Re: [CAD_CAM_EDM_DRO] Re: welding machine as a power supply

To: CAD_CAM_EDM_DRO@yahoogroups.com
From: "caudlet" <info@...>
Date sent: Sun, 21 Mar 2004 16:00:19 -0000
Subject: [CAD_CAM_EDM_DRO] Re: welding machine
as a power supply
Send reply to: CAD_CAM_EDM_DRO@yahoogroups.com


One feature of a welding transformer, and the more expensive
the machine the more it has this feature.
At low/no current they have a high secondary voltage to make
it easy to strike the are, Then as the current increases, the
voltage goes down. A good ransformer welder will have
and have a 'open circuit secondary voltage rating'. It seems
like the old/new Lincoln adjustable 250 amp welders had
an open circuit voltage of 70 volts which went down to about
23 when the arc was struck. Some of the cheap 'cracker box'
welders were not near that wide range and were hard to start
an arc. So, you might want to look at a cheap, small, cracker
box welder for you power supply.

Bob McKnight
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--- In CAD_CAM_EDM_DRO@yahoogroups.com, "ftomazz"
<filipetomaz@p...>
wrote: > Hello. I need about 65V to feed 3 motors that take 10Amp
each
(max > ratings). > > There is any good reason not to use a welding
machine (or more than > one) in a serial configuration to reach the
65V@30Amp? I can get old > (not inverter welding machines). > >
Probably a bad ideia...

Actually a 65V 30A supply IS a welding machine! If the 10A ratings
are peak then you probably don't need to design using that number.
Most servo's average power consumption is 50% of peak. I suspect
that
a 65V 15A design with a 30A bridge rectifier would do fine. While I
don't see anything wrong with using parts from a surplus Welder
provided you can get the correct AC secondary voltages, it sure would
be a big hummer! Magnetic and most passive components can live
long
productive lives if you stay within their ratings on average. Example:
You can hit a 10W wirewound power resistor with 100W for one
second
every 11 seconds and it will get warm but not burn up. Semiconductors
are less tolerant of short peaks over their ratings. While they will
let you pulse them with higher currents over short duty cycles they
will create blue smoke if you exceed max voltages ratings for even
short durations.

What kind of machine is this going on and what are the motors? What
are the secondary voltages of the transformers in question? You might
be able to get a lower secondary voltage if it has a dual or tapped
primary that would allow you to use the 230v configuration but run it
at 120VAC. That would make the secondary voltage 1/2 of the full
rating. You might have to derate the transformer if they use two
separate windings for the primary and expect them to be in parallel
for 120V operation. The wiresize of the primary winding becomes the
limiting factor. I doubt this would be a factor for something like a
100A or larger welder transformer.

There might well be other components in the welder that you can use
as
well. They might have some large diodes you could use but they would
probably need to be rewired to form a full wave rectifier circuit. If
it is a wirefeed it will have a variable speed gearhead motor. While
it's not much use in CNC it does make a dandy variable speed power
drive for a manual mill! My first mill/drill still has the motor,
gearbox and original motor controller from an old miller wirefeed as
the X axis power feed. Saved a whole lot of hand cranking.





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