JPH0985136A - High voltage generator for electrostatic coating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively prevent sparks from being generated by setting the capacity of each of capacitors so that a length of the distance between the spray gun and the object, at which the applied voltage begins to decrease at the time of gradually bringing a spray gun for coating close to an object to be coated, is longer than another length of this distance, at which the current passing through between the spray gun and the object reaches a prescribed value or higher. SOLUTION: In this generator, a spray gun 2 for coating is provided with a high voltage supply device 3 consisting of a combination of a step-up transformer 33 and a Cockcroft-Walton circuit 34. In the Cockcroft-Walton circuit 34, the secondary voltage is rectified by diodes 34b and stored in each of capacitors 34c to increase the secondary voltage to an n-fold high voltage value at a high voltage terminal 34a. Then, when the distance L between the spray gun 2 and an object to be coated W becomes shorter and the current passing through between them reaches a prescribed value or higher, the power supply to the high voltage generating circuit is cut off to stop the high voltage application. The capacity of each of the capacitors 34 is determined so that a length of the distance L, at which the applied voltage begins to decrease is longer than the length of the distance L, at which the high voltage application is stopped.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a Cockcroft-Walton circuit that uses a capacitor to generate a high voltage, applies a high voltage between a coating gun and an object to be coated, and simultaneously applies the coating gun and the object to be coated. The present invention relates to a high voltage generator for electrostatic coating, which stops application of high voltage when a current flowing between and exceeds a predetermined value.

[0002]

2. Description of the Related Art As a conventional high voltage generator for electrostatic coating, for example, Japanese Patent Application Laid-Open No. 58-61852 discloses that a coating gun detects a current value flowing between a coating gun and an object to be coated. It is known that a high voltage is stopped when a current approaches a coating object and the current value exceeds a predetermined value to prevent sparks between the coating gun and the coating object.

[0003]

SUMMARY OF THE INVENTION In the above-mentioned prior art, even if the coating gun approaches the object to be coated until the current value exceeds a predetermined value, the voltage applied between the coating gun and the object to be coated is It is set to maintain a high voltage. Therefore, as in the case where the approach speed of the coating gun to the object to be coated is fast, or when it is delayed to detect that the above current value has exceeded the predetermined value, the coating gun is still applied with the high voltage. There is a risk that sparks will occur if the object gets closer to the object to be coated.

The present invention has been made in view of the above-mentioned problems of the prior art, and the coating gun is used until the current value flowing between the coating gun and the article becomes a predetermined value or more. It is an object of the present invention to provide a high voltage generator for electrostatic coating in which the occurrence of sparks does not occur even if the timing of stopping the application of the high voltage is slightly delayed when approaching to.

[0005]

In order to achieve the above object, a high voltage generator for electrostatic coating of the present invention has a Cockcroft-Walton circuit for generating a high voltage by using a capacitor, and a coating gun and In a device that applies a high voltage between the object to be coated and stops the application of the high voltage when the current flowing between the coating gun and the object to be coated reaches a predetermined value or more,
The distance at which the applied voltage begins to drop when the coating gun is brought close to the object to be coated while a high voltage is applied is longer than the distance at which the current flowing between the coating gun and the object to be coated exceeds a specified value. In addition, the capacitance of the capacitor of the Cockcroft-Walton circuit is set.

The higher the voltage applied between the coating gun and the object to be coated is, the more sparks are generated even if the distance between the two is large. On the other hand, the output performance of the high voltage generator is determined by the capacity of the condenser of the Cockcroft-Walton circuit, and the output performance is represented by the product of the applied voltage between the coating gun and the object to be coated and the current between them. That is, when the coating gun approaches the object to be coated, the applied voltage remains constant and the current value between the two increases, but when the current value increases to some extent, the applied voltage between the two increases after that. Is reduced. In the conventional one, the output performance of the high-voltage generator is set to a large value so that the distance at which the applied voltage begins to decrease becomes short,
Although the applied voltage is not lowered even if the current flowing between the coating gun and the object to be coated exceeds a predetermined value, in the present invention, the capacitance of the capacitor is adjusted so that the current flowing between the two has a predetermined value. By the time the above is reached, the applied voltage has already begun to decrease, so that sparks do not occur due to the decreased applied voltage even when the coating gun further approaches the object to be coated while the high voltage application stop timing is delayed. did.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Referring to FIG. 1, a coating gun 2 mounted on an arm 1 of a coating robot is rotated at a high speed by an air motor (not shown) incorporated in the coating gun to atomize the coating. A bell cup 21 is provided at the tip.
In order to apply a high voltage of, for example, -90 kV to the bell cup 21, a high voltage supply device 3 is attached to the coating gun 2, and an input terminal 31 for connecting to a power source is provided on the base end side thereof. ing. On the other hand, the output side (output terminal) 32 of the high voltage supply device 3 is electrically connected to the bell cup 21 and is configured to apply a high voltage to the grounded object W to be coated. ing.

The high voltage supply device 3 is a combination of a step-up transformer 33 and a Cockcroft-Walton circuit 34, and a high voltage is generated between the ground terminal 33a and the high-voltage terminal 34a of the step-up transformer 33. High voltage terminal 3
4a is connected to the output terminal 32 of the high voltage supply device 3 via a protection resistor 35. The protection resistor 35 is provided for the purpose of protecting the circuit when the high voltage supply device 3 is short-circuited with a load, and is set to 260 MΩ in this embodiment.
Cockcroft-Walton circuit 34 includes diode 3
4b and the capacitor 34c are one set of circuits, and a plurality of sets of circuits are connected in multiple stages. The secondary side AC voltage of the transformer 33 is rectified by each diode 34b and accumulated in each capacitor 34c. It Therefore, in the Cockcroft-Walton circuit 34 in which the diode 34b and the capacitor 34c are connected in n stages, the secondary side voltage of the transformer 33 is doubled to a high voltage of n times at the high voltage terminal 34a. However, the size of the capacitance of each capacitor 34c is not related to the output voltage and affects the output performance of the Cockcroft-Walton circuit 34. That is, when the capacitance of the capacitor 34c is large, the output performance is increased and the output voltage is not reduced even when the output current value is increased. However, as the capacitance of the capacitor 34c is reduced, the output performance is reduced and the output voltage is reduced. The current value to start becomes small. Therefore, how to determine the capacitance of the capacitor 34c will be described with reference to FIG.

Assuming that the distance between the coating gun 2 and the object W to be coated is L, as shown in the upper graph of FIG. 2, when the voltage applied to the coating gun 2 is -90 kV, normal electrostatic coating is performed. The standard distance L is about 300 mm shown by L1. When the distance L is L1, a current of 10 to 20 μA is normally output from the Cockcroft-Walton circuit 34,
It flows between the coating gun 2 and the object W to be coated. The applied voltage must be maintained at -90 kV in order to perform good electrostatic coating, and the applied voltage is maintained at -90 kV when the distance L is around 300 mm shown by L1.

On the other hand, when the distance L becomes smaller, the value of the current flowing between the coating gun 2 and the object W to be coated increases, and the distance L becomes L2.
When the current becomes smaller than about 150 mm and the current exceeds 120 μA shown by the predetermined value A, the overcurrent detection device (not shown) immediately cuts off the power supply to the high voltage generation circuit 20 and stops the application of the high voltage. Then, L0, which is the distance at which the applied voltage starts decreasing due to the increase in current, is the coating gun 2 and the object W to be coated.
It is set to be larger than the distance L2 at which the application of the high voltage between and is stopped. By the way, in order to determine the capacitance of the capacitor 34c in this manner, the capacitance of the capacitor used in the conventional Cockcroft-Walton circuit is set so that the applied voltage does not decrease even when the distance L becomes L2. Although it is necessary to reduce the capacitor capacity, for example, a Cockcroft-Walton circuit 3 including 20 stages of diodes 34b and capacitors 34c.
If it is 4, the capacitance of the 1st to 4th stages on the transformer side is set to 150pF as in the conventional case, and the capacitance of the 5th to 20th stage capacitors is reduced to 82pF. However, in the present invention, the capacitor 3 of the Cockcroft-Walton circuit 34 is
Since the starting point L0 of the voltage drop can be controlled by the total capacitance of 4c, the capacitances of all the capacitors 34c may be set to the same capacitances smaller than the conventional capacitances.

By the way, when the capacitance of the capacitor is set smaller than that of the conventional one, the residual charge from when the output of the high voltage generator 20 is stopped until the applied voltage becomes the spark generation minimum voltage V1 (-10 kV) or less. The discharge time T1 can be made shorter than that of the conventional one, which is further advantageous in preventing sparks.

[0012]

As described above, according to the present invention, when the coating gun approaches the object to be coated and the operation of the high voltage generating circuit is stopped, the applied voltage has already started to drop, so that Even if the operation of the voltage generator is delayed, there is no risk of sparking.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a high voltage generator.

FIG. 2 is a graph showing changes in applied voltage and current with respect to a distance L between a coating gun and an object to be coated (upper part), and a graph showing changes with time in applied voltage and current from the time when the operation of the high voltage generator is stopped. (beneath)

[Explanation of symbols]

 2 Coating gun 3 High voltage supply device 32 Output terminal 33 Transformer 34 Cockcroft-Walton circuit 34b Diode 34c Capacitor W W Coated object L0 Distance at which applied voltage starts decreasing L1 Standard distance between coating gun and coated object L2 High voltage Distance to stop applying

Claims (1)

[Claims] 1. A Cockcroft-Walton circuit for generating a high voltage by using a capacitor, wherein a high voltage is applied between a coating gun and an object to be coated and a current flowing between the coating gun and the object to be coated. When the voltage exceeds a predetermined value, the application of high voltage is stopped.The distance at which the applied voltage begins to decrease when the coating gun is brought close to the object to be coated with the high voltage applied is the distance between the coating gun and the object to be coated. A high voltage generator for electrostatic painting, wherein the capacitance of the capacitor of the Cockcroft-Walton circuit is set so that the current flowing between the two is longer than a predetermined value.