EP2276323A1

EP2276323A1 - Electrical power supply circuit

Info

Publication number: EP2276323A1
Application number: EP09425287A
Authority: EP
Inventors: Marco Passoni
Original assignee: Calamari SpA
Current assignee: Calamari SpA
Priority date: 2009-07-16
Filing date: 2009-07-16
Publication date: 2011-01-19

Abstract

There is described a power circuit (10) to supply and control at least one heating element (5), having a single-phase resistive load in alternating voltage, the circuit (10) having a three-phase alternating voltage source (6) comprising: a rectifier (3) suitable to transform the alternating voltage of the three-phase source into a single-phase direct voltage; a frequency converter (4) suitable to transform the single-phase direct voltage into single-phase alternating voltage for the heating element (5); a PLC suitable to control said frequency converter (4), according to a periodic sequence of alternating active phases and recirculation phases such that the effective voltage output from said converter (4) can be regulated by varying the duration of said phases.

Description

The present invention relates to an electrical power supply circuit to supply and control at least one load, for example an induction heating element for industrial machines for the extrusion of metal section bars.
In the industrial sector for producing extruded metal section bars, a metal billet is fed longitudinally through an electric induction furnace capable of heating it to increase the plasticity and facilitate deformation thereof during extrusion through a die which defines the final section.
Among furnaces suitable for the purpose, some are known which are divided longitudinally into subsequent sections controlled with different temperatures dictated by the plastic deformation requirements of the billet material along the extrusion path thereof.
Due to the high service powers required by these devices, they are supplied by means of an electrical power supply circuit with three-phase voltage. Generally, being supplied with three-phase voltage at fixed frequency, the aforesaid furnaces are divided into sections, each section being supplied with alternating current directly by a respective pair of phases.
The aforesaid prior art has some important drawbacks.
In fact, the decision to connect each section to a respective pair of phases means that areas are divided preferably into multiples of three, as by using fewer the system would be unbalanced.
Moreover, during heating some sections are progressively deactivated, causing unbalance in the system.
Another drawback of such prior art system is that the prior art three-phase power circuit produces a phase shift between voltage and current and therefore a reduced cosϕ value, thus requiring the use of a bank of power factor correction capacitors.
The object of the invention is therefore to provide an electrical power circuit able to substantially overcome the aforesaid drawbacks.
Another object of the present invention is to provide a power circuit able to regulate the power supplied to each heating element separately.
A further object of the present invention is to provide a power circuit which is flexible and simple to use for multiple power distribution configurations to more than one furnace.
Yet another object of the present invention is to provide a power circuit which is able to achieve a high power factor cosϕ, also for low supply powers of the heating elements.
One more object of the present invention is to provide an electrical power circuit with a configuration suitable to allow cooling of the components of which it is composed, in a simple and inexpensive manner, while ensuring safe electrical insulation.
A further object of the present invention is to provide a power circuit which is able to ensure high reliability and duration in time.
These and other objects are achieved by a power circuit to supply and control at least one heating element as claimed in the attached claim 1.
Preferred embodiments are highlighted in the dependent claims.
The invention will now be illustrated with the description below of an embodiment thereof, provided by way of non-limiting example, with reference to the accompanying drawings wherein:

Fig. 1a shows a possible diagram of a power circuit according to the invention, of the type with 6 pulses;
Fig. 1b shows part of a possible diagram of a power circuit according to the invention, of the type with 12 pulses;
Fig. 2 shows the waveform output from the three-phase transformer of the type with 12 pulses;
Fig. 3 shows the voltage and current trend output from the rectifier;
Fig. 4 shows a simplified diagram of a frequency converter according to the invention;
Fig. 5 shows the instantaneous voltage and current trend output from the frequency converter;
Fig. 6 shows the voltage and current trend downstream of the power factor correction reactors;
Fig. 7 shows two possible instantaneous voltage trends at the output of the converter varying the duration of the active phases;
Fig. 8 shows a possible configuration of a circuit according to the invention, in which two furnaces or series of inverters can be controlled through the same rectified voltage; and
Fig. 9 shows a possible embodiment of a sealed container block according to the invention, containing a rectifier and a converter according to the invention. With reference to the figures listed above, a possible embodiment of a power circuit according to the invention is indicated as a whole with the number 10.

It is suitable to electrically supply an industrial furnace for heating metal billets, for example made of aluminium, to facilitate plastic deformation thereof during feed through a die which defines the final shape of the section of the extruded section bar obtained from said billets.
Generally, prior art furnaces are provided with heating elements 5 comprising coils 19 wound in the form of solenoid and arranged aligned upstream of the die, inside which the billet is fed to heat it through the known mechanism of electromagnetic induction.
As shown in Fig. 1, the power circuit 10 comprises an electrical power source 6, of the alternating current three phase type, a rectifier 3 suitable to transform the alternating voltage of the three-phase source 6 into a single-phase direct voltage downstream of the rectifier 3, a frequency converter 4, arranged downstream of the rectifier 3, suitable to transform the single-phase direct voltage downstream of the rectifier 3 into single-phase alternating voltage downstream of the converter 4, suitable to supply the heating element.
The circuit 10 also comprises a PLC, not shown in the figure, suitable to control the frequency converter 4, in such a manner as to supply downstream of the converter 4 a desired value of frequency and of single-phase effective voltage to supply the heating element 5.
Advantageously, the circuit 10 comprises a three-phase transformer 1 suitable to adapt the power supply voltage of the source 6 to a voltage value suitable to supply the furnace 5.
The three-phase transformer 1 also performs the function of galvanic separation between the power supply of the source 6 and the supply circuit 10.
In particular, the three-phase transformer 1 has a double secondary and therefore has two separate secondary windings, one of which is star, or "Y", connected, and one delta, or "D", connected, from which a first three-phase line 7 and a second three-phase line 8 extend respectively at the same voltage, which share the line power equally, and which reunite upstream of the rectifier 3. In this manner, the advantage of limiting line harmonics is achieved.
In particular each line 7 and 8 has a respective switch 9. Moreover, each line can have respective precharge resistors 12.
Advantageously, a three-phase transformer 1 of the rectifiable type with 12 pulses is selected, in order to achieve a substantially sinusoidal waveform downstream of the transformer 1, as shown in Fig. 2, where the wave 21 output from the transformer is compared with the ideal sinusoidal wave 20.
The three-phase voltage output from the two lines 7 and 8, is thus input to the rectifier 3.
This rectifier 3 advantageously comprises uncontrolled diodes 13, connected as shown in Fig. 1 in order to obtain the maximum angle of flow and minimum harmonic pollution coming from the three-phase voltage upstream. In this manner, a constant direct voltage 22 proportional to the input three-phase line voltage 23 is output from the rectifier 3, as shown in Fig. 3.
The frequency converter 4, shown in Fig. 1, advantageously comprises at least one semiconductor transistor 14 of IGBT type (Insulated Gate Bipolar Transistor), with switch on and switch off controllable through a signal sent by said PLC. These transistors are connected in an H-shaped bridge configuration, in such a manner as to selectively and alternately connect the positive and negative pole of the single-phase direct voltage output from the rectifier 3 to a respective end of the resistive load of the heating element 5.
More in detail, each transistor 14 is equivalent to a switch which can be closed or opened through an electrical open/close signal sent by the PLC. In other words, the converter 4 of Fig. 1 can be schematized as in Fig. 4, in which Vin is the direct voltage downstream of the rectifier 8, Vout is the single-phase alternating voltage sent to the heating element 5, and the switches 15 represent the transistors 14.
As shown in Fig. 1, the heating element 5, besides having a coil 19, also comprises two reactors 16 and 17 and a capacitor 18, suitable to adapt the stepped alternating voltage output from the convertor to a sinusoidal alternating voltage input to the coil 19.
In fact, Fig. 5 shows the voltage 25 and current 24 trend output from the converter 4, and Fig. 6 shows the corresponding voltage 27 and current 26 trend output from the power factor correction reactors in input to the coil 19.
In the case in which the furnace comprises a plurality of heating elements 5 in sequence, the circuit 10 is provided with a plurality of frequency converters 4 connected in parallel downstream of the rectifier 3, each frequency converter supplying a respective heating element 5. In this case the PLC is capable of controlling each single converter 4, and therefore each heating element 5, separately.
An example of such configuration is described in Fig. 8, in which twelve heating elements 5 are provided, divided into two groups 42 and 43 of six elements each. Each group 42 and 43 is managed by a respective PLC 40 and 41 capable of controlling the heating groups 5 separately by means of a respective frequency converter 4. The system is supplied by a single source with three-phase alternating voltage 6, and comprises a three-phase transformer 1 with double secondary, of which one is star connected and the other delta connected. A respective three- phase line 7 and 8, at the same voltage, i.e. 520V, extends from each secondary, and reunite in the rectifier 3. A single-phase direct voltage line, i.e. at 780V, extends downstream of the rectifier 3, and on which all the frequency converters 4 are connected. The PLC 40 and 41 supplies these converters 4 with a preset sequence of pulses on which the frequency and effective voltage supplied to each heating element 5 depends, according to a function described below.
The aforesaid two groups of six heating elements can each belong to two separate furnaces, allowing high flexibility in the possibility of multiple distribution of the power, applying it simultaneously to more than one furnace. In practical cases, as high powers are fed through the components of such circuit, they tend to become hot due to power losses, and therefore require a continuous cooling process. In this energy conversion, the largest losses and consequently the most heat are localized in the power transformer 1, in the rectifier 3, in the converter or inverter 4 and in the filter reactors 16 and 17.
In particular, the transformer 1 must be maintained at a temperature of around 40-45 °C, while the PLC and the regulation electronics must be maintained at a temperature of 30-35°C.
As shown in Fig. 8, according to the present invention, this object is achieved by integrating several components in a single block 50 in such a manner as to use a single heat exchanger.
In particular, in a preferred embodiment of the invention, the rectifier 3 and the frequency converter 4 are contained in a same sealed container 51 electrically insulated and provided on a first face 52 with a electrically insulated and thermally conductive heat sink layer 55 which can face a heat exchanger, not shown. A second face of the sealed container comprises the electrical connections 53, 54 and 56.
More in detail, the layer 55 comprises a thin layer of electrically insulating material interposed between the components contained and the outside, externally coated by a copper or aluminium heat sink substrate. In particular, such configuration allows the components, for example the rectifier and the converter, to be buried directly in an insulating matrix, for example made of plastic or ceramic materials, which forms the container 51.
The present invention can also be implemented in other embodiments, in which the sealed container 51 can also include other components therein, selected from the three-phase transformer, filter reactors or yet other components. Operation of a circuit according to the invention and in particular of the frequency converter 4, the structure of which is described above, is as follows. With reference to Figs. 1 and 4, analyzing the converter 4, commonly known as "inverter", each of the four transistors 14, schematized with a switch 15, has two possible states, ON and OFF, for a total of sixteen theoretical configuration combinations. Of these configurations, four are with only one transistor 14 on and do not involve energy exchanges, seven are to be avoided as they cause a short circuit on the direct current input line, one is the null configuration with all the transistors 14 switched off, while the remaining four configurations are those possible for operation of the inverter 4. Of these four possible configurations, two are active in which there is an energy exchange between input and output of the converter 4 and two configurations are of recirculation in which the instantaneous voltage output from the converter 4 is null. In the two active configurations two transistors positioned on one of the two diagonals of the H-shaped bridge are switched on, so that the positive pole 23 and the negative pole 29 in input are alternately inverted onto the output voltage Vout according to the active diagonal. An inversion of the output voltage corresponds to each alternation of the two active configurations, so as to achieve an alternating voltage, but in steps. In this manner, if the two configurations are cyclically altered with the same duration a null, or alternating, mean output voltage is achieved, the frequency of which is variable simply by varying the period of the cycle and therefore the duration of the configurations.
A similar trend of voltages output from the inverter 4 is described in Fig. 7, which shows, in a first graph, an instantaneous voltage trend 29 corresponding to active phases 31 and 32 of maximum duration and recirculation phase 33 of null duration, and, in the second graph, an instantaneous voltage trend 30 with active phases 31 and 32 of lesser duration and recirculation phase of greater duration, of the same period 28 with respect to the first graph. The first graph corresponds to a maximum power supplied to the heating element, while the second corresponds to an intermediate power.
In particular, switching from one active configuration 31 to the other 32 passes through a recirculation configuration 33 which introduces a null level on the instantaneous output voltage 29, 30, which has a total of three levels: the null level 33, where the instantaneous voltage is equal to zero and two active levels 31 and 32 of an amplitude equal to the output voltage of the rectifier 3 and with opposite polarity.
During operation, the four possible configurations of the transistors are alternated with the operating frequency of the furnace, which is thus established unequivocally through the cycle period 28. After establishing the period 28, it is possible to modulate the effective voltage Ve output from the converter 4 varying the duration of the recirculation configurations 33 and therefore of the active levels 31 and 32. For example, by increasing the duration of the recirculation configurations 33 and consequently reducing the duration of the active configurations 31 and 32, the effective output voltage is reduced. In this way, it is possible to vary the effective voltage output from the inverter 4 from zero to a maximum equal to the rectified input voltage. Therefore, the inverter 4 performs the two effective frequency and voltage regulations of the system.
The invention achieves important advantages.
In fact, the power circuit according to the invention allows all the heating elements of a furnace for extrusion machines to be controlled separately, in such a manner as to optimize the temperatures in the different subsequent sections in complete freedom, without causing unbalances in the system. Another important advantage of the present invention is that of being able to use any number of heating elements and not necessarily multiples of three.
An important advantage of the present invention is to allow high flexibility and simple implementation for configurations of multiple distribution of the power generated, applying it simultaneously to more than one furnace.
A further advantage of the present invention is that of allowing simple and inexpensive cooling of the components through an air/water heat exchanger which does not require high qualities of the water used, due to a large surface of the heat sink layer.
One more advantage of the present invention is the fact that it achieves a high power factor cosϕ, between 0.98 and 1, which makes the use of a bank of power factor correction capacitors unnecessary.
Another advantage of the present invention is that of reaching the nominal cosϕ value already at 20% of the power regulation.
Moreover, an advantage of the present invention is reduced harmonic distortion.
As a consequence of said advantages, it is possible, as mentioned previously, to connect the circuit 10 to an even number, in particular to two, induction furnaces for heating billets.
These furnaces can be synchronized so that each one, or each group, has a high power heating period and a temperature maintenance period at a lower power, in particular around one fifth of the high power.
Said heating and maintenance periods are preferably alternated, so that while one group of furnaces, or a single furnace, is in heating mode, a second group of furnaces, or single furnace, is in maintenance mode.
In fact, induction furnaces for billets require a first heating period at maximum power, during which the whole billet is heated homogeneously, and a second maintenance period, during which only some areas of the billets are heated, while other areas are maintained at a constant temperature.
Consequently, this process allows 100% of the energy available always to be used and makes it possible to heat a greater number of billets in the unit of time, with the same available power.
The description above of a specific embodiment is able to show the invention from a conceptual perspective so that others, using prior art, may modify and/or adapt this specific embodiment in various applications, without further research and without departing from the inventive concept and, therefore, it is intended that these adaptations and modifications may be considered as equivalents of the specific embodiment. The means and the materials to implement the various functions described may be of different types without however departing from the scope of the invention. The expressions or the terminology used are intended as purely descriptive and therefore non limiting.

Claims

A power circuit (10) to supply and control at least one heating element (5) having a single-phase resistive load with alternating voltage, said circuit (10) having a three-phase alternating voltage source (6) and being characterized in that it comprises: a rectifier (3) suitable to transform the alternating voltage of said three-phase source into single-phase direct voltage; a frequency converter (4), suitable to transform said single-phase direct voltage into single-phase alternating voltage for said heating element (5) a PLC suitable to control said frequency converter (4), according to a periodic sequence of alternating active phases and recirculation phases such that the effective voltage output from said converter (4) can be regulated by varying the duration of said phases .
The power circuit according to claim 1, wherein said frequency converter (4) comprises at least one semiconductor transistor (14) of IGBT type (insulated Gate Bipolar Transistor), with switch on and switch off controllable through a signal sent by said PLC.
The power circuit according to claim 2, wherein said frequency converter comprises four transistors of IGBT type (14) in an H-shaped bridge configuration, in such a manner as to selectively and alternately connect the positive and negative pole of said single-phase direct voltage to a respective end of said resistive load.
The power circuit according to one or more of the preceding claims, wherein said rectifier (3) comprises uncontrolled diodes.
The power circuit according to one or more of the preceding claims, wherein said three-phase source (6) comprises a three-phase transformer (1).
The power circuit according to claim 5, wherein said three-phase transformer (1) is of the type with double secondary, having a first secondary star, or "Y", connected, and a second secondary delta, or "D", connected, suitable to give rise to a first three-phase line (7) and a second three-phase line (8), which reunite upstream of said rectifier (3).
The power circuit according to claim 6, wherein at least one between said first and second three-phase lines (7, 8) comprises a switch (9,11).
The power circuit according to claim 5, wherein said three-phase transformer 18) is of the type suitable to supply a rectifier with twelve pulses or with twenty-four pulses.
The power circuit according to one or more of the preceding claims, comprising at least one filter reactor (16, 17) arranged downstream of said frequency converter (4) and upstream of said resistive load (8), suitable to transform said single-phase alternating voltage output from said frequency converter to a sinusoidal single-phase alternating voltage for said resistive load.
The power circuit according to one or more of the preceding claims, wherein at least one between said rectifier (3) and said frequency converter (4) is contained in an insulated sealed container (51) provided on a first face (52) with an electrically insulating (55) and thermally conductive layer which can face a heat exchanger, and on a second face with electrical connections (53, 54, 56).
The power circuit according to claim 10, wherein both said rectifier (3) and said frequency converter (4) are contained in the same said sealed container (51).
The power circuit according to claim 10 or 11, wherein said insulated sealed container (51) contains therein at least one between: said three-phase transformer (4); said at least one filter reactor (16, 17).
An induction furnace for heating billets comprising at least one heating element (5) having a single-phase resistive load in alternating voltage and a power circuit (10) to supply and control said at least one heating element according to one or more of the preceding claims.
A process to supply and control a plurality of induction furnaces, comprising at least one heating element (5) having a single-phase resistive load with alternating voltage and connected to a power circuit (10) to supply and control said at least one heating element (5), said process comprising a high power heating period and a temperature maintenance period at lower power to said high power and being characterized in that said furnaces are divided into two groups of furnaces, each including at least one furnace, in that said furnaces are activated simultaneously and in that one of said groups is activated in said heating period and the other of said groups is activated in said maintenance period.