FR2535917A1 - METHOD FOR SUPPLYING CONTINUOUS HIGH VOLTAGE TO AN ELECTROSTATIC SPRAYING APPARATUS AND DEVICE FOR CARRYING OUT THE METHOD - Google Patents

Abstract

IN THIS DEVICE, THE LOW VOLTAGE 1, 4 SUPPLY IS SIZED SO AS TO PROVIDE A CONTINUOUS LOW VOLTAGE WHICH HAS, IN THE ABSENCE OF CURRENT, A VALUE V SENSITIVELY HIGHER THAN THAT OF THE STABILIZED CONTINUOUS LOW VOLTAGE V NORMALLY DELIVERED THE VOLTAGE STABILIZER CIRCUIT 6 WHEN THE CURRENT INTENSITY IS LESS THAN A THRESHOLD VALUE I; A PASSIVE CIRCUIT ELEMENT 11 IS MOUNTED UPSTREAM OF CIRCUIT 6 SO AS TO GRADUALLY DECREASE LOW VOLTAGE V SUPPLIED TO CIRCUIT 6 IN SUCH A WAY THAT LOW VOLTAGE V PASSES, DECREASING, BY A VALUE SENIABLY EQUAL TO THE VALUE OF LOW STABILIZED CONTINUOUS VOLTAGE V WHEN THE CURRENT INTENSITY REACHES THE SAID THRESHOLD VALUE I, THIS LAST CORRESPONDING TO A NORMAL OPERATING DISTANCE OF THE ELECTROSTATIC SPRAYING EQUIPMENT FROM AN OBJECT TO EARTH.

Description

The present invention relates to a method for supplying high voltage

  an electrostatic spraying device from a continuous low voltage supply and a high voltage generator supplying a continuous high voltage substantially proportional to the continuous low voltage supplied by said low voltage supply, the latter having a current-voltage characteristic such that the voltage decreases when the intensity of the current exceeds a preset threshold value-i 7 The invention also relates to a device for setting

implementing this process.

  The electrostatic painting or powdering process

  is now well known he uses the action of an electric field

  intense stick on a droplet of paint or a solid particle carrying an electric charge to ensure the transport and the deposit of the droplet or the particle on the object

to paint or powder.

  In electrostatic spraying installations

  (paint or electrostatic powdering), the high voltage

  required to obtain the electric field is obtained

  by means of a generator which usually includes a supply

  stabilized mentation delivering a low voltage direct current at constant voltage and a properly high voltage generator

  says that converts the low continuous voltage delivered by the ali-

  ment stabilized in a continuous high voltage of the

  dre de 40 1 _ 20 Wk V Typically, the high voltage generator itself includes a high frequency oscillator (a few tens of k Hz), a high frequency transformer which raises the output voltage of the high frequency oscillator

  at a medium frequency high frequency, and an adder-

  voltage rectifier or multiplier, of the "Villard" chain type, which rectifies and raises the voltage obtained across the secondary terminals of the high frequency transformer to provide the desired high DC voltage as output The output of the high voltage generator is generally connected by in-

  through a very high value resistance to an election

  charging station which is located next to the spray nozzle

  paint or powder of the spraying device,

  for example a spray gun, for charging electric

  only paint droplets or powder particles emitted from the nozzle and to produce the electric field

  for transporting said droplets or said particles

  from the nozzle to the object to be covered with paint or

powder.

  In addition, a security device is usually.

  mounted in the generator low voltage circuit in order to provo-

  quer the fall of the high voltage when the current delivered by

  the generator becomes too large. Such a device for

  curity is necessary to avoid striking an electric arc

  dangerous between the high voltage electrode and the mass (object to be painted or covered with powder, or any other grounded object) Indeed, in the case where the spraying device

  is used in a flammable or explosive atmosphere (

  air and paint mist, with or without solvent vapors) striking an arc could cause ignition

  or the explosion of said atmosphere Likewise, there is reason to evi-

  ter that an ignition could occur between the high voltage electrode and a worker who could otherwise suffer a serious

electrical shock.

  The only way to detect the imminence of a strike is to detect an abnormal increase in the intensity of the current delivered at high voltage. Indeed, an increase in the intensity of the current delivered indicates that the high voltage electrode is approaching. dangerously from an object to ground and that, therefore, it is necessary to reduce or cancel the high voltage This information is generally taken from the return current from ground to the generator It is also possible to take this information from the low voltage power supply which is roughly proportional or at least a function

  increasing current output at high voltage.

  Until now, the manufacturers of high voltage generators for electrostatic spraying have endeavored to produce generators having a current-voltage characteristic of rectangular shape, that is to say that the voltage remains substantially constant as long as the intensity of the current does not exceed a preset threshold value, then drops abruptly beyond said threshold value In this regard, reference may be made to French patents n 1,568,060 (page 1, left column, last paragraph), n 1,568,060 (page 2, right column, line 27) n 1,562,917 (page 1, column

  right, penultimate paragraph) and N O 2 036 794, the latter short-

  vet even flattering itself to provide an absolutely rectan diagram

  gular. In addition, it is obvious that the action of the electrostatic forces is all the more important as the gradient of the electric field potential is high Until now, it was thus considered that the maintenance at a high value of the high voltage ensured the ' maximum potential gradient and that this corresponded to the best use of possibilities

  of the high voltage generator The safety device

  should therefore be adjusted so as to act at the moment when the maintenance of the high voltage becomes dangerous, that is to say when the distance between the high voltage electrode and an object to ground becomes less than a critical distance beyond which the strike of an arc became highly probable This

  critical distance, which can vary depending on the value of the height

  the voltage used and depending on the type of spray gun

  sation used, is generally of the order of 5 cm, while the normal operating distance is generally between

and 30 cm.

  However, the use of safety devices

  as currently produced have two drawbacks

  The first drawback is that the device

  security is usually made up of an active electronic element.

4 2535917

  If he does not react or if he reacts badly to the information

  security is no longer guaranteed The se-

  cond disadvantage comes from the fact that high voltage is now

  naked at its maximum value until the intervention of the safety device Or, the high-voltage generator comprises, by construction, elements having a significant electrical capacity The energy stored in these capacities can feed a dangerous disruptive discharge if it is not evacuated very quickly at the time of the danger Or, current high-voltage generators-conserve this energy at its maximum value

  until the intervention of the safety device.

  If the discharge of these capacities by the high-voltage electrode is too slow, the ignition of an arc may take place despite the intervention of the safety device. The danger of ignition by discharge of the capacities is so real that has chosen to protect itself by various means, for example by inserting a very high value resistor immediately upstream of the high voltage electrode, by reducing as much as possible, by construction, the capacities of the high voltage generator and of the elements metal under high voltage, and / or by providing an additional safety device acting

  case of rapid variation of the intensity of the current delivered indi-

  as for the fast approach of the high voltage electrode compared to

wear to a grounded object.

  On the other hand, the adjustment of the threshold for which the device

  security must intervene is quite complicated Indeed, the current delivered at high voltage includes two components:

  1) The ion current delivered by crown effect between the elect

  trode and the object to be painted which is grounded Part of this

  current ensures the electrical charge of the sprayed paint.

  This current is increasing when the potential gradient between

the electrode and the mass increases.

  ) The leakage current flowing from the spray nozzle

  sation to ground in the paint tank through the paint itself which is brought from said tank to the spray gun This leakage current depends

  of the conductivity of the paint.

2535917

  The safety device is adjusted.

  in general, with the paintless spray gun,

  so with the ion current alone For example, we set gener-

  the safety device so that it intervenes when the intensity of the current delivered corresponds to the aforementioned critical distance of 5 cm between the high-voltage electrode and the ground. Thus, in service, with a non-conductive paint, the safety device works for a distance of 5 cm,

  but it will work for a greater distance 'if the paint-

  re used is conductive, this distance being all the greater that the resistivity of the paint is lower In the latter case, it may happen that the safety device cuts the high voltage while the high voltage electrode is at a distance still suitable for operation

  normal, for example 15 cm For some conductive paints

  these, we are therefore led to make special adjustments so as to prevent the safety device from intervening for too great electrode-ground distances.

  are particularly delicate since they must

  may be carried out by trial and error and the high continuous voltage must be maintained at its maximum value until the

critical distance mentioned above.

  The main object of the present invention is to

  remedy the disadvantages of generators previously.

  known by providing a method and a device for powering

  in high voltage continuous an electro-

  static The object of the present invention is also to provide a device for supplying high voltage to an electrostatic spraying device, including the safety device.

  is less expensive and more reliable than electronic devices

  safety assets of previously known generators.

  To this end, the method according to the present invention is

  characterized in that it consists in decreasing gradually-

  the voltage produced by the low voltage power supply as soon as.

  the intensity of the current reaches a heat-corresponding to a normal operating distance of the sprayer

6 2535917

  electrostatic with respect to a grounded object.

  The present invention is based on the remark that, if

  it is good to maintain the high voltage at a maximum value cons-

  aunt at normal operating distances of the spraying device, it is not necessary to keep it at its constant maximum value when the high-voltage electrode approaches the object to be painted or any other object to ground, contrary to what was sought with previously known generators Indeed, while with known generators delivering a high constant maximum voltage up to

  the aforementioned critical distanceg we were looking for an increase

  rapid tation of the potential gradient when the 13 high voltage electrode approaches the objects to be painted, while remaining at a greater distance than the aforementioned critical distance,

  the Applicant has found that it is not necessary to search for

  expensive such a large growth in the potential gradient and that a small growth in the gradient is sufficient to

  ensure a good electrostatic effect for medium and low

  bles spraying distances In other words, the Applicant has found that, in order to obtain a good electrostatic effect at medium and low spraying distances, it is sufficient for the potential gradient to increase reasonably (and with it the current output) without reaching the gradient disruptive for which a priming can take place between the electrode and

the mass.

  The generator of the present invention is therefore designed

  to supply a high continuous voltage having a maximum value

  male constant up to a normal spraying distance, then a high voltage whose value decreases progressively for medium and low spraying distances when the intensity of the current delivered increases, therefore when the distance -between the high voltage electrode and the mass decreases From the point of view of safety, the generator of the present invention has the advantage of ensuring a progressive decrease in the high

  voltage 'even before the high voltage electrode reaches a.

  distance from an object to earth equal to the critical distance mentioned above Thus, when the danger of an ignition could arise as a result of a raonrochn'en tt iç i orta-nt g rqpide of the high voltage electrode compared to a grounded object, the voltage is low enough that a possible ignition

is not dangerous.

  Furthermore, with the generator of the present invention, when a conductive paint is used, it is still possible

  work at normal and medium spraying distances

  tion with a potential gradient which is certainly lower, but which is still 'very acceptable,' - whereas, under the same conditions, we can no longer work with known generators whose safety device completely cuts the high voltage In the case where highly conductive paints are used, the intensity threshold beyond which the high voltage decreases progressively can also be adjusted so that the high voltage begins to decrease from an equal distance to that for which high voltage begins

  decrease when using a non-conductive paint.

  However, in this case, the setting of the intensity threshold is

  less delicate to carry out than in the case of generators

  laughing known since the setting corresponds to a normal spraying distance whereas before, it corresponded

  at the critical spraying distance below which

  priming was highly likely.

  The generator of the present invention comprises, so

  known, an alternative low voltage source, a rectifier circuit

  seur connected to said source, a voltage stabilizer circuit connected to the output of the rectifier circuit, a high voltage generator connected to the output of the voltage stabilizer circuit to provide a high DC voltage substantially proportional to the output voltage of the voltage stabilizer circuit,

  and a means for decreasing the output voltage of the circuit

  cooked voltage stabilizer and therefore high voltage

  continuous high voltage generator output, when the intensity

  sity of the current reaches a preset threshold value According to the invention, said source is dimensioned to provide a low AC voltage which has a value such that the low DC voltage obtained at the output of the rectifier circuit in the absence of ' current has a value substantially higher than that of the stabilized continuous low voltage which is normally delivered by the voltage stabilizer circuit when the current intensity is less than said preset threshold value. In addition, said means is constituted by an element of passive circuit, which is mounted upstream of the voltage stabilizer circuit so as to decrease

  gradually the low voltage supplied to the stabilization circuit

  tor depỉs said higher value up to a sensitive value

  considerably lower than the said value of the low voltage conti-

  naked stabilized, so that the low voltage passes, decreasing, by a value substantially equal to the value of the low voltage continuous-stabilized when the intensity of the

  current reaches said preset threshold value, the latter

  re corresponding to a normal operating distance of the electrostatic spraying device from a

grounded object.

  As will be seen later, said passive element can be constituted by a simple resistor - which is much less expensive and much more reliable than the active electronic security devices which were usually

  included in previously known generators.

  Other 'features and advantages of this

  invention will appear at 7 u during the description which follows

  of a preferred embodiment of the present invention,

  given as an example purely indicative and in no way limited

  , with reference to the appended drawings in which: FIG. 1 shows the diagram of a high voltage generator for an electrostatic spraying apparatus, in

  which the present invention is implemented.

  Figure 2 is a diagram to explain the

  operation of the generator of figure 1.

  The generator of FIG. 1 comprises, in a known manner, a transformer l, the primary of which is connected to a low-voltage alternating current source at industrial frequency, for example the sector 220 V, 50 Hz The alternating voltage V 1

  available at the terminals of the secondary winding 3 of the transformer

  matrix i is rectified by a rectifier circuit, constituted for example by a di'sp Qsiti% to di 4 pdeg 4, p'is filtered by a filtering capacitor 5 The DC voltage V 2 available at the terminals of the

  capacitor 5 is applied to the input of a stabilization circuit

  voltage generator 6 which delivers on its output a stabilized low continuous voltage V 4 of for example 10 Ve The voltage V 4 is applied to the input of a high voltage generator 7, which delivers on its output another continuous voltage VHT, for example 40,000 to 12,000 V, which is proportional to the voltage V 4 o The output of the high voltage generator 7 is connected by

  through a resistor 8 of very high value (more

  tens of megohms) at the high voltage electrode 9

  a spraying device, for example a spray gun

  rization (not shown) As is known, the resistor 8 and the high voltage generator 7 can be housed in the spray gun, while the other elements

  1 to 6 of the generator are located away from the spray gun

  connection and are connected to it by a low tension cable.

  sion In Figure 1 i is also shown schematically-

  an object 10 to be painted or covered with powder, which is

to ground.

  All the elements which have been described above are well known and it is therefore not considered useful to describe them in detail. It will simply be noted that the high voltage generator 7 can advantageously include a high frequency oscillator (a few tens of k Hz ), a high frequency transformer to raise the output voltage of the high frequency oscillator

  at a medium frequency high frequency, and an adder-

  rectifier or voltage multiplier, of the rectifier type and

  with capacitors to rectify the medium voltage high fre-

  quence supplied by the transformer and to raise it to the desired high DC voltage VHTO As is well known,

  the use of a high frequency reduces the capacity

  cited capacitors of the voltage multiplier and, consequently, to miniaturize the latter, which allows it to be housed in the spray gun This miniaturization also makes it possible to reduce the internal electrical capacity of the high-voltage generator and consequently, reduce the capacitive energy

  stored in said capacity, which is particularly important

  both for safety in the event of a grounding.

  According to the present invention, the secondary winding

  daire 3 of the transformer, 1 is dimensioned to provide an AC low voltage VI having a value such that the

  DC low voltage V 2 obtained 'at no load, that is to say in the ab-

  sence of current, after rectification by diode 4 and filtering by capacitor 5, has a value significantly higher than that of the stabilized low-voltage continuous Vs which is normally delivered by the voltage stabilizer circuit 6 For example, if the stabilizer circuit of voltage 6 is

  designed to deliver a stabilized voltage of 10 V, the winding

  secondary ment 3 of transformer 1 can be dimensioned so that the voltage V 2 obtained when empty is equal to 24 V.

  Furthermore, in accordance with the present invention, a re-

  resistor 11 is connected in series between diode 4 and the voltage stabilizer circuit 6 The resistor 11 has the effect of gradually decreasing the low voltage V 3 supplied to the stabilizer circuit 6 from the value V 2 to a

  value significantly lower than the value of the low ten-

  stabilized continuous voltage (10 V) when the intensity of the current increases The value of the resistance is chosen so that the voltage V 3 decreases by a value

  appreciably equal to the value of stable low voltage

  read VA (at the voltage drop Av near in the stabilizing circuit

  voltage sator 6) when the current intensity reaches a preset threshold value Is, the latter corresponding to

  normal operating distance of the pulverizer

  electrostatic verification with respect to a grounded object.

  For example, with a voltage drop LÄN of around 3 V in the voltage stabilizer circuit 6, which corresponds to a typical voltage drop in the stabilizer circuits usually used, if the threshold Is is chosen at 0.5 A in l 'no paint (as shown in figure 2),

  which corresponds to a distance of about 10 cm between the elect

  high voltage trode 9 and object 10 to be painted, resistance

It will have a value of 22 ohms.

  In real operating conditions, that is

  say with a more or less conductive paint, a leakage current flows to the ground through the paint This current is added to the ion current between the electrode 9 and the object 10. It follows that the threshold 1 S will actually correspond to a distance greater than 10 cm, the value of this distance

  may vary depending on the conductivity of the paint.

  However, in most cases, this will not matter.

  tance because, when the current intensity exceeds the threshold value Is, the generator of the present invention provides a high direct voltage which still has a sufficient value

  to obtain a gradiexft of potential sufficient for painting.

  In any case, in borderline cases, for example when we

  uses highly conductive paints, it is always possible to

  ble to adjust the Isde threshold so that it corresponds to an electrode / ground distance of between approximately 10 cm and approximately 30 cm This can for example be obtained by adjusting the value of the voltage V 1, therefore the value of the voltage V 2, for example by means of a transformer with secondary multiple-take-up winding, and / or by adjusting the slope of the straight line V 3 (fig 2) by changing the value of the resistance 'l I which, at this

  indeed, can be a variable resistance.

  Thus, as long as the electrode 9 is at a distance from the object 10 to be painted greater than the distance corresponding to the threshold Is (o 10 cm or more) the voltage V 3 applied to the voltage stabilizer circuit 6 remains greater than the value of the voltage (13 V in the example considered) which is necessary for the stabilizing circuit 6 to function correctly, and the

  voltage V 4 at the output of circuit 6 therefore remains constant and equal

  le at 10 V in the example considered High voltage continues

  VHT, which is proportional to the voltage V 4 also remains cons-

  aunt On the other hand, when the distance between the electrode 9 and the object 10 becomes less than the distance corresponding to the rqel'ici threshold Is, the voltage V 3 becomes insufficient for the stabilizer circuit 6 to be able to function normally, so that its output voltage V 4 decreases linearly eni according to the voltage V 3 to within A V as the intensity of the current increases, therefore as the electrode 9 approaches the object The high continuous voltage VHT, which is proportional at the voltage V 4 decreases in a similar manner, and this before the electrode 9 arrives at the critical distance (approximately 5 cm) for which an ignition could occur between the electrode of 9 and the object 10 o Thus, when the electrode 9 arrives at said critical distance, the high continuous voltage VT is already sufficiently low for a possible ignition to be

not dangerous.

  Although, beyond the threshold Is, the high continuous voltage VHT decreases, the potential gradient remains at a value sufficient to paint, the decrease in the high voltage being compensated by a decrease in the electrode distance 9 /

object 10.

  From the point of view of security, it is advantageous to have a fairly large voltage difference between the voltage V 1 t therefore the voltage V 2; and the stabilized voltage V 4 o Indeed, this makes it possible to use a resistor 11 whose value is suitable for ensuring a fairly rapid fall in the voltage V 3 (slope of the line V 3 in fig 2), therefore also a fairly rapid fall of

  the voltage V 4 and, consequently, of the high continuous voltage VHT.

  when the intensity of the current exceeds the Is threshold On the other hand,

  so that the potential gradient retains a sufficient value

  te beyond the threshold Is, it is not necessary that the voltage V 3, therefore the voltage V 4 and the high continuous voltage VHT decrease too quickly The choice of the value of the voltage V 2 and of the value of the resistance 11 will therefore have to be a compromise between the above considerations and will also depend

  of the value of the desired threshold Is, therefore of the value of the distance

  this from which we wish to decrease progress-

  the high voltage continues VHT.

  It goes without saying that the embodiment of the present invention which has been described above has been given by way of purely indicative and in no way limiting example, and that numerous modifications can be easily made by those skilled in the art. art without departing from the scope of the present invention Thus, in particular that, instead of placing

  resistor 11 in series between diode 4 and voltage stabilizer circuit 6, it could be placed in series between

  the secondary winding 3 of the transformer 1 and the diode 4 as shown in dotted lines in FIG. 1 Likewise, the resistor 11 could be eliminated and a transformer 1 with saturated core could be used, the characteristic of which is to supply a voltage inversely proportional to the current output In this case, the voltage V 1 would vary in the same way as the voltage V 3 In addition, in the generator of FIG. 1, the transformer 1 and the diode 4 could be replaced by a battery of for example 24 Vo

Claims (7)

R E V E N D I C A T I O N S   1. Method for supplying high voltage to an Eledtrostatic spraying device from a power supply   continuous low voltage and a high voltage generator   providing a high DC voltage substantially proportional to the low DC voltage supplied by said power supply   low voltage, the latter having a current characteristic   voltage such that the voltage decreases when the current intensity exceeds a preset threshold value, characterized in that it consists in gradually decreasing the voltage produced by the low voltage supply as soon as the current intensity reaches a corresponding value at a normal distance   of the electrostatic spraying device   than with respect to an object to ground.   2. Method according to claim 1, consisting in supplying a low AC voltage, in rectifying the low AC voltage in order to obtain a continuous low voltage, in stabilizing the low voltage will continue at a predetermined value and in   convert the stabilized continuous low voltage into a high voltage   continuous voltage substantially proportional to the stabilized continuous low voltage, characterized in that it consists in supplying an alternating low voltage which has a value such that, after rectification, the continuous low voltage a, in the absence of   current, a value significantly higher than the pre-   determined of the stabilized continuous low voltage, and gradually decreasing the low voltage, before it is stabilized, from said significantly higher value   up to a value significantly lower than the predefined value   end of the stabilized continuous low voltage corresponding   - dance with the increase in the intensity of the current, so that the low continuous voltage passes, decreasing, by a value substantially equal to the stabilized low continuous voltage when the current intensity reaches said value corresponding to the normal operating distance. 3. Device for supplying high voltage to an electrostatic spraying apparatus, comprising a   continuous low voltage power supply (i'4) g, a stabilizing circuit   voltage sator '(6) connected to the power supply output   continuous low voltage, a high voltage generator (7) connected   tee at the output of the voltage stabilizer circuit (6) to provide a high DC voltage substantially proportional to the output voltage of said voltage stabilizer circuit, and means (11) for decreasing the output voltage of the voltage stabilizer circuit (6) and, consequently, the high-voltage continuous output of the high-voltage generator (7),   when the current intensity reaches a pre-threshold value   regulated, characterized in that said continuous low voltage power supply (1, 4) is dimensioned so as to provide a low continuous voltage which has, in the absence of current, a value (V 2) substantially higher than that of the low stabilized direct voltage (V 4) which is normally delivered by the voltage stabilizer circuit (6) when 12 intensity of the current i is less than said preset threshold value (Is), and in that said means (11) is constituted by a passive circuit element, which is mounted upstream of the voltage stabilizer circuit (6) so as to gradually decrease the low voltage (V 3) supplied to the stabilizer circuit (6) from said higher value (V 2) up to a value significantly lower than said value of low continuous voltage   stabilized (V 4), so that the low voltage (V 3) -   decreases by a value substantially equal (to within L & V) to the value of the stabilized continuous low voltage (V 4 when the current intensity reaches said value   preset threshold (Is), the latter corresponding to a 16 2535917   normal operating tance of the sprayer   electrostatic with respect to a grounded object -   4. Device according to claim 3, in which said low-voltage direct current supply comprises a low-voltage alternating source, a transformer (1) and a rectifier circuit (4), characterized in that the secondary winding (3) of the transformer ( 1) is dimensioned to supply a low AC voltage which has a value (Vy) such that the low DC voltage obtained at the output of the   rectifier circuit (4) at said higher value (V 2).   5. Device according to claim 4, characterized   in that said passive circuit element (11) is a resistor   tance connected in series between said rectifier circuit (4) and   said voltage stabilizer circuit (6).   6 Device according to claim 4, characterized   in that said passive circuit element (11) is a resistor   tance connected in series between the secondary winding (3) -and said rectifier circuit (4).   7. Device according to claim 4, characterized in that said passive circuit element (11) is said   transformer (1), which is a saturated core transformer.