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INVERTER SQUEEZE-TYPE RESISTANCE SPOT WELDERS

December 20, 2004 - Many squeeze-type resistance spot welding (STRSW) machines use alternating current (AC), transformer based power sources, and supply an AC secondary welding current to the electrode tips. These types of welders are still available and viable for collision repair.

Advancements in technology have led to inverter power sources that use a high frequency AC voltage to supply a direct current (DC) secondary welding current. Before the differences between these two types of welders can be explained, a basic understanding of the principals of STRSW is required.

Principals Of Resistance Welding

There are three primary factors involved in making a spot weld: the pressure applied by the electrode tips, the amount of current that passes through the parts, and the length of time that the current flows. As the name of the process implies, the heat used for welding is generated by the current flowing through a point of resistance. The point of maximum resistance, where the maximum heat is generated, is where the two pieces being welded touch each other (see Figure 1). The amount of heat generated is a function of the current in amps, the resistance through the work pieces, and the amount of time the current flows. During welding, enough heat is generated to bring the metal to a molten state. This, combined with the pressure applied by the electrode tips and the cooling of the weld after the current flow stops, fuses the metal between the electrode tips together.

Heat losses through radiation and conduction must also be considered. The longer the current flows, the more heat that is generated, but also the more heat that is lost through radiation and conduction. To minimize these heat losses, it is usually preferred to have a high welding current with a short weld time. Welding heat is proportional to the square of the welding current, meaning that when you double the current you quadruple the amount of heat generated in the same time. This means that when secondary welding current through the electrodes is increased that weld time can be reduced. This will reduce heat losses to the atmosphere and the surrounding metal, which helps to keep the heat-effect zone of the weld smaller.

Creating a good spot weld requires a consistent secondary current output through the electrode tips. This requires consistent, adequate pressures between the electrode tips, good part fit-up, and little or no variances in the line voltage supplied to the welding machine. Of these, a technician can control part fit-up and squeeze pressures (see Figure 2), but fluctuations in incoming line voltage typically cannot be controlled. To compensate for fluctuations in the incoming line voltage to the welder, feedback systems were put on welders. These systems may look at voltage and current fluctuations on just the primary side, or on both the primary and secondary side and supply information to the controller for the welding machine. The controller can then make adjustments to compensate for the line voltage variances and maintain consistent output.

Inverter Power Supplies

What does an inverter power supply do? Inverter power supplies take the incoming low frequency (50–60 Hz) AC line voltage and, through a rectifier, changes it to DC voltage. This DC voltage is then passed through a filter to remove any high frequency modulation that was introduced. The DC voltage is then sent through a bridge where it is cycled on and off by switches to create a high frequency (typically more than one kHz) simulated AC voltage. This high frequency AC voltage is sent through a transformer to create a low voltage (about 13 volts), high current (as much as 12,500 amps) AC voltage on the secondary side. The high frequency AC voltage is then sent through an output rectifier where it is changed to DC voltage that is used for welding (see Figure 3).

Let’s examine some of the benefits of this one at a time. Why use a DC output for making the weld? With an AC output welding machine, the output current begins at zero, increases to a peak value, decreases back through zero to a peak negative value, and then increases back to zero where the cycle starts over again. This happens at the same rate that the input current cycles (50–60 times a second). The pieces being welded are heated on the positive half cycle and cooled on the negative half cycle. This can cause problems when welding thin material where the weld is made quick and only goes through a few cycles. The metal may be weakly bonded together on the first or second half cycle. If the weld cools sufficiently on the negative half cycles, this bond can cause a loss of resistance and the remaining cycles will not produce enough heat to make a good weld. This will require a longer weld time with more heat input into the surrounding metal. A DC output will produce a positive current flow throughout the weld time and overcome the negative effects of cycling and inductive heat losses.

Another advantage of a high frequency inverter power source is better information supplied to the welder feedback system. Since the incoming AC line voltage cycles at 50–60 times a second, and AC transformer machines do not change this, welding thin material may only involve a few cycles. The feedback system may not have enough time to make accurate adjustments. With a high frequency inverter machine, the feedback system can operate at more than 1,000 cycles per second instead of 60. The result is much finer control of the output current, especially when welding thin material that only requires a weld time of milliseconds. This results in a smoother and more consistent output current.

Inverter power sources also provide increased efficiency. Transformers operate more efficiently when supplied with high frequency voltage. Because of this, the transformer in an inverter STRSW welder can be smaller than that in an AC machine with similar ratings. This also allows for squeeze-type resistance spot welders with higher current output capabilities, which means that shorter weld times with less heat-effect to the surrounding metal are possible. Ensure that there is adequate power available where the welder will be used. Even the best equipment will not perform correctly if the source voltage and amperage is inadequate.

Conclusion

While STRSW machines that use inverter DC have higher efficiency and increased current capabilities compared to non-inverter STRSW machines, both types of equipment are viable for collision repair.

When using any type of STRSW equipment, always be sure to follow the vehicle maker’s recommendations. Also, be sure to make practice welds on the same type and thickness of metal to be welded on the vehicle, and visually inspect and destructively test the welds before making welds on the vehicle (see Figure 4). This will help ensure that the welding machine being used is viable for the procedure being done.

Current Events/News: Advantage Online: 2004 Archives

Principals Of Resistance Welding

Inverter Power Supplies

Conclusion