Welding apparatus for resistance welding

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Components of a welding device for resistance spot and projection welding

Welding equipment for resistance welding are technical facilities for performing various resistance welding process. They consist of a mechanical machine body and an electrical system, which includes a power unit and a controller, a power generation and cooling system.

The mechanical structure can be designed as a stationary welding machine or spot welding gun of different sizes and different designs. Stationary welding machines are used for spot, projection and roller seam welding or butt welding in workshop production. The respective welding process determines the type of construction and the electrical and mechanical requirements for the machines EN ISO 669. Welding guns are mobile welding devices for spot welding that are guided to the welding location by hand or by manipulators such as industrial robots .

The power unit and an adapted welding transformer generate a high welding current with a low welding voltage from the high mains voltage with a comparatively low mains current. The force and the welding current are set by a control system according to a time sequence specified by the welding task.

Mechanical construction

The properties of the welding machine, among other factors, influence the quality of the connection , as well as the productivity and availability of the equipment. How they can be integrated into a given manufacturing environment depends on their properties.

For spot and projection welding, a double stroke of the force generation system is generally required, which enables a larger feed movement and a small stroke for welding. Electrodes should be positioned and re-positioned as smoothly and without bouncing as possible. The devices should be stiff and not bend open. The bending up causes eccentricity and pushing of the electrodes during spot welding, which negatively affects the spot quality. Especially for resistance projection welding , a high resetting speed is required in order to securely join the collapsing projection.

Mounting and repositioning a resistance welding machine

Stationary welding machines

Spot, projection and roller seam welding

Resistance spot welding machine in operation
Elements of spot, projection and seam welding equipment according to ISO 669

With different sizes and designs, the welding machines are made up of similar components. Within a stable frame with upper and lower arm, there is a power source of different types of electricity (symbolized by the transformer in the picture) to generate electricity . The secondary line for the welding current leads to the welding tools. Since the upper arm is movable, a flexible electrical tape is used. The welding tools are the electrodes in electrode holders for spot welding, clamping plates, on which the respective electrode systems are clamped , for projection welding , and a motor-driven roller head with the roller electrodes for roller seam welding . The welding force is applied via a force generation system that is attached to the upper arm.

In the welding operation with spot and projection welding, the upper electrode is moved down after inserting the welding parts. The force is applied and the current is switched on. During the heating process, a spot weld forms which, when cooled, leads to a firm connection. With roller seam welding, the upper arm is also lowered, force is applied and the rollers are driven. The parts to be connected are moved by the rollers. By continuously or repeatedly switching the current on and off, a chain of welding points is set which together form a continuous weld seam.

Welding machine for roller seam welding (schematic)

Butt welding

Elements of a butt welding device according to ISO 669

As essential elements of a resistance butt welding machine, clamping devices with clamping jaws are placed on a stable frame. A power source of different types of current (symbolized by the transformer in the picture) is connected to the secondary side with the clamping jaws . The clamping jaws are closed by the clamping device and clamp the workpieces during welding. One of the clamping devices is mounted on a slide, which is moved by a slide drive and generates the required upsetting force.

Resistance welding guns

Spot welding guns are tools for resistance spot welding and are used to produce resistance welded joints.

design type

Spot welding guns differ in the external or integrated installation of the transformers required for welding.

If the transformer is left external, the welding gun has a compact and lightweight design. Accessibility in tight work areas is good. The disadvantage is the relatively long secondary line, which is reduced when using direct current. In cooperation with welding robots , the freedom of movement is very restricted.

Welding guns with an integrated transformer have short secondary lines, which means they are more efficient. In robot operation, they offer quick and easy replacement options thanks to plug-in primary cables.

Scheme of a robot welding gun in C design
Spot welding guns

Another distinction is the basic mechanical construction. A distinction is made between so-called X-tongs (they work on the scissors principle) and C-tongs.

With the X-pliers, the pliers move the electrodes via a joint, as does the pliers compensating movement. The X-tongs tend to undesirably "push" the electrodes.

With the C-tongs, all movements, such as the infeed of the electrode and the balancing of the tongs, run in a straight line. The cylinder transfers the welding force directly to the counter electrode.

Together with the power supply lines, cooling water lines, compressed air lines and control lines are fed to the spot welding gun in the so-called hose package.

Manual welding guns

Manual welding guns can be transformer guns or guns with an external transformer and are designed in X or C design. They are manually positioned at the welding point by an operator and switched on for welding. In the case of manual welding guns, the hose package is usually hung on a jib crane (load balancing aid) due to its weight, in order to be able to work relatively comfortably despite the weight.

Robotic welding guns

Modern robot tongs are usually designed as transformer tongs in X or C design. They have pliers bodies of various sizes and designs. The base body is used to accommodate the application-specific electrode arms and the units for generating force (pneumatic cylinder, hydraulic cylinder, electric motor). The transformer is integrated in the pliers body and connected to the electrode arms via flexible lamellar strips, which also perform the movement of the electrode arm.

Robots can change welding tongs automatically. For this purpose, changing pliers are used, in which the mounting flange and the supply lines must be designed accordingly.

Controls and power units

Control and power section of a resistance welding device (schematic) according to

The different requirements for the welding process result in a wide range of welding control functions. They relate to the organization of the production processes - such as welding at manual workstations or using automated systems - but requirements for quality assurance and data management must also be met.

Controls have a controlling and regulating effect on the power unit and are structured very differently with different functionality. In cooperation with the power unit, the control enables the setting or regulation of the welding current, the electrode force and the process times . It synchronizes the timing of the welding current and electrode force with the required machine processes. The parameter setpoints of the welding current, the electrode force and the process times are set point-related on the control, actual values ​​such as welding current, electrode voltage, welding energy, electrode force and path are monitored and archived if necessary. Parameter changes, status and error messages can also be logged. The welding process can be visualized by showing the voltage, current, resistance and force curves.

Power parts

The power unit is connected to the mains. The output is connected to the primary winding of the welding transformer, on whose secondary winding the low welding voltage is available for welding (3 to 16 V). The voltage present at the output results from the interaction with the welding control.

Depending on the type of current , a distinction is made between several versions:

  • Mains frequency power sources (50 or 60 Hz)
  • Direct current
    • rectified from the single or 3-phase network
    • Transistor-regulated power source
    • Capacitor discharge
  • Medium frequency inverter (typical working frequency a few 100 Hertz to a few kHz)
    • High frequency inverter (from 20 kHz)

Their use depends on the properties.

Mains frequency power sources

Welding control with 50 Hz welding power level
Voltage and current curve with phase control
The phase control, voltage and current curve at mains frequency

In mains frequency power sources, anti-parallel connected thyristors are used as AC power controllers . This allows the voltage to be changed continuously via an ignition point delay so that only part of each voltage half-wave reaches the welding transformer. This phase control makes the rms value of the secondary voltage variable and the welding current adjustable. The current flow is controlled by connecting two thyristors in anti-parallel. After the current has passed zero, further current flow is blocked until the corresponding thyristor receives an ignition pulse. From this point on, the consumer is supplied with energy until the next zero crossing. The later the respective thyristor is ignited, the lower the average power.

However, the current is delayed by the inductance of the transformer and circuit, as is the quenching of the thyristor and there is an inductive voltage overhang. The counter thyristor can only re-ignite after this voltage overhang has been reduced.

DC power sources

Direct current from one phase

The power components of a 1-phase DC welding system are constructed like AC power components, with the welding transformer being equipped with rectifier diodes on the secondary side and having a center tap.

Direct current from 3 phases

The power units of a 3-phase DC welding system are 3-phase connected to the supply network. All three phases are loaded symmetrically during welding, their currents are smaller compared to the AC power part, because the draw is divided between the three phases of the network.

3-phase direct current power units are usually equipped with 3 single-phase transformers that have a center tap on the secondary side. Three thyristor controllers are arranged on the primary side. They switch the welding current on and off and control the phase angle. A 6-pulse rectifier rectifies the voltage on the secondary side of the three welding transformers. The ripple of the direct current generated in this way has six times the network frequency. The welding current is smoothed by an inductance in the secondary circuit in order to avoid ripples that are detrimental to the welding process, even with low powers with a long ignition delay.

Inverter power sources
Inverter for medium frequency welding (box in the top of the control cabinet); the transformer is external.
Principle of an MF inverter

In the case of inverter power sources, the welding transformer is fed with an alternating voltage of 1 kHz to 4 kHz ( medium frequency ) or around 20 kHz ( high frequency ).

The three-phase alternating voltage is converted into a direct voltage by means of a 6-pulse rectifier. This intermediate circuit voltage is smoothed by capacitors. A transistor inverter (H-bridge made up of four IGBTs ) converts the DC voltage into a single-phase square-wave AC voltage, which is fed into the welding transformer on the primary side. The control is carried out by changing the pulse width by changing the phase position of the two half bridges to one another. This allows the rms value of the voltage and thus the current strength in the secondary circuit to be changed. On the secondary side, rectification is carried out directly on the transformer in order to keep inductive voltage losses small.

The significantly higher control speed is advantageous (with 1 kHz inverters a response time of the power controller of 0.5 ms is achieved compared to 10 ms with 50 Hz welding current controllers). Another technological advantage of inverter welding is the lower mass of the welding transformers, so that it is possible to integrate them into the welding guns. a. there is a small secondary circuit with low resistance.

Several inverter power units can be connected in parallel in what is known as a master-slave arrangement in order to increase the welding current or the welding power if required. Several power units (slaves) are connected to a welding control (master). The first inverter in the chain is the master inverter, which ensures a synchronized start and execution of the same welding programs in all subsequent slave inverters, i.e. H. the master controls the power units of all slaves synchronously and can display any status messages of the individual slaves. Via the connection to the master, the slaves report e.g. B. their "ready for operation" and the respective current actual value to the master.

Transistor power sources
Principle of a transistor power source for resistance welding

Transistor power parts are used today, especially in the field of small part welding, for currents up to 5 kA and welding times up to 100 ms. There is no residual ripple.

In these power units, transistors regulate the welding current in the welding circuit, with the transistors serving as adjustable resistors. The transistor reduces the power consumed to the amount required in the welding point. There is a high loss of energy, which is converted into heat in the transistor. This results in the lower maximum duty cycle.

Power sources for capacitor pulse welding

Schematic diagram of a power source for capacitor pulse welding

In principle , the power sources used for capacitor pulse welding are direct current sources. In a departure from this, however, circuits for reversing the polarity of the current direction have also been developed, which has the advantage of reversing magnetization for the transformer.

The power source basically consists of the following components:

  • DC generation
  • Memory (capacitor battery)
  • Thyristor switch
  • special transformer.

After the thyristor is triggered, the capacitors are suddenly discharged via the welding transformer and the welding point. There is a high output at the beginning of the welding up to a peak value and a subsequent decay of the current (see picture: KE - welding: current and voltage curve). Numerous variants of power generation and control for pulse-wise discharge with fixed or variable pulse shape are known; For example, several partial pulses with different energies are recommended.

Mains connection

The special characteristics of the power consumption of resistance pressure welding machines due to intermittent, often single-phase operation with high load peaks and frequently non-sinusoidal current and voltage curves require special attention when connecting to the mains. These special features are described in detail in DVS bulletin 2918.

Cooling system

The cooling system can consist of several cooling circuits for the electrodes or welding tools, the transformer and the controller. When connecting, ensure that there is sufficient inflow and outflow. The respective inlet temperature should be below 25 ° C.

Individual evidence

  1. a b c DIN ISO 669: 2001-06: "Resistance welding equipment - Mechanical and electrical requirements"
  2. a b c d e DVS : "Controls and power units for resistance welding" , DVS data sheet 2904, 2010
  3. DVS : "Recommendations for the selection and comparison of resistance point, projection and seam welding devices as well as resistance point and seam welding devices" , DVS Merkblatt 2907, 1991
  4. DVS 2937: "Resistance welding with industrial robots" ; DVS bulletin 2937, 1990
  5. DVS 2937-2: "Welding energy systems for resistance welding robots , the design of the secondary circuit and the periphery" , DVS bulletin 2937-2, 2001
  6. Patent US6321167 : RESISTANCE-WELDING POWER SUPPLY APPARATUS. Published on 1998 , Inventors: Takashi Jochi, Mikio Watanabe.
  7. Patent DE102009008373 : Process for capacitor discharge welding to connect metallic components and welding unit to carry out the process. Published on 2009 , Inventors: M. Hirn, R. Hirn.
  8. DVS : "Resistance pressure welding machines, network connection and energy conversion" , DVS Merkblatt 2918, 1979