HOW TO SET THE WELDING CURRENT

by Sam Belkin, MSEE

Setting the welding current with ammeter and voltmeter

Setting the welding current with ammeter

Setting the welding current by "Trial and Error" method

The next problem awaiting the welder: how to apply the obtained parameters to real welding process? The contact tip diameter can be easily measured, electrode force can be determined from the machine's specifications, or from known air cylinder area and air pressure. Welding time is set with a controller and usually does not depend on the weld itself. However how would the welder set the required welding current? This is not a trivial task since the resistance of the welded parts is unknown. Three different methods can be used to solve this problem.

Setting the welding current with ammeter and voltmeter

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Figure 1. Circuit for measurement of resistance of the welded parts

The first way is to measure the unknown resistance that requires the True RMS Ampere/Volt Meters with a Hold feature. For measuring the resistance a few welds are needed to be made with the measurements of current flow through electrodes and voltage across the welded parts according to the circuit on Figure 1. Then the ratios of voltage over current has to be calculated and an arithmetic mean from these values be taken.

The Voltmeter must be connected to the welded parts, not to the electrodes! The Clamp Ammeter may be connected either to the primary or to the secondary circuit. Since the current range for such meters is limited to 2000 A, the primary circuit connection is only the case if the welding current is more than 2000 A. At that, the meter's readings must be multiplied by the transformer ratio:

Is = Ip x n

where Is is the secondary (welding) current,

Ip is the primary current and

n is the transformer ratio, which is equal to the ratio of the primary voltage over the secondary open circuit voltage.

The best, but more expensive way of measurements is to use Digital Storage Oscilloscope (DSO) with isolated channels Tektronix THS710A (the cheapest one), THS720A or THS720P (the best for power measurements). In this case, the Clamp Current Probe is connected to one channel of DSO and its other channel is connected in place of the Voltmeter as on the Figure 1. For a current range from 0 to 2000 A Tektronix A621 Clamp Current Probe is the best choice and it can be found for about $400. The Extech 380905 model works well for 50 - 2000 A range. This probe is much cheaper (about $100), but is noisy below 50 A. Furthermore other brand name models are available for the up to 10000 A.

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How to set Digital Storage Oscilloscope

The DSO must be set properly. In the case when the current probe is connected to channel 1, set the vertical menu for this channel to AC Coupling, Position 2 div., Probe Type 1 mV/A. Channel 2 vertical menu: AC Coupling, Position - -2 div., Probe Type x1 or x10 depending on the scope's probe. Push the Acquire button, then choose Stop After Single Sequence, Single Acquisition Sequence. Push the Trigger Menu button and set the Trigger Source Ch.1, Mode Norm & Holdoff. For the Horizontal Menu button select Trigger Position 10%. The trigger level needs to be set to approximately 10 - 20 % of expected current value. In the MEAS button menu: push "Select Measrmnt", Ch1 then with the Page button choose Pk-Pk. If the crest factor for current and voltage functions is unknown which is the most common case, Pk-Pk measurements will provide better results. Select the same type of measurement for Ch2. Adjust desirable vertical gain and time.

If the THS 700 family DSO was elected, the voltage probe's ground wire can be connected to the circuit safely. For any another DSO suggested point of grounding of the probe must be checked for electrical potential above the ground before the connection. Remember that only THS 700 family DSO has the floating optically isolated channels. Usage of any other DSO may be extremely DANGEROUS.

The sought resistance will be found as a ratio of obtained current and voltage peak-to-peak values. Knowing Vw, Iw, and Rw values allows to calculate the internal resistance of transformer as a current source Rt = (Vo - Vw)/Iw, where Vo is transformer's open circuit voltage and Vw is the voltage between welded parts. Because Rt and Vo are constant for a given transformer, the welding current can be easily set by measuring Vw with a voltmeter, which can be calibrated in amperes by using the formula

Vw = Vo - (Rt x Iw).

For example Vo = 5V and Rt = 1mOhm (0.001Ohm). Then for the welding current Iw = 1,000A Vw will be 4V, for Iw = 2,000A Vw = 3V and so on.

Furthermore, the phase shift between current and voltage may be checked, and this may say a lot about the welding transformer properties. DSO also allows to save acquired plots on the computer and to print them. This feature is very helpful for troubleshooting. Moreover, the plots can be sent to a manufacturer or to a consultant for further assistance. Knowledge of the values of resistance, current, and voltage is very important for getting the best welds in various situations.

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Setting the welding current with ammeter

The second method is to measure the welding current the same way as it was described above (see Figure 1). This is applicable if the suggested welds are always done with the same machine and set up. Measure the welding current for several different settings and find one of them that is closest to the required value. In this case it is possible to calibrate the machine's current settings for frequently used parts.

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Setting the welding current by "Trial and Error" method

The third way is an old, well-known "Trial and Error" method, which involves welding two parts together, tearing them apart and checking the weld quality. If the weld nugget is too small, the welding current needs to be increased. But if the molten metal squirts out from a spot weld, the welding current must be decreased. Continue to make test welds and tear them apart until the required quality of weld and nugget size are achieved.

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