The technology is known that controls a welding electric current in a high precision by performing a primary electric current control through an inverter in a welding transformer of resistance welding device (referring to a Patent document 1). Further, through devising its coil structure, a further high speed control is attempted (referring to a Patent document 1).
Resistance welding technology is widely used for the manufacture line of a car or general industries. In those industries, in order to overcome competition on a world level, improvement in productivity is indispensable.
Furthermore, development of the energy-saving technology which cuts down CO2 from the viewpoint of earth-environment protection is also an urgent subject. However, the conventional resistance welding method for the steel plate with bad weld ability was presenting the aspect which moved against productivity or energy saving. The various resistance welding methods that it was suitable for such various uses are developed (referring to a Patent document 1,2,3,4).