US2074276A - Apparatus for arc welding - Google Patents

Description

March 16, 1937. R 1 SCHMERBER 2,074,276

APPARATUS FOR ARC WELDING Filed Feb. 28, '1935 Patented Mar. 16, 1937 UNITED STATES mime PATENT GFFCE signor lto La Soudure Electrique Autogne Socit Anonyme, Brussels, Belgium Application February 28, 1935, Serial No. 8,774 In France March 5, 1934 l Claim.

In previous patent application No. 677,721 led 4the 26th of June 1933, (now Patent No. 1,997,238)

there is described a method of feeding an arc welding circuit with alternating current consist- 5 ing in applying across the electrode and the object to be welded at each alternation a difference of potential the open circuit curve of which exhibits at the beginning of the alternation only and during a small fraction of the total duration 1o of the alternation a peak which corresponds to the voltage which is necessary and sufficient for striking the arc and subsequently during nearly the whole of the remainder of the alternation a substantially horizontal stretch which corresponds to the voltage which is necessary and sufficient for maintaining the arc.

This method has been carried out, when feeding from a three-phase source, by employing two transformers, one of which is termed the principal or main transformer and is capable of developing on open circuit its secondary a difference of potential merely suicient to maintain the arc which is assumed to have been struck. The other, which is termed the auxiliary transformer, has its primary fed by the phase which is 120 electrical degrees in advance relatively to the phase which supplies the primary of the principal transformer and is capable of developing on open circuit in its secondary, a peak of voltage which is sufficient to strike the arc. In the said previous patent application, there is described an apparatus of this type in which the secondaries of the two transformers are connected in parallel.

The present invention has for object an apparatus which has a more effective action than the apparatus described in the said previous patent application.

To this end, according to the invention, one or 40 more generators or transformers capable of developing on open circuit a difference of potential sumcient for maintaining an arc after having been struck is or are connected in series with one or more generators or transformers capable 45 of developing .on open circuit a peak of voltage which is suiicient forstriking the arc. This connection in series is eiected in such a manner that the generated electromotive forces are added together with the supplementary condition that 50 these electromotive forces are out of .phase so that the peak voltage which serves to strike the arc occurs at the beginning ofthe alternation of the voltage for maintaining the arc.

In the case where the apparatus comprises two transformers, the primaries of which are connected as aforesaid, and the seccndaries may supply the differences of tension also as aforesaid, the secondary of the auxiliary transformer,

- according to the invention, is connected in series with the secondary of the principal transformer in such a manner that the electromotive `forces generated'in the two windings are added to gether.

By this arrangement, not only is the action of the auxiliary transformer more effective, as it has been proved by experience, but the circulating currents between the seccndaries are automatically suppressed.

It suices that the two transformers have drops of tension compatible with a stable operation of the arc, or in other words, that the difference of potential at the terminals of the latter drops to about 25 volts on closed circuit, which can be easily carried out by any competent welder.

An apparatus according to the invention may be provided by employing for the principal transformer and for the auxiliary transformer two single-phase transformers, the primaries of which are in V-connection to the terminal of the supply network which is assumed to be a three-phase network.

But it is preferred to use as the main transformer a three-phase-single-phase transformer the primary of which is in V-connection with the network and as auxiliary transformer a singlephase transformer, of which the primary is connected to the terminals at the extremities of the V. Y

'Ihis latter form of connection has the advantage that the phases of the supply network are better balanced. In effect, the primary of the auxiliary transformer forms the third phase which exists only virtually in thel ordinary form of V-connection. The three line currents can thus be in the ratio 0.81-0.8, whereas with ordinary three-phase-single-phase transformers the ratio is 05-1-0.5.

The invention will now bedescribed with the aid of the accompanying drawing, which shows diagrammatically and by way of example two embodiments of an apparatus according to the invention.4

In the drawing:-

Figure 1 shows diagrammatically one form of apparatus according to the invention comprising two single-phase transformers the primaries of which are in v-connection with the terminals of a three-phase supply network;

Figures 2, 3 and 4 are curves showing renary metallic sheets.

spectively the open circuit voltage during one alternation of the supply source at the terminals of the secondary of the main transformer, at the terminals of the secondary of the auxiliary transformer and across the electrode and the object to be welded.

Figure 5 illustrates diagrammatically another form of apparatus in accordance with the invention which comprises a three-phase singlephase main transformer, of which the primary is in V-connection with the terminals of a threephase supply network, and an auxiliary singlephase transformer of which the primary is connected to the ends of the V.

Figure 6 represents the curves of the tension on closed circuit and of the welding current of an apparatus according `to the invention, the curves having been registered by an oscillograph.

In the various iigures like numerals represent like parts.

Referring to Figure 1, l, 2 and 3 represent the three supply lines of a three-phase network of which the sequence of the phases is I-2, 2-3, 3--l. The primary 4a of a single-phase transformer 4, hereinafter termed the auxiliary transformer, is connected to the phase I-2 which is electrically 120 in advance of the phase 2 3.

The power of this auxiliary transformer is relatively small and is, for example, of the order of 30-40% of the power of the main transformer which is to be used with it. These gures, how- Vthat it develops an open circuitvoltage which is necessary and sufcient to maintain the arc. On open circuit the voltage from the two transformersis for example 45 to 55 volts. The voltage striking the arc may be, for example, 2 to 2.5 times the voltage read on the voltmeter.

The transformer 5, like all ordinary welding transformers, is a transformer having a drooping load characteristic. The difference of potential which is maintained on loadby the two transformers at the terminals of the arc is about 25 volts.

The curve which represents the open circuit voltage of the secondary is for example of the kind represented by the curve l in Figure 2.

This curve should be as nearly as possible' sinusoidal.

The auxiliary transformer 4 is an ordinary transformer having two cores and two yokes of which the core of the secondary is constructed in such a way that it is rapidly saturated by the flux produced by the current in the primary. For this purpose the vcore is made for example of a metal having a high magnetic permeability such as permalloy. In this way the primary sinusoidal wave is transformed into a secondary wave of peaked form.

The auxiliary transformer can also be constructed by providing a recess in the magnetic circuit which can then be constructed with ordi- A third` core;y which serves as a magnetic shunt may be provided in this case in order to permit the leakage flux to be adjusted by the variation of the size of an air-gap, provided in the core.

The curve which represents the open circuit voltage in the secondary of the auxiliary transformer is for example similar to the curve 22 illustrated in Figure 3.

The secondary winding 5b of the main transformer is connected in series with a secondary winding 4b of the auxiliary transformer in the welding circuit which comprises the electrode 8 and the object 9 to be Welded (see Figure 1). This connection is made in such a way that the voltages generated are added together. The secondary winding 4b must obviously be able to take the welding current. The welding current can be regulated by any suitable regulating means, for example by a self-induction coil 23 provided with contacts 24 over which an arm 25 can be moved.

Owing to the fact that the auxiliary transformer is supplied by the phase which is electrically 120 in advance of the phase which supplies the principal transformer, there is obtained an open circuit voltage curve of the type represented by the curve I4 in Figure 4, which shows that the peak voltage is produced at the beginning of the alternation. 'I'his curve is an enlargement of an oscillograph recordmlt must be mentioned that, owing to the two curves of Figures 2 and 3 being out of phase and having diiferent form factors, the secondary voltages of the two transformers are not added arithmetically, and that these voltages should be in such a ratio that the resultant wave can be represented by the curve indicated at I4 in Figure 4.

Experience has proved vthat the passages by zero of the voltage curve on closed circuit are more quick when the secondaries of the two transform'- ers are connected in series than when they are connected in parallel at the terminals of the arc and that on the other hand the stability during welding is increased.

In practice, it is advantageous to employ,` as

vthe main transformer, a three-phase-singlephase transformer of which the primary winding is in V-connection with the terminals of the supply and, as the auxiliary transformer, a singlephase transformer of which the primary is connected to the extremities of the V.

An apparatus of this type is illustrated diagrammatically in Figure 5. The phases |-2 and 2-3 supply the primary a of the principal transformer 5 which is in V-connection. The phase 3-l supplies the primary 4a of the auxiliary transformer 4. 'Ihe two primaries are thus connected in A-connection to the supply network. The secondaries 5b and 4b remain connected in series in the welding circuit and in such a way that their voltages are added together. In this way a better balance of the load on the three phases of the supply network is obtained than with ordinary three-phase-single-phase transformers with which the currents in the three phases are in the ratio 0.5-l-0.5.

In Figure 6, there is represented an oscillograph record of the tension on load at the terminals of the arc and of the welding current.

'It will be noted at the point l'la of the curve I1 which represents the tension on load a drop of tension produced by the passage of a drop of metal from the electrode to the object to be welded, this drop of metal having produced a partial short-circuit. It will also be noted at the point 26a of the curve 26 which represents the welding current the increase of current which has been produced by this partial short-circuit. It can also be noted the action of the auxiliary transformer' at this moment, which almost instantaneously has brought back the arc voltage to its normal value.

In additions the we or sinking inem" and keeping it stable in spite of the low voltage, the

apparatus in accordance with the invention etiects great `economy in power as compared with 5 ordinary welding transformers and -the power factor of the installation is considerably improved.

What I claim is:

An apparatus for feeding an arc welding circuit from a three-phase source of alternating current,

10 comprising a threefphase single phase principal transformer capable of developing on open circuit v r in its secondary, for each alternation oi the source of current, a difference of potential sumcient for maintaining an arc after having been struck, a 15 single-phase auxiliary transformer capable oi' developing on open circuit in its secondary, for each alternation of the source oi' current, a dierence of potential represented by a curve with one sharp peak whose apex corresponds to a difference oi potential sullicient to strikethe arc, a connec- 5 tion for branching the primary of the principal transformer in V-connection with the threephase source of current, a connection for branching the primary of the auxiliary transformer to the terminals at the extremities of the V, and l04 connections for connecting in series the secondaries of the two transformers in such a manner that the generated electromotive forces are added together.

RAOUL JEAN SCHMERBER. 15

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