Abstract:
A driving arrangement for rolling stands of a rolling mill, is described. The driving arrangement includes a control system which prescribes desired values for the rolling stands via control units, the rolling stands having at least one electric motor which is supplied with electric energy by a power supply system via at least one converter having turn-off power semiconductors. The converter has an air-cooled design. The cooling power is dimensioned such that the temperature of the turn-off power semiconductors does not exceed a critical temperature limit for continuous operation. The converter includes heat sinks which have an optimum design. At least some of the heat sinks are connected thermally in parallel.

Description:
This application is a 371 of PCT/DE97/00945 filed May 9, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a driving arrangement for rolling stands having an air-cooled converter. 
     BACKGROUND INFORMATION 
     It is known to design self-commutated converters for drives of rolling stands in a water-cooled fashion. Such designs, however, are expensive to produce and maintain. 
     SUMMARY OF THE INVENTION 
     It is the object of an present invention to provide a converter for drives of rolling stands which, in contrast with conventional converters in this power range, can be produced more easily. 
     It is also an object of the present invention to provide a converter for drives of rolling stands which, in contrast with conventional converters, is simpler and more cost-effective to operate and maintain. 
     The object is achieved according to the present invention by providing an air-cooled converter. Previously, air cooling was believed to be unsuitable for converters for drives of rolling stands which, in particular, are operated in a power range of 1 to 20 MW. However, it has proved that converters can be designed with air cooling in the power range designated above. In this case, by contrast with known water-cooled converters in the above-named power-range, such air-cooled converters have proved to be particularly cost-effective and easy to maintain. 
     In an advantageous embodiment of the present invention, heat sinks electrically connect individual turn-off power semiconductors. This electrical connection likewise represents a good thermal connection, with the result that the heat occurring in the power semiconductors is adequately dissipated. In this case, it has further proved to be advantageous to use heat sinks having such a high thermal capacity that the heat sinks react thermally with a time delay in the case of peak loads. 
     The driving arrangement according to the present invention is suitable, in particular, for converters having low operating frequencies. In the case of converters in a three-point circuit such as is shown, for example in FIG. 6, the drive system according to the present invention is used with particular advantage in the case of mean operating frequencies of the converter &lt;250 Hz. In driving arrangements having a motor in a tandem connection, the drive system according to the present invention is suitable in a particularly advantageous fashion for mean operating frequencies &lt;100 Hz. 
     In a further advantageous embodiment of the present invention, the converter has a blower which feeds ambient air or precooled air to the heat sinks, or advantageously sucks ambient air through the heat sinks. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a converter-fed motor having a diode rectifier on the line side, 
     FIG. 2 shows a converter-fed motor having self-commutated rectifier and inverter, also on the line side, 
     FIG. 3 shows a-converter arrangement having an automation device connected via optical fibers, 
     FIG. 4 shows a three-point inverter having GTO thyristors (main circuit without a protective circuit), 
     FIG. 5 shows a three-point inverter module having RC-GTOs and a protective network, 
     FIG. 6 shows a converter arrangement for feeding a three-phase AC motor having a component converter in a three-point circuit on the line and machine sides, 
     FIG. 7 shows a converter arrangement for bilateral feeding of a three-phase AC motor having an open winding having a component converter in a three-point circuit, 
     FIG. 8 shows the mechanical design of a converter according to the present invention, and 
     FIG. 9 shows the use of a converter according to the present invention in a rolling mill. 
    
    
     DETAILED DESCRIPTION 
     A converter-fed motor  15  having a diode rectifier  2  on the line side is represented in FIG.  1 . The converter arrangement is preferably designed as a series circuit of two  26  diode bridges. The line-side interfacing is performed via a transformer  1  having two secondary winding systems, preferably offset electrically by  301 , for achieving a 12-pulse system perturbation. Diode rectifier  2  is connected on the DC voltage side to machine-side inverter  4  via voltage link  3 . The link connection is preferably performed via three poles, the positive and negative link poles and the DC voltage center point. Machine-side inverter  4  is designed as a self-commutated inverter in a three-point circuit to whose output side three-phase AC motor  15  is connected via three conductors. 
     A converter-fed motor  9  having self-commutated rectifier and inverter  6  and  8 , also on the line side, is represented in FIG.  2 . The rectifier arrangement comprises a line-side self-commutated rectifier  6 , which is connected on the DC voltage side to machine-side inverter  8  via voltage link  7 . Two component converters  6  and  8  are designed in a three-point circuit and the link connection is preferably performed via three poles, the positive and negative links and the DC voltage center point. Line-side self-commutated rectifier  6  is connected to the line via transformer  5 . The circuit is preferably of the same design as that of machine-side inverter  8  and permits operation both as rectifier and as inverter for energy recovery, for example, in the braking operation of motor  9 . The machine-side inverter is connected on its output side to the three-phase AC motor via three conductors. 
     FIG. 3 shows a converter arrangement having an automation device  14  for controlling converter  16 , the entire information exchange being performed via an optical fibre connection  13 . Converter  16  has a line-side component converter  10 , a voltage link  11  and a machine-side component converter  12 . The power connections of the component converters to the line and the motor can be executed, for example, according to FIG.  1  and FIG.  2 . Converter  10  contains all the sensors required for operation and monitoring, with the result that no further connection is required to the environment. It is not shown that both the converter and the automation device require an auxiliary power supply or battery. 
     FIG. 4 shows the main circuit of a three-point inverter. Connected in series between positive DC voltage pole  56  and negative DC voltage pole  57  are P-side link capacitor  54  and N-side link capacitor  55 . Their tie point forms DC voltage center point  58 . Phase modules  50 ,  51 ,  52 , in each case having four series-connected GTOs and anti-parallel-connected freewheeling diodes, are respectively connected between the positive and negative DC voltage poles. The tie point between the first and second GTOs of a phase module and the third and fourth GTOs of a phase module is respectively connected via an additional two diodes connected in series, and in antiparallel fashion, with the GTOs; the center point of these two diodes is, in each case, connected to DC voltage center point  58 . The tie point between the second and third GTOs of a phase module forms the respective output terminal, which is connected to motor  53   
     FIG. 5 shows a three-point-inverter module having RC-GTOs and a protective network. The series circuit of an inductor LI, four RC-GTOs (Reverse Conducting Gate-Turn-Off-Thyristors) V 1 , V 2 , V 3 , V 4 , and inductor L 2  between positive DC voltage pole  24  and negative DC voltage pole  26 , together with two center point diodes V 15  and V 16 , form the main circuit of phase module of a three-point inverter. In this circuit, V 15  is connected, by way of the anode, to DC voltage center point  25  and, by way of the cathode, to the tie point of first RC-GTO VI to second RC-GTO V 2 . V 16  is connected, by way of the cathode, to DC voltage center point  25  and, by way of the anode, to the tie point of third RC-GTO V 3  to fourth RC-GTO V 4 . The tie point between second RC-GTO V 2  and third RC-GTO V 3  forms the AC voltage output of phase module V (V or W). 
     L 1  and L 2  function to limit the rate of current rise, and protective networks V 21  and V 22 , together with C 7  and C 1 , as well as V 24  and V 23 , together with C 17  and C 11 , function to limit the rate of voltage rise when the GTOs are switched. The energy stored in the respective protective network per switching operation is converted into heat in resistors R 3  and R 4 , and the overloading of capacitors C 1  and C 11  is prevented or fed back. 
     Two RCD protective networks R 11 , C 9 , V 25  and R 21 , C 19 , V 26  function as additional protective circuits of two middle RC-GTOs V 2  and V 3 . They are advantageously used in the case of high-power converters, consequently having large mechanical dimensions, in order to prevent overvoltages on design-related parasitic inductances of GTOs V 2  and V 3 . 
     FIG. 6 shows a converter arrangement for feeding a three-phase machine, line-side component converter  33  and motor-side component converter  34  being designed, identically in each case, having GTOs in a three-point circuit. The main circuit of a phase module is represented in each case with its protective network  40  and  41 , respectively. P-side link capacitor  37  forms, together with N-side link capacitor  39 , the DC link via which the two component converters are connected. P-side protective circuit charge reversal resistor  36  and N-side protective circuit charge reversal resistor  38  are connected to the respective side of protective networks  40  and  41 , respectively. Line-side component converter  33  is connected on the output side to line  30  via transformer  31  and circuit breaker  32 . Machine-side component converter  34  is connected on the output side to three-phase AC motor  35 . 
     In the arrangement in FIG. 7, a first converter  74  and a second converter  75  are connected on the output side respectively to a side  71  and  72  of the open three-phase winding of three-phase AC motor  73 . In addition to doubling the power, this arrangement results in a particularly advantageous operational performance since, assuming an appropriately tuned pulse procedure, a largely sinusoidal current characteristic is achieved in the motor having a low harmonic load even in the case of a low operating frequency of the GTO thyristors. 
     On the line side, first converter  74  is connected to power supply system  60  via an optional line-side additional inductor  63  and a first transformer  61 , for example in a star/delta connection. Second converter  75  is connected to power supply system  60  via an optional line-side additional inductance  64  and a second transformer  62 , advantageously offset (for example in a star/star connection) electrically by  300  with respect to first transformer  61 . This arrangement produces particularly favorable system perturbations on the line, in particular when, as in the present example, the converters include component converters in a three-point circuit. The result in this case is a sinusoidal current characteristic with a very low harmonic content, even in the case of fundamental loading of the self-commutated line converters. 
     Two converters  74  and  75  each respectively have line-side component converters  66  and  65  and machine-side component converters  69  and  70 , which are respectively connected via a DC link  67  and  68 . Two DC links  67  and  68  are separated from one another electrically. All component converters  66 ,  65 ,  69 ,  70  are designed in a three-point circuit, preferably with RC-GTOs. 
     FIG. 8 shows the mechanical design of an air-cooled rectifier according to the present invention. The semiconductor elements are accommodated, in the present exemplary embodiment, on a removable rectifier module  81 . Rectifier  10  module  81  can be inserted into a carrier  82 . Carrier  82  is shown in FIG. 8 without side walls and without doors. Cooling is performed via an air flow which is produced by means of fans  80  and flows through carrier  82  and inserted rectifier module  81 . The semiconductors of rectifier module  81  are advantageously arranged between heat sinks  83 , which are cooled in parallel by the air flow. 
     FIG. 9 shows a converter  95 ,  96 ,  97 ,  98  according to the present invention in a rolling mill. The material to be rolled  103  is rolled in rolling stands  104 ,  105 ,  106 ,  107 , which are driven by electric motors  99 ,  100 ,  101 ,  102 . Motors  99 ,  100 ,  101 ,  102  are fed by a power supply system  90  via in each case one transformer  91 ,  92 ,  93 ,  94  and in each case one converter  95 ,  96 ,  97 ,  98  according to the present invention.