GB2162704A

GB2162704A - Arc welding generator set

Info

Publication number: GB2162704A
Application number: GB08514376A
Authority: GB
Inventors: Roberto Rasotto; Corrado Leggieri
Original assignee: MOSA SpA
Current assignee: MOSA SpA
Priority date: 1984-06-08
Filing date: 1985-06-07
Publication date: 1986-02-05
Also published as: FR2565517B1; DE3520572A1; ES8609843A1; GB2162704B; ES544598A0; FR2565517A1; GB8514376D0

Abstract

The generator set comprises a high frequency, variable reluctance power generator 10, capacitive means 20 for power factor connection, and a controlled AC/DC converter 30 supplying the welding arc via a filter 40. The converter 30 may be phase-angle controlled by a unit 90 to give selected desired output V/I characteristics (Figure 8). The unit 90 has inputs from manual control 100, 200, 300, 600, 700, and inputs 40a, b representing the welding voltage and current. A variable reluctance auxiliary generator 10' supplies auxiliary equipment and via an exciter unit 70 provides constant current D.C. excitation for the generator 10,10'. A synchronous or asynchronous auxiliary generator may alternatively or additionally be provided (Figures 13,15). A three phase main power generator may be formed by three single phase generators keyed to the same shaft and with their stator or rotor teeth mutually displaced by 120 DEG C electrically. <IMAGE>

Description

SPECIFICATION A generator set for generating d.c. electric power The present invention relates to a generator set for generating direct current electric power suitable for supplying circuits for use in electric arc welding.

Generator sets for use in electric arc welding are known in a variety of structures, dimensions and characteristics. Ideally such a generator set should possess simultaneously and in combination a series of features which comprise: the capacity to carry out efficiently all types of welding, including both electrode welding and unshielded metal arc welding; high flexibility in operation so that it can adjust itself automatically to all welding conditions; be economical to manage, that is consume a minimum quantity of electrical power for carrying out a specific type of work; have structural simplicity and advantageously small bulk having regard to the power generated; in general: be economical to manufacture and operate, and therefore have low unit costs of production; the possibility of generating a voltage or several auxiliary voltages in direct current or alternating current or both, simultaneously or not with the welding operation, for the operating of various and different electrical tools from the welding circuit.

In the art, various generator sets are known, some of which possess one or more of the features listed above, but none are known to us which possess all of them and are appropriate for many if not all industrial requirements.

According to the invention there is provided a generator set for the generation of direct current electrical power in which comprises an electrical power generator having variable reluctance, capacitive means for power factor correction and an electronic circuit for converting alternating current into rectified current with phase angle control.

Preferably, the capacitive power factor correction means are so designed as to reduce the slope of the voltage/current characteristic curve of the generator, at least for a significant portion of the curve, bringing it close to a condition of constant function. In particular the generator set according to this invention comprises a generator for supplying alternating current at a frequency not less than 2000 Herz and preferably higher. As is known, the frequency of the current generated by a generator depends upon the number of poles, upon its magnetic construction and upon the speed at which the generator is made to operate. The generator is associated for its functioning with an internal combustion engine, such as a diesel engine.Such engines, by their nature, having an optimum operating speed for which they are designed and dimensioned, which for the most part may lie in the range 1500 to 3600 rpm. This said, for a generator of variable reluctance, the frequency f of the cur rent produced is determined by the formula f = np 60' where n denotes the rotational speed (revolutions per minute) and p the number of poles of the generator.

To obtain the desired frequencies above 2000 Herz, taking account of the number of poles with which variable reluctance generators are normally equipped and which can be, for exmaple, 24, it is usually necessary to provide a step-up gear enabling the rotational speed of the engine to be increased, bringing it, for example, to values around 6000 to 7000 rpm or above.

According to a preferred form of the invention, the above- mentioned capacitive means are designed so as to impart to the characteristic curve of the power generator a slope less than a specific limit. For the purpose of defining quantitatively this limit, said characteristic curve is considered and on it there is established, for standardization purposes, a point corresponding to a current value of 80A, at said point the slope of the curve, which will be called "standardized slope" and will be measured in volts/amps, shall not be greater than 0.2 V/ A, and preferably not greater than 0.1 ViA. At the limit, the standardized slope may be substantially zero. The electronic conversion circuit referred to it based upon semiconductor valves or devices of the controlled type. Said valves, and the circuits in which they are incorporated, may vary very widely.

The semiconductor valves may be, for example, controlled diodes and said diodes desirably should have a low extinction time, preferably less than 40 Fs and still more preferably less than 20 microseconds. Again, the conversion circuit may comprise one or more transistors. The controlled diodes or transistors may be replaced by components of other types having similar functional characteristics. Among said components, which are covered by the term "controlled semiconductor valves" mention may be made by way of example of the type ASCR (asymmetrical structure controlled diode).According to a particular aspect of the invention, the generator set forming its subject comprises control means for modifying, during the operation of the set as a direct current supply for supplying circuits for electric arc welding, the supply characteristic of the power generator in response to the welding voltage and current signal.

As will be explained later, the characteristics of the direct current supplied to the welding circuit may be extremely varied depending upon the type of welding and the welding conditions. For example, welding with electrodes, in particular cellulosic electrodes, requires a characteristic which is almost vertical or at least of a very steep slope, whereas unshielded metal arc welding demands a characteristic which is almost horizontal or at least of a very flat slope.The generator set may be controlled by the operator in such a way as to supply a characteristic of this class, but it must not be forgotten that in the operating phase, that is during welding, the conditions of formation of the arc change continuously and, consequently, if the functioning of the set is to be efficient, it is desirable that there shall be means which enable the set to adjust itself automatically to the instantaneous welding conditions. The aforementioned control means are intended to achieve this.Basically, it may be stated that the operator arranges in ad vance, depending upon the type of work which he has to carry out, a static supply characteristic, but that the dynamic characteristic, that is to say the characteristic present at every instant of the welding operation, is determined by the static characteristic and by the action of the control means, which provide the variations required at every instant. It should be remembered moreover that said control means are themselves known, in that it is possible to apply to the generator set forming the subject of this invention control means which can be used with other existing sets, and consequently a detailed definition and description of said control means is not necessary. Accordingly, the description of the control means or, as it is termed in the art, "of the control chart" which follows will be relatively brief.According to a preferred and special development of the present invention, the generator set comprises, in addition to the power generator mentioned, an auxiliary generator. In general and preferably, the auxiliary generator is distinct from the power generator: it is composed of a different stator and a different rotor, having the required electromagnetic characteristics. The auxiliary generator supplies a current which can be used via appropriate outputs for the operation of electrical tools or other electrical equipment separate from the welding circuit, for the purpose of carrying out auxiliary operations as is known in the art. Nevertheless, preferably, the current produced by the auxiliary generator is also used, after appropriate processing, for supplying the excitation both of the power generator and of the auxiliary generator itself.The auxiliary generator can also be of variable reluctance and in such a case will preferably be keyed onto the shaft of the power generator, or it can be a generator of some different type, either synchronous or asynchronous, and in such a case will have its own, separate alternator. If the auxiliary generator is absent, the excitation of the power generator will be obtained in some manner known in the art, for example by self-excitation of same.

For the purpose of obtaining an excitation current for the power generator, from the output current of the auxiliary generator, the output current may be processed by suitable transforming for adjusting its voltage to values appropriate for the purpose to be achieved, rectification of the current and a control of same in the manner to be stated.

The direct current obtained by the conversion and control means is then supplied to the excitation windings of the power generator and of the auxiliary generator arranged in series, provided that the latter is of the variable reluctance type. The processing of said current, before supplying it to the excitation windings, is effected by means of electronic circuits, the object of which is to maintain substantially constant said current independently of the welding conditions. These circuits are incorporated in an "excitation diagram", a preferred example of an embodiment of which will be described below.

These and other features and characteristics of the invention will be better understood from the description of a preferred example of an embodiment thereof, with reference to the attached drawings, in which: Figure 1 is a block diagram which illustrates genrally and schematically an example of embodiment of the generator set according to the invention; Figures 2 and 3 are sections through the power generator on a plane perpendicular to the axis of same, but could also be interpreted as sections through the auxiliary generator in the case where it also is of variable reluctance because, in such a case, the two generators differ basically in their axial development;Fig. 2 illustrates by hatching the stator and Fig. 3 the rotor; Figures 4 and 5 illustrate schematically, respectively the excitation winding and the power winding of the power generator, said figures being also capable of interpretation as illustrating respectively the excitation and supply windings of the auxiliary generator, always in the case where the latter is of variable reluctance; Figure 6 illustrates schematically an example of a conversion circuit and its application to the power generator; Figure 7, in its variants a, b, c, d, illustrates possible variants of the conversion circuit, the generator not being shown here schematically; Figure 8 illustrates some possible characteristic curves of the welding current, plotted as direct current, that is after the conversion means;; Figure 9 is a simplified circuit diagram of an example of an embodiment of the invention according to the block diagram of Fig. 1; Figure 10 shows schematically a control diagram according to a preferred example of the embodiment; Figure 11 shows schematically an excitation diagram of a preferred example of an embodiment of the invention according to the block diagram of Fig. 1; Figure 12 shows an example of the voltage-current characteristic curve of the power alternator; Figure 13 is a block diagram similar to that of Fig. 1, but illustrating another example of embodiment; Figure 14 is a circuit diagram analogous to that of Fig. 9, but relating to the example of the embodiment of Fig. 13; Figure 15 shows a block diagram of the complete generator set according to another embodiment of the invention; ; Figure 16 is a simplified circuit diagram embodying the block diagram of Fig. 15; Figure 17 shows the circuit of a control diagram of a preferred embodiment of the invention, according to the arrangement of Fig. 15; Figure 18 shows schematically an excitation diagram in a preferred example of the embodiment according to the arrangement of Fig. 15.

Referring now to Fig. 1, the generator set according to one embodiment of the invention, comprises a power alternator 10 which is of the variable reluctance type and supplies a high-frequency current. For example, in one form of the embodiment of the invention, the alternator may have 24 poles and may be driven, by means of an engine and transmission, at 6500 revolutions per minute, producing a current having a frequency of 2600 Hz.

The alternating voltage obtained from the power alternator is supplied to the capacitive means composed, in this case, of power factor correction capacitors in series 20, which apply to the voltage generated a characteristic which approaches the horizontal. One particular case of said curve is illustrated in Fig. 12, in which the current values in amperes are plotted as abscissae and the voltage values in volts are plotted as ordinates. Said characteristic curve could be plotted in alternating current or direct current, but since we assume that it is measured immediately after the electronic conversion circuit, we may consider it in direct current.

From the diagram, the ordinate at 80A is shown, in relation to which the standardized slope of the curve is measured. The tangent to the characteristic curve at the point in which said ordinate intersects the curve has a slope which is represented in the diagram by the angle a. Given that, as stated, the abscissae of the diagram represent amperes and the ordinates volts, said slope is measured in volt/ampere. In the particular case of the figure, this slope has the value 0.02 V/A. As has been stated, it should not be greater than 0.2 V/A and preferably not greater than 0.1 VIA, and still more preferably it is less than said values, as in the case illustrated.

The current produced, after passing through the power factor correction capacitors 20, passes to the electronic conversion circuit 30 for converting it from alternating current to rectified current 30, with regulation and power factor correction which, in this particular case, is a single-phase bridge with controlled diodes. It passes successively to further circuit means 40, which may comprise filtering inductances and/or capacitances, and in the case illustrated is composed of an inductive filter. From this point, the current is supplied to the welding circuits, not illustrated in Fig. 1.

On the block diagram being described, there is indicated also the auxiliary alternator 10', also of variable reluctance type, which, as already stated, is generally separate from the power alternator, although keyed onto the same shaft. It is, moreover, also possible to construct a single alternator, from which to take a power current for carrying out welding and an auxiliary current for the purposes which will now be described.In every case the current produced by the auxiliary alternator passes via appropriate capacitive power factor correction means 50 located in a branch and, preferably, via a transformer 500, to the excitation unit 70, which comprises means for converting the current from alternating current to rectified current and means for controlling the current itself and thereby maintaining it constant independently of the welding operations and in general of the supply of the set.

The excitation unit receives also the current produced by a flywheel magneto 80, for starting up the set. The direct, constant current obtained from the excitation unit passes to the excitation windings, connected in series, of the power alternator and of the auxiliary alternator. The current generated by the auxiliary alternator, via the capacitive power factor correction means 50, is taken as a supply for auxiliary power, after being rectified and filtered by the unit 60. The voltage generated by the power alternator, after passing through the capacitive power factor correction means 20, is supplied to the control unit 90. The control unit also receives voltage and current signals from the line which conducts the supply of the power generator to the welding circuits (conductors 40a and 40b).

Furthermore there is provided a manual regulator indicated on the diagram by the block 100, for the welding arc; a manual regulator of the voltage, block 200, and of the welding current, block 300, which can be used separately by means of a switch, block 400; and a manual remote control, block 600. The control unit 90 also receives at its input a supply alternating voltage taken from the generating circuit of the auxiliary alternator via the transformer 500, and supplies at its output the control signals for the electronic conversion circuit 30.

A description will now be provided of the construction of the power alternator, which is substantially also the construction of the auxiliary alternator, according to a preferred embodiment of the invention, in which the latter also is of variable reluctance.

The blocks 10 and 10' (Fig. 1) each comprise a variable reluctance alternator, composed of a stator 11 (Fig. 2) having sixteen salient teeth 13 and a rotor 12 (Fig. 3) having twenty4our salient teeth 13a, in this way observing a more general ratio of 3!2 considered optimal for the production of an alternating current at high frequency: in the example at 2600 Herz frequency with 6500 revolutions per minute of the rotor. On the rotor 12 there are no electrical windings, with the obvious advantages of reliability and economy. On the stator 11 of each of the two alternators there is incorporated, in accordance with known techniques, an excitation winding, respectively 14 and 18 (Fig. 4) and a power winding, respectively 16 and 15 (Fig. 5).

Fig. 6 illustrates a preferred example of a conversion circuit, showing schematically the power generator 60 comprising the excitation winding 61, the power winding 62, the conventional capacitive filtering means 63 in parallel with the excitation winding, the capacitive power factor correction means 64 in series with the power winding, and a bridge referenced generally as 65, which constitutes a means for converting alternating current into direct current. In the specific example of the embodiment shown in Fig. 6, said bridge comprises two controlled diodes 66, disposed on an arm of the bridge as illustrated in the figure, and two diodes 67 disposed on the other arm. The inductance 68 serves for equalizing the rectified current leaving the bridge.

Fig. 7 illustrates possible modified forms of the conversion circuit, the schematic representation of the generator having been omitted from this figure.

In the preferred case of Fig. 7, the conversion cir cuit comprises two controlled diodes 78 having a common cathode, being the two arms of the bridge completed by the two diodes 79; in this case there may also be provided a free circulation diode 76.

In Fig. 7b, the circuit comprises four controlled diodes 80 and the free circulation diode 81 may also be provided.

In Fig. 7c, the circuit comprises a single controlled diode 77 and three diodes 77a; this solution permits control of a single half-wave.

Finally, in Fig. 7d, the circuit comprises four diodes 70 arranged as a bridge; an equalizing filter comprising a reactance 71 and a capacitance 72; a transistor 73 made to function in force switching at a high frequency (for example 20 KHz), a free circulation diode 74 and a filter reactance 75 at the output.

Fig. 8 illustrates, in direct current, various possible characteristic curves of the welding current which can be obtained by the generator set making use of various signals received from the control unit 90. Because the complex composed of the conversion means and of the control means constitutes a system reacting in a closed ring, different curves can be obtained depending upon the different signals used. If only the ampere-reading signal is used, constant current characteristics are obtained as indicated by the curve 1.

If only the voltage-measuring signal is used, constant voltage characteristics are obtained as indicated by curve 2. If both the signals are used, characteristics having a constant slope are obtained, as indicated by curve 3; but it is also possible, and this case does occur in practice, to obtain welding characteristics composed of different portions having different slopes, as indicated in Fig. 8 by curve 4. The letter a indicates a reference curve NEMA, which does not need to be described since it is an internationally recognised reference curve for electrode welding processes.

Making specific reference now to the succeeding Figures 9, 10 and 11, a description will now be given of the details of a representative but not limiting example of an embodiment of the electric circuit of the general diagram (Fig. 9), of the circuits of the electronic control unit 90 (Fig. 10), and of the circuit arrangement of the excitation electronic unit 70 (Fig. 11), all relating to the example of embodiment illustrated schematically in the block diagram of Fig. 1.From the power winding 16 of said generator 10 there is obtained an alternating current, which is transmitted to the input of the group of capacitors 20 connected in series with said winding 16, said group being composed of two capacitors C1 and C2, for power factor improvement of the alternating current generated by said generator 10, that is of compensating the high leakage reactance of the alternator, which would be the cause of a very modest obtainable current, or low efficiency of the generator itself. By compensating said reactance, it is possible to make high powers available in relation to the small dimensions of the alternator itself, enabling its output characteristic to become very similar to that of a voltage generator.

The alternating current subjected in this way to power factor correction is supplied to said conversion electronic circuit 30, which converts it from alternating current into direct current, that is to say into a form which can be used for supplying welding means. Said conversion circuit 30 is composed of a single-phase bridge, comprising the controlled diodes SCR1 and SCR2, and the diodes D1 and D2.

The bridge, which is known in the art and of which many other embodiments are possible, functions according to the principle of power factor correction regulation.

In fact, said controlled diodes are made conductive when the ignition command signal arrives from the control unit 90 via the conductors 90a. By retarding the ignition point of the controlled diodes relative to the natural ignition point (passage through zero of the alternating voltage at the input to the bridge 30), it is possible to control the mean values of voltage andlor current supplied to the welding means. The conversion circuit is, moreover, equipped with all the devices necessary for its correct functioning, such as impulse over-voltage suppression devices, composed of the resistors R1, R2, R3, R4 and of the capacitors C4, C5, C6, C7, heat dissipation devices not shown in the figure and the like.The rectified current obtained in this manner is caused to pass via a current equalizing circuit 40, comprising an inductive filter formed by the inductance L1; said filter having the task of damping the current transients which occur during welding and of making the output current from said bridge as continuous as possible. Thus, at the output of the generator set, there is a rectified and equalized direct current of high frequency and having a given characteristic suitable for a specific arc welding process.

In the case of motor-generators used for arc welding, it is necessary to predetermine the voltage-current characteristic curve forming the basis of the welding process used and also the current or voltage values necessary in the various welding situations. In particular, welding with a coated electrode (termed according to the International standards MSAW = Metal shielded arc welding) and welding with a non-consumable electrode (TIG = Tungsten inert gas), require electrical characteristics like that of curve 1 in Fig. 8, whereas welding with a bare electrode and continuous wire (ivilG = Metal inert gas; MAG = Metal active gas) require electrical characteristics of the type illustrated in curve 2 of Fig. 8.

In order to obtain such characteristics, it is necessary suitably to control the ignition points of said controlled diodes.

To this end, the example of the embodiment of the set provides for a control unit 90, which receives at its input: the current and voltage available downstream of said filter 40, respectively by means of the conductors 40a and 40b; gnals coming from the manual welding arc controller 100; signals coming from the welding current controller 300, from the manual welding voltage controller 200 (controllers which in this example of the embodiment coincide) via a switch 400 or from a re mote controller 600 mounted on the welding equipment; a synchronisation alternating voltage obtained downstream of said power factor correction unit 20 by means of the conductors 20a and a supply alternating voltage obtained from said transformer 500 via the conductors 50a.

The control unit 90 is illustrated in Fig. 10 which shows a preferred form of the embodiment and is composed of the blocks described below.

The alternating voltage obtained from the transformer 500 is rectified in the feed block 91 by means of the diode bridge 91c, is filtered by means of the capacitors 91d, regulated to a fixed positive and negative value respectively via integrated voltage controllers 91a and 91b, and then filtered via the capacitors 91e. The voltage values obtained and indicated by p if positive and n if negative, serve to feed the electronic circuits constituting the unit. A signal proportional to the current supplied in welding arrives via the conductors 40b at the differential amplifier-filter 92, with which are associated resistors and capacitors in a manner known and not further described. By this block the signal is amplified and filtered to remove superimposed harmonics.

The output voltage signal in welding arrives via the conductors 40a at the amplifier 93, with which are associated resistors and capacitors in a known manner not further described here. The signals, after filtering by means of the capacitor 93a, are amplified and supplied to a divider 94, composed of resistors 94a, 94b and 94c, where it is algebraically summated to the current signal from the circuit 92. The diode 93b, in conjunction with the value of the resistors of the circuit 93, enables the voltage signal to intervene in the algebraic sum solely for output voltages less than the values of the NEMA reference curve leading to characteristics of type 4 of Fig. 8. In the algebraic sum, moreover, there are different weightings between the current signal and the voltage signal, depending upon the short circuit value imposed upon the welding arc controller 100.The output signal from said divider 94 is supplied to the input of the error amplifier block 95, composed of the impedance adaptor 95a and of the amplifier 95b, with which are associated resistors and capacitors in a manner known and not further described here. In this stage, the summated current-voltage output signal is compared with a current reference signal provided by a divider composed of the block 95c and of said welding current manual controller 300. The error signal, amplified at the output of said block 95, is supplied to the integrated circuit 96a forming part of said block 96 for power factor correction control. The integrated circuit 96a is supplied also with the signal for synchronisation with the controlled diodes of said conversion circuit 30, via the conductors 20a and the transformer 98a.On the basis of the value of the error signal, the integrated circuit 96a supplies the ignition command to the two controlled diodes after amplification and electrical insulation via the transistors 97 and the transformers 98, with a delay regulated with respect to the natural switching instant of the controlled diodes themselves. On the basis of the position selected by the operator both on said welding current controller 300 and on said welding arc controller 100, the desired output characteristics for the welding processes with electrodes are obtained.

In the case in which it is desired to carry out unshielded metal arc welding, it is necessary to operate the switch 400 in such a manner that, automatically, the welding arc controller 100 is inhibited and the manual welding current controller (300) becomes the controller for the welding voltage (200).

The conductors 90b at the input to block 99 are connected to the control push-button 700 located on the MIG-MAG welding means in such a way that, at the instant at which the operator decides to interrupt welding, the generator set produces no power. The block 99 has the function of inhibiting, in this case, the integrated circuit 96a from supplying the ignition command to the controlled diodes.

The control unit 90 described constitutes the return arm of a closed ring feed back system in which said conversion means 30 constitute the controlled circuit.

Said generator set therefore enables the operator to have electrical output characteristics which can be selected on the basis of the welding process used and values of current and voltage adapted to the conditions in which welding is carried out, from one instant to the next. The most suitable values of voltage andior current having been selected by the operator, the generator set functioning as a closed ring keeps under control, at every instant, the electrical output signals and varies the internal parameters of electrical type in such a manner as to respond to the variations in the output signals relative to the electrical reference signals imposed by the operator.

As a result of the use of a variable reluctance alternator generating an alternating voltage at high frequency (e.g. 2600 Hz), extremely rapid response times of the generator set in relation to the variations of the controlled output signals are obtained.

Referring to Figures 1, 9 and 11, it can be seen that the set comprises also the excitation unit block 70, the circuit arrangement of which is now described.

In particular, there is present, a single-phase, controlled diode bridge 71, which receives at its input the alternating output voltage from the transformer 500 via the conductors sob. The ignition commands for the controlled diodes 79 are supplied via the conductors 78a. The bridge is equipped with all the means necessary for its correct functioning, including the capacitance 71a and resistance 71b, which form the attenuation filter for impulse over-voltages. Downstream of the bridge 71 there is provided a filter 72, which supplies at its output the excitation current to said windings 14 and 18 of said alternators 10 and 10' via the conductors 14a. The filter is composed of the inductance 72b and the capacitances 72a.

At said excitation winding 18 there arrives, also, the current generated by the fly wheel magnet 80, via the conductors 80a and after rectification by means of the diode 72c. The diagram comprises also a supply circuit, referenced 73, composed of a diode bridge 73a and filter capacitances 73b. The positive and negative voltages thus obtained serve to supply the electronic circuits. The block 74 is composed of a differential amplifier 74a having also the function of a filter, which amplifies the output current signal capped by means of the shunt 77 and filters it to remove superimposed harmonics. The output signal of the amplifier 74a is supplied to the block 75, composed of an error signal amplifier 75a; said signal is compared with a reference signal obtained by means of a Zener diode 76.The output of the error amplifier is supplied to block 78, composed of an integrated circuit 78c, adapted for the phase angle control of the controlled diodes 79. The synchronisation of the ignition commands for the controlled diodes with the alternating voltage at the input to the bridge 79 is obtained by means of the conductor 78b. The function of the circuits forming part of the excitation unit is to maintain constant, during variations in the operating conditions of the generator set, the current supplied to the excitation windings of the alternators.

This is achieved by retarding the ignition instant of the controlled diodes 79 with respect to the natural ignition instant (passage through zero of the input alternating voltage to the bridge), on the basis of the excitation current value supplied, measured by means of the shunt 77.

In the diagram of the generator set of Fig. 9, the power factor correction capacitor C3 appears in the block 50 branching from said power winding 15 of said alternator 10'.

The alternating voltage is rectified by means of the single-phase bridge 60, comprising four diodes D4, D5, D6, D7, the resistors R6, R7 and the capacitor C8 for filtering the switching over voltages. The rectified voltage at the output serves for supplying auxiliary equipment for the welding operations, for lighting or the like during welding.

According to another preferred embodiment of the invention, illustrated in the block diagram of Fig. 13, the auxiliary generator is an asynchronous generator, referenced 10". In Fig. 13, the blocks which are the same as those in Fig. 1 are given the same reference numerals. The asynchronous generator, fairly similar in its construction to a normal asynchronous cage motor, well known in the art, is composed of a stator, on which are incorporated the windings intended for producing the rotating field by means of excitation capacitors, and of a rotor, in which are housed appropriate conductors, the ends of which are short-circuited among themselves to allow free circulation of the induced currents. Such a rotor has a rotational speed of 1500 or 3000 revolutions per minute, depending upon the motor used.The alternating current supplied by the asynchronous generator at industrial frequency (50 or 60 Hz) is used: directly for supplying various electrical tools from the welding circuit and, preferably via the transformer 500, for supplying the excitation unit 70' and the control unit 90.

The direct current, constant independently of the welding operations and in general of the supply of the set, obtained at the output from the excitation unit 70', is supplied to the excitation winding only of the power generator 10. The functioning of all the other units forming a part of the example of embodiment of Fig. 13, in particular of the variable reluctance generator 10 and of the electronic conversion circuit 30, is identical to that described for the example of embodiment in Fig. 1.

Fig. 14 is a circuit diagram relating to the block diagram of Fig. 13. Because it is analogous to the diagram of Fig. 9, the components which remain unchanged are given the same reference numerals.

In particular, in the diagram of Fig. 14, it may be noted: that the asynchronous generator 10", threephase in this example, comprises the excitation capacitors C10, C11, C12: that the auxiliary current supplied is alternating at industrial frequency; that the excitation unit 70' supplies direct current to the excitation winding of the power generator 10.

The modifications applied to the electronic circuits of the excitation unit 70' in Fig. 14 as compared with the diagram of Fig. 11 do not need to be illustrated separately, in that they consist essentially in the elimination of one of the two capacitors 72a and of the diode 72c.

The description given above of the preferred embodiments of the invention presuppose a supply on the welding side of the rectified current which is obtained from the single-phase alternating current issuing from the power generator according to this invention. It is, however, possible also to obtain the welding current by rectification of a threephase current obtained from three power generators according to this invention, keyed onto the same shaft with the teeth of the rotors or stators of each generator phase-displaced by 120 electrically with respect to those of the other two generators.

In this case the electronic conversion circuit will be three- phase, that is composed of three rectification branches. Such a conversion circuit may be of the semicontrolled type, each of the three arms being composed of a diode and of a controlled diode and a diode of the totally controlled type, each arm thus being formed of two controlled diodes.

A further example of embodiment of the invention is shown schematically in the block diagram of Fig. 15, in which it will be seen that the supply arm for welding current, comprising the blocks, 10, 20, 30, 40, remains unchanged, whereas the construction of the feed arms for the current to the auxiliary services and for excitation of the power generator is simplified by the elimination of some components, as will be described below.

The block 10' represents the excitation alternator, which is of the variable reluctance type and is generally separate from the power alternator, although keyed onto the same shaft of the motor M. The supply from the excitation alternator 10' passes via the excitation unit 70, which comprises means for converting the current from alternating to rectified current, and at which also there arrives the current generated by a flywheel magneto 80, for starting up the set. The direct current obtained from the excitation unit passes to the excitation windings of the power alternator 10 and of the excitation alternator 10'.

The voltage generated by the power alternator, after having passed through the capacitive power factor correction means 20, is supplied to the control unit 90, which also receives voltage and current signals from the line which leads from the power generator output to the welding circuits (conductors 40a and 40b). In addition, a manual control is provided, indicated on the diagram by the block 100, for the welding arc; also a manual control of the welding voltage and/or current, block 300, capable of being used separately by means of a switch indicated with the block 400; also a remote manual control, block 600. The control unit 90 receives also at its input an alternating supply voltage tapped from the output circuit of the excitation alternator 10' and supplies, at its output, the command signals for the electronic conversion circuit 30.There is also present a block 10", constituting the auxiliary current generator driven by said internal combustion engine M and supplying the auxiliary services not shown in the figure.

Making special reference now to the succeeding Figures 16, 17, 18, a description will be given in detail of a representative but non-limiting example of embodiment of the electric circuit of the general diagram (Fig. 16), of the circuits of the electronic control unit 90 (Fig. 17) and of the circuit arrangement of the excitation electronic unit 70 (Fig. 18), all of these relating to the example of embodiment illustrated schematically in the black diagram of Fig. 15. As stated in the case of motor- generators used for arc welding, it is necessary to predetermine the voltage-current characteristic curve on the basis of the welding process used and also the values of current or voltage necessary in the various welding situations, in order to obtain electrical characteristics of the constant current type and electrical characteristics of the constant voltage type.

For obtaining such characteristics, it is necessary appropriately to control the ignition points of said controlled diodes.

To this end, the example of embodiment of the set provides a controlled unit 90, which receives at its input: the current and the voltage available downstream of said filter 40, respectively via the conductors 40a and 40b; signals coming from the manual welding arc controller 100; signals coming from the welding current and voltage controller 300 (controllers which, in this example of embodiment, coincide) via a switch 400 or from a remote controller 600 located on the welding means: an alternating synchronization voltage tapped downstream of said power factor correction unit 20 by means of the conductors 20a and an alternating supply voltage obtained from the excitation alternator 10' via the conductors 50a.

Said excitation generator 10' comprises an excitation winding 18 and three working windings, respectively 15a for the supply to the unit 90 via the conductors 50a, 15b for the supply of the excitation current to the power generator with rectification previously carried out in the excitation unit 70 and 15c which provides the self excitation current of said generator 10' after processing in the unit 70.

There is also provided a flywheel magneto 80, which supplies the start-up current for the excitation generator.

The auxiliary generator 10" is, in this example, of the asynchronous type, fairly similar in construction to a normal asynchronous cage motor and is composed of a stator, on which are incorporated the windings intended for producing the rotating field by means of excitation capacitors C10, C11, C12, and of a rotor, in which there are housed appropriate conductors, the ends of which are short-circuited among themselves for the free circulation of the induced currents.

Said rotor has a rotational speed of 1500 or 3000 revolutions per minute, depending upon the motor used. The alternating current generated by the asynchronous generator at industrial frequency (50 or 60 Hz), is used: directly for supplying various electrical tools from the welding circuit.

Said control unit 90 is illustrated in Fig. 17 according to a preferred form of embodiment, composed of the blocks described below.

The alternating voltage obtained from the excitation alternator 10' is rectified in the supply block 91 by the diode bridge 91c, is filtered by the capacitors 91d, is regulated to a positive and negative fixed value respectively by the integrated voltage regulators 91a and 91b, and is then filtered by the capacitors 91e.

The voltage values obtained and referenced p if positive and n if negative, serve for supplying the electronic circuits constituting the unit. The signal proportional to the current supplied in welding arrives via the conductors 40b at the differential amplifier-filter 92, with which are associated resistors and capacitors in a known manner not further described here. By this block 92, said signal is amplified and filtered to remove the superimposed harmonics.

The output voltage signal in welding arrives via the conductors 40a at the amplifier 93, with which are associated resistors and capacitors in known manner, not further described here. Said signal, after filtering by the capacitor 93a, is amplified and supplied to the divider 94 composed of the resistors 94a, 94b and 94c, where it is algebraically summated to the current signal issuing from the block 92. The diode 93b, in conjunction with the value of the resistors of the block 93, enables the voltage signal to intervene in the algebraic sum solely for output voltages less than the values of the NEMA reference curve.

In the algebraic sum moreover, there are different waitings between the current signal and voltage signal depending upon the short-circuit value imposed upon the welding arc controller 100. The output signal from said divider 94 is supplied to the input of the error amplifier block 95, composed of impedance adaptor 95a and of the amplifier 95b, with which are associated resistors and capacitors in known manner, not further described here. In this stage, the summated current-voltage output signal is compared with the current reference signal given by the divider composed of the block 95c and of the manual welding current controller 300.

The error signal, amplified at the output from said block 95, is supplied to the block 996. At block 996 there arrives also the signal proportional to the current from block 92, which passes to the amplifier 996b, with which are associated resistors and capacitors in known manner, not further described.

Said signal is compared with the maximum current reference signal given by the divider 996c. The output signal from the amplifier 996a, via the diode 996e, passes together with the signal coming from the block 95, via the diode 996a, to the integrated circuit 96a, through the divider 996d, forming part of said block 96 for the phase angle control. To said integrated circuit 96a there is also supplied the signal for synchronization with the controlled diode of said conversion circuit 30, via the conductors 20a and the transformer 98a.

On the basis of the value of the error signal, the integrated circuit 96a supplies the ignition command to the two controlled diodes after amplification and electrical insulation via the transistors 97 and the transformers 98, with a delay regulated with reference to the natural switching instant of the controlled diodes themselves. On the basis of the position selected by the operator both on said welding current controller 300 and on said welding arc controller 100, the desired output characteristics for the welding processes with electrodes are obtained.

In the case in which it is desired to carry out unshielded metal arc welding, it is necessary to operate the switch 400 in such a way that, automatically, the welding arc controller 100 is inhibited, the manual welding current controller (300) becomes welding voltage controller and that the signal coming from the divider 94 is replaced by the signal coming from the block 998. The block 998 causes the voltage signal to intervene in the algebraic sum with the current signal, coming directly from the block 92, for any value of output voltage.

The conductors 90b at input to the block 99 are connected to the control button 700 located on the MIG-MAG welding means in such a manner that, at the instant at which the operator decides to interrupt welding, the generator set produces no power. The block 99 has the function of preventing, in this case, the integrated circuit 96a from supplying the ignition command to the controlled diodes.

The control unit 90 described constitutes the return arm of a closed ring feed back system, in which said conversion means 30 constitute the controlled circuit.

The generator set forming the subject of the present invention, constructed according to the examples of embodiment described above, possesses to a high degree the qualities required of an industrial motor welding set adapted for supplying circuits for electric arc welding.

In particular the use of a power generator with variable reluctance which produces an alternating current at high frequency, equal to 2600 Hz in the examples described, in conjunction with the use of the electronic conversion circuit, enables excellent static and dynamic characteristics to be obtained, both for welding with electrodes and for unshielded metal arc welding.

In fact, the complex composed of the electronic conversion circuit and of the electronic circuits of the control unit constitute a closed ring reaction system which, operating at high frequency, is capable of responding to the variations which take place during welding in the current and/or voltage signals, relative to the reference values imposed by the operator, in very short times, less than 10 ms and preferably less than 5 ms. Such response times are 10 to 30 times shorter than the response times obtained with motor welding sets which use, as power generator, an alternator functioning at industrial frequency (50 to 60 Hz).

Moreover, the use of a power generator of variable reluctance at high frequency enables a motor welding set to be obtained of reduced bulk and weight as compared with a machine which, for equivalent power, uses a generator operating at industrial frequency. It is therefore possible to achieve a notable economy in manufacture and use.

Claims

1. A generator set for generating direct current electric power suitable for supplying circuits for electric arc welding, comprising a variable reluctance electric power generator, capacitive power factor correction means, and an electronic circuit for converting from alternating current to rectified current with automatic control.

2. A generator set according to claim 1, wherein the converting circuit is provided with automatic phase angle control.

3. A generator set according to claim 1 or 2, further comprising means for the generation of power for the supply of auxiliary services.

4. A generator set according to claim 1, 2 or 3, in which the capacitive means are so designed as to reduce the slope of the voltage-current characteristic curve of said generator, at least for a significant portion of this curve, bringing it close to a condition of constant voltage.

5. A generator set according to claim 1, 2, 3 or 4 comprising control means for modifying, during the operating of the set as a source of direct current for supplying circuits for electric arc welding, the output characteristic of the power generator in response to welding voltage and current signals.

6. A generator set according to any one of the preceding claims, comprising motor means for driving the generator such that the alternating current produced by the generator as a function of the number of poles of same, has a frequency of at least 2000 Herz.

7. A generator set according to any one of the preceding claims, comprising inductive and/or capacitive equalizing circuit means and/or filtering circuit means at the output of the conversion circuit.

8. A generator set according to any one of the preceding claims, in which the conversion circuit comprises valves having controlled semiconductors.

9. A generator set according to claim 8, in which said semiconductor valves are controlled diodes which have an extinction time of less than 40 microseconds.

10. A generator set according to any one of the preceding claims, comprising an auxiliary generator, which may be of variable reluctance or of other type, synchronous or asynchronous.

11. A generator set according to claim 10, in which the current generated by the auxiliary generator is used, after rectification, as excitation current for the power generator and for the auxiliary generator, when the latter is of variable reluctance.

12. A generator set according to claim 10 or 11, in which the supply circuit of the auxiliary generator comprises means for transforming and rectifying the current generated.

13. A generator set according to claim 12, in which the auxiliary generator is a variable reluctance generator, and the excitation windings of the power generator and of the auxiliary generator are connected in series with one another and are supplied through said rectification means for the current generated by the auxiliary generator.

14. A generator set according to claim 13, comprising electronic circuit means for maintaining substantially constant the direct current supplied to the excitation windings of the power generator and of the auxiliary generator, independently of the welding conditions.

15. A generator set according to any one of claims 10 to 14, comprising means for obtaining from the auxiliary generator direct current for supplying various circuits from, or other than, the welding circuits.

16. A generator set according to any one of the preceding claims, in which the supply circuit of the power generator comprises one or more capacitors in series.

17. A generator set according to any one of the preceding claims, in which the supply circuit of the power generator comprises one or more capacitors in shunt.

18. A generator set according to any one of the preceding claims, comprising one or more capacitors connected in parallel with the heads of the excitation winding of the power generator.

19. A generator set according to any one of the preceding claims, in which the conversion circuit comprises at least one controlled diode of asymmetrical structure (ASCR).

20. A generator set according to any one of claims 1 to 17, in which the conversion circuit comprises at least one transistor.

21. A generator set according to any one of claims 5 to 20, in which the control means for modifying the supply characteristic of the power generator comprise circuit elements adapted for receiving voltage-measuring and current-measuring signals from the output which supplies the welding circuit and circuit means adapted for permitting the operator, at his choice, alternatively or simultaneously, to predetermine the voltage and/or current of the voltage-current welding curve, the insertion of specific welding strategies, the limitation of the maximum current, and the interruption of the functioning of the conversion circuit.

22. A generator set according to any one of claims 1 to 9, further comprising an excitation generator for said power generator and an auxiliary generator for the supply of the auxiliary services, said generators being driven by the same power source but separately from one another and from the power generator.

23. A generator set according to claim 22, in which said independent excitation generator is of the variable reluctance type.

24. A generator set according to claim 22, in which said excitation generator is of the self-excitation type.

25. A generator set according to claims 20 to 22, in which said excitation generator comprises an excitation winding and three working windings, of which the first is for the excitation of the power generator, the second for self-excitation, and the third for the supply of the electronic conversion circuits.

26. A generator set according to claim 22, in which said auxiliary generator is of the synchronous or asynchronous type.

27. A generator set according to claim 26, in which the output current from said auxiliary generator is at grid frequency.

28. A generator set substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.