Installation; Plasma Basics - Forney 325P Manual De Instrucciones

Installation

Plasma Basics

WHAT IS PLASMA?
One common description of plasma is to describe it as the fourth state of matter. We normally think of
the three states of matter as solid, liquid and gas. For water, the three states are ice, liquid and steam.
The difference between these states relates to their energy levels. When we add energy in the form
of heat to ice, the ice melts and forms water. When we add more energy, the water vaporizes into
hydrogen and oxygen, in the form of steam. By adding more energy to steam these gases become
ionized. This ionization process causes the gas to become electrically conductive. This electrically
conductive, ionized gas is called plasma.
HOW PLASMA CUTS THROUGH METAL?
The plasma cutting process, as used in the cutting of electrically conductive metals, utilizes this
electrically conductive gas to transfer energy from an electrical power source through a plasma cutting
torch to the material being cut. The basic plasma arc cutting system consists of a power supply, an arc
starting circuit and a torch. These system components provide the electrical energy, ionization space
capability and process control that is necessary to produce high quality, highly productive cuts on a
variety of different materials.
The power supply is a constant current DC power source. The open circuit voltage is typically in the
range of 240 to 400 VDC. The output current (amperage) of the power supply determines the speed
and cut thickness capability of the system. The main function of the power supply is to provide the
correct energy to maintain the plasma arc after ionization.
The arc starting circuit is a high frequency generator circuit that produces an AC voltage of 5,000 to
10,000 volts at approximately 2 megahertz. This voltage is used to create a high intensity arc inside
the torch to ionize the gas, thereby producing the plasma.
The Torch serves as the holder for the consumable nozzle and electrode, and provides cooling (air) to
these parts. The nozzle and electrode constrict and maintain the plasma jet.
PLASMA CUTTER OPERATION
The power source and arc starter circuit are connected to the torch via interconnecting leads and
cables. These leads and cables supply the proper gas flow, electrical current flow and high frequency to
the torch to start and maintain the process.
1. A start input signal is sent to the power supply. This simultaneously activates the open circuit
voltage and the gas flow to the torch. Open circuit voltage can be measured from the
electrode (-) to the nozzle (+). Notice that the nozzle is connected to positive in the power
supply through a resistor and a relay (pilot arc relay), while the metal to be cut (work piece)
is connected directly to positive. Gas flows through the nozzle and exits out the orifice. There
is no arc at this time as there is no current path for the DC voltage.
2. After the gas flow stabilizes, the high frequency circuit is activated. The high frequency
breaks down between the electrode and nozzle inside the torch in such a way that the
gas must pass through this arc before exiting the nozzle. Energy transferred from the high
frequency arc to the gas causes the gas to become ionized, therefore electrically conductive.
This electrically conductive gas creates a current path between the electrode and the nozzle,
and a resulting plasma arc is formed. The flow of the gas forces this arc through the nozzle
orifice, creating a pilot arc.
3. Assuming that the nozzle is within close proximity to the work piece, the pilot arc will attach
to the work piece, as the current path to positive (at the power supply) is not restricted by
a resistance as the positive nozzle connection is. Current flow to the work piece is sensed
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