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Re: Inside A Plasma Cutter (Must See Pictures for SSTCers!)

Original poster: Jim Lux <jimlux-at-earthlink-dot-net> 

At 07:30 AM 6/11/2004 -0600, you wrote:
>Original poster: "Jim Mitchell" <Electrontube-at-sbcglobal-dot-net>
>Hello John,
>That might explain their rating of 1200v,  I didn't look to see if they were
>in series,  but 1200X6 would give 7200v of rectfication,  so perhaps the
>transformer gives 5kv? out.
>Something I've noticed,  that even IF the ground clamp isn't connected to
>the work,  I can still gouge the metal with the torch.  This also lead me to
>believe that the output is high frequency as well, because we all know that
>a unrectified flyback will arc to ungrounded metal,  but a rectified flyback
>will not (or just make small blue sparks to the metal)
>I guess I can still gouge the metal without being grounded, because the arc
>is being started in the torch head, and then blown out by the air stream.
>It seems plasma cutters are kind of a "mystery device" ;-)
>Regards - Jim Mitchell
>----- Original Message -----

The plasma cutter I used to use (Thermal Dynamics, as I recall), was about 
300V open circuit with inductive "ballasting" + a HV/HF circuit (similar to 
a TC or TIG welder) to start and stabilize the arc.  There were three wires 
out of the supply: the ground clamp, the main annular electrode, and the 
center electrode.  There's a fairly good arc between the center and annular 
electrode, creating the plasma that does the cutting (which is blown into 
the workpiece by the pressurized air (in my case.. they also use H2 in some 
cutters apparently).  There may also be current flow to and through the 
workpiece to help heat it so that the air blast can blow it out of the way 
(which is really how a plasma cutter works.. melt the metal and blow it out 
of the way: compare to a cutting torch, which actually burns the metal with 

They're really pretty simple devices.  The "art" is in the design of the 
"torch" to create a stable arc and air flow so that it's controllable and 
precise.  There's a LOT of interaction between the electrical properties of 
the power supply (in particular, the I/V curve) and the fluid mechanics of 
the torch, so that the arc is stabilized.  If you look at the inserts in 
the torch head, there are little slots at an angle and the profile of the 
inside of the outer electrode looks simple.  IN reality, I suspect that a 
LOT of trial and error went into that design.