One: The generation and characteristics of the plasma arc:
Plasma arc is composed of plasma, plasma is the fourth state of matter, is fully ionist gas, wherein the number of positive and negative chargeises equal, in its overall is neutral. Plasma arc is the ordinary arc of forced compression, concentrated and obtained higher than the ordinary arc temperature (up to 15000 to 30000 degrees), more concentrated energy heat source.
Plasma arc is obtained by three forms of compression effect: the arc between tungsten and workpiece through the fine hole of the nozzle, producing a mechanical compression effect, and the nozzle is forced to cool through water, producing a thermal compression effect; High-temperature plasma arc flame with high energy concentration. At the same time, the cooling gas around the arc column inside the nozzle hole is heated by the arc column, the high temperature and high pressure gas is formed in the nozzle hole, and the high-speed spewing from the nozzle to the atmosphere, so that the flame flow of the plasma arc has a high speed (can reach the speed of sound or supersonic) in the hole, showing a strong impact force.
The principle and application of plasma cutting
Cutting, generally refers to metal cutting. Plasma arc cutting is a method of using extremely fine and high temperature plasma arcs to make local metals melt quickly, and then blow the molten metal away with air flow. Plasma arc cutting because of the cutting efficiency, low loss, wide range of applications and other advantages have been widely used in various types of engineering construction, manufacturing and other industries.
Third, plasma cutting arcing mode
Plasma arc cutting generally has two ways to arc:
1. Contact: that is, the nozzle insulated from the pole needle is attached to the workpiece (connecting cutting power supply positive end), and then add high-frequency high-voltage current to the electrode needle (tungsten needle) at the negative end of the joint power supply, so that the pole needle spews out the arc, the arc in the voltage, air pressure, magnetic field under the action of the formation of a plasma arc, through a large current to maintain the plasma arc stable combustion, and then slightly raise the nozzle (avoid the hot workpiece damage nozzle), start cutting. This cutting method is mostly suitable for small current and small power cutters
2. Transfer arc (dimensional arc): that is, the power supply positive end through a certain resistance and relay to the nozzle, so that the pole needle and the nozzle to form an arc (due to resistance limit flow, the arc is small), and then the nozzle close to the direct connection to the power supply positive end of the workpiece, the pole needle and the workpiece will form a greater energy arc, the arc is compressed to form a plasma arc, and the nozzle power supply connection is broken. The transfer arc cutting method avoids the arc from straying from the center of the nozzle under the action of air pressure and damaging the nozzle. This method is suitable for high-power cutters.
Fourth, the technical parameters of the cutting machine:
Plasma arc cutting machine with a steep drop of external characteristics of the DC power supply, power supply empty voltage is generally between 150 to 400 volts, the power supply type is dedicated silicon rectifier power supply. Cutting machine output requires a higher voltage (generally 100 to 180 volts) and the current is small. (Generally 30 to 160 amps). The technical parameters of the cutter have changed relative to the welder:
1) The main transformer has a smaller turn ratio to obtain a higher output voltage
2) Slow (slow) longer start time to ensure gas supply and air pressure
3) The feedback gain is smaller, and the feedback run-in gain changes from open-loop gain to closed-loop gain;
4) Large duty ratio (more than 90%) to get a higher output voltage
5) The impedance of the impedance becomes large to prevent high-frequency interference and ensure reliable shutdown of the control relay.
Fifth, the electrical control part:
Cutting power supply includes the main circuit and control circuit two parts, the main circuit includes contactor, high leakage resistance three-phase power transformer, three-phase bridge rectifier, high-frequency arc coil and protection elements. The out-of-power feature sedated by high leakage resistance to a steep drop. The control circuit completes the entire cutting process through a button switch on the torch:
Pre-ventilation - main circuit power supply - high frequency arc - cutting process - arc - stop. The power supply of the main circuit is controlled by the contactor, the gas is controlled by the solenoid valve, the high frequency oscillator is controlled by the control circuit, and the high frequency stops working after the arc is established. In addition, the control circuit still has the following internal locking function: thermal switch action, stop working.
Six, the working principle of plasma cutting machine:
After switching on the power switch, the three-phase leakage anti-transformer, the rectifier bridge after the output l20 to 180V (model s the empty DC voltage of different voltages is in standby. After turning on the work switch, the control component is electrically charged, if the cutting switch is pressed at this time (attached to the head of the nozzle of the gas supply, electricity), the gas supply electromagnetic gate opens the gas supply, and the power supply of the high-voltage inverter assembly is switched on. If no cutting at this time, discharge clearance will release part of the high pressure to avoid breaking through the work rectifier, if at this time has been connected to the workpiece and the nozzle has been close to the metal products to be cut, the air flow is ionized under the action of the high-voltage electric field. More than one hundred volts of voltage is along the electric ion path to form an arc, in the ion, voltage, arc, air pressure under the action of the metal rapidly melting, so as to achieve the purpose of cutting metal.
Seven, plasma cutting machine failure and maintenance:
1. There is no high-frequency guide arc.
Check the high frequency arc circuit, first check 110VAC and observe the presence of discharge sparks between G1, G2. If not, generally 110VAC did not come or fixed G1, G2 glue board moisture, after the tide G1, G2 can not discharge and produce high pressure. With electric blowing dry glue board, restore 110VAC. If there is still no guide arc, should check the high frequency arc, due to the high frequency skin collection effect, high-frequency line and the conductor ring in the nozzle contact is not good or because of the seal ring reasons and cooling water short circuit will not have high frequency. Removing a cutting gun, tightening the high-frequency line or replacing the seal softener generally solves the problem.
2. There is no cutting arc.
Check for 400VDC open-circuit voltages when high-frequency sparks are observed. If not, check that the three-phase power supply is out of phase. Then check the high-power scrare in the power box and trigger the circuit board. If the power supply is normal, open the PLC control box and check the PLC's input and output signal. Inputs include cooling water, cutting water flow signals, nitrogen, oxygen pressure signals. There is no cooling water, cutting water flow signal, should replace the cooling water pump, cutting water pump, no nitrogen, oxygen pressure signal should detect nitrogen, oxygen source, and check the line for leakage. If the start-up conditions are met, to check the cutting gun, the electrode rod seal ring or nozzle seal ring damage will seep water into the cavity between the electrode, nozzle, resulting in dc power supply and nozzle short circuit, can not form a circuit with the workpiece. Replacing the seal and reassembling the cutter gun will solve the problem.
3. Poor cutting quality.
The performance of cutting not wearing workpieces, or hanging slag more, uneven cutting, this is the main arc has not been sufficient compression, arc column thickening, insufficient penetration caused, the main reason is insufficient cutting gas pressure or cutting gas pipe leakage. Check the combined solenoid valves, combined switches and trachea that control cutting gas, and if a imitation nozzle is used, the parameters are not correct, resulting in a disturbance of air flow between the electrode nozzles.
4.40A plasma cutter. Only cut very thin iron plate, cutting thicker iron plate can only hit a pit point after the dead fire.
Because of other normal, it is determined that the cutting current is insufficient, focus on leak-resistant transformers and rectifiers. After careful examination, the rectification part is normal, found that the three-phase leakage anti-transformer sub-edge winding inner layer has been blacked out. This transformer is a silicon steel sheet stacked into three core columns and the upper and lower two cross-bar combination welded, the use of the grinding wheel to cut open the welding port to remove the cross-bar to remove the coil (measured 2mmx4mm flat aluminum wire 86 thumps). After purchasing the same type of aluminum wire as is the same winding, after drying after the immersion paint assembly (note: assembly coil and iron core should have good insulation, and to be installed as tightase as possible, silicon steel sheet to use the clamp press interface to weld, so as not to increase the loss). When complete, the test machine is all right. The cause of the analysis of damage is that the coil is loose and rubs with the iron core under electromagnetic force, causing the coil to short-circuit and burn.
5.40A plasma cutters cannot be ignited.
The test machine checks for no high - voltage fire. This machine has been used for more than a year, with compressed air after spraying the dust inside. It is seen that the high-pressure package high-voltage rectification output of the filter porcelain plate capacitor (two strings, 4kW/2200pF) under the two have been cracked, the above two of the two have leaked electricity (measured with 2500V rocker. Only 0.5Mn). There is no such capacitor on the market, removing the 2000pF/10kV polypropylene capacitor from the old stun gun and replacing it with a rear test machine normally (Note: remove whether there is high pressure or not.) all to discharge first).
6.40A plasma cutting machine, work suddenly extinguished, about half a minute after the paint smell, and low frequency humming.
This machine is a new machine, only use more than ten days, still in the warranty period, because the user has been self-disassembled, dealers do not want to warrant. Dismantling machine inspection, leakage transformer on the middle core column on the original secondary coil has slightly discolored (indicating that the temperature has been too high), test boot ten seconds after the heat is very large, concluded that there is a short circuit or heavy load, with insulation screwdriver rod for clearance discharge (this to remember), remove the secondary winding to the rectifier bridge of the connection, and then power the test machine, the transformer winding is no longer hot. Try to press the cutting switch, high pressure and gas supply normal. After shutting down and discharge, it is detected that one of the two three-phase rectifier half bridges has been broken. The test machine is normal after the change.
7. Spark generator can't automatically break arc
When the plasma cutting machine is working, the first step is to ignite the plasma arc, by the high-frequency oscillator to stimulate the gas between the electrode and the inner wall of the nozzle, produce high-frequency discharge, so that the gas local ionization and form a small arc, this small arc by the role of compressed air, from the nozzle to ignite the plasma arc, which is the main task of the spark generator. Under normal circumstances, the working time of the spark generator is only 0.5 to 1s, the reason for not automatically breaking the arc is generally to control the circuit board component imbalance, spark generator discharge electrode clearance is not suitable. The spark generator should be checked frequently to keep the electrode on a flat surface, adjust the discharge electrode clearance (0.8 to 1.2mm) of the spark generator in due course, and replace the control plate if necessary.
8. Poor contact between the ground and the workpiece
Grounding is an essential preparation before cutting. The non-use of special grounding tools, insulation on the surface of the workpiece and the long-term use of severe aging ground, etc., will make the ground wire and the workpiece of poor contact. Special grounding tools should be used and insulation should be checked to affect ground wire contact with the workpiece surface to avoid the use of aging ground wires