EP0462929B1 - Method and apparatus of starting the automatic control of the operating current of a current source on switching-on the circuit - Google Patents

Abstract

To start the automatic control of the operating current of a current source when the circuit is switched on, it is proposed to control the current source by presetting the circuit parameters in such a manner that the current produced on switch-on of the circuit tends towards a preset nominal value. The current control is only brought into action when the difference between the actual value and the nominal value of the current drops below a particular limit value. For this purpose, a control device is used which exhibits a voltage controller (7), a current controller (8) and a switch-over device (9) with threshold circuit. The switch-over device (9) generates a switching signal (s) which effects the switching-over from the initial voltage control to the current control as soon as the current exceeds a particular threshold value after the switching-on of the circuit. This makes it possible effectively to avoid switching-on current surges even when the circuit exhibits a current source controlled with an intermittent adjusting effect, for example a current source controlled by phase-control of valves. This effect can be improved further by the current control initially coming into action with an impressed actual value corresponding to the nominal value of the operating current during switch-on. <IMAGE>

Description

The invention relates to a method for the automatic control of the operating current of a current source when the circuit is closed and a control device for carrying out the method.

When a circuit is closed, the operating current of which is regulated automatically, the dynamic behavior of the control circuit, the switching means for closing the circuit and / or the apparatus operated with the current may result in current peaks which give rise to faults, in particular harmful effects on the can exercise current-carrying components.

In general, by suitable design of the control device, such current peaks can be kept within narrow limits or avoided entirely during a switch-on process. Under However, in certain circumstances there are difficulties in achieving this goal by the known means. One of these circumstances may be that a control method is used in which the manipulated variable that intervenes in the controlled system is only effective intermittently, rather than continuously.

An example of this is the regulation of electrical valves based on a gate control. As is known, a controlled electric valve of this type leads from the ignition point to current up to the zero crossing of the alternating voltage or to the transition of the current to another valve, e.g. in the case of a multi-phase rectifier, without the value of this valve current being able to be influenced at the moment. The current value cannot be changed until the next ignition of this valve by shifting the ignition point. In the meantime, the current can assume uncontrollable values, i.e. As a result of the control delay, settling processes can occur which cannot be easily controlled with a control of this type.

The reason for the difficulties mentioned is that the current control normally starts at the beginning of the current flow and therefore the controller has to process a relatively large difference between the setpoint and actual value of the current. The problem should therefore be solvable if the difference between the target and actual value of the current at the time the use of the control system is as small as possible, ie if the actual value of the current has already reached the target value at the start of the control action, that is to say to a certain extent has previously been raised to the target value. This possibility is provided that the current used when closing the circuit can be effectively limited by other measures until the control effect begins.

The method according to the invention is based on this finding and consists in first controlling the current source by presetting the circuit parameters so that the current used when closing the circuit does not exceed a preset setpoint value, and that the current control is only brought into effect when the Difference between the actual value and the setpoint of the current falls below a certain limit.

From DE-A-3 406 251 it is known in an arc welding device to limit the current used in the arcing for a certain period of time and to remove the limitation after this period has elapsed. However, no current regulation is provided, and no method for the current-dependent activation of such is specified.

A relatively simple, in certain cases sufficient embodiment of the method according to the invention can consist in that before the circuit is closed Open-circuit voltage of the current source is regulated on the basis of the preset current value to a value which, when the circuit is closed, leads to a current in the operating current which is at least approximately the same and that the voltage regulation is switched off when the current regulation is brought into effect. In this case, the current control only begins when the difference between the Actual value and the target value of the current is practically zero.

Under certain circumstances, however, it may be desirable to activate the current control at an earlier point in time, e.g. even when the operating current has only reached a fraction of its target value. In this case, another embodiment of the method according to the invention can be expedient, which is characterized in that the open circuit voltage of the current source is controlled to an upper limit value before the circuit is closed and that the current control when switched on initially with an impressed, the desired value of the Operating current corresponding actual value comes into effect. This means that the current control initially counteracts the rising current.

The application of the control method according to the invention in a circuit which has a current source controlled with intermittent actuation, e.g. a power source with electric valves controlled by gate control, e.g. Thyristors has the advantage that the current rise can be controlled perfectly when the circuit is closed and, in particular, no more current peaks exceeding the nominal value of the operating current occur in any phase of the short switch-on process.

A special application relates to a power supply unit for the operation of industrial gas discharge devices, for example for the operation of a plasma torch or for the optional operation of several plasma torches of different outputs. The commissioning of such devices with independent gas discharge, i.e. the closing of the circuit by igniting the gas discharge, presents difficulties insofar as the correct ignition of the gas discharge and the maintenance of a stable operating state of the plasma have different requirements, to which the circuit parameters must be adjusted. In addition, the transition from the ignition phase to the operating phase should be carried out as smoothly as possible, to which a switching of the circuit parameters that is as delay-free as possible can contribute.

The use of the method according to the invention for avoiding current peaks during the ignition phase has various advantages. On the one hand, this solution contributes to the stability of the ignition process; on the other hand, excessive stress on the electrodes on the gas discharge device is thereby avoided. In particular in the case of smaller plasma torches, an excessive current load even with a short exposure time has the consequence that electrode material is removed and the life of the electrodes is reduced.

The invention also relates to a control device for carrying out the method according to the invention in a power supply unit, in particular for the operation of a plasma torch, which is equipped with a rectifier formed by gate valves controlled by gate control, for example thyristors, a headset for timing the ignition of the valves, and a control arrangement influencing the headset consisting of a current and a voltage regulator, which the latter is designed so that the power supply unit works as a constant voltage source when idling and as a constant current source in operation.

A control device of this type is known from the patent DE-A-27 16 332. In this case, the current regulator is subordinate to the voltage regulator and the rest of the arrangement is such that the current is regulated to a predetermined open circuit voltage and with flowing current to the maximum achievable voltage. There are no special measures to reduce or avoid current peaks that can arise from overshoot of the current regulator.

Another proposal has become known from DE-A-20 17 261. There is a start circuit for a constant voltage control, which is about avoiding current peaks when the system is switched on. According to that DE-A-2017261, this should be achieved in that the regulation (in this case a voltage regulation) does not take effect immediately after switching on, but only after a period of time is determined by an RC link. In contrast to the solution according to the invention, the control is therefore switched on not as a function of the actual value of the current, but rather as a function of the time constant of an RC element.

In contrast, the control device according to the invention is characterized in that the voltage regulator and the current regulator can be optionally switched on by a changeover switch, the changeover switch being controlled by a signal which is dependent on the difference between the actual value and the desired value of the current, and in that means are provided with which the setpoint value of the open circuit voltage on the voltage regulator and the setpoint value of the current on the current regulator can be set to a value corresponding to the setpoint value of the operating current.

The time of switching from the voltage regulator to the current regulator depends on the process variant used. If the control device according to the invention is used in a power supply unit for operating industrial gas discharge devices, in particular a plasma torch, this changeover is preferably carried out immediately after the ignition phase has ended, i.e. at a point in time when the operating current has only reached a fraction of its desired value. This fraction of the setpoint, hereinafter referred to as the initial value of the operating current, is a quantity gained through experience and, for example, through a correspondingly set threshold switching detectable.

When a consumer is connected to a circuit, the full operating current required to operate the consumer usually flows immediately. The current surge associated therewith, especially at high current values, can trigger various types of interference, for example, generate excessive mechanical forces on the power lines and induce electromagnetic fields in neighboring electrical systems. Furthermore, it has been shown that the ignition of a gas discharge becomes critical if the current flow initiated with the gas discharge exceeds a certain level. For these reasons, it has proven expedient to limit the operating current to a certain extent in an introductory phase when the circuit is closed and then to gradually regulate it to the full value in order to avoid the disadvantages mentioned. With regard to the operation of plasma torches or other gas discharge devices, this measure also has the advantage that an operating current limited to the same degree can initially be selected for the optional operation of a plurality of plasma torches of different powers, so that the various plasma torches have a single, constant basic setting of the Control device put into operation and for this basic setting the cheapest, optimal parameters for stable ignition of the plasma can be selected in each of these plasma torches.

Fig. 1

Blockschema einer Stromversorgungsanlage für einen Plasmabrenner, mit einer erfindungsgemässen Regeleinrichtung;

Fig. 2

Schaltungsschema des Stromreglers;

Fig. 3

Schaltungsschema eines stromabhängigen Umschalters für die Umschaltung von Spannungsregelung auf Stromregelung;

Fig. 4

Zeitlicher Verlauf des Einschaltstroms beim Zünden eines Plasmabrenners unter Anwendung eines bekannten Regelverfahrens; und

Fig. 5

Zeitlicher Verlauf des Einschaltstroms beim Zünden eines Plasmabrenners unter Anwendung des erfindungsgemässen Regelverfahrens in einer Stromversorgungsanlage nach Fig. 1.

An exemplary embodiment of the control device according to the invention is explained in more detail below with reference to the accompanying drawing. In the drawing:

Fig. 1

Block diagram of a power supply system for a plasma torch, with a control device according to the invention;

Fig. 2

Circuit diagram of the current regulator;

Fig. 3

Circuit diagram of a current-dependent switch for switching from voltage regulation to current regulation;

Fig. 4

Time course of the inrush current when igniting a plasma torch using a known control method; and

Fig. 5

Time course of the inrush current when igniting a plasma torch using the control method according to the invention in a power supply system according to FIG. 1.

1 comprises a rectifier 1 connected to a three-phase network and a plasma torch 2 fed from the rectifier 1. The rectifier 1 contains controllable valves, which are arranged, for example, in a three-phase bridge circuit. The controllable valves are preferably thyristors, the throughput of which is regulated by gate control. The working circuit also contains means (not shown) for measuring the actual values of current and voltage required for the control device, the current being measured primarily on the AC side and the voltage being measured at the connection terminals of the plasma torch 2.

The control device comprises circuit devices 3, 4, 5 and 6 for the formation of signals which correspond to the target and actual values of voltage and current Us, Ui, Is and Ii in the working circuit, as well as a voltage regulator 7, a current regulator 8 and a changeover switch 9 for switching from voltage to current control. The resulting control signal is sent via an amplifier 10 to a control unit 11 for timing the firing of the thyristors of the rectifier 1.

The setpoints Us and Is of voltage and current are predetermined signals in the form of voltages with a positive sign, for example, which are matched in value to the type of consumer, in the present example to a number of plasma torches of different powers. The circuit devices 3 and 6 essentially contain an inverter, so that their output signals have negative potential to have. In addition, the circuit device 6 contains an integrator which, in the event of an abrupt change in the current setpoint value Is, causes a relatively slow change in the operating current. The circuit device 4 essentially contains an impedance converter and the circuit device 5 contains a rectifier. Both circuit devices 4 and 5 deliver signals in the form of voltages with a positive sign. The comparison of the respective negative setpoint signals and positive actual value signals can thus be carried out both in the voltage regulator 7 and in the current regulator 8 by simple signal addition at a node.

The switch 9 responds to the actual value of the current and generates a switching signal S which, by changing the polarity, causes the switch from voltage to current control as soon as the operating current exceeds a certain threshold value. For this purpose, the changeover switch 9 contains a threshold circuit.

The circuit structure of the current regulator 8 is shown in FIG. 2. The basic element of the same is an operational amplifier OP1 operating as a PI controller with an integrating element arranged in the negative feedback circuit, which is formed by a capacitor C1 and a resistor R4. The negative signal Is representing the nominal value of the current and the positive signal Ii representing the actual value of the current are at the inverting input of the operational amplifier OP1 linked together, while its non-inverting input is connected to ground via a resistor R5. A zener diode Z1 with a series resistor R3 limits the setpoint signal Is. A first electronic switch S1 is used to switch the negative feedback circuit on and off. The capacitor C1 can be connected to a voltage source via a second electronic switch S2, the voltage value being adjustable using a potentiometer P. These electronic switches are self-blocking field-effect transistors, an transistor of the N-channel type being used for the switch S1 and a transistor of the P-channel type being used for the switch S2. The two switches S1 and S2 are controlled jointly by the switching signal S supplied by the changeover switch 9, each via a resistor R2 or R1. As long as this switching signal S is negative, switch S1 is in the open state and switch S2 is in the closed state. With this switch position, the negative feedback circuit is interrupted and the signal at the output of the operational amplifier has no control effect. At the same time, a positive voltage is present across capacitor C1, which keeps capacitor C1 in a certain state of charge. As soon as the switching signal S changes its sign, the switch S1 closes and the switch S2 opens. As a result, the operational amplifier OP1 begins to work as a PI controller, specifically with the actual value specification impressed by the state of charge of the capacitor C1.

Simultaneously with the activation of the current regulator 8 in the manner described, the voltage regulator 7 is switched off. The same switching signal S can be used for this. The details of the circuit arrangement with regard to the voltage regulator 7 and its switching on and off are not discussed in detail here, since these are definitely within the scope of the person skilled in the art.

The basic element of the switch 9 shown in FIG. 3 is an operational amplifier OP2, which operates as a threshold switch and at whose output the switching signal S appears. The signal Ii corresponding to the actual value of the current reaches the non-inverting input of the operational amplifier OP2 via the resistor R8. At the inverting input of the same there is a positive reference voltage which is taken from a voltage divider R6, R7. The Zener diode Z2 serves to stabilize the reference voltage and the Zener diode Z3 serves to limit the signal Ii. A capacitor C2 is connected in parallel with the Zener diode Z3. The output signal is fed back to the inverting or non-inverting input of the operational amplifier OP2 with the resistors R9 and R10. If the current-dependent signal Ii is small, the output signal representing the switching signal S is negative. The switching signal S changes its sign as soon as the operating current I and thus the signal Ii exceeds the threshold value given by the reference signal, and remains positive as the current I increases.

The mode of operation of the control device described with reference to FIGS. 1 to 3 is as follows: When the power supply unit is switched on, a preparation phase is initiated. During this, the rectifier 1 is connected to the operating voltage. Furthermore, the predetermined setpoint signals Us and Is for voltage and current are fed to the control device. Because of the switching signal S which is negative when current I is still missing, the voltage regulator 7 starts the control operation, while the current regulator 8 does not yet perform any control function. The open circuit voltage at the plasma torch 2 is limited to a specific value by the voltage regulator 7. At the same time, the capacitor C1 of the current regulator 8 is charged to a specific voltage value via the switch S2 which is still closed. As a result, the current regulator 8 is practically given an actual value which preferably corresponds to the initially desired setpoint of the operating current. This completes the preparatory phase.

The ignition of the plasma torch 2, which is accomplished with separate means, not shown, closes the working circuit, ie a current I begins to flow, which rises very rapidly. As soon as this current exceeds the previously mentioned threshold value, the voltage regulation is switched off and the current regulation is switched on due to the polarity change of the switching signal S. The current controller 8 begins to work with the impressed actual value, which corresponds, for example, to the setpoint value Is of the operating current. Of the Current controller 8 thus initially works in accordance with an actual value which the operating current I has not yet reached at this moment. As a result, the rectifier 1 is regulated back at the moment, ie it is counteracted until the actual actual value of the operating current can come into effect in the control loop. As a result of this control delay, overshoot of the current regulator 8 is practically impossible.

This is clear from the traced oscillogram according to FIG. 5, which shows the time course of the inrush current when the plasma torch is ignited. The current I increases from zero to the preset nominal value of 150 A, for example, and is kept at this value on average by the current controller 8. After just a few tenths of a second, the nominal value of the current can be raised (or possibly also lowered) to the current value required for the operation of the plasma torch 2 (for example 1000 A), the current I changing only gradually as a result of the timing element in the switching device 6 Fig. 5 can be seen.

In contrast, FIG. 4 shows the current increase under otherwise identical conditions, but without the above-mentioned mode of operation of the control device. Current peaks of the order of 400 A are to be expected.

Claims (7)

1. Method for starting the automatic control of the operating current of a current source on switching on the current, characterized in that at first the current source is controlled by the pre-setting of the circuit parameters so that the current being set on switching on the circuit does not exceed a pre-set desired value and that the current control is only brought into effect when the difference between the actual value and the desired value of the current falls below a certain limit value.
2. Method according to Claim 1, characterized in that, before the circuit is switched on, the open-circuit voltage of the current source is controlled on the basis of the pre-set current value to a value which, on switching on the circuit, leads to a current at least approximately the same as the operating current current in the latter, and that the voltage control is switched off when the current control is brought into effect.
3. Method according to Claim 1, characterized in that the open-circuit voltage of the current source before switching on the circuit is controlled to an upper limit value and that the current control on switching on comes initially into effect at an impressed actual value corresponding to the desired value of the operating current.
4. Control apparatus for carrying out the method according to Claim 1, in a power supply unit, especially for the operation of a plasma burner, which is equipped with a rectifier (1) formed from electrical valves controlled by phase control, e.g. thyristors, a gate control set (11) for the time control of the ignition insert of the valves, and a control arrangement which influences the gate control set, containing a voltage and a current controller (7 or 8), the last-named of which is constructed so that the power supply unit in open-circuit operation operates as a constant voltage source and, in operation, as a constant current source, characterized in that the voltage controller (7) and the current controller (8) can optionally be switched on by a transfer switch (9), the transfer switch (9) being controlled by a signal dependent on the difference between the actual value (Ii) and the desired value (Is) of the current, and that means (3, 6) are provided, by which the desired value (Is) of the open-circuit voltage at the voltage controller (7) and the desired value (Is) of the current at the current control (8) can be set at a value corresponding to the desired value of the operating current (I).
5. Control apparatus according to Claim 4 for carrying out the method according to Claim 3, characterized in that the current control (8) is an operational amplifier (OP1) working as a PI-controller with an integrating member (C1, R4) located in the negative-feedback circuit, that a first electronic switch (S1) is provided which switches on and off the negative-feedback circuit, that a second electronic switch (S2) is provided, by which the capacitor (C1) of the integrating member (C1, R4) can be connected to a voltage source (P) with variable voltage, and that the two electronic switches (S1, S2) can be controlled by a switching signal (S), so that in each case always the one switch is switched off and the other is switched on.
6. Control apparatus according to Claim 5, characterized in that self-locking field-effect transistors are provided as electronic switches (S1, S2), of which one is a P-channel type and the other an N-channel type, and that the two switches are controlled by a switch signal (S) of alternating polarity.
7. Control apparatus according to Claim 6, characterized in that a changeover apparatus (9) is provided, which responds at a threshold value of the operating current (I) and emits a switch signal (S) which changes its polarity when the threshold value is exceeded.

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