JPH0380069B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of the Invention> The present invention provides a direct current low voltage for supplying a high voltage to an electrostatic spray (or atomization or atomization) device, and the direct current low voltage is previously stabilizing to a predetermined value and setting the magnitude of the current below a predetermined threshold corresponding to a predetermined distance between the electrostatic spray device and the grounded object. , reducing the DC low voltage when the magnitude of the current exceeds the threshold, and converting the DC low voltage into a DC high voltage approximately proportional to the DC low voltage; Relating to a method for supplying. The invention also relates to a device for carrying out the method.

Description of the Prior Art Electrostatic atomization spray painting methods are currently well known. In this method, the action of a large electric field on paint droplets or solid particles with an electrolytic caustic agent is used to ensure the transport and adhesion of the paint droplets or particles to the object to be painted or sprayed.

In electrostatic spraying (electrostatic painting or electrostatic atomization) equipment, the high DC voltage required to generate the electric field is typically coupled to a regulated power supply that provides a low voltage DC current under constant voltage; It has a high voltage generator that converts the low DC voltage supplied by the stabilized power supply into high DC voltage in the range of 40 to 120 KV (kilovolts). In a typical example, a high-voltage generator in the original sense consists of a high-frequency (several tens of kilohertz) oscillator, a high-frequency transformer that boosts the output voltage of the high-frequency oscillator to a high-frequency intermediate voltage, and a high-frequency transformer. "VILLARD" rectifies and boosts the voltage obtained from the secondary side terminal of the device to generate the desired DC high voltage at the output.
cascade-connected voltage summing rectifiers or voltage multipliers. The output of this high-voltage generator is generally connected via a resistor of very large value to a charging electrode, which is located on the side of the atomizing device, e.g. the paint or powder atomizing nozzle of an atomizing spray gun. arranged to charge the paint droplets or powder particles ejected by the nozzle and transport the droplets or particles from the nozzle to the object to be coated with the paint or powder; It functions to generate an electric field that forms an electric field.

Furthermore, safety devices are usually provided in low voltage generation circuits in order to cause a drop in the high voltage when the current output by the generator becomes too large. Such safety devices prevent the formation of a dangerous electrical arc between the high voltage electrode and earth (the object being painted or covered with powder or any other object placed on the ground). necessary to avoid. In fact, when using an atomizing device in a flammable or explosive atmosphere (a mixture of air and paint vapor with or without solvent vapor), arcing can cause combustion or explosion of the atmosphere. Similarly, possible short circuits between high voltage electrodes and personnel must be avoided.
Otherwise, the worker would receive a large electric shock.

A means of detecting the risk of the occurrence of a spark discharge is to detect an abnormal increase in the strength or magnitude of the current output under high voltage. In fact, an increase in the magnitude of the output current indicates that the high voltage electrode has come dangerously close to a grounded object and therefore indicates that the high voltage must be reduced or disabled. . This information is typically obtained by sampling the return current from ground to the voltage generator. However, it is also possible to obtain information about the current of the low voltage power supply which is approximately proportional to, or at least a function of, the increase in current generated at the high voltage.

To date, manufacturers of high voltage generators for electrostatic spraying have developed rectangular current-voltage characteristics, i.e., the voltage remains approximately constant as long as the magnitude of the current does not exceed a preset threshold; It was common practice to build power generation equipment that had the characteristic of dropping rapidly as soon as it was exceeded. Regarding this point, French Patent No. 1568060 (page 1, right column, last section)
French Patent No. 1568060 (page 2, right column,
(line 27), French Patent No. 1562917 (line 1)
(before the last paragraph in the right column of the page) and French patent no.
See specification No. 2036794. It should be noted that the last-mentioned specification shows a completely rectangular current-voltage characteristic curve.

By the way, it goes without saying that the effect of electrostatic force becomes stronger as the potential gradient of the electric field becomes larger. Until now, it has therefore been believed that maintaining the high voltage at a large value will thereby ensure a maximum potential gradient, thereby making better use of the capacity of the high voltage generator. Therefore,
The safety device is activated at the point when maintaining the high voltage becomes dangerous, in other words when the distance between the high voltage electrode and the grounded object becomes less than a critical distance at which the probability of spark discharge increases. It was controlled. This critical distance, which can vary according to the value of the high voltage used as well as the type of atomizing gun used, is generally on the order of 5 cm, while other conventional distances or spacings are generally between 10 cm and 30 cm.

In any case, there are two drawbacks to the use of safety devices currently in use. A first drawback stems from the fact that safety devices are usually constituted by active electronic components.
If you fail to respond or respond incorrectly to information regarding the output current, safety is no longer guaranteed. A second disadvantage is based on the fact that the high voltage is maintained at its maximum value until the safety device intervenes. By the way, the high voltage generator is
Structurally, it includes elements with non-negligible capacitance. If the energy stored in such capacitances is not released very quickly at the point of danger, dangerous destructive discharges can occur. The high voltage generators currently in use use this energy to
The maximum value is saved until the safety device intervenes. Therefore, if the discharge from these capacitors via the high-voltage electrodes is very slow, there is a risk of sparking due to capacitive discharge, where sparking can occur despite the intervention of safety devices. This is a very practical problem, such as inserting a very large value resistor immediately upstream of the high voltage electrode, or increasing the capacitance of the high voltage generator and the metal element to which the high voltage is applied during manufacturing. Protection measures are taken by various means, including the provision of supplementary safety devices that act upon rapid changes in the magnitude of the output current, which may represent a rapid approach of the high-voltage electrode to a grounded object. That is the reality.

On the other hand, the control of the threshold at which the safety device should intervene is very complex. In fact, the output current under high voltage includes two components: (1) Ionic current generated by corona action between the electrode and the grounded object to be coated. A portion of this current charges the pulverized paint. This current increases as the potential gradient between the electrode and ground increases.

(2) Leakage current flowing from the spray nozzle to the paint reservoir in the paint container via the paint itself that is supplied from the container to the spray gun. This leakage current depends on the composition of the paint.

Control of the safety device is generally carried out using an atomizing spray gun in which no paint is present and therefore only with ionic current. For example, safety devices are generally controlled to intervene when the magnitude of the output current corresponds to a critical distance of 5 cm between the high voltage electrode and earth. Therefore, during work, if non-conductive paint is used, the safety device will operate at a distance of 5 cm, but if the paint used is conductive, it will operate at a correspondingly larger distance. The safety device will work. This distance further increases as the resistivity of the paint decreases. In the latter case, a situation may arise in which the high voltage electrode interrupts the high voltage even when it is at a normal operating distance of, for example, 15 cm. For this reason, when using certain conductive paints, special controls are implemented to avoid intervention of safety devices at very large electrode-to-earth distances. Such special control is a particularly delicate control determined by trial and error, and the DC high voltage must be maintained at its maximum value up to the above-mentioned critical distance.

<Summary of the invention> The essential object of the present invention is to eliminate the drawbacks of conventional high voltage generators and supply high DC voltage to electrostatic sprayers while ensuring great safety in terms of spark generation relative to ground. The object of the present invention is to provide a method and apparatus for doing so. It is also an object of the present invention to provide a safety device for applying high voltage to an electrostatic sprayer that is less expensive and more reliable than the active electronic devices used in the safety devices of previously known voltage generators. The objective is to provide equipment for supplying

In order to achieve the above object, the method according to the invention provides a DC low voltage with a value significantly larger than a predetermined value of the stabilized DC low voltage at no current (no load). supply from a power source, and in response to an increase in the magnitude of the current, the power source direct current low voltage is gradually increased from the substantially high value to a value substantially lower than the predetermined value of the stabilized direct current constant voltage. a predetermined value of the DC low voltage, thereby stabilizing the power supply DC low voltage when, upon a decrease in the power supply DC low voltage, the magnitude of the current reaches a predetermined threshold; The DC low voltage is stabilized at the predetermined value when the power supply DC low voltage is larger than the predetermined value, and the magnitude of the current is made to pass through a value approximately equal to the predetermined value. This feature is characterized in that when a threshold value is exceeded, the stabilization is released and the amount is reduced.

The present invention provides that, unlike what has been attempted with conventional voltage generators, the high voltage electrodes can be used on objects to be coated or This is based on the recognition that it is not necessary to maintain the high voltage at a constant maximum value when approaching all grounded objects. In fact, in conventional voltage generators that generate a constant maximum high voltage up to the critical distance mentioned above, the potential gradient is created when the high voltage electrode approaches the object to be coated while maintaining the distance greater than the critical distance. Although attempts have been made to increase the potential gradient rapidly, the inventor has determined that it is not necessary to increase the potential gradient very much and that a small increase in the potential gradient will ensure a good electrostatic effect for intermediate and small spray distances. It was confirmed that this was sufficient. In other words, the inventor has found a reasonable way to obtain a good electrostatic effect for intermediate and small spray distances without creating a breakdown gradient that would create a spark between the electrode and the ground. They confirmed that increasing the potential gradient (using the output current) is sufficient.

Therefore, the high voltage generator according to the invention supplies a DC high voltage with a constant maximum value up to the normal spraying distance, and when the magnitude of the output current increases, in other words, the high voltage electrode and the ground and is configured to supply gradually decreasing values of high voltage for intermediate and small spray distances as the distance between them decreases. From a safety point of view, the high voltage generator according to the invention has the advantage that the high voltage can be gradually reduced before the high voltage electrode reaches a distance to the ground object equal to the aforementioned critical distance. . Therefore, when a risk of sparking arises due to a very large and rapid approach of the high-voltage electrode to a grounded object, the voltage is already at a sufficiently low level to eliminate such a risk.

Furthermore, with the high-voltage generator according to the invention it is possible to work at normal and intermediate spraying distances with admittedly small but tolerable potential gradients, even when using electrically conductive paints. can. In contrast, known high-voltage generators of the type in which the safety device completely shuts off the high voltage cannot be operated under the same conditions. In the case of using very highly conductive paints, the threshold at which the high voltage is gradually reduced starts from a distance equal to the distance at which the high voltage begins to decrease when using a non-conductive paint. It can be controlled so that the voltage starts to decrease. In this case, in any case, the control of the threshold value is not as delicate or precise as in the case of conventionally known high voltage generators. This is because this threshold or adjustment is
This is because, whereas conventional spraying has been carried out in correspondence with a critical spraying distance where the probability of spark generation is high, spraying is carried out in correspondence with a normal spraying distance.

The high voltage generator according to the present invention includes a DC low voltage power supply circuit, a voltage stabilization circuit connected to the output end of the DC low voltage power supply circuit, and a voltage stabilization circuit connected to the output end of the voltage stabilization circuit. A high voltage generator supplies a high DC voltage approximately proportional to the output voltage of the circuit, and reduces the output voltage of a voltage stabilization circuit when the magnitude of the current reaches a predetermined threshold and then the high voltage generator output It has means for reducing the DC voltage. According to the invention, the low voltage power supply circuit provides a stabilized direct current low voltage normally generated by the voltage stabilization circuit when the current magnitude becomes smaller than a predetermined threshold during no-load conditions. is configured to supply a low DC voltage having a value significantly higher than the value of . Further, the means are configured to gradually reduce the low voltage supplied to the stabilization circuit from the substantially high value to a value substantially lower than the value of the stabilized DC low voltage. Consisting of passive circuit elements or elements installed in the voltage supply circuit, so that when the low voltage decreases, when the magnitude of the current reaches the above-mentioned predetermined threshold, a stabilized DC low voltage is generated. The low voltage passes through a value approximately equal to the value of . Note that the predetermined threshold value corresponds to a predetermined operating distance of the electrostatic spray device with respect to a grounded object.

As will be explained later, the passive elements described above are constructed using simple resistors that are much cheaper and have much higher reliability than the active electronic safety devices used in conventional voltage generators. Can be configured.

DESCRIPTION OF THE EMBODIMENTS Other features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, which are illustrated in the accompanying drawings.

The voltage generator shown in FIG. 1 is equipped with a transformer 1 similar to known ones, and its primary side 2 is, for example,
Connected to a low-voltage AC power source with a commercial frequency such as 220 volts, 50 Hz. The alternating current voltage V ₁ obtained at the terminals of the secondary winding 3 of the transformer 1 is rectified by a rectifier circuit constituted by a device consisting of a diode 4, for example, and then by a smoothing capacitor 5.
filtered by The DC voltage V ₂ obtained at the terminals of the capacitor 5 is applied to the input end of the voltage stabilization circuit 6 . The voltage stabilizing circuit 6 has at its output terminal,
Generates a stabilized DC low voltage V ₄ of eg 10 volts. This voltage V ₄ is applied to the input of a high voltage generator 7, which at its output outputs a voltage proportional to the voltage V ₄ , e.g.
Generates a DC high voltage _VHT of 120000V (volts).
The output terminal of the high voltage generator 7 has a very large value (several
It is connected via a resistor 8 (10 megohms) to a high voltage electrode 9 of a comminution device, such as an atomizing gun (not shown). As is known, the resistor 8 and the high-voltage generator 7 can be housed within the atomizing spray gun, while the other elements 1 to 6 of the voltage generator can be spaced apart from the atomizing spray gun. and connected to the spray gun by a low voltage cable. FIG. 1 schematically shows an object 10 to be painted or coated with powder, which object 10 is grounded.

All the elements mentioned above are well known and therefore need not be explained in detail. To briefly describe the high voltage generator 7, this generator 7 has a high frequency (several 10 K)
Hz) oscillator, and a transformer for increasing the output voltage of the high frequency oscillator to a voltage intermediate to the high frequency;
Advantageously, it consists of a summation rectifier or voltage multiplier of the type comprising a rectifier and a capacitor in order to rectify the high-frequency intermediate voltage supplied by the transformer and increase it to the desired high DC voltage _VHT . It is. Also, as is well known, the use of high frequencies makes it possible to reduce the capacitance of the voltage multiplier capacitor, thereby making it possible to reduce the size of the capacitor and accommodate it in the atomizing spray gun. becomes possible. This miniaturization also makes it possible to reduce the internal capacitance of the high-voltage generator and reduce the capacitive energy stored in it, which is particularly important for potential grounding. This is important for ensuring safety in the event of a short circuit.

According to the present invention, the secondary winding 3 of the transformer 1 receives the no-load (i.e., no current) DC low voltage V ₂ obtained after rectification by the diode 4 and smoothing by the capacitor 5 through the voltage stabilizing circuit 6. It is designed to supply an alternating current low voltage V ₁ of such a value that it can have a value considerably higher than the value of the stabilized direct current low voltage V ₄ that is normally generated. for example,
If a stable voltage of 10 volts is to be supplied by the voltage stabilizing circuit 6, the secondary winding 3 of the transformer 1 is designed in such a way that the resulting no-load voltage V ₂ is equal to 24 volts. can do.

Further according to the invention, resistor 11 is connected in series between diode 4 and voltage stabilization circuit 6. This resistor 11 changes the low voltage V ₃ supplied to the voltage stabilizing circuit 6 from the value V ₂ to a stabilized DC low voltage (10V) as the magnitude of the current increases.
It has the effect of gradually decreasing to a value considerably lower than the value of . The value of the resistor 11 is such that the voltage V ₃ is approximately equal to the value of the stabilized DC low voltage V ₄ (voltage stabilization _circuit 6 ΔV plus the voltage drop within ΔV) is chosen to pass when decreasing. The threshold value I _S corresponds to the normal usage or operating interval of the electrostatic atomization device on grounded objects. For example, with a voltage drop ΔV of approximately 3 volts in the voltage stabilization circuit 6, corresponding to the typical voltage drop in commonly used stabilization circuits,
The threshold value I _S corresponds to a distance of approximately 10 cm between the high voltage electrode 9 and the object to be painted 10, and when no paint is present (second
As shown in the figure), the resistance of resistor 11 would be given a value of 22 ohms.

Under practical operating conditions with paints that are more or less conductive, leakage currents flow through the paint to earth. This current flows between the electrode 9 and the object 1
It is added to the ionic current between 0 and 0. the result,
The threshold value _IS will in practice correspond to a distance greater than 10 cm, and the value of this distance or spacing can vary depending on the conductivity of the paint. However, in most practical cases this is not important. This is because when the magnitude of the current exceeds the value of the threshold I _S , the voltage generator of the present invention supplies a high DC voltage with a value sufficient to obtain a sufficient potential gradient for painting. be. Similarly, in limited cases, for example when using paints with very high conductivities, the threshold I _S is about 10 cm and about 30 cm.
It is always possible to control the threshold I _S in such a way that it corresponds to an electrode-earth distance lying between cm. This can be done, for example, by changing the value of the voltage V ₁ and therefore also the value of V ₂ using, for example, a transformer with a multi-tapped secondary winding and/or a resistor 1
This can be achieved by controlling the slope of the voltage line V ₃ (FIG. 2) by changing the value of 1.

Similarly, when the electrode 9 is located away from the object 10 at a distance (10 cm or more) slightly greater than the distance corresponding to the threshold value _IS , the voltage V ₃ applied to the voltage stabilizing circuit 6 is The voltage value (in this embodiment, 13 volts) required for the stabilization circuit 6 to operate correctly remains at a value exceeding that value, so the output voltage V 4 of the stabilization circuit 6 remains constant, and in this embodiment, the output voltage V ₄ of the stabilization circuit 6 remains constant. remains equal to 10 volts. Further, the DC high voltage V _HT which is proportional to the voltage V ₄ is also constant. Conversely, the distance between the electrode 9 and the object 10 is the threshold I _S
When _the distance corresponding _to As the magnitude of the current increases, it decreases linearly from the point where there is a difference of ΔV with respect to the voltage _V3 . The DC high voltage V _HT , which is proportional to the voltage V ₄ , decreases as well. This high voltage V _HT
The decrease occurs before the electrode 9 reaches a critical distance (approximately 5 cm) at which a short circuit between the electrode 9 and the object 10 can occur. When the electrode 9 reaches the critical distance, the high DC voltage V _HT has reached a sufficiently low value that a possible short circuit or grounding is no longer dangerous.

Even if the threshold I _S is exceeded and the current high voltage V _HT is reduced, the potential gradient remains at a value sufficient for painting.
This is because the reduction in the high voltage _VHT is compensated by the reduction in the distance between the electrode 9 and the object 10.

From a safety point of view, it is advantageous to set a sufficiently large voltage difference between the voltage V ₁ and therefore also the voltage V ₂ and the stable voltage V ₄ . In fact, in this case a resistor 1 of an appropriate value is used to ensure a sufficiently rapid drop of the voltage V ₃ (linear slope V ₃ in FIG. 2).
1 can be used, thereby ensuring a sufficiently rapid drop of the voltage V ₄ and therefore also of the high DC voltage V _HT when the magnitude of the current exceeds the threshold _IS . On the contrary, if the potential gradient has a value that is sufficiently higher than the threshold value _IS , the voltage V ₃ and therefore also the voltage V ₄ and the high DC voltage V _HT do not have to decrease in a very rapid manner. The selection of the value of the voltage V ₂ as well as the value of the resistor 11 is therefore made as a compromise between the two mentioned above, and the desired threshold I _S and therefore also the gradual reduction of the DC high voltage V _HT . This will depend on the value of the distance used as the starting point.

The embodiments of the invention described above are given by way of example only and are not intended to be limiting in any way, and numerous modifications will readily occur to those skilled in the art without departing from the scope of the invention. Needless to say, it is possible. For example, instead of providing the resistor 11 in series between the diode 4 and the voltage stabilizing circuit 6,
The secondary winding 3 of the transformer 1 is shown as a dotted line in FIG.
It is also possible to provide the diode 4 in series between the diode 4 and the diode 4. Alternatively, the resistor 11 could be omitted and a saturated iron core type transformer 1 having the characteristic of supplying a voltage inversely proportional to the generated current could be used. In this case, the voltage V ₁ changes in the same way as the voltage V ₃ .
Furthermore, in the voltage generator shown in FIG.
It would also be possible to replace the transformer 1 and the diode 4 by a 24 volt accumulator, for example.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram schematically showing a high voltage generator for an electrostatic spraying device to which the present invention can be applied, and FIG. 2 is a circuit diagram for explaining the operation of the voltage generator shown in FIG. FIG. 1...Transformer, 2...Primary winding, 3...Secondary winding, 4...Diode, 5...Capacitor, 6...
...Voltage stabilization circuit, 7...High voltage generator, 8,1
1...Resistor, 9...High voltage electrode, 10...Object to be painted.

Claims (1)

[Claims] 1. In order to supply a high voltage to an electrostatic spray device, a DC low voltage is supplied, the DC low voltage is stabilized at a predetermined value, and the magnitude of the current is The DC low voltage is set to a value lower than a predetermined threshold value corresponding to a predetermined distance between the electrospray device and a grounded object, and when the magnitude of the current exceeds the threshold value, the DC low voltage and converting said DC low voltage to a DC high voltage approximately proportional to said DC low voltage, said method for supplying a high voltage to an electrostatic spray device, said supplying a DC power supply low voltage having a value significantly higher than a predetermined value of the stabilized DC low voltage at a time) and increasing said DC power supply low voltage in response to an increase in current strength; From a fairly high value,
gradually reducing the stabilized DC power supply low voltage to a value significantly lower than a predetermined value, and when the DC power supply low voltage decreases, the magnitude of the current reaches the predetermined threshold; The DC power supply low voltage passes through a value approximately equal to a predetermined value of the stabilized DC low voltage, and when the value of the DC power supply low voltage is greater than the predetermined value, The DC low voltage is stabilized at a predetermined value, and when the magnitude of the current exceeds the predetermined threshold, the DC low voltage is released from stabilization and reduced. A method of supplying high voltage to electrostatic spray equipment. 2. In order to supply high voltage to the electrostatic spraying device, power supply circuits 1 and 4 that supply low voltage DC, a voltage stabilization circuit 6 connected to the output end of the low voltage DC power supply circuit, and a high voltage generator 7 connected to the output end of the voltage stabilizing circuit 6 for supplying a direct current high voltage approximately proportional to the voltage of the voltage stabilizing circuit; , means 11 for reducing the output voltage of the voltage stabilizing circuit 6 and therefore also the output DC high voltage of the high voltage generator 7, wherein the DC low voltage power supply circuits 1, 4 have a current DC low voltage having a value V 2 at no load which is considerably higher than the stabilized DC low voltage V ₄ normally supplied by said voltage stabilization circuit ₆ when the magnitude of is smaller than its predetermined threshold I _S , and said means 11 are configured to reduce the low voltage V ₃ supplied to said stabilizing circuit 6 from said considerably high value V ₂ to a value considerably greater than said value V4 of the stabilized low voltage. The low voltage V 3 is constituted by passive circuit elements provided in the DC low voltage power supply circuits 1 and 4 in such a way that the low voltage V ₃ is gradually decreased to a low value.
Upon its decrease, the magnitude of the current passes through a value (different by ΔV) approximately equal to the value of the stabilized DC low voltage when it reaches the predetermined threshold I _S and reaches the predetermined threshold I S I _S is a device for supplying a high voltage to an electrostatic spray device, characterized in that it corresponds to a predetermined operating distance of the electrostatic spray device with respect to a grounded object. 3. A DC low voltage power supply circuit includes an AC low voltage power supply, a transformer 1, and a rectifier circuit 4, and the secondary winding 3 of the transformer 1 is connected to the output of the rectifier circuit 4. supplying a high voltage to an electrostatic spraying device according to claim 2, which is configured to supply an alternating current low voltage having a value V ₁ such that the direct current low voltage applied has said large value V ₂ ; equipment for. 4. For supplying high voltage to the electrostatic spraying device according to claim 3, wherein the passive circuit element 11 is a resistor installed in series between the rectifier circuit 4 and the voltage stabilization circuit 6. equipment. 5. For supplying high voltage to the electrostatic spraying device according to claim 3, wherein the passive circuit element 11 is a resistor connected in series between the secondary winding 3 and the rectifier circuit 4. equipment. 6. A device for supplying high voltage to an electrostatic spray device according to claim 3, wherein the passive circuit element 11 is constituted by a transformer 1 that is a saturated iron core type transformer.