Due to the concentration of plasma arc energy, high temperature, great mechanical impact force, and stable arc, plasma arc cutting has the following characteristics:
The composition of plasma arc cutting equipment is shown in Figure Schematic diagram of plasma arc cutting equipment, which usually consists of power supply, high-frequency generator, gas supply system, cooling water (gas) system, control system (control box), cutting torch and cutting table and other devices. component composition. The function and composition of its main devices and components are listed in Table Function and composition of main devices and components of manual plasma arc cutting equipment.
Plasma cutter adopts DC power supply, it should meet the following basic requirements:
(1) It has a steep drop external characteristic curve.
(2) It has high no-load voltage and working voltage.
(3) The current cannot rise too fast when igniting the main arc.
According to the static characteristics of the plasma arc and the frequent fluctuation of the arc length during the cutting process, in order to maintain a stable plasma arc, the external characteristics of the power source should have a steeper curve close to the constant current characteristic than the electrode arc welding power source. Figure——Typical external characteristic curve of power supply for Plasma cutter shows the typical external characteristic curve of the power supply for plasma arc cutting.
As shown in Figure 2-20, if the external characteristic is a slow-down characteristic, once the working gas flow increases and the arc voltage increases, the arc static characteristic curve may move up to the top of the external characteristic curve of the power supply and the arc extinguishing phenomenon occurs. In addition, in the process of plasma arc cutting, the anode spot frequently jumps up and down in the incision, causing the arc length to change continuously, and the change of the torch height will also cause the arc length to change. In the case of the slow-down characteristic, the current fluctuation value ΔI2 is relatively large. , it is easy to make the cutting process unstable. In the case of steep drop characteristics, the current fluctuation value ΔI1 is very small (Figure Influence of Arc External Characteristics on Arc Stability), which can ensure the stability of cutting parameters, and can also reduce the possibility of double arcs caused by a sudden increase in current, which is beneficial to prevent electrode nozzles. of burnout.
Since diatomic gases such as ammonia, oxygen, and hydrogen are usually used for cutting, these gases must absorb a lot of decomposition heat before ionization, and the ionization voltage is high. In addition, the diameter of the plasma arc is small and the current density is extremely high. The voltage drop is also large, so the working voltage of plasma arc is higher than that of arc welding, which is 100 ~ 200V. In order to maintain the stable combustion of this high-voltage arc, the no-load voltage of the power supply should generally be 2 times the working voltage, usually between 150 ~ 400 V (varies with the working gas used), and the large power supply for cutting large thickness plates is used. The power supply requires a no-load voltage of up to 500 V.
The existing power supplies for plasma cutter cutting are:
(1) Three-phase magnetic saturation amplifier silicon rectifier power supply.
(2) Three-phase moving iron split magnetic rectifier power supply.
(3) Saturable reactor rectifier power supply.
(4) Thyristor bridge rectifier power supply.
(5) Magnetic leakage transformer plus tap reactor rectifier power supply.
(6) Transistor inverter power supply.
At present, the main types of plasma arc cutting power supply are: transistor inverter power supply is used for working current below 100A; thyristor bridge rectifier is used for current 100~700A; saturable reactor power supply is used when current is greater than 700A.When it is necessary to use high current to cut thick parts, 2 sets of plasma arc cutting power sources of the same type can be used in parallel.
Plasma cutter control box mainly includes program control relay, contactor, high frequency oscillator, solenoid valve, water pressure switch, etc. The plasma arc cutting program diagram is shown in Figure Plasma arc cutting program diagram, it is controlled according to the required program, and its action points are as follows:
(1) The gas flow and the speed of the trolley can be adjusted before cutting (when automatic cutting), and the concentricity of the electrode and the nozzle can be checked by the high-frequency discharge spark.
(2) Advance gas supply and delay gas stop to protect the electrode from being oxidized.
(3) The non-transfer arc is ignited with a high-frequency oscillator, and the high frequency is automatically cut off after the non-transfer arc is established.
(4) Reliably transition from non-transfer arc to transfer arc, once the transfer arc is established, the non-transfer arc extinguishes itself.
(5) When the cooling water is not connected to the person and the flow is insufficient or the water is interrupted in the middle, the cutting machine cannot be started. If it has been started, it should stop working immediately to prevent the nozzle from burning out.
(6) When the cutting ends or the arc is extinguished due to other reasons, the control circuit can be automatically cut off.
(7) When the cutting power supply is short-circuited or overloaded, the protection device can automatically cut off the network power supply.
The nozzle and electrode are cooled with water, and water-cooled resistors and water-cooled wires that limit the non-transfer arc current are also attached to ensure that the cutting torch can work stably and continuously. According to the way of cooling the electrodes, there are usually two types of cooling water circuits: indirect water-cooled electrodes and direct water-cooled electrodes.
Fig. Schematic diagram of the water circuit system of the indirect water-cooled electrode. The cooling water enters from the lower part of the nozzle, and after cooling the nozzle, it passes through the upper cavity and then indirectly cools the electrode. The water flow should be controlled above 3L/min, the water pressure should be 0.15 ~ 0.2 MPa, and the cooling water can be tap water or circulating water.
High-power plasma arc torches that require intensive cooling must cool the nozzle and electrode separately to prolong the service life of the electrode and the nozzle. The water flow should be above 10 L/min. At this time, a water pump should be used for circulating cooling. Generally, a 11/2PC-3 type (head 41.3 m, flow rate 4 m3/h) or a similar type of water pump can be used. Figure schematic diagram of the water circuit system of the direct water-cooled electrode
In order to prevent the burnout accident of the nozzle and electrode when the water is not passed or the water flow is too small, a water flow transmitter should be set in the water system so that it can automatically cut off the power supply and cannot ignite the arc or cut. .
Plasma arc gas cutting has certain requirements on the water quality and cleanliness of cooling water. Debris in unclean water will accumulate in the waterway and torch, affect the cooling effect, and corrode the joints of cables and pipes. In general plasma arc cutting, a transparent hose should be used for the water inlet pipe, so that no floating objects or other colors can be seen in the water. In order to prevent ionized substances in the water, the method of measuring the resistivity can be used to check, and the resistivity is greater than 5kΩ·cm.
The function of the gas is to compress the arc as the medium of the plasma arc, prevent the electrode from oxidizing and protect the nozzle from being burned out. Stable and continuous supply of gas is one of the important conditions to ensure stable plasma arc cutting, so it is necessary to ensure that the gas circuit system is unobstructed.
Depending on whether the working gas uses a single gas or a mixed gas, the gas supply system has two types: a single gas circuit (Fig. Single gas gas circuit system diagram) and a mixed gas circuit (Fig. mixed gas system diagram). The air storage cylinder is arranged in the air circuit to reduce the impact of the air flow before starting the cutting, which is convenient for arc ignition; and at the end of the cutting, the gas can be stopped after a delay to protect the electrode from being oxidized. Gas mixing is simply used to mix two gases evenly. The pipeline for transporting gas should not be too long, and the gas pipeline can be made of hard rubber. The gas working pressure is generally adjusted to 0.25 ~ 0.3 MPa. The flowmeter should be installed behind each air valve. The selection of the flowmeter depends on the cutting thickness and the common flow rate. Generally, a larger margin should be selected. The flow rate used should not exceed half of the full scale value of the flowmeter used to avoid electromagnetic When the valve is turned on, the shock damages the flowmeter.
When using hydrogen, it is necessary to strictly prevent the pipeline from leaking. It is best not to pass the hydrogen pipeline through the control box, so as to avoid the hydrogen and air mixing in the sealed control box when the gas leaks. sparks and explode.