JPS622909B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a power supply device for an automatic spot welding device such as a spot welding robot.

It is well known that spot welding robots are used in many welding operations, including automobile body assembly lines, but in recent years, welding transformers have been used to save energy in spot welding and to prevent wear and tear on water cooling cables for welding current circuits. The use of so-called transformer-incorporated guns, which incorporate a welding head and a spot welding head, is increasing.

Previously, welding current was supplied to the spot welding gun attached to the tip of the robot arm using a long water-cooled cable with a large cross-sectional area from a large welding transformer suspended from the ceiling. . Therefore, the robot has to move at high speed by swinging around this thick cable, and in order to control the attitude of the welding gun at each welding point,
Cables are subject to sudden twists and bends, and they have become a type of consumable item.

On the other hand, with the transformer built-in gun mentioned above, the secondary coil side of the welding transformer is integrated with the welding head with a pressurizing mechanism, so there is no need for a thick water-cooled cable for the welding current circuit, and a large welding current Since the impedance of the circuit through which the welding transformer flows becomes extremely small, the electric capacity of the welding transformer can be reduced, making it possible to make the welding transformer extremely compact and lightweight.

However, a new problem arose when this gun with a built-in transformer was attached to the tip of a welding robot arm to perform welding work.

In other words, in this system, instead of the conventional low-voltage, high-current circuit cable, a high voltage circuit rated at 200V or 400V is routed around by a robot, and although the cable itself is thinner and its lifespan is longer, the round-trip power circuit is This increases the risk of a short circuit between the two cable core wires that form the cable core wire, or of insulation breakdown between the cable core wire that is at high potential and the robot arm, jig, etc. that is at ground potential, and these accidents occur. The damage caused by sparks to peripheral equipment and the impact on power supply systems has become a serious threat that cannot be overlooked.

These risks seriously hinder the practical application of the transformer-equipped gun, which has many other advantages, but the present invention provides a drastic solution to these problems.

The figure is an electric circuit diagram showing one embodiment of the present invention. A comparative explanation of the prior art and the present invention using the figure will be as follows.

In general, the transformer built-in gun 1 is fixed to the tip of a movable arm of a spot welding robot 2 (not shown), and is sequentially moved according to the spot welding position of the workpiece.
The gun is transferred at high speed, and the attitude of the gun itself changes rapidly depending on the shape of the workpiece.

Therefore, the movable cable 3 connected to the primary side of the welding transformer is forced to undergo extremely severe twisting, bending, etc. movements and gradually wears out.

Conventionally, this movable cable 3 is directly connected to a welding power source 10 rated at 200V or 400V via an electronic switch 7 such as a thyristor/contactor that controls welding energization. - When the contactor 7 is in a conductive state, a high welding power source voltage is directly applied between the cable core wires of the pair of movable cables 3, and for example, the thyristor contactor 7
Even when the cable core is in a non-conducting state,
In some cases, it will be kept at a high potential with respect to ground.

As a result, a strong dielectric breakdown force acts even if the insulator that protects the outer circumference of the cable 3 is slightly damaged, and once a short circuit between cables or a short circuit to ground occurs, it can be extremely dangerous. There is a risk that a large short-circuit current will flow.

When such a short circuit causes a large current, the breaker 9 installed on the welding power source will operate and cut off the power, but the large spark caused by the cable short may damage peripheral equipment such as robot arms and jigs. This may cause damage, and in some cases may even affect power supply equipment such as substations, leading to major accidents such as factory power outages and fires.

On the other hand, so-called earth leakage circuit breakers are effective against deterioration of the cable core wire's insulation against ground, but in the case of water-cooled thyristors, etc., which are currently widely used,
If water scale accumulates on the water cooling hose and the insulation resistance of the electrical circuit decreases, the earth leakage circuit breaker may malfunction. This required complicated processing such as the use of special insulated thyristors.

In contrast, in the present invention, a step-down insulation transformer 4 is installed between the welding power source 10 and the transformer built-in gun 1, so that the movable cable 3
Since the voltage applied between the core wires can be reduced, the possibility of a short circuit between cables is reduced, and even if a short circuit occurs between cables in the movable cable 3, the internal impedance of the isolation transformer 4 will prevent welding. It is possible to limit the short-circuit current flowing into the power supply.

On the other hand, by keeping the secondary coil side of the isolation transformer 4 in a floating state with respect to the ground,
Even if the insulation on one side of the movable cable 3 breaks down and a part of the cable core comes into contact with a component at ground potential, such as a robot arm or jig, no ground current will flow, and sparks will cause damage to the robot equipment. There will be no damage or adverse effects on the power supply equipment.

However, if the electric potential of the electric circuit including this movable cable 3 is kept in a floating state with respect to the ground, leakage between the primary coil and the secondary coil of the insulating transformer 4, impedance, etc. will cause the primary coil to close to the secondary coil side. For example, when replacing the movable cable 3, if a person touches the electric circuit, there is a risk of receiving a severe electric shock.

To prevent this, it is necessary to connect the electrical circuit in this floating state to a suitable low impedance 5, e.g.
It can be connected to ground 6 via a resistor of about 100Ω.

Such grounding can sufficiently reduce the current flowing through the ground even in the event of a ground fault in the non-grounded core wire of the movable cable 3, and can reliably prevent damage to contact parts due to sparks or the like.

Furthermore, if the secondary voltage of the step-down isolation transformer 4, that is, the voltage supplied to the welding transformer, is too low, the movable cable 3 will become thick, resulting in poor workability and economic efficiency. become. Depending on the balance between these and safety, an optimal voltage of, for example, several tens of volts will be selected.

[Brief explanation of the drawing]

The figure is an electrical circuit diagram showing an embodiment of the present invention. 1...Gun with built-in transformer, 2...Spot welding robot, 3...Movable cable, 4...Isolation transformer for step-down, 5...Grounding impedance, 6...
Earth, 7...thyristor contactor, 8...timer, 9...breaker, 10...200V or 400V rated welding power source.

Claims (1)

[Claims] 1. In automatic spot welding equipment such as spot welding robots that use a so-called transformer built-in gun that integrates a welding transformer and a spot welding head, welding via an electronic switch that controls welding current flow. A step-down isolation transformer is installed between the power source and the welding transformer, and the secondary coil side of the isolation transformer and the primary coil side of the welding transformer are connected with a flexible cable, and one point of the electrical path is connected. A welding power supply device for a gun with a built-in transformer, characterized in that the is grounded with a low impedance.