006 15:22:27 GMT, Glen Walpert <gwalpert@xxxxxxxxxx> wrote:
On Tue, 16 May 2006 04:54:40 GMT, Ignoramus29705
<ignoramus29705@xxxxxxxxxxxxxxxxxxxx> wrote:

Glen, I solved the mystery. (about the firing system tripping on
overcurrent with small currents, on a 200A welder)

Congratulations!

Thanks. I am going to get a full night of sleep tonight.

Here's what it was all about:

The current sensor (connected to DC shunt) was NOT, contrary to a
statement by their engineer, a differential sensor.

What it was actually measuring was difference between the negative bus
and the plus side of the shunt. Even though I properly wired two ends
of the shunt where they belonged on the firing system, the sensor was
not taking the difference between them, it was taking the difference
between the + side of the shunt and the negative bus, ignoring the
minus side of the shunt.

It is completely contrary to the impression that they were trying to
make.

The problem was that there was too much wire between the minus bus and
the old shunt in the welder, which was connected to the current meter
and was located near the commutator. Between the negative bus and the
shunt, there was an interphase transformer and also the "reactor". Too
much resistance was all a part of the "virtual shunt" that was in
reality being sensed.

Hmm, how did the interphase transformer get in the circuit; it is
there to balance current in the 6-phase configuration and is of no use
to your 3-phase rectifier, which should not even be using the neutral
???

It is of no use, but it simply provides some convenient wires to hook
up the negative side. It is merely useless, but not harmful, right?

Remember, I wired the rectifier the way you suggested, using only half
of the secondaries, wye connected. The common point of the wye is
connected to the interphase transformer, simply because "the wires are
there".

Maybe I should take it out and try to sell on ebay, but if I do not
take it out, there should not be any harm from just having it.

Also you should be doing your voltage sensing on the rectifier side of
the reactor, to provide some hope of control stability while welding.

You are 100% right, as I am now realizing. Thank you.

That is indeed the case, voltage sensing is done right off the copper
bars bolted on the rectifier output.

Consider the voltage swings on either side of the reactor during
short-circuit mode welding, where a drop of liquid metal contacts the
workpiece before separating from the wire.
Looking at your figure 3, the inductor needs to be in the load
circuit; you should consider it to be part of your load as far as the
regulator is concerned. You do *not* want the capacitor connected
directly to the work leads after the inductor as shown on your Figure
3, which I must not have looked at very carefully the first time.

Do I need a capacitor at all?

MIG welding sometimes is called a "constant voltage" process because
the I/V curve for the power supply at normal welding currents is
closer to constant voltage than to constant current. The MIG welding
process (as opposed to power supply) is however actually approximately
constant current, with the current adjusted with the welding wire feed
speed control. You might want to find some MIG welder I/V curves, and
possibly consider a means of adjusting voltage droop with current. I
am not sure if droop is necessary for good arc stability or just an
artifact of the construction of the old welders I have seen curves
for.

Thanks. I know very little about MIG, but I heard that people weld
from car batteries. I will try to read up about it.

My plan is to make a super ridimentary control panel with two pots for
voltage and current control, a on off switch, and a RESET pushbutton.

My first goal is to get stick welding to work.

I was thinking of setting voltage to 90 volts and current to desired
current, and try to weld.

Should I try to drop voltage as current increases, even when it is
below the desired current set by the pot? (that would make the machine
not quite a CC power supply, but, perhaps, it is necessary for proper
welding?)

Or should voltage be constant until current reaches the p[reset
amount, and then should steeply drop to maintain the desired current
through the arc?

My solution: put another shunt right on the negative bus. It is a
300A, 50mV shunt, so the system can produce more current than allowed,
I will have to adjust for it, but it happily puts out A LOT of amps.

I pronounce this firing system to be perfectly functional.

I am very happy right now.

I think that I can adjust the firing board (there is a trimpot for
current sensing), so that it would produce exactly 200A when the
current control potentiometer is at the max. I will do that before
going to be tonight.

When everything works, I will buy a huge box of donuts for PCTI.

Is that when it works into your test load, or into an arc?

When I get a sensible looking weld with 6013 electrodes, I think.

A side comment here is that what I am building is not just a welder,
it is a universal DC power supply capable of 12 or so kilowatts of
power, with range from 0 to 300 volts and 0 to 200 amps. I may revisit
the amp issue later, but right now reality is that 200 A welding is
the max that my 60 amp garage circuit would allow.

i

.

Relevant Pages

  • Re: Strange control problem -- SOLVED!!! -- 3 phase "firing system"
    ... the old shunt in the welder, which was connected to the current meter ... to provide some hope of control stability while welding. ... voltage sensing is done right off the copper ...
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