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A method of measuring the heat generation parameters in the resistance spot welding in single phase AC machines

In a number of cases, for example, in setting of resistance welding machines for the optimum conditions, and in scientific research investigations, it is necessary to measure the main parameter, characterizing the quality of the welded joints, i.e. the amount of heat, generated in the welding cycle in the electrode–electrode section. In direct measurement by the method of calorimetric measurements of the specimens and welding electrodes it is necessary to ensure optimum experimental conditions, use special electrodes, and switch off water cooling of the electrode sections. In this case, the error of measurements is high because of the small value of heat input (1–4 kJ) and heating of the electrodes during the passage of welding current through them. The currently available methods (Author's Certificate No. 967729, 1109291, 1281356, USSR) of parametric measurement of the heat input in the electrode–electrode section are based on the measurement of the instantaneous values of the voltage drop between the electrodes and the current in the welding circuit. Integration of the product of these quantities makes it possible to evaluate the energy generated in the welded section. However, if the measurement of current in the secondary circuit is a technically feasible task, the measurement of voltage in the electrodes is associated with a number of difficulties. Firstly, in welding magnetic materials, the voltage in the electrode–electrode section includes the voltage drop in both the active and reactive components of the total resistance, whereas useful information is provided only by the active component. Secondly, it is not possible to measure the voltage directly in the end section of the electrode, and the measurement of the voltage at a certain distance from the ends causes an error because of the voltage drop in the electrodes. Thirdly, fixing the voltage sensors to the electrodes results in additional contacts in the measurement circuit, and the resistance of the contacts must be efficiently measured and, if possible, taken into account in the measurement results. Therefore, the authors of this article have proposed a method of parametric measurement of heat generation in the electrode–electrode section in which the measurement of current in the secondary circuit is accompanied by the application, as a diagnostic feature, of the duration λ of the welding current pulse of the given angle α of activation in every half cycle. Ignoring the variation of the resistance R e–e of the electrode–electrode section during a single cycle of welding current, the heat generation in this period in accordance with the Joule–Lentz law is: