GB2132832A

GB2132832A - Supplying electrostatic spraying apparatus with high DC voltage

Info

Publication number: GB2132832A
Application number: GB08328155A
Authority: GB
Inventors: Henri Perrin
Original assignee: SKM SA
Current assignee: SKM SA
Priority date: 1982-11-04
Filing date: 1983-10-21
Publication date: 1984-07-11
Also published as: DE3340010A1; GB8328155D0; FR2535917A1; JPS5998754A; JPH0380069B2

Abstract

A low voltage supply (1) provides a low DC voltage (V3) which, in the absence of current, has a value substantially higher than that of the stabilized low DC voltage (V4) normally delivered by a voltage stabilizer circuit (6) to a high voltage generator (7) when the intensity of the current is less than a threshold value. A passive circuit element (11) eg. a resistor is connected upstream of the stabilizer circuit (6) to cause the low voltage supply to the circuit to decrease progressively so that the low voltage passes, while decreasing, through a value substantially equal to the value of the stabilized low DC voltage (V4) when the intensity of the current reaches the threshold value, this latter corresponding to a normal operating distance of an electrostatic spray gun (9) from a grounded object (10) being sprayed. The resistor (11) may be omitted if a saturated core transformer is used. <IMAGE>

Description

SPECIFICATION A process for supplying an electrostatic spraying apparatus with a high DC voltage and a device for implementing this process.

Background of the Invention 1. Field of the Invention The present invention relates to a process for supplying an electrostatic spraying apparatus with a high voltage, consisting in supplying a low DC voltage, stabilizing the low DC voltage to a predetermined value as long as the intensity of the current remains less than a predetermined threshold value corresponding to a predetermined distance between the electrostatic spraying apparatus and a grounded object, causing the low DC voltage to decrease when the intensity of the current exceeds said threshold value and converting the low DC voltage into a high DC voltage substantially proportional to the low DC voltage. The invention also relates to a device for implementing this process.

2. Description of the PriorArt The electrostatic process for paint or powder spraying is now well known. It uses the action of an intense electric field on a paint drop or a solid particle carrying an electric charge so as to transport and deposit the drop or the particle on the object to be painted or powdered.

In electrostatic spraying installations (electrostatic paint or powder spraying), the high DC voltage required for obtaining the electric field is obtained by means of a generator which usually comprises a stabilized power supply delivering a low voltage DC current at constant voltage and a high voltage generator properly speaking which converts the low DC voltage delivered by the stabilized power supply into a high DC voltage of the order of 40 to 120 kV.

Typically, the high voltage generator properly speaking comprises a high frequency oscillator (a few tens of kHz), a high frequency transformer which boosts the output voltage of the high frequency oscillator to a high frequency medium voltage and a voltage adder-rectifier or multiplier, of the "Villard" chain type which rectifies and boosts the voltage obtained at the terminals of the secondary of the high frequency transformer so as to output the desired high DC voltage.The output of the high voltage generator is generally connected through a very high value resistor to a charge electrode which is situated at the side of the paint or powder spraying nozzle of the spraying apparatus, for example a spray gun, to charge electrically the paint drops or the powder particles emitted by the nozzle and so as to produce the electric field for transporting said drops or said particles from the nozzle to the object to be coated with paint or powder.

In addition, a safety device is usually connnected in the low voltage circuit of the generator so as to cause a drop in the high voltage when the current supplied by the generator becomes too high. Such a safety device is required for avoiding the production of a dangerous electric arc between the high voltage electrode and ground (object to be painted or coated with powder, or any other grounded object). In fact, in a case where the spraying apparatus is used in an inflammable or explosive atmosphere (mixture of air and paint mist, with or without solvent vapors) the production of an arc could cause the ignition or explosion of said atmosphere. Similarly, there is also need to prevent arcing between the high voltage electrode and an operator who, otherwise, might receive a severe electric shock.

The only means of detecting the imminence of an arc is to detect an abnormal increase in the intensity of the current supplied at a high voltage. In fact, an increase of the intensity of the current supplied indicates that the high voltage electrode is dangerously close to an object connected to the ground and that, consequently, the high voltage must be reduced or cancelled. This information is generally taken from the return current coming from ground to the generator. It is also possible to take this information from the low voltage supply current which is substantially proportional or at least an increasing function of the current supplied at high voltage.

Up to now, the constructors of high voltage generators for electrostatic spraying have endeavoured to produce generators having a rectangular shaped current-voltage characteristic, i.e. the voltage remains substantially constant as long as the intensity of the current does not exceed a preset threshold value, then drops sharply beyond said threshold value. in this connection, reference may be made to French patents n" 1 568 060 (page 1, left-hand column, last paragraph), n" 1 568060 (page 2, right-hand column, line 27), n 1 562917 (page 1, right-hand column, last but one paragraph) and n 2 036 794, this latter patent even going so far as to provide an absolutely rectangular diagram.

Moreover, it is evident that the action of the electrostatic forces is all the higher the higher the potential gradient of the electric field. Up to now, it has therefore been considered that maintaining the high voltage at a high value ensured the maximum potential gradient and that that corresponded to the best use of the possibilities of the high voltage generator. The safety device had then to be set so as to act at the time when maintenance of the high voltage became dangerous, that is to say when the distance between the high voltage electrode and a grounded object became less than a critical distance beyond which the production of an arc became highly probable.This critical distance, which may vary depending on the value of the high voltage used and on the type of spray gun used, is in general of the order of 5 cm, whereas the normal operating distance is generally between 10 and 30 cm.

However, the use of safety devices such as they are at present constructed comprise two disadvantages. The first disadvantage comes from the fact that the safety device is usually formed by an active electronic element. If it does not react or if it reacts badly to the information concerning the current supplied, safety is no longer ensured. The second disadvantage comes from the fact that the high voltage is maintained at its maximum value until the safety device comes into action. Now, the high voltage generator comprises, because of its construction, elements having a not inconsiderable electric capacity. The energy stored in these capacitites may supply a dangerous disruptive discharge if it is not removed very rapidly at the time of the danger. Now, present high voltage generators keep this energy at its maximum value up to the moment when the safety device comes into action.If the discharge of these capacities through the high voltage electrode is too slow, an arc may be produced despite the action of the safety device. The danger of arcing through discharge of the capacities is so real that efforts have been made to provide protection therefrom by different means, for example by inserting a very high value resistor immediately upstream of the high voltage electrode, by reducing as much as possible during construction the capacities of the high voltage generator and of the live metal elements under high voltage and/or by providing an additional safety device acting should a rapid variation of the intensity of the current supplied occur indicating the rapid approach of the high voltage electrode to a grounded object.

Furthermore, setting of the threshold at which the safety device should come into play is fairly complicated. In fact, the current supplied at high voltage comprises two components: 1) The ionic current supplied by corona effect between the electrode and the object to be painted which is connected to ground. A part of this current provides the electric charge for the sprayed paint.

This current is increasing when the potential gradient between the electrode and ground increases.

2) The leak current which flows from the spraying nozzle to ground in the paint reservoir through the paint itself which is brought from said reservoir to the spray gun. This leak current depends on the conductivity of the paint.

Setting of the safety device is generally carried out with the spray gun without paint, so with only the ionic current. For example, the safety device is generally set so that it comes into play when the intensity of the current supplied corresponds to the above-mentioned critical distance of 5cm between the high voltage electrode and ground. Thus, in use, with a non conducting paint, the safety device operates for a distance of 5cm, but it will operate for a greater distance if the paint used is conducting, this distance being all the greater the lower the resistivity of the paint.In this latter case, it may happen that the safety device cuts off the high voltage when the high voltage electrode is at a distance still suitable for normal operation, for example 15cm. For some conducting paints, special settings are therefore effected to prevent the safety device from coming into action for electrode-ground distances which are too great. These special settings are particularly delicate since they must be effected by trial and error and since the high DC voltage must be maintained at its maximum value up to the above-mentioned critical distance.

Summary of the Invention The present invention has essentially as aim to overcome the disadvantages of prior art generators by providing a process and a device for supplying an electrostatic spray apparatus with high DC voltage, with increased security with respect to arcing to ground. Another aim of the invention is to provide a device for supplying an electrostatic spray apparatus with high voltage, whose safety device is less costly and more reliable than the active electronic safety devices of previously known generators.

To this end, the process of the present invention consists in supplying a low DC supply voltage which has, in the absence of current, a value substantially higher than the pre-determined value of the stabilized low DC voltage, and in causing the low DC supply voltage to decrease progressively from said substantially high value to a value sustantially lower than the pre-determined value of the stabilized low DC voltage in correspondence with the increase of the intensity of the current, so that the low DC supply voltage passes, while decreasing, through a value substantially equal to the pre-determined value of the stabilized low DC voltage when the intensity of the current reaches said pre-determined threshold value, the low DC voltage being stabilized at the pre-determined value as long as the value of the low supply DC voltage is greater than said predetermined value, and ceasing to be stabilized and decreasing when the intensity of the current exceeds said pre-determined threshold value.

The present invention is based on the remark that, though it is suitable to maintain the high voltage at a constant maximum value for normal operating distances of the spraying apparatus, it is not necessary to maintain it at its maximum constant value when the high voltage electrode draws near to the object to be painted or to any other grounded object, contrary to what was sought with previously known generators.In fact, whereas, with known generators delivering a high constant maximum voltage up to the above-mentioned critical distance, a rapid increase of the potential gradient was sought when the high voltage electrode draws close to the objects to be painted, while remaining at a distance greater than said critical distance, the applicant has discovered that it is not necessary to have as high an increase of the potential gradient and that a low increase of the gradient is sufficient to provide a suitable electrostatic effect for medium and small spraying distances. In other words, the applicant has discovered that, to obtain a suitable electrostatic effect at medium and small spraying distances, it is sufficient for the potential gradient to increase reasonable (and with it the current supplied) without reaching the disruptive gradient at which arcing may take place between the electrode and ground.

The generator of the present invention is therefore designed to supply a high DC voltage having a maximum constant value up to a normal spraying distance, then a high voltage whose value decreases progressively for medium and small spraying distances when the intensity of the current supplied increases, so when the distance between the high voltage electrode and the ground decreases. From the safety point of view, the generator of the present invention has the advantage of ensuring progressive reduction of the high voltage even before the high voltage elctrode reaches a distance from a grounded object equal to said critical distance. Thus, when a danger of arcing might be present because the high voltage electrode approaches too closely and too rapidly a grounded object, the voltage is sufficiently low for possible arcing not too be dangerous.

Furthermore, with the generator of the present invention, when conducting paint is used, working may still take place at normal and medium spraying distances with a potential gradient which is certainly smaller but which is still very acceptable, whereas, under the same conditions, working can no longer continue with known generators whose safety device completely cuts off the high voltage. In the case where very conductive paints are used, the current threshold beyond which the high voltage decreases progressively may also be set so that the high voltage begins to decrease from a distance equal to that at which the high voltage begins to decrease when using a non conducting paint.However, in this case, setting of the intensity threshold is less delicate than in the case of prior known generators since the setting corresponds to a normal spraying distance whereas before it corresponded to the critical spraying distance beyond which arcing was highly probable.

The generator of the present invention comprises, in a way known per se, a low DC voltage supply circuit, a voltage stabilizing circuit connected to the output of the low DC voltage supply circuit, a high voltage generator connected to the output of the voltage stabilizer circuit to supply a high DC voltage substantially proportional to the output voltage of the voltage stabilizer circuit and means for causing the output voltage of the voltage stabilizer circuit and, consequently, the high DC voltage output of the high voltage generator to decrease when the intensity of the current reaches a pre-determined threshold value.According to the invention, said low DC voltage supply circuit is dimensioned so as to provide a low DC voltage which has, in the absence of current, a value substantially greater than that of the stabilized low DC voltage which is normally delivered by the voltage stabilizer circuit when the intensity of the current is less than said predetermined threshold value.Furthermore, said means are formed by a passive circuit element which is connected in the low DC supply circuit so as to cause the low voltage supplied to the stabilizer circuit to decrease progressively from said higher value to a value substantially lower than said value of the stabilized low DC voltage, so that low voltage passes, while decreasing, through a value substantially equal to the value of the stabilized low DC voltage when the intensity of the current reaches said pre-determined threshold value, this latter corresponding to a pre-determined operating distance of the electrostatic spraying apparatus with respect to a grounded object.

As will be seen further on, said passive element may be formed by a simple resistor which is much less costly and much more reliable than the active electronic safety devices which were usually included in prior known generators.

BriefDescription of the Drawings Other characteristics of the present invention will appear from the following description of a preferred embodiment of the present invention, given by way of example with is purely indicative and in no wise limiting, with reference to the accompanying drawings in which: Figure 1 shows the diagram of a high voltage generator for an electrostatic spraying apparatus, in which the present invention is incorporated.

Figure 2 is a diagram for explaining the operation of the generator of Figure 1.

Description of the Preferred Embodiment The generator of Figure 1 comprises, in a way known per se, a transformer 1 whose primary is connected to a low voltage AC source at an industrial frequency, for example the 220V, 50 Hz mains. The AC voltage V1 available at the terminals of the secondary winding 3 of transformer 1 is rectified by a rectifier circuit formed for example by a diode device 4, then smoothed by a smoothing capacitor 5.

The DC voltage V2 available at the terminal of capacitor 5 is applied to the input of a voltage stabilizer circuit 6 which delivers at its output a stabilized low DC voltage V4, for example of 10 volts.

Voltage V4 iS applied to the input of a high voltage generator 7 which delivers at its output a high DC voltage VHT, for example 40 000 to 120 000 volts, which is proportional to the voltage V4. The output of the high voltage generator 7 is connected through a very high value resistor8 (several tens of megohms) to the high voltage electrode 9 of a spraying apparatus, e.g. a spray gun (not shown). As is known, resistor 8 and the high voltage generator 7 may be housed in the spray gun, whereas the other element 1 to 6 of the generator are at a distance from the spray gun and are connected thereto by a low voltage cable. In Figure 1 there has also been shown schematically an object 10 to be painted or coated with powder which is connected to ground.

All the elements which have been described above are well known and it has therefore been thought pointless in describing them further. It will simply be noted that the high voltage generator 7 may advantageously comprise a high frequency oscillator (a few tens of kHz), a high frequency transformer for boosting the output voltage of the high frequency oscillator to a high frequency medium voltage, and a voltage adder-rectifier or multiplier, of the rectifier and capacitor type, for rectifying the high frequency medium voltage supplied by the transformer and for boosting it to the desired high DC voltage VHT. As is also well known, the use of a high frequency allows the capacity of the capacitors of the voltage multiplierto be reduced and consequently, this latter to be miniaturized, which allows itto be housed in the spray gun. Such miniaturization further allows the internal electric capacity of the high voltage generatorto be reduced and, consequently, the capacitive energy stored in said capacity to be reduced, which is particularly important for the sake of safety in the case of a possible arcing to ground.

In accordance with the present invention, the secondary winding 3 of transformer 1 is dimen sioned to supply a low AC voltage V1 having a value such that the low DC voltage obtained off-load, i.e. in the absence of current, after rectification by the diode 4 and smoothing by capacitor 5, has a value substantially higher than that of the stabilized low DC voltage V4 which is normally delivered by the voltage stabilizer circuit 6. For example, if the voltage stabilizer circuit 6 is designed to deliver a stabilized voltage of 10 volts, the secondary winding 3 of transformer 1 may be dimensioned so that the voltage V2 obtained off-load is equal to 24 volts.

Furthermore, in accordance with the present invention a resistor 11 is connected in series between diode 4 and the voltage stabilizer circuit 6. The purpose of resistor 11 is to cause the low voltage V3 supplied to the stabilizer circuit 6 to decrease progressively from the value of V2 to a value substantially lower than the value of the stabilized low DC voltage (10 volts) when the intensity of the current increases.The value of resistor 11 is chosen so that the voltage V2 passes, while decreasing, through a value substantially equal to the value of the stabilized low DC voltage V4 (ignoring the voltage drop A V in the voltage stabilizer circuit 6) when the intensity of the current reaches es a preset threshold value Is, this latter corresponding to a normal operating distance of the electrostatic spraying apparatus with respect to a grounded object.For example, with a voltage drop A V of about 3V in the voltage stabilizer circuit 6, which corresponds to a typical voltage drop in the stabilizer circuits usually used, if threshold 15 is chosen at 0.5 amp in the absence of paint (as is shown in Figure 2), which corresponds to a distance of about 1 Ocm between the high voltage electrode 9 and the object to be painted, resistor 11 will have a value of 22 ohms.

Under actual operating conditions, i.e. with paint which is conducting to a greater or lesser extent, a leak current flows to ground through the paint. This current is added to the ionic current between the electrode 9 and the object 10. The result is that threshold 15 will correspond in reality to a greater distance than 1 Ocm, the value of this distance varying depending on the conductivity of the paint.

However, in the majority of cases, that will be of no importance for, when the intensity of the current exceeds the threshold value 15, the generator of the present invention supplies a high DC voltage which has again a sufficient value to obtain a sufficient potential gradient for painting. In any case, in the limit cases, for example when very conductive paints are used, it is still possible to adjust the threshold 15 so that it corresponds to an electrode/ground distance between about 1 Ocm and about 30cm.This may for example be obtained by adjusting the value of voltage V1, so the value of voltage V2, for example by means of a transformer with secondary winding having multiple tappings and/or by adjusting the slope of the straight line V3 (Figure 2) by changing the value of resisitor 11 which, for this purpose, may be a variable resistor.

Thus, as long as electrode 9 is at a distance from the object lotto be painted greater than the distance corresponding to threshold Ig (10cm or more) the voltage V3 applied to the voltage stabilizer circuit 6 remains greater than the value of the voltage (13V in the example considered) which is required for the stabilizer circuit 6 to function correctly, and the voltage V4 at the output of circuit 6 remains then constant and equal to 10V in the example considered. The high DC voltage VHT, which is proportional to the voltage V4 also remains constant.On the other hand, when the distance between the electrode 9 and object 10 becomes less than the distance corresponding to threshold 15, the voltage V3 becomes insufficient for the stabilizer circuit 6 to be able to function normally, so that its output voltage V4 decreases linearly while following the voltage V3, ignoring A V, as the intensity of the current increases, so as the electrode 9 draws near to the object 10. The high DC voltage VHT, which is proportional to voltage V4, decreases similarly and this before electrode 9 reaches the critical distance (about 5cm) at which arcing might occur between the electrode 9 and object 10. Thus, when electrode 9 reaches said critical distance, the high DC voltage VHT iS already sufficiently low for possible arcing not to be dangerous.

Although, beyond the threshold 15, the high DC voltage VHT decreases, the potential gradient is maintained art a sufficient value for painting, the reduction of the high voltage being compensated for by the electrode S/object 10 distance.

From the safety point of view, it is advantageous to have a fairly high voltage difference between the voltage V1, so voltage V2, and the stabilized voltage V4. In fact, that allows a resistor 11 to be used whose value is suitable for providing a fairly rapid drop of voltage V3 (slope of the straight line V3 in Figure 2), so also a fairly rapid drop in voltage V4 and, consequently, in the high DC voltage VHT when the intensity of the current exceeds threshold iS. On the other hand, so that the potential gradient keeps a sufficient value beyond threshold 15, voltage V3, SO voltage V4 and the high DC voltage VT should not decrease too rapidly.The choice of the value of voltage V2 and of the value of resistor 11 will then have to be a compromise between the abovementioned considerations and will also depend on the value of the desired threshold 15, and so on the value of the distance from which it is desired to cause the high DC voltage VHTto decrease progressively.

It goes without saying that the embodiment of the present invention which has been described above has been given by way of example purely by way of indication and is in no wise limiting, and that numerous modifications may be readily made by a man skilled in the art without for all that departing from the scope of the invention. Thus, more particularly, instead of placing resisitor 11 in series between diode 4 and the voltage stabilizer circuit 6, it could be placed in series between the secondary winding 3 of transformer 1 and diode 4, as is shown with a broken line in Figure 1. Similarly, resistor 11 could be omitted and a saturated core transformer 1 used whose characteristic is to supply a voltage inversely proportional to the current supplied. In this case, the voltage V1 would vary in the same way as voltage V3.

Furthermore, in the generator of Figure 1, transformer 1 and diode 4 could be replaced by a battery of for example 24 volts.

Claims

1. A process for supplying an electrostatic spraying apparatus with high voltage, consisting in providing a low DC voltage, stabilizing the low DC voltage at a predetermined value as long as the intensity of the current remains less than a predetermined threshold value corresponding to a predetermined distance between the electrostatic spraying apparatus and a grounded object, causing the low DC voltage to decrease when the intensity of the current exceeds said threshold value and converting the low DC voltage into a high DC voltage substantially proportional to the low DC voltage, which process further consists in providing a low DC supply voltage which, in the absence of current, has a value substantially higher than the predetermined value of the stabilized low DC voltage and causing the low DC supply voltage to decrease progressively from said substantially higher value to a value substantially lower than the predetermined value of the stabilized low DC voltage in correspondence with the increase in the intensity of the current, so that the low DC supply voltage passes, while decreasing, through a value substantially equal to the predetermined value of the stabilized low DC voltage when the intensity of the current reaches said predetermined threshold value, the low DC voltage being stabilized at the predetermined value as long as the value of the low DC supply voltage is greater than said predetermined value, and ceasing to be stabilized and decreasing when the intensity of the current exceeds said predetermined threshold value.

2. A device for supplying an electrostatic spraying apparatus with high voltage, comprising a low DC voltage supply circuit, a voltage stabilizer circuit connected to the output of said low DC voltage supply circuit, a high voltage generator connected to the output of said voltage stabilizer to supply a high DC voltage substantially proportional to the output voltage of said voltage stabilizer circuit and means for causing the output voltage of said voltage stabilizer and, consequently, the high DC output voltage of said high voltage generatorto decrease when the intensity of the current exceeds a predetermined threshold value, wherein said low DC voltage supply circuit is dimensioned so as to supply a low DC voltage which, in the absence of current, has a value substantially higher than that of the stabilized low DC voltage which is normally delivered by said voltage stabilizer circuit when the intensity of the current is less than said predetermined threshold value, said means being formed by a passive circuit element which is connected in the low DC voltage supply circuit so as to cause the low voltage supplied to said stabilizer circuit to decrease progressively from said higher value to a value substantially lower than the value of said stabilized low DC voltage, so that the low voltage passes, while decreasing, through a value substantially equal to the value of the stabilized low DC voltage when the intensity of the current reaches said predetermined threshold value, this latter corresponding to a predetermined operating distance of the electrostatic spraying apparatus from a grounded object.

3. The device as claimed in claim 2, wherein said low DC supply voltage comprises a low AC voltage source, a transformer and a rectifier circuit, the secondary winding of said transformer being dimensioned so as to provide a low AC voltage which has a value such that the low DC voltage obtained at the output of said rectifier circuit has said higher value.

4. The device as claimed in claim 3, wherein said passive circuit element is a resistor connected in series between said rectifier circuit and said voltage stabilizer circuit.

5. The device as claimed in claim 3, wherein said passive circuit element is a resistor connected in series between said secondary winding and said rectifier circuit.

6. The device as claimed in claim 3, wherein said passive circuit element is said transformer, which is a saturated core transformer.