JP3092049B2 - High voltage generator for electrostatic coating - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art As a conventional high voltage generator for electrostatic coating, for example, Japanese Unexamined Patent Publication No. 58-61852 discloses a method of detecting the value of a current flowing between a coating gun and an object to be coated. There has been known an apparatus in which the application of a high voltage is stopped when the current value becomes equal to or more than a predetermined value when approaching a coating object to prevent a spark between the coating gun and the object to be coated.

[0003]

In the above conventional apparatus, the voltage applied between the coating gun and the object to be coated does not increase even if the coating gun approaches the object until the current value exceeds a predetermined value. It is set to be maintained at a high voltage. Therefore, as in the case where the speed at which the coating gun approaches the object to be coated is high, or when it is delayed to detect that the current value has become equal to or higher than the predetermined value, the coating gun remains in a state where the high voltage is applied. However, there is a possibility that sparks may be generated when the object further approaches the object to be coated.

[0004] The present invention has been made in view of such problems of the prior art, and the coating gun is operated until the value of a current flowing between the coating gun and the object to be coated exceeds a predetermined value. It is an object of the present invention to provide a high voltage generator for electrostatic coating which does not cause sparking even if the timing of stopping the application of the high voltage is somewhat delayed when approaching.

[0005]

In order to achieve the above object, a high voltage generator for electrostatic coating according to the present invention has a Cockcroft-Walton circuit for generating a high voltage using a capacitor, and has a coating gun and In applying a high voltage between the object to be coated and stopping the application of the high voltage when the current flowing between the coating gun and the object to be coated becomes a predetermined value or more,
The distance at which the applied voltage begins to decrease when the coating gun is brought close to the workpiece with the high voltage applied is longer than the distance at which the current flowing between the coating gun and the workpiece becomes a predetermined value or more. In addition, the capacitance of the capacitor of the Cockcroft-Walton circuit is set.

[0006] The higher the voltage applied between the coating gun and the object to be coated, the more spark is generated even if the distance between the two is greater. On the other hand, the output performance of the high voltage generator is determined by the capacitance of the capacitor 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 the two. That is, when the coating gun approaches the object to be coated, the applied voltage is kept constant and the current value between the two increases, but after the current value increases to some extent, the applied voltage between the two increases with the current value thereafter Decrease. In the conventional one, the output performance of the high-voltage generator is set to be large so that the distance at which the applied voltage starts to decrease becomes short,
Although the applied voltage did not decrease even if the current flowing between the coating gun and the object to be coated exceeded a predetermined value, the present invention adjusts the capacity of the capacitor so that the current flowing between the two can be reduced to a predetermined value. At this point, the applied voltage has already started to decrease, so that the spark does not occur due to the decrease in the applied voltage even if the coating gun approaches the work further with the application stop timing of the high voltage delayed. did.

[0007]

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) built 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 supply is provided at a base end 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 so that a high voltage is applied between the bell cup 21 and the grounded workpiece W. ing.

The high-voltage supply device 3 is a combination of a step-up transformer 33 and a Cockcroft-Walton circuit 34, and generates a high voltage 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 the protection resistor 35. The protection resistor 35 is provided for the purpose of protecting the circuit when the load of the high-voltage supply device 3 is short-circuited, and is set to 260 MΩ in the present embodiment.
The Cockcroft-Walton circuit 34 includes a diode 3
4b and a capacitor 34c as one set of circuits, and a plurality of sets of circuits are connected in multiple stages. The secondary AC voltage of the transformer 33 is rectified by each diode 34b and stored in each capacitor 34c. You. Therefore, in the Cockcroft-Walton circuit 34 in which the diode 34b and the capacitor 34c are connected in n stages, the secondary voltage of the transformer 33 is doubled to n times the high voltage at the high voltage terminal 34a. However, the magnitude of the capacitance of each capacitor 34c is not related to the output voltage but affects the output performance of the Cockcroft-Walton circuit 34. That is, when the capacity of the capacitor 34c is large, the output performance increases and the output voltage does not decrease even when the output current value increases. However, as the capacity of the capacitor 34c decreases, the output performance decreases and the output voltage decreases. The starting current value decreases. Therefore, how to determine the capacitance of the capacitor 34c will be described with reference to FIG.

When the voltage applied to the coating gun 2 is -90 kV, as shown in the upper graph of FIG. Is set to about 300 mm shown in L1. When the distance L is L1, a current of usually 10 to 20 μA is output from the Cockcroft-Walton circuit 34,
It flows between the coating gun 2 and the workpiece W. In order to perform good electrostatic coating, it is necessary to maintain the applied voltage at -90 kV. When the distance L is about 300 mm shown by L1, the applied voltage is maintained at -90 kV.

On the other hand, when the distance L decreases, the value of the current flowing between the coating gun 2 and the work W increases, and the distance L becomes L2.
When the current is reduced to about 150 mm and reaches a current of 120 μA or more, which is a predetermined value A, an overcurrent detection device (not shown) immediately shuts off power supply to the high-voltage generating circuit 20 and stops applying high voltage. L0, which is the distance at which the applied voltage starts decreasing due to an increase in the current, is determined by the coating gun 2 and the workpiece W.
Is set to be longer than the distance L2 at which the application of the high voltage is stopped. By the way, in order to determine the capacitance of the capacitor 34c in this way, the capacitance of the capacitor used in the conventional Cockcroft-Walton circuit, which is set so that the applied voltage does not decrease even when the distance L becomes L2, is determined. Although it is necessary to reduce the capacitance of the capacitor, for example, a Cockcroft-Walton circuit 3 including a 20-stage diode 34b and a capacitor 34c
In the case of 4, the capacitance of the first to fourth stages on the transformer side is set to 150 pF as in the conventional case, and the capacitance of the fifth to 20 stages is reduced to 82 pF. However, in the present invention, the capacitor 3 of the Cockcroft-Walton circuit 34 is used.
Since the start point L0 of the voltage drop can be controlled by the total capacity of 4c, the capacities of all the capacitors 34c may be set to be the same as the capacities of the conventional capacitors.

By the way, if 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 equal to or lower than the spark generation minimum voltage V1 (-10 kV) is obtained. The discharge time T1 can be made shorter than that of the prior art, which is further advantageous for spark prevention.

[0012]

As described above, according to the present invention, the applied voltage has already started to decrease when the coating gun approaches the object to be coated and the operation of the high voltage generating circuit is stopped. Even if the stoppage of the operation of the voltage generator is delayed, there is no possibility of spark generation.

[Brief description of the 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); (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 Coating object L0 Distance at which applied voltage starts to decrease L1 Standard distance between coating gun and coating object L2 High voltage Distance to stop applying

Claims (1)

(57) [Claims] 1. A cockcroft-Walton circuit for generating a high voltage by using a capacitor, a high voltage is applied between a coating gun and a workpiece, and a current flows between the coating gun and the workpiece. When the application of the high voltage is stopped when the value is equal to or more than a predetermined value, the distance at which the applied voltage starts 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 coating, characterized in that the capacitance of the capacitor of the Cockcroft-Walton circuit is set so that the current flowing therethrough is longer than a distance at which the current flows over a predetermined value.