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Plasma Cutter/ Tesla Coil

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Ryan

Tue Mar 10 2009, 12:11AM

Registered Member #1606 Joined: Fri Jul 25 2008, 02:40PM
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Posts: 71

Go to 3 or more minutes into the video, it shows a schematic. Does this schematic represent that of a tesla coil somewhat?

Considering a plasma cutter is basically a jet stream of air, sparked with a HV ignition, I am just wondering what the similarities might be between a TC and plasma cutter.

Any guess if that HV spark would be of high frequency?

Thanks, Ryan.

Billybobjoe

Tue Mar 10 2009, 12:51AM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

Ryan wrote ...

Go to 3 or more minutes into the video, it shows a schematic. Does this schematic represent that of a tesla coil somewhat?

Thanks, Ryan.

Yes, search for "high frequency arc starter" on these forums. I've made one for use with arc/TIG welding and I hope to use it for a plasma cutter in the future.

The high frequency, high voltage is just for starting the arc - the actual arc is low voltage (maybe 80-100 open circuit volts) at fairly high current (20-60 amps). The key is arc stabilization - this is usually done by creating a vortex with the stream of compressed air.

I bought 4 (used) plasma torches mainly to inspect their insides in hopes of making my own power supply. Newer models use a liqhtweight inverter. I was planning on going old school with an iron-cored transformer.

Ryan

Tue Mar 10 2009, 06:58PM

Registered Member #1606 Joined: Fri Jul 25 2008, 02:40PM
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Posts: 71

So just so I am correct on the function of a plasma cutter, one the air is forced through the initial arc in the gun, there is no more spark required and plasma is continually fed air via external source (thus keeping the plasma going) correct?

I guess what I am getting at is, what part is actually creating the plasma, is it the high voltage arc or high frequency?

Thanks, Ryan.

Billybobjoe

Tue Mar 10 2009, 07:56PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

Ryan wrote ...

on[c]e the air is forced through the initial arc in the gun, there is no more spark required and plasma is continually fed air via external source (thus keeping the plasma going) correct?

I'm not too sure what you mean here . . . the high frequency/high voltage current (produced by the tesla-style tank circuit) is usually turned off once the main "power arc" (high current arc) is established. The HV/high frequency is used to initiate the arc by ionizing the air and providing a conductive path for the main arc.

There are some torches that do not use a high frequency starting. They have two electrodes, that when compressed air is applied, separate and essentially strike an arc.

Ryan wrote ...

I guess what I am getting at is, what part is actually creating the plasma, is it the high voltage arc or high frequency?

The high voltage is creating the plasma. High frequency is used because . . . current in a plasma cutter is DC, and usu. DC or low frequency AC is used in a stick/TIG welder (although there are some adjustable freq. squares waves, etc . . .) If you attempted to superimposed DC or 60hz AC onto the large welding/cutting current, there would be nothing to prevent your welder or power source from being damaged by the high voltages across its terminals. If high frequency is used, and "injected" in series with the high current, it can be easily routed with a capacitor across the terminals of the power source.

(see here -

- an "arc starter" is used for TIG/stick welding but it the same concept as HF starting for plasma cutting )

I'm really surprised that no one here has (as far as I know) attempted to make their own plasma cutting system, whether it be an iron-cored or switching inverter type.

Ryan

Tue Mar 10 2009, 08:08PM

Registered Member #1606 Joined: Fri Jul 25 2008, 02:40PM
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Posts: 71

Great so the HF (such as that of a tig or plasma cutter) is used to start the arc by basically sending out streamers to the work peice. But, does the arc inside the gun (high voltage high amperage) stay lit during cutting or is the plasma process self sustaining once a plasma is established?

-Ryan

TheBoozer

Tue Mar 10 2009, 11:32PM

Registered Member #1535 Joined: Wed Jun 11 2008, 11:37PM
Location: Northeastern Pennsylvania - USA
Posts: 117

Arc inside the gun? Hmmm. Once transfer is established, the workpiece is made positive through the use of a "ground clamp" and and the torch electrode is made negative. Plasma is quite conductive so the current flows quite easily. Plasma cutters are quite similar to tig welders. They are a "constant current" type power supply. Nothing amazing.....

I happen to be a service technician in this field and have seen MANY different designs. We do warranty repairs for all major US manufacturers. I like nosing around inside these because I'm curious. Sometimes I even get to see patented/proprietary prints. Newer stuff is nearly all inverter based, without the use of "high-frequency" start. This gets only a hair more complex. I could explain if need be.

I've actually taken the "high-freq" components out of scrap units and made table top Tesla coils.

Ryan

Wed Mar 11 2009, 03:25AM

Registered Member #1606 Joined: Fri Jul 25 2008, 02:40PM
Location:
Posts: 71

So once the electrodes create an establishing arc to get the plasma going (via HV/HF), the plasma is self sustaining from the gun to the work peice (as it is conductive and being contanly fed supply air)

Do I have it right? haha

Also, what kinds of frequencies do the HF compoenents run at to create initial plasma?
-Ryan

Billybobjoe

Wed Mar 18 2009, 09:44PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

Ryan wrote ...

So once the electrodes create an establishing arc to get the plasma going (via HV/HF), the plasma is self sustaining from the gun to the work peice (as it is conductive and being contanly fed supply air)

Do I have it right? haha

Also, what kinds of frequencies do the HF compoenents run at to create initial plasma?
-Ryan

This topic is a little old but I missed this response . . . the plasma is self sustaining because it is constantly being fed a high current from the power supply. Moving air, if anything, will serve to disrupt the plasma under most circumstance (which is why it, of a dielectric gas like SF6 is used to forcefully extinguish some HV circuit breakers). The moving air is necessary for the function of the cutting part of the plasma cutter (the arc provides the heat), and to keep the torch itself from melting.

Frequencies are dependent on the actual component values in the circuit, but tend to be in the 100's of kHz to 1 MHz range -like a SGTC.

Ryan

Thu Mar 19 2009, 10:13PM

Registered Member #1606 Joined: Fri Jul 25 2008, 02:40PM
Location:
Posts: 71

Ahhh, ok I got it, thanks!

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