EP0135424B1 - Device for the fast discharge, without an arc, of the capacity of a high tension source - Google Patents

Description

The invention relates to a device for the rapid discharge, without arc, of a direct high voltage source, the capacitance of which, coupled to the impedance of a use circuit, has an excessive value, device whereby two electrodes , connected respectively to the two poles of the source, at least one is projected towards the other by a triggering means following a path of approach until coming into contact, (see FR-A 1 137824).

The present invention has emerged in the context of electrostatic spraying, where a jet of sprayed material is projected towards a part to be covered, the jet being electrostatically charged at a few tens of kilovolts.

For obvious safety reasons, the electrodes that charge the jet must be discharged after the sprayer has stopped, without an arc blowing, because the sprayed material is quite generally flammable, containing organic solvents.

Different processes are known to be usable at the discharge of the source: the corona discharge from the high voltage electrodes themselves, and the discharge by short circuit between the terminals of the source, or through a leakage resistor permanently connected to the source.

Patent document FR-A-1,137,824 describes a circuit-breaker device for the short-circuit discharge of the capacity of a coaxial cable connecting a high voltage source of around 40 kV to the electrode of charge of the jet of an electrostatic spray gun. The circuit breaker has a pneumatic control with a piston which moves, driving an axial rod, in a cylinder between two positions. In a first position one end of the rod is in abutment on a ground contact; in the second position the other end of the rod is in abutment on an output terminal of the high voltage source. The cylinders, piston and rod are connected to the core of the coaxial cable for connection to the charging electrode. The operation of the piston is controlled by the gun control so that the spray is loaded, and that the capacity of the cable is discharged to the ground when the spraying is stopped. The charge energy of the cable, which is between approximately 0.01 and 0.1 joule, is suddenly dissipated, on discharge, in an energy spark.

In some cases the charge of the spray of sprayed material is done by means of corona "emanations emitted by charging electrodes which have the shape of needles. Typically a needle of 10 mm in length, and a few tenths of a millimeter diameter emits, in the direction of a flat conductor (ground plane) 10 cm apart, a current of about 60 µA when the potential difference between needle and plane is 60 kV. This corresponds to an apparent resistance of 1 Gil During the discharge, if the needle is gradually brought closer to the ground plane, the apparent discharge resistance remains of the same order of magnitude.

The needle's own capacity with respect to the ground conductor plane at 10 cm is approximately 1 pf, which would determine a time constant of the order of a millisecond; it would be very acceptable.

However, when, as in the device described in FR-A-1 137824, the capacity to discharge has a significantly higher value, for example of the order of 100 pf, the discharge time increases accordingly. Furthermore, when the charging electrode does not produce an effluvium by corona effect, or ceases to produce it before the capacity is discharged, the discharge time can reach several seconds, which is unacceptable, and saw that the free short circuit discharge caused an energy spark.

One could have thought of using the corona created by corona effect to discharge without capacity the capacity of high voltage sources. We know, for example, from patent document US-A-3 935 508, an apparatus capable of discharging static electricity accumulated in the body of an operator in a dry atmosphere, by friction on highly insulating surfaces, without the operator feels an unpleasant jolt, and that a spark produces parasites capable of disturbing the operation of delicate electronic machines. The device essentially comprises in a bellows insulating body, a tapered tip connected to a conductive button at the top of the bellows, for the support of the operator's finger, and opposite a plate connected to ground by a resistance. When the operator places his finger on the key, a corona effluvium occurs at the end of the tapered tip, and discharges the capacity formed by the operator's body. The deformation of the bellows brings the tip of the wafer closer, so that the scent is maintained until the operator has fully discharged, with the tip touching the wafer. The resistance which connects the wafer to ground is determined to prevent the scent from degenerating into a spark.

If we analyze the operation of this device, we see that it is unable to solve the problem posed by the spark-free discharge of the capacity of a high voltage source.

Indeed, in the vicinity of a point, the electric field, at a given voltage, is not inversely proportional to the distance which separates the point from an electrically antagonistic surface. In order for a tip not to be a source of corona fragrance at the voltages currently used in electrostatic spraying, it would be necessary for the stroke of this tip for discharging the high voltage source capacity to be prohibitively long.

It will be noted moreover that the question does not arise in the application object of the patent of the United States, because to the isolation between point and plate comes to be added, when the discharge is not sought, isolation in fact between the operator's finger and the button connected to the tip. Furthermore, in the application provided for by the United States patent, it would be advantageous for a scent to arise as soon as the operator's tension with respect to the mass reaches a troublesome value.

It is in view of the problem which has just been exposed that the invention proposes a device for the rapid discharge, without arc, of a direct high voltage source, the capacity of which, coupled with the impedance of a circuit d 'use, presents an excessive value, device where of two electrodes connected respectively to the two poles of the source, at least one is projected towards the other by a triggering means following a path of approach until coming into contact, characterized in that the electrodes in a dielectric gas are in the form of domes centered on the trajectory, at least one of them being equipped with an erectile needle adapted to protrude out of the dome in the direction of the trajectory during approximation, so that a scent spurts from the needle into the gas and discharges a substantial part of the charge of the capacity, before contact.

While the general domed shape of the electrodes makes it possible to arrange them, in the dielectric gas, at a sufficiently small distance so that the trajectory to contact is traversed quickly without, however, emanations emanating from the electrodes in the gas, the protrusion of the erectile needle promotes, during the approximation, the emission of scent with sufficient intensity. The source discharge time constant can thus be short enough. The reduction in the distance between electrodes on standby, authorized by their shape, is favorable to efficient confinement of these electrodes in the dielectric gas.

Preferably, the electrodes, each equipped with an erectile needle, are produced in the form of pistons capable of sliding in an insulating cylinder, a pipe of pressurized gas, which the triggering means releases into the atmosphere, opening into the middle zone. of the cylinder between the piston-electrodes, so that the gas pressure keeps the electrodes apart and their needles retracted against return means.

Thanks to this arrangement, the electrodes are kept apart by the gas under pressure. The disruptive field in the gas increasing with the pressure, one obtains a suitable insulation of the electrodes under tension in spite of a relatively small variation. The venting of the pipeline which brings the gas under pressure, not only causes the needles to be erected and the electrodes to be thrown towards each other, but also causes the pressure of the gas to drop between the electrodes, this which is favorable to the emission of scents.

Preferably, the piston-electrodes are equipped with auxiliary internal pistons integral with the needles, on which the return means act, vents being formed in the auxiliary pistons and piston-electrodes for adjusting the pressure between piston-electrodes and auxiliaries, and their relative displacements, and to allow the circulation of gas under pressure in order to avoid the accumulation of ions in the space between the electrodes.

The return means may be metal springs compressed between the auxiliary pistons and end cylinders, pierced with a vent and traversed by a connection connecting to a pole of the source. They can also be formed by pressurizing the spaces between cylinder bottoms and auxiliary pistons, by means of a triggering device constituted by a four-way valve. Pressure variations at the rear of the auxiliary pistons protrude and retract the needles.

In a preferred arrangement, the needles have complementary ends, one being a tapered point, while the other has a blind hole with an end bevel. Thus one can obtain a frank contact between the needles, without the ends deteriorating.

• la figure 1 représente en coupe axiale un dispositif selon l'invention, en position d'attente ;
• la figure 2 représente le dispositif de la figure 1, en position de court-circuit ;
• la figure 3 est une vue perspective d'un pulvérisateur à bol tournant, équipé d'un dispositif selon l'invention ;
• la figure 4 représente, en coupe, une variante de dispositif ;
• la figure 5 représente un détail d'exécution des aiguilles érectiles.

The characteristics and advantages of the invention will moreover emerge from the description which follows, by way of example, with reference to the appended drawings in which:

• Figure 1 shows in axial section a device according to the invention, in the standby position;
• Figure 2 shows the device of Figure 1, in the short circuit position;
• Figure 3 is a perspective view of a rotating bowl sprayer, equipped with a device according to the invention;
• Figure 4 shows, in section, a variant of the device;
• Figure 5 shows a detail of the erection needles.

According to the embodiment chosen and shown in FIGS. 1 and 2, the device comprises a cylindrical body 1 made of insulating material, provided at its two ends with bottoms 10 and 11. The interior surface 1a of the cylinder 1 is precisely bored and partly comprises median a groove 1c c where an oblique channel 1b opens. which can be connected to a source of pressurized air.

In the cylinder 1 are mounted two metal pistons 2 and 3, the opposite ends of which 2a and 3a respectively are in the form of a dome in a spherical cap, with a base diameter less than that of the surface 1a of the cylinder 1. These domes 2a, 3a are connected to the lateral surface of the pistons 2, 3, by rounded shoulders 2d. 3d. O-rings 16, 17, engaged in grooves made in the surface 1a near the bottoms 10, 11, form rear stop for the stroke of the pistons 2, 3.

These pistons 2, 3 are internally equipped with auxiliary metal pistons respectively 4, 5 coaxial and stopped in the rearward race by rings 4b, 5b housed in the interior of the piston skirts 2, 3. The auxiliary pistons 4, 5 carry, at the center of their front face, needles 6, 7 which are engaged in wells 2b, 3b formed in the axis of the domes 2a, 3a, the tip of these needles 6, 7 being however set back from the domes 2a, 3a when the auxiliary pistons 4, 5 are in rear stop on the rings stop 4b, 5b. Metal springs 8, 9 are arranged between the bottoms 10, 11 and the rear face of the auxiliary pistons 4, 5 respectively, these springs 8, 9 being capable of relaxing until bringing the ends of the needles 6, 7 into contact, as shown in figure 2.

These springs 8, 9 which make contact with the auxiliary pistons 4 and 5, are connected to bushings 12 and 13, which pass through the bottoms 10 and 11, in order to allow the junction of the piston-electrodes at the two terminals of a high voltage source, not shown.

Vents 2c, 3c were made through the pistons 2, 3, whose domes 2a, 3a form electrodes, 4a, 5a, through the auxiliary pistons 4, 5, and 14.15 through the bottoms 10, 11, respectively.

When the channel 1b is connected to a source of compressed air, under a pressure of 3 to 6 bars, the air pressure will force the pistons 2 and 3 towards the bottoms 10, 11 of the cylinder 1, until the support on the O-rings 16, 17. Furthermore, the air passing through the vents 2c, 4a, 14 on one side, and 3c, 5a and 15 on the other gradually relaxes, so that the pressures are staged from the atmosphere, beyond the vents 14, 15 to the pressure in the central region of the cylinder, below the vents 2c, 3c, in the areas between the bottoms 10, 11 and the auxiliary pistons 4, 5, and between the auxiliary pistons 4, 5 and the electrode pistons 2, 3. In practice, the vent diameters are adjusted so that the consumption of compressed air is not excessive. In this way, as long as the source pressure remains applied to the central zone of the cylinder 1, not only the pistons 2, 3 are applied against the O-rings 16, 17, but also the auxiliary pistons 4, 5 are applied against their rings stop 4b, 5b, the springs 8, 9 being compressed. This ensures that, without excessive consumption of air, the space between the electrodes is sufficiently ventilated.

When, on the other hand, the trigger system releases the pipe which extends the channel 1b, the force of the springs 8, 9 being exerted on the rear of the auxiliary pistons 4, 5, the relative overpressure in the space between piston electrodes 2, 3 and auxiliary pistons rapidly disappearing, the air flowing through the vents 2c, 3c and 4a, 5a, the auxiliary pistons 4, 5 are pushed towards the domes 2a, 3a, and the needles 6, 7 protrude in front of the domes 2a, 3a. The sets of piston electrodes 2, 3 and auxiliary pistons 4, 5 are propelled towards one another under the expansion of the springs 8, 9, to which is added the residual pressure in the bottoms of cylinder 1. The needles 7, 8 come into contact according to the representation of FIG. 2.

After having exposed the mechanical operation, we will describe the electrical operation.

When the device is in the position of FIG. 1, with the central zone under pressure from 3 to 6 bars, the piston-electrodes 2, 3 in abutment on the O-rings 16, 17 and the needles 6, 7 retracted behind the front surface of the domes of electrodes 2a and 3a, the application of a high voltage between the bushings 12 and 13, and consequently between the electrodes 2 and 3 causes practically no leakage of current, due to the increased dielectric strength of the air under pressure, and of the rounded shape with a high radius of curvature of the electrodes, which avoids any concentration of the electrostatic field on the surface of the electrodes, and therefore any emission of emanation. It is known in fact that the concentration of the electrostatic field in the vicinity of projections with a very small radius of curvature causes the acceleration of free electrons, which causes the scent by ionization of the air.

On the other hand, when the triggering device releases the pipe which supplies the central zone of the cylinder 1 to the atmosphere, on the one hand the air pressure between the electrodes 2 and 3 drops, on the other hand the erectile needles 6 and 7 protrude, so that the electric field is concentrated locally in the vicinity of their tips; a relatively intense scent arises in the vicinity of the points of needles with passage of a current between them. Of course, it is preferable that the triggering device only comes into action after the power supply to the HV generator has been cut off, so that the current flowing between the needles 6, 7 comes solely from the discharge of the capacity of the high voltage electrode. As the pressure decreases, the needles get closer, and the smell remains until the needles come into contact, the tension gradually decreasing.

The shape of the erectile needles 6 and 7, according to the representation of FIG. 5, has been studied to allow the effective emission of effluvium, by reducing the radii of curvature of the terminal surfaces, while avoiding the matting of the tips during contact franc. As can be seen, the needle 6 has a cylindrical end portion 6a of reduced diameter, terminated by an acute cone 6b. For its part, the needle 7 ends in a hollow cone 7a, of the same angle at the apex substantially as the cone 6b, and connecting to the cylindrical surface of needle 7 by a bevel or tapered edge 7b. This hollow cone 7a is extended by a drill bit 7c, of diameter smaller than the diameter of the terminal part 6a of the needle 6. When the needles 6 and 7 come into contact the terminal part 6a of the needle 6 enters the hollow cone 7a, the contact occurring between the sides of the cones 6b and 7a. Thus the tip of the needle 6 and the edge of the needle 7 are protected from mechanical blasting.

FIG. 3 represents a sprayer 20 for electrostatic painting, equipped with a distributor member 20a rotating at high speed (a few hundred revolutions a second), commonly known as a bowl. This sprayer 20 is brought to a high voltage of a few tens of kilovolts (typically 80 kV) by the high voltage source 30 supplied with low voltage by the connection 31. This bowl 20a has an edge which constitutes a charging electrode for a sprayed liquid by centrifugal force.

A rapid discharge device 1 is mounted on the high voltage source 30, with a connection 12 connected to the sprayer 20 through the resistor 21 for limiting the discharge current, while the connection 13 is connected to the cylinder head 30b of the high source tension 30, cylinder head which is grounded. The rapid discharge device is supplied with pressurized air through line 1 and the pneumatic turbine of the sprayer 20 through line 32.

It will be appreciated that the rapid discharge device is well associated with a sprayer with a rotating distribution member, which has a relatively high capacity, due to the dimensions of the parts brought to high voltage and directly connected to the high voltage source.

The variant shown in Figure 4 uses air pressure to project the electrodes towards each other as well as to separate them. In the cylinder 101 are arranged two piston electrodes 102, 103 with a dome in a spherical cap 102a, 103a. Here the domes 102a, 103a are connected directly to the side wall of the pistons 102, 103, which facilitates machining but has the disadvantage that the device is more sensitive to the risk of discharges which creep on the internal wall of the cylinder. In the axis of the domes 102a, 103a are made wells 102b, 103b for erectile needles 106, 107 mounted on auxiliary pistons 104, 105. It will be noted that the auxiliary pistons 104, 105 are pierced with vents 104a, 105a, while the pistons 102, 103 are pierced with vents 102c and 103c.

Rods 104b, 105b limit rearward movement of the auxiliary pistons 104, 105, while rods 116, 117 limit the movement of the piston electrodes 102, 103. The cylinder bottoms 110, 111 do not have vents .

It will be noted that the needles 106, 107 have their ends formed like the needles 6, 7, as shown in FIG. 5. Furthermore, the auxiliary pistons 104, 105 are connected by flexible connections 108, 109 to current bushings in the bottoms 110, 111, for the connection to the terminals of the source to be discharged.

A pipe for pressurized gas 101 b opens into the middle zone of the cylinder 101, and two pipes 138, 139 open in the vicinity of the bottoms 110, 111 respectively.

A four-way reversing valve 120 is arranged to operate the device. A path 120a leads to the atmosphere, a path 120b is connected to a source of pressurized air, a path 120c feeds the pipe 101b, while the path 120d jointly feeds the pipes 138 and 139. A rotating bushel connects, in the arrangement shown, channel 120c to channel 120b, and channel 120d to channel 120a. By rotation of a quarter turn. bushel 121 will connect track 120c to track 120a, and track 120d to track 120b.

The arrangement shown corresponds to waiting, the high voltage source being in service. The air pressure is applied in the middle zone between the pistons 102 and 103. supported on the stop rings 116, 117. When the high voltage electrode must be discharged, the plug 121 turns a quarter of a turn. Pressure is applied to the rear of the groups of pistons 102, 104 and 103, 105. The latter are pushed forward and protrude the needles 106, 107, and the pistons 102 and 103 are pushed towards each other. 'other. The needles 106, 107 come into contact.

In addition, when thereafter, for the re-establishment of the high voltage, the plug 121 will again turn a quarter of a turn, putting the pipes 138 and 139 into the atmosphere and the pipe 101b under pressure, the chambers between pistons -electrodes 102, 103 and auxiliary pistons 104, 105 will be in overpressure relative to the ends of cylinder 101, and the auxiliary pistons 104, 105 will be pushed on the stop rods 104b, 105b, while the pistons 102, 103 are pushed back towards the cylinder bottoms 110, 111 by the pressure applied to the central region of the cylinder.

Of course, it is possible to provide an annular groove in the interior surface of the cylinder 101, at the outlet of the pipe 101b, to cut the lines of leakage on this interior surface. And it goes without saying for a person skilled in the art that the layout and the nature of the pipes 101b, 138 and 139 will have to be studied in order to provide leakage lines for high voltage, in order to avoid uncontrolled losses of current; and it may be necessary to choose, for the production of the cylinder, an insulating material with a lower resistivity than that of the best known insulators, to avoid that the surface regions charge erratically, and give rise to surface micro-discharges as precursors generalized landfills.

Of course, the invention is not limited to the examples described. In particular, one of the piston electrodes 2, 102 or 3, 103 could be replaced by a fixed dome-shaped electrode, this fixed electrode preferably being equipped with an erectile needle. It goes without saying that, if an electrode is fixed, the outlet of the central channel 1b, 101b would be disposed near the fixed electrode.

Claims (8)

1. Apparatus for the rapid discharge, without an arc, of a high dc voltage source whose capacitance, coupled to the impedance of a user circuit, is excessive in value, in which apparatus of two electrodes (2, 3) which are respectively connected to the two poles of the source, one at least is projected towards the other by a tripping means (120) along a trajectory providing for relative approach movement until contact occurs characterised in that the electrodes, in a dielectric gas, are in the form of domes (2a, 3a, 102a, 103a) centred on the trajectory, one at least thereof being provided with an erectile needle (6, 7, 106, 107) adapted to project out of the dome (2a, 3a, 102a, 103a) in the direction of the trajectory in the course of the approach movement, in such a way that a brush discharge issues from the needle (6, 7, 106, 107) in the gas and discharges a substantial part of the charge of the capacitance, before contact.

2. Apparatus according to claim 1 characterised in that the electrodes (2a, 3a, 102a, 103a) which are each provided with an erectile needle (6, 7, 106, 107) are in the form of pistons (2, 3 ; 102, 103) capable of sliding in an insulating cylinder (1, 101), a passage (1 b, 101 b) for compressed gas which communicates the tripping means (120) with atmosphere opening into the central zone of the cylinder (1, 101) between the piston electrodes (2, 3, 102, 103), in such a way that the gas pressure holds the electrodes (2a, 3a, 102a, 103a) apart and their needles (6, 7, 106, 107) retracted against return means.

3. Apparatus according to claim 2 characterised in that the piston electrodes (2, 3 ; 102 ; 103) are provided with auxiliary internal pistons (4, 5, 104, 105) which are fixed with respect to the needles (6, 7, 106, 107), on which internal pistons return means act, vents (4a, 5a, 104a, 105a) being provided in the auxiliary pistons (4, 5, 104, 105) and vents (2c, 3c ; 102c, 103c) being provided in the piston electrodes (2, 3 ; 102 ; 103) for adjustment of the pressure between the piston electrodes (2, 3 ; 102 ; 103) and the auxiliary pistons (4, 5, 104, 105) and their relative displacements.

4. Apparatus according to claim 3 characterised in that the return means are metal springs (8, 9) which are compressed between the auxiliary pistons (4, 5) and end plate portions (10, 11) of the cylinder (1), which are apertured with a vent (14, 15) and through which passes a connecting means (12, 13) for providing a connection between a source Pole and a respective spring (8, 9).

5. Apparatus according to claim 3 characterised in that the cylinder (101) is provided with passages (108, 109) opening into the terminal zones, the tripping means (120) being adapted in the ready condition to connect to a compressed gas source (120b) the passage (101 b) which opens in the central zone, and the passages (138, 139) which open in the terminal zones to atmosphere (120a) and, in the triggering mode, to connect said passages (138, 139) to the compressed gas source (120b) while the passage (101b) which opens in the central zone is connected to atmosphere (120a).

6. Apparatus according to any one of claims 2 to 5 characterised in that the electrode domes (2a, 3a) which are smaller in diameter than the diameter of the cylinder (1) are connected to the piston electrodes (2, 3) by a rounded shoulder (2d, 3d).

7. Apparatus according to any one of claims 2 to 6 characterised in that the needles (6, 7) have complementary ends (6a, 7a), one end (6a) being in the form of a tapered point (6b) while the other comprises a hollow cone (7a) terminated by a sharp end edge (7b).

8. Apparatus according to any one of claims 2 to 7 characterised in that an annular groove (1c) is provided in the internal surface (1a) of the cylinder (1), at the outlet of the compressed air passage (1 b).

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