Electric heating – Metal heating – For bonding with pressure
Patent
1987-05-19
1988-05-24
Shaw, Clifford C.
Electric heating
Metal heating
For bonding with pressure
320 1, B23K 1126
Patent
active
047467835
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to capacitor-operated spot welders, and more particularly to a spot welder in which the electrolytic capacitor is prevented from being reversely charged.
BACKGROUND ART
FIG. 3 is an electrical circuit diagram showing a conventional capacitor-operated spot welder indicated in a capacitor-operated spot welder catalog (May 1982) published by "Origin Denki Shoji Kabushiki Kaisha (Origin Electric Commercial Co. Ltd.)" in Japan. In FIG. 3, reference numeral 1 designates a charging and discharging electrolytic capacitor; 2, a back-to-back type thyristor switch for obtaining current from the capacitor 1; 3, a welding transformer for increasing the current; 4, materials to be spot-welded by the output of the welding transformer; 5, a DC source for charging the charging and discharging electrolytic capacitor 1; and 6, a charging resistor for suppressing the charging current.
The conventional spot welder is constructed as described above. Therefore, when, under the condition that the electrolytic capacitor 1 has been charged to a voltage V by the DC source 5, the thyristor switch 2 is turned on, the electrolytic capacitor 1 is discharged through the welding transformer 3 which is low in impedance, so that a current i having a large peak value flows. The parts (a) and (b) of FIG. 4 are explanatory diagrams showing the terminal voltage V of the electrolytic capacitor and the waveform of the discharge current, respectively. The components of the circuit have a loss. As a result, both the terminal voltage and the discharge current are of an oscillatory waveform which damps with time. The current, which is substantially proportional to the current i in the part (b) of FIG. 4 in accordance with the winding ratio of the welding transformer 3, is applied to the materials 4 through the welding transformer 3. As shown in the part (b) of FIG. 4, the oscillatory current i is interrupted at the current zero point t.sub.off after the thyristor switch is turned off. Thereafter, the electrolytic capacitor 1 is charged by the DC source 5 again; that is, the welding circuit is placed in the initial state. In the spot welder, the pulse-shaped oscillatory current is repeatedly caused to flow to spot-weld the materials.
However, the above-described conventional capacitor-operated spot welder suffers from difficulties that, since the terminal voltage of the electrolytic capacitor becomes negative as indicated by the shaded regions 10a and 10b in the part (a) of FIG. 4, the electrolytic capacitor 1 is deteriorated, so that the welding performance is gradually lowered.
DISCLOSURE OF THE INVENTION
Provided according to the invention is a spot welder in which a main circuit is made up of: a first electrolytic capacitor connected in parallel to a DC source; a second electrolytic capacitor which is connected in series to the DC source and is substantially equal in capacitance to the first electrolytic capacitor and which is shunted by a resistor; a switch connected in series to the second electrolytic capacitor; and a welding circuit connected in series to the switch to weld materials to be welded, and after the first electrolytic capacitor only is charged by the DC source, the switch is turned on to cause the charge current of the first electrolytic capacitor to flow in the welding circuit. In the welder of the invention, the main circuit includes not only the first electrolytic capacitor connected in series to the DC source but also the second electrolytic capacitor which is connected in series to the DC source and is substantially equal in capacitance to the first electrolytic capacitor and which is shunted by the resistor. Therefore, when, after the first electrolytic capacitor only is charged, the switch is turned on, the charge current of the first electrolytic capacitor charges the second electrolytic capacitor, and therefore charging the first and second electrolytic capacitors in opposite directions is prevented, i.e., the voltages of the capacitors are maintained unchanged in polarity.
Acc
REFERENCES:
patent: 2184628 (1939-12-01), Watson et al.
Catalogue of Origin Electric Co., Ltd., issued May 1982.
Shibuya Yoshikazu
Toya Hideaki
Mitsubishi Denki & Kabushiki Kaisha
Shaw Clifford C.
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