Arc extinguishing measures in the hottest plasma n

Arc extinguishing measures in ion nitriding power supply

Abstract: several common arc extinguishing methods in ion nitriding power supply are discussed, their working principles are introduced, and their arc extinguishing performance is analyzed and compared

key words: ion nitriding power arc extinguishing

1 introduction

ion nitriding is a surface strengthening technology that uses the principle of gas glow discharge to bombard the steel surface with nitrogen ions, heat it, and make nitrogen atoms penetrate into the surface of the workpiece. In the process of ion nitriding, glow discharge is changed into arc discharge for some reason, which will stop the nitriding process, and in serious cases, it can melt and burn the workpiece surface, and even damage the power supply equipment. Although the cleaning, baking and other measures on the surface of nitriding parts can reduce the number of arc discharge, arc discharge will inevitably occur at the early stage of nitriding, when enterprises will be provided with reliable test service in time. Therefore, the power supply for ion nitriding must be equipped with arc extinguishing measures, and the arc extinguishing performance is one of the important indicators of ion nitriding equipment [1]

2 detection of arcing signal

normal ion nitriding process works in the glow discharge area, but when arcing, all the current is concentrated in a small area, causing the current density to increase here. If the arc is concentrated at a certain point, it shows that the inter electrode voltage drops rapidly, while the current rises sharply, and the voltage drops to about 200V. According to the phenomenon during arcing, the arcing signal can be judged by the following parameters [2]:

(1) current upper limit signal

(2) current rise differential signal di/dt

(3) voltage lower limit signal

(4) voltage drop differential signal dv/dt

experiments show that the voltage drop is rapid, which is ahead of the arrival time of the maximum current, and the time difference is about 1ms ~ 1.5ms. Usually, multiple detection methods are used at the same time. When arc rotation occurs, the voltage decreases and the current increases slightly, and the voltage differential and current signals are not easy to be taken out. Generally, Schmidt trigger is used to take the lower voltage limit signal to extinguish the arc

3 principle and characteristics of several common arc extinguishing methods

3.1 series high resistance resistance

series high resistance resistance in the anode and cathode circuit of nitriding power supply. When arcing occurs, the current increases sharply, resulting in a large voltage drop on the resistance, so that the voltage between the positive and negative electrodes is lower than the ignition voltage, so that the arc cannot be maintained, and also limits the arc current from being too large. This is feasible for medium and small power, but for high-power equipment, the power consumption on the series resistance is too large, so the arc extinguishing of high-power equipment cannot only rely on the series resistance [3]. In order to reduce the power consumption on the glow stabilizing resistor, it is changed to variable, that is, when the glow just starts, the arc striking is frequent, and a large resistor can be connected in series. After the glow discharge is stable, the resistor is reduced to obtain a larger current. Even so, there are still some difficulties in the application of equipment with a rated current of more than 10A in production

3.2 current cut-off negative feedback arc extinguishing

strictly speaking, the current cut-off negative feedback with a fixed value is not arc extinguishing, because in high-power equipment, the current cut-off value is often more than dozens of amperes, so a large current is enough to maintain a stable arc discharge without automatic extinction. When the current reaches a certain upper limit, the control circuit turns off the trigger pulse and maintains it for a certain time. After the arc source is eliminated, it is reopened. Since the thyristor can be turned off only after voltage commutation after triggering, the current value may reach a large value in half cycle. The arc extinguishing time of this arc extinguishing method is about 10ms. Since the general power supply has a certain overload capacity, it is effective to protect the power supply, so this current limiting method is essential when using thyristor power supply. Adopting the method that the cut-off value can be adjusted with the working current can advance the reaction time and reduce the arc discharge current. If the current differential negative feedback is used to control the change of current in advance, the stability of glow discharge can be significantly enhanced

3.3 LC oscillation arc extinguishing

lc oscillation arc extinguishing circuit is shown in Figure 1 [4]

the capacitor has a voltage of hundreds of volts during normal operation. When arc discharge occurs, the voltage of the cathode and anode suddenly decreases from hundreds of volts to tens of volts. At this time, the capacitor C is discharged through the coil L and the cathode and anode. At this time, l, C and the conductive gas between the anode and cathode form L, C and R series oscillation circuit, and oscillate at its natural oscillation frequency. In the first cycle of oscillation, when the voltage Uc on the capacitor becomes a reverse voltage and the current becomes zero, the arc light can be extinguished. At this time, the capacitor has been charged in reverse to hundreds of volts, but then the power supply charges the capacitor through the current limiting resistor R, so that the voltage on the capacitor gradually changes from reverse to forward and reaches the ignition voltage, and the glow is regenerated. The voltage and current waveform during oscillation is shown in Figure 2. If the factors that make the transition from glow discharge to arc discharge have disappeared at this time, a stable glow discharge will be obtained. If the factor of transition to arc discharge still exists, the capacitor will discharge and extinguish the arc again

the above circuit extinguishes the arc when the oscillation current crosses zero, so when selecting circuit parameters, it should ensure that the current can cross zero. According to relevant derivation, the following conditions should be met:

where: R - current limiting resistance, Ω

r - discharge circuit resistance, including the equivalent resistance between cathode and anode during arc discharge, the resistance of inductance L and the sum of conductor resistance (generally small, about 0.1 Ω), Ω

l - inductance of arc extinguishing inductance coil, h

c - capacity of arc extinguishing capacitor, F

the arc extinguishing time is half a period of oscillation, and its value is (usually at 100 μ S order of magnitude)

the time to regenerate glow discharge after arc extinguishing is the time required for the power supply to make the capacitor extinguish the arc light through the current limiting resistance to establish the positive voltage and reach the ignition voltage. Therefore, the time to regenerate glow (usually 1000 μ S order of magnitude)

LC arc extinguishing circuit has almost no energy loss during normal glow discharge, and its response is relatively sensitive. However, the instantaneous current of arc discharge can still reach a large amount, which is detrimental to both work and power supply. Especially in the case of continuous arc light, the power supply is in a long-term overload state, so LC arc extinguishing and sweeping method is adopted: therefore, there are certain precautions for its operation. For repair, the power supply must be additionally protected

3.4 thyristor bypass arc extinguishing

ion nitriding equipment with high power (above 100kW) usually adopts thyristor bypass arc extinguishing, and its circuit is shown in Figure 3. A thyristor V is connected in parallel between the anode and cathode of the circuit. Before the transition from glow discharge to arc discharge, the power supply charges the capacitor C1 to the power supply voltage through resistance R1. When the arc discharge, the voltage between the anode and cathode suddenly drops, and the capacitor C1 discharges. Pulse current is generated in the primary and secondary sides of the pulse transformer TM, making V trigger on and short circuit between the anode and cathode, so the arc is extinguished. At this time, LC forms an oscillation circuit. When the discharge current flowing through V is zero (the voltage has been reversed), V is automatically turned off. The power supply recharges the capacitor C1 to the voltage required for glow discharge through the current limiting resistor R and the filter inductor l0, and the glow starts again. Because the conduction time of V only needs more than a dozen μ s. Therefore, the arc discharge is bypassed only after a short time, and the arc discharge intensity is greatly weakened

this circuit also has its disadvantages: in the case of high power, a small current limiting resistance will produce a large power loss. The thyristor bypass arc extinguishing takes the sudden drop of voltage between the anode and cathode as the trigger signal. During the lighting, if the lighting voltage is high, the voltage will suddenly drop after the lighting, resulting in the false triggering of the thyristor. Therefore, a lighting cathode must be added to reduce the lighting voltage. The lighting cathode is led out from the cathode, 2mm ~ 3mm away from the anode

3.5 electronic switch arc extinguishing

on the basis of thyristor V1 bypass arc extinguishing, the sound of boom and bust that often appears in the process of tensile loading in the main circuit is connected in series with a thyristor V2 to form an electronic switch high-efficiency arc extinguishing circuit, as shown in Figure 4. When arcing occurs, immediately trigger V1 and V2 to bypass the current. Because LC (l2-c) oscillates, turn off V1 and V2 in reverse

ordinary thyristor is adopted, which is suitable for high current isolation capacitor energy storage. The principle of parallel series thyristor electronic switch circuit is shown in Figure 5. Since the capacitor of this circuit has been charged with voltage with opposite polarity at both ends of the furnace when it works, V1 and V2 are turned on when arcing, forcing the furnace current to drop, and turning off V3 at the same time, a faster arc extinguishing speed can be obtained

3.6 pulse power supply arc extinguishing

pulse power supply is a newly developed ion nitriding power supply in recent years. It changes the original DC power supply mode into high-frequency pulse mode, and the voltage and current naturally cross zero, which greatly inhibits the generation of arc discharge. When arc striking occurs, it can quickly turn off the controllable devices to extinguish the arc, and the arc extinguishing speed can reach several μ s。

4 phenomenon and situation comparison of various arc extinguishing methods

the comparison of various arc extinguishing methods is shown in Table 1

references

1 Zhou Xiaozhong, Chen Dakai Plasma heat treatment technology China Machine Press, 1990

2 Xu Bin et al A new type of ion source control circuit Metal heat treatment, 1999 (7): 39 ~ 41

3 heat treatment electrotechnics People's education press, 1977

4 heat treatment equipment and design Shandong people's publishing house, 1977 (end)