Lee,
The plasma torch internally raises the temperature of air to its ionization temperature....when a gas reaches ionization...it becomes more electrically conductive. The ionized air is passed through a nozzle orifice, which constricts the flow of air an increases its velocity....then DC current is applied to this ionized stream of air...and the current fllows the path of air to the plate...(connected to the work or ground clamp) completing the circuit. The air is superheated to over 24,000 degrees fahrenheit....which melts pretty much anything in its path.
Air is comprised of approximately 20% oxygen.....and when you are using an air plasma to cut materials that oxidize easily....(steel) an exothermic reaction takes place...adding further cutting power to the process.
So plasma cutting is mostly thermal melting of the plate (when cutting stainless and aluminum) and is a combination of exothermic (like oxy-fuel cutting) and thermal when cutting easily oxidisable materials such as steel.
For your grounding questions:
1. Plasma requires a workpiece ground connection. For rotating workpieces some sort of a brush type connection of adequate design to handle the Amperage level being used (plasma cutters output power range from 12 amps to 1000 amps).....the more sophisticated systems I have seen (such as tube mills that produce irrigation pipes) have used rotational mercury type contacts to maintain a good ground path.
2. On any type of cnc equipment that is used with plasma it is very important to have a good nearby earth ground....each component (plasma, drives, cnc, machine frame, etc.) should be directly connected to this ground source (no daisy chaining of connections) with adequately sized wiring. This protects low voltage signals from being affected by the relatively high plasma voltages. High end industrial plasmas (and some small entry level plasmas) also use a high frequency starting voltage in the 15kV @ 2 megaherts range....this starting voltage is active for a few miliseconds at the beginning of each cut cycle to ionize the plasma gas (air).....CNC equipment nees to be protected through proper grounding and isolation when used with plasmas with high frequency starting.
3. As far as output failure of a Hypertherm plasma...there of course are different types of failures....in theroy there could be a failure that causes the output to be "full on".....this type of failure will quickly be detected by the overcurrent detection in the plasma (any of Hypertherm's systems)...and will cause the unit to shut down. If for some reason this circuitry does not detect and output problem....then there also would be an increase in input current...which will trip the input circuit breaker quite rapidly.
Regardless...plasma is a very safe process. It is virtually impossible with Hypertherm's systems to contact cutting voltage, or open circuit voltage under normal use. I look at some of the import (Chinese mostly) plasma systems and they have exposed torch connections with open circuit voltage of over 300 VDC....very dangerous. I do not understand how these systems pass regulatory tests...
Jim Colt  |