TECHNICAL FIELD

The present invention relates to a resistance annealing furnace for annealing a metal wire, strand, string, wire rod or strap.

In particular, the present invention is advantageously, but not exclusively applied to an in-line resistance annealing furnace, i.e. placed directly at the outlet of a machine for manufacturing a metal wire or wire rod, e.g. a drawing machine, to which explicit reference will be made in the following description without because of this losing in generality.

BACKGROUND ART

A direct current resistance annealing furnace adapted to be arranged in-line with a drawing machine normally comprises at least two, and in particular three, electric axes, provided with respective pulleys and motorized to feed the metal wire, a plurality of idle or motorized transmission rolls and a motorized outlet pull ring. The transmission rolls and the outlet pull ring are arranged so as to define a given path for the wire, which starts about a first electric axis, turns about the other two electric axes and the transmission rolls and ends about the outlet pull ring.

The annealing furnace comprises an electric apparatus for generating a direct current voltage which is applied between the second electric axis and the other two electric axes, i.e. the positive potential of the electric voltage is applied to the second electric axis and the negative potential of the electric voltage is applied to both the first and the third electric axis. The annealing process occurs by Joule effect due to the current passage in the first wire lengths between the second electric axis and the other two (first and third) electric axes.

The path of the wire is divided into a first pre ^¬ heating stretch, which goes from the first electric axis to the second electric axis, a real annealing stretch, which goes from the second electric axis to the third electric axis, and a cooling stretch, which goes from the third electric axis to the outlet pull ring. The pre-heating stretch is longer than the annealing stretch so that the temperature of the wire in the pre-heating stretch is lower than in the annular stretch.

The electric voltage applied between the annealing axes and the corresponding electric current which circulates in the wire are commonly known as "annealing voltage" and "annealing current", and in general depend on the length of the pre-heating and annealing stretches, on the feeding speed of the wire along the path and on the section of the wire. In particular, it is known to represent the dependence between annealing voltage and feeding speed of the wire by using a so-called annealing curve. According to the annealing curve, the required annealing voltage increases as the feeding speed increases. Furthermore, the annealing current, in general, increases as the cross section of the wire increases. Over given wire section values, the maximum wire speed value is determined by various factors, such as, for example, the cooling capacity of the cooling stretch. It derives that the speed may be high for small cross sections of the wire, to which low annealing currents correspond, and thus the annealing voltage must be high. On the other hand, the speed must be lower for large cross sections, to which high annealing current correspond, and thus the annealing voltage must be lower .

The electric apparatus comprises a three-phase transformer, in which the primary circuit is supplied by the three-phase network, e.g. the 400 V and 50 Hz three- phase network, and a controlled rectifier circuit, which is coupled to the secondary circuit of the transformer to supply the annealing voltage. In order to reach the required annealing temperatures (a few hundreds of degrees Celsius), the transformer is sized to supply an alternating current voltage to the secondary circuit having an amplitude in the order of size of the maximum annealing voltage to be obtained and a maximum annealing current which depends on the overall features of the annealing furnace (wire path length and wire feeding speed) and on the cross section of the wire. For example, the transformer is sized to supply an alternating current voltage of approximately 70 V for a power of approximately 1000 kVA.

The rectifier typically consists of a thyristor bridge (SCR) . The modulation of the annealing voltage is obtained by varying the firing angle of the thyristors. In other words, the voltage reduces, starting from the maximum value, with the reduction of the firing angle of the thyristors. However, the firing angle decreases the power factor of the apparatus, i.e. increases the reactive power which is exchanged by the apparatus with the electric network. A high reactive power results in a power engagement of the electric network which does not result in a creation of active work. Furthermore, the national authorities which control the distribution of electricity on the power network normally apply penalties when the reactive power exceeds a given percentage of the delivered active power.

A further disadvantage of the apparatus described above is the cumbersome size of the transformer, which is in fact oversized for its use because it never supplies the maximum current at the maximum voltage to the secondary circuit .

An electric apparatus which overcomes some of the drawbacks of the apparatuses described above is known. This other apparatus differs from the described one substantially in that it comprises a transformer with a plurality of tap points on the primary circuit. The tap point of the primary circuit which allows to maximize the firing angle of the thyristors of the rectifier and thus to minimize reactive power is selected according to the section of the wire to be annealed. However, the transformer with multiple tap point primary circuit is also oversized, and in all cases more complicated and costly than a transformer with a simple primary circuit. Furthermore, it is economically inconvenient to construct large-sized transformers (e.g. 70 V for 1000 kVA on the secondary circuit) with more than four tap points on the primary circuit.

A known architecture alternative to the use of a transformer with multiple tap point primary circuit comprises a simple primary circuit transformer and an AC/AC inverter coupled to the primary circuit of the transformer to adjust the power voltage of the primary circuit to a higher number of levels and thus correspondingly adjust the voltage supplied by the secondary circuit. This solution allows to reduce the reactive power further, but the drawbacks related to large sized transformer remain.

DISCLOSURE OF INVENTION

It is the object of the present invention to make a resistance annealing furnace to anneal a metal wire, which furnace is free from the drawbacks described above and which is at the same time easy and cost-effective to make.

In accordance with the present invention, a resistance annealing furnace for annealing a metal wire, strand, string, wire rod or strap is provided as defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the accompanying drawings, which show a non- limitative embodiment thereof, in which:

- figure 1 schematically shows the resistance annealing furnace made according to the present invention;

- figure 2 shows the annealing voltage generator of the furnace in figure 1 by means of a block chart;

- figure 3 shows the voltage wave forms in various intermediate points of the voltage generator of the figure 2 ; and

- figures 4 and 5 illustrate two respective inner stages of the voltage generator in figure 2 in greater detail .

BEST MODE FOR CARRYING OUT THE INVENTION

In figure 1, reference numeral 1 generically indicates, as a whole, a direct current resistance annealing furnace for annealing a metal wire, the latter indicated by reference numeral 2, for example a copper or aluminum wire. The annealing furnace 1 is of the type adapted to be inserted in-line, i.e. at the outlet of a drawing machine (not shown) . The wire 2 exits from the drawing machine and enters into the annealing furnace 1 by moving forward in direction 3 and exits from the annealing furnace 1 in direction 4.

With reference to figure 1, the annealing furnace 1 comprises three electric axes 5, 6 and 7, which are provided with respective pulleys 8, 9 and 10, two transmission rolls 11 and 12, which are either idle or motorized and are arranged between the first two electric axes 5 and 6, and a motorized outlet pull ring 13. The transmission rolls 11 and 12 and the outlet pull ring 13 are arranged so as to define a given path for the wire 2, which starts about pulley 8 of the electric axis 5, turns about the transmission rolls 11 and 12 and the pulleys 9 and 10 of the other two electric axes 6 and 7, and ends about the outlet pull ring 13. The wire 2 runs along such a path pulled by the outlet pull ring 13. Advantageously, electric axes 5-7 are also motorized to aid the pulling of the wire 2.

The annealing furnace 1 comprises a DC voltage generator 14, which can be supplied with an AC voltage, and in particular with the three-phase voltage Uac supplied by a three-phase electric network 15, to generate a DC voltage, the so-called "annealing voltage", indicated by Uann in the figures, which is applied between the electric axis 6 and the two electric axes 5 and 7. In other words, the positive potential of the voltage Uann is applied to the electric axis 6 and the negative potential of the voltage Uann is applied to the other two electric axes 5 and 7. The annealing process occurs by Joule effect because of the passage of electric current in the wire lengths between the electric axis 6 and the two electric axes 5 and 7.

The path of the wire 2 is divided into a pre-heating stretch, which is indicated by reference numeral 16 and goes from electric axis 6 to electric axis 5 passing through the transmission rolls 11 and 12, an real annealing stretch, which is indicated by reference numeral 17 and goes from electric axis 6 to electric axis 7, and a cooling and drying stretch, which is indicated by reference numeral 18 and goes from electric axis 7 to the outlet pull ring 13. In the case of the considered example, in which the wire 2 is made of copper or aluminum, the pre-heating stretch 16 is longer than the annealing stretch 17 so that a current Iprh, which is lower than the current Iann that circulates in the wire portion 2 along the stretch 17, circulates in the portion of wire 2 along the stretch 16, the section of the wire 2 being equal. In such a manner, the temperature of the wire 2 in stretch 16 will be lower than that of the wire 2 in stretch 17. The cooling and drying stretch 18 crosses a tank full of cooling liquid and is provided with drying devices, the tank and the drying devices being known per se and thus not shown.

With reference to figure 2, according to the present invention, the voltage generator 14 comprises an active supplying stage 19 supplied with the three-phase voltage Uac to generate an intermediate DC voltage, indicated by Udc, a pulse width modulating stage 20, or more simply a PWM modulating stage, to transform an intermediate voltage Udc into a first PWM voltage, which is indicated by Uml, and has a zero mean value and an amplitude substantially equal to the intermediate voltage Udc, a high-frequency voltage transformer 21 with transformation ratio higher than 1 to transform the voltage Uml into a corresponding second PWM voltage, which is indicated by Um2 but has non ^¬ zero mean value and an amplitude smaller than that of the voltage Uml, and an passive rectifier stage 22 to transform the voltage Um2 into the annealing voltage Uann.

Figure 3 shows, in a qualitative manner and by way of example only, the wave forms of the various voltages Uac, Udc, Uml, Um2 and Uann.

With reference to figure 4, the active supply stage 19 comprises an input low-pass filtering stage 23 adapted receive the three-phase voltage Uac, a voltage wave form distortion detector unit 24, which is connected to a node 25 of the low-pass filtering stage 23 and is adapted to measure the instantaneous values of the three-phase voltage Uac and to supply a signal indicative of the distortion of the three-phase voltage Uac with respect to an ideal three- phase voltage (sinusoidal at the nominal frequency of the three-phase voltage network 15) as a function of the instantaneous measured values, an AC/DC converter stage 26 connected to the output of the low-pass filtering stage 23 to convert the filtered voltage into the intermediate voltage Udc, and a control unit 27 configured to measure the intermediate voltage Udc and to control the AC/DC converter stage 26 as a function of the signal supplied by the voltage wave form distortion detector unit 24 and of the measured intermediate voltage values Udc so as to reduce the reactive power which engages the three-phase electric network 15 and which flows through the input 19a of the active supplying stage 19. The active supplying stage 19 performs the function of a so-called Active Front End Rectifier (AFE Rectifier) .

The low-pass filtering stage 23 is an LCL filter, known per se and therefore not illustrated in greater detail. The low-pass filtering stage 23 comprises at least one capacitor with a terminal connected to the node 25. The active supplying stage 19 comprises a power contactor or power switch 28 arranged upstream of the low-pass filtering stage 23 and a pre-charging circuit 29 connected between the input 19a and the node 25 for pre-charging said capacitor. The capacitor is pre-charged with the power contractor 28 open.

The AC/DC converter stage 26 is of the substantially known type, and thus not shown in detail, and comprises a plurality of IGBT devices and a set of capacitors to level the intermediate voltage Udc and inject reactive current harmonics which are required by the load constituted by the stages connected downstream.

The control unit 27 comprises voltage measuring means comprising an A/D converter 30 connected to the output of the AC/DC converter stage 26 for measuring the value of the intermediate voltage Udc according to known techniques. The control unit 27 controls the switching on and off of the IGBT devices of the AC/DC stage converter 26 as a function of the measured values of the intermediate voltage Udc and of the signal supplied by the voltage wave form distortion detector unit 24.

By way of example, assuming that the three-phase voltage Uac is 400 V and 50 Hz, the active supplying stage 19 supplies an intermediate voltage Udc equal to approximately 600 V, impressing on the three-phase electric mains 15 a substantially sinusoidal three-phase current, i.e. guaranteeing a power factor greater than 0.95.

With reference to figure 5, the PWM modulating stage 20 comprises a bridge H of electronic switching devices 31, and in particular IGBT devices, supplied by the intermediate voltage Udc, and a controller 32, which is configured to control the bridge H 31 so as to generate the voltage Uml and modulate the width of the pulse of the voltage Uml in a manner correlated with the ratio between the current feeding speed of the wire 2, indicated by Vw in figures 2 and 5, and the difference between the maximum value and the minimum value of the feeding speed. The maximum and minimum values of the feeding speed of the wire 2 depend on the features of the annealing furnace 1. The voltage frequency Uml is predetermined according to the performance of the IGBT devices and of the voltage transformer 21.

At each value of speed Vw corresponds a desired annealing voltage, hereinafter named "annealing setpoint" Uref . The annealing voltage can be calculated by multiplying the square root of the feeding speed of the wire 2 by a constant K, which depends on the overall features of the annealing furnace 1 and which can be determined according to known techniques. The controller 32 receives the speed Vw of the wire 2 from the external device 33, for example the control unit of the drawing machine connected to the inlet of the annealing furnace 1 or a speed acquisition unit coupled to one of the members rotating at the speed of the wire 2 (a transmission roll 11, 12, an electric axis 5, 6, 7 or the pull ring 13) . The controller 32 is configured to calculate the annealing setpoint Uref by multiplying the square root of the speed Vw by the constant K. So, the annealing setpoint Uref varies between a minimum value Urefmin and a maximum value Urefmax .

More in detail, the controller 32 controls the bridge H 31 by adjusting the conduction offset, i.e. the conduction delay of one side (half) of the bridge H 31 with respect to the other, proportionally to the ratio between the annealing setpoint Uref and the difference between Urefmin and Urefmax. Thus, the modulated signal Uml has a duty cycle which varies between 0 and 0.5 as a function of the conduction delay set by the controller 32. In particular, the minimum value Urefmin corresponds to the duty cycle equal to a 0 and the maximum value Urefmax corresponds to the duty cycle equal to a 0.5 (square wave with zero mean value) .

The controller 32 comprises voltage measuring means comprising an A/D converter 34 connected to the outlet of the passive rectifier stage 22 to measure the annealing voltage value Uann according to known techniques. The controller 32 controls the bridge H 31 by adjusting the conduction offset also as a function of the measured values of the annealing voltage Uann so that the annealing voltage Uann follows the annealing setpoint Uref . Indeed, during annealing, the current which circulates in the wire 2 varies as a function of the work-hardening state of the material of the wire 2 and of the quality of the contact between the wire 2 and the pulleys 8-10.

The voltage transformer 21 is a single-phase, high- frequency power transformer, i.e. capable of operating at frequencies higher than 5 kHz. This allows to program the PWM modulating stage 20 so that it generates the voltage Uml at a frequency higher than 5 kHz, and preferably equal to a 8 kHz.

Furthermore, the voltage transformer 21 has a secondary circuit winding with central zero so as to transform the voltage Uml with zero mean value into the voltage Um2 with non-zero mean voltage, and has a nominal transformation ratio which is predetermined as a function of the intermediate voltage Udc and of the maximum value Urefmax. Assuming a maximum value Urefmax equal to a 100 V, which allows to anneal a wide range of section values of the wire 2 and a wide range of feeding speeds of the wire 2, and assuming that an intermediate voltage is equal to 600 V, the nominal transformation ratio is equal to 6.

The voltage transformer 21 described above is much smaller and thus more costly of the voltage transformers of the known electric apparatuses for generating the annealing voltage, the materials used being equal.

The rectifier stage 22 is of the non-controlled, passive type, and in particular comprises two diodes, each of which is associated to a respective half of the secondary circuit of the voltage transformer 21 to operate as a half-wave rectifier, and a low-pass filter LC connected downstream of the diodes.

It is worth noting that the voltage generator 14 is not limited to the use in in-line resistance annealing furnaces for wires, but is also adapted for use in resistance annealing furnaces for metal strands, strings, wire rods or straps, fed either in-line or off-line, i.e. fed wound as a simple skein or about a coil or a metal or cardboard drum.

Furthermore, the voltage generator 14 can be generically used also in annealing furnaces 1 having only two electric axes, i.e. without the pre-heating stretch of the wire, strand, string, wire rod or metal strap.

The main advantage of the annealing furnace 1 described above is to minimize the reactive power exchanged with the three-phase electric network 15 by virtue of the presence of the active supplying stage 19 placed at the input of the voltage generator 14. Furthermore, the annealing furnace 1 may be easily configured for annealing metal wires, strands, strings, wire rods or straps having a cross section variable in a wide range of values and in a wide range of feeding speeds of the metal wire, strand, string, wire rod, or strap by virtue of the presence of the PWM modulator 20 connected between the active supplying stage 19 and the voltage transformer 21. Finally, the high- frequency single-phase voltage transformer 21 is considerably more compact and cost-effective than a 50 Hz three-phase transformer, typically used in known annealing furnaces .