Re: welding machine as a power supply

--- 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.