Electric heating – Metal heating – By arc
Patent
1992-10-13
1994-08-23
Shaw, Clifford C.
Electric heating
Metal heating
By arc
363151, B23K 910
Patent
active
053409639
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns an alternating current power source for welding, in particular for TIG and electrode welding, for generating an alternating current from a three-phase current.
In the welding of metals, which form nonconducting and/or high-melting oxides, by the TIG process, it is necessary for the purpose of removing the oxide skin to operate either with reversed polarity (positive pole at the electrode), which is possible, however, only with low current intensities in view of the excessive generation of heat at the welding torch, or with an alternating current, which corresponds to the current practice.
In electrode welding with a direct current, blow effects are frequently encountered, which render the control of the arc impossible. Here, again, an alternating current must be used.
According to the prior art, welding transformers are used for this purpose, which are connected on the primary side to two phases of the three-phase supply network and wherein the welding current is regulated by means of scatter yoke cores, transductors or thyristors with phase-gating control.
Power sources of this type have the disadvantage of a high power requirement, which in the case of the welding currents usually required rapidly exceeds the available connected load. Another disadvantage is the asymmetrical loading of the three-phase supply network.
In order to eliminate these disadvantages, more recently power sources have been developed, which are connected to all three phases of the network and contain a three-phase bridge rectifier, together with a subsequent thyristor inverter or transistor inverter. As against conventional transformers with thyristors, such power sources are naturally complicated and expensive.
SUMMARY OF THE INVENTION
It is therefore the object of the invention to provide at a comparatively low cost an alternating current power source which loads all three phases of the three-phase supply network symmetrically.
According to the invention, this object is achieved by a circuit for an AC power source for welding, in particular for TIG (tungsten inert gas) and electrode welding, which generates an alternating current from a three-phase current and includes a three-phase transformer, 6 electronic switches, preferably thyristors and a throttle with center tapping. The three secondary windings of the transformer have center taps, which when switched together form a welding current connection. Three unidirectional windings starts lead to the cathodes of three thyristors, the anodes whereof are combined and lead to a winding start of the throttle. The three unidirectional winding ends lead to the anodes of three thyristors, the cathodes whereof are combined and lead to the other winding end of the throttle. The center tap of the throttle forms the second welding current connection. The thyristors are triggered sequentially so that the actuation of the thyristor at the start of the winding of the first transformer leg is followed by the actuation of the thyristor at the winding end of the second leg and subsequently by the actuation of the thyristor at the winding start of the third leg, followed by the actuation of the thyristor at the winding end of the first leg, etc.. The thyristor act as welding current control elements by means of phase shift control and the phase angle is limited to max 120.degree. in order to prevent internal power source short circuits between the phases. An alternating current is generated in this manner on the secondary side, and all three phases of the network are uniformly loaded on the primary side. Thus, the power source essentially consists of a three-phase transformer, an inductor and six expensive thyristors with a low off-state voltage for system frequency applications and with a circuit for their sequential triggering.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated in the drawing and is described in more detail hereinbelow.
FIG. 1 shows the circuit diagram of the alternating current power source according to the i
REFERENCES:
patent: 2337932 (1943-12-01), Rogers
patent: 2534754 (1950-12-01), Boyer et al.
patent: 3959712 (1976-05-01), Kelley, Jr. et al.
patent: 4435632 (1984-03-01), Risberg
Miller Electric Mfg. Co.
Shaw Clifford C.
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