-Early plasma torch designs used a high frequency discharge (think of the energy used to fire a spark plug in a gas engine) to ionize the plasma forming gas (either nitrogen or air in early systems) to allow the DC arc to transfer to the material being cut. With these torches the operator had to move the torch very close to, or even make contact with the material. These systems had a hard time getting the arc to start when the material surface was dirty, corroded or painted. You can still buy imported plasma cutters with contact or scratch start technology....very cheap and almost useless! They should not be used on any cnc machine that uses a laptop or PC as the controller as the high frequency discharge will cause electrical noise interference and possible damage to the computer and electronics.
- The major manufacturers (in the late 1960's) then developed high frequency - pilot arc circuitry that allowed a strong pilot arc that would fire the torch without being in close proximity to the plate. This used a strong high frequency discharge inside the torch that ionized the gas, allowing a pilot arc that would transfer to the material regardless of dirt, rust or paint. These systems were widely used for hand held as well as mechanized cutting applications. On cnc machines there had to be careful grounding as well as isolation to protect the system cnc controller from damage caused by the high frequency and its electrical noise emission.
- In the early 1990's Hypertherm developed the first air plasma torch that had a strong pilot arc that could transfer to the metal regardless of surface condition, yet did not use a high frequency discharge to ionize the gas. This technology was called Blow Back start technology and was patented by Hypertherm. For hand cutting this made for a lighter and smaller power supply, and eliminated a lot of internal circuitry that was necessary with high frequency start plasma cutters. This technology allowed Hypertherm (relatively small in the 90's!) to become the largest producers of plasma cutters in the world.
Blowback start uses a moving, spring loaded electrode that "blows back" away from the nozzle at the beginning of each cut cycle using the cutting air pressure to make the electrode move against the spring force. Early designs had a spring loaded plunger, the newest Hypertherm torch has a spring mounted on the electrode (better reliability over time). This technology (no high frequency) allowed for a new wave of lower cost cnc plasma cutting machines that could be driven by standard office computers and laptops (as opposed to high cost / complexity cnc controllers designed to work with high frequency start plasma cutters). Jim Colt
