ZA200405343B

ZA200405343B - Mains current regulation for a resistance multispot welding machine comprising a rechargeable energy store.

Info

Publication number: ZA200405343B
Application number: ZA200405343A
Authority: ZA
Inventors: Klaus Ritter
Original assignee: Evg Entwicklung Verwert Ges
Priority date: 2002-11-12
Filing date: 2004-07-05
Publication date: 2005-06-06
Also published as: ATE514514T1; WO2004043641A1; AT501410A1; EP1560675A1; EP1560675B1; AT501410B1

Description

"e2004/53 3 ®

MAINS CURRENT REGULATION FOR A RESISTANCE MULTISPOT WELDING

MACHINE COMPRISING A RECHARGEABLE ENERGY STORE.

The invention relates to a method and arrangement for producing a periodically changing welding current of high power density with any curve shape for a fixed- cycle resistance welding machine operated with single-phase alternating or direct current, wherein the welding current is produced with the aid of a controllable, pulse-width modulated inverter circuit from a three-phase mains voltage source.

With the welding current controls for resistance welding machines used at present, in order to obtain a high power density preferably thyristors in anti- parallel connection are used as alternating current controllers. With this, the required welding current is obtained directly from the three-phase mains.

Especially when welding gratings or wire mesh mats with the aid of multispot resistance welding machines, this causes a high and in most cases unsymmetrical current loading of the mains current source by the welding current, whereas during the welding intervals the electricity network is loaded only slightly by the auxiliary equipment of the resistance welding machine. This periodically occurring mains load causes considerable fluctuations in the mains voltage, as a result of which problems may occur in respect of the operation of other machines and installations. Furthermore, when operating a multispot resistance welding machine the power supply network is loaded with a high idle current portion. This requires the use of expensive idle current compensation systems. "From WO - 98/21001 a method is known for producing a welding current of high power density for multispot resistance welding machines, with which from the three-phase mains by means of a bridge rectifier a direct voltage is formed,

® which supplies an inverter. The output of the inverter supplies alternating current of adjustable frequency with any, preferably sinusoidal curve shape to one or several welding transformers. With this a uniform current distribution over all mains phases occurs only from a welding current frequency that amounts to three times the mains frequency. The mains current, however, is not sinusoidal, and there is a small portion of idle current. Also with this arrangement, the same as with the welding current control by thyristors, the mains current flows only when the inverter supplies a welding current. Also with this arrangement, therefore, the problem of a pulsating mains loading occurs.

It is the object of the invention to avoid the aforementioned disadvantages of the known methods and arrangements and to ensure an operation of the resistance welding machine with a uniform loading of the mains voitage source. According to the invention this is achieved with a method of the type indicated at the outset in that the welding current is taken for the greater part from a rechargeable energy store arranged between the mains voltage source and the user and only a small portion is taken from the mains voltage source, during the welding intervals the energy store is charged from the mains voltage source with a constant mains current to a selectable, maximum voltage value which is greater than the maximum value of the mains voltage, the mains current also during the welding is regulated to a constant value independently of the size and course of the welding current, wherein the mains current assumes a sinusoidal curve shape and is in- phase with the mains voltage, and with the aid of a current regulation for the inverter circuit the welding current is regulated to a preselected value.

The invention furthermore relates to an arrangement for the implementation of the method comprising at least one intermediate circuit condenser, a controllable inverter, a welding transformer, a control computer, an input computer and a machine control unit, characterised in that in order to charge the intermediate circuit condensers, per mains phase a power choke is provided as well as an input

® bridge controllable by a control computer controlled with the aid of the input computer and the machine control unit, which input bridge comprises per mains phase several power transistors connected in pairs together with parallel connected diodes, a driver as well as at least one attenuation condenser; in that the primary welding current can be measured with a current measuring device, the measured values of which are transmitted at least to the control computer of the input bridge and to a control computer for the inverter; in that the mains current can be measured with a current measuring device and the intermediate circuit voltage with a voltage measuring device, wherein the measured values are transmitted at least to the input computer of the input bridge; in that the control computer of the input bridge based on the measured values sends corresponding control signals to the drivers of the input bridge, as a result of which the power transistors of the input bridge switch an intermediate circuit current required by the inverter for the making available of the welding current in a pulse-width modulated time and the mains current also during the welding cycle is regulated to a constant value independently of the size and course of the welding current; and in that the control computer of the inverter comprises a circuit for keeping the primary welding current constant.

By using the welding current control according to the invention, the connect load of the resistance welding machine is reduced considerably. This permits the operation of the resistance welding machine with the aid of a current mains that can be designed for a lower capacity. This results in cost savings for the mains transformers, the switchgear and the cabling. Moreover, because of the uniform current load, the power supply of a resistance welding machine by a diesel unit can take place with considerably fewer problems and more cost-effectively.

The invention also relates to a multispot resistance welding machine for manufacturing gratings or wire mesh mats consisting of longitudinal and transverse elements that cross one another perpendicularly, comprising an

® arrangement characterised in that to the intermediate circuit consisting of an input bridge with associated control computer and at least one intermediate circuit condenser at least one welding current circuit consisting of a controllable inverter and a welding transformer can be connected.

Further characteristics and advantages of the invention will be explained in more detail in the following description of an exemplified embodiment with reference to the drawings, wherein: Fig. 1a is a schematic circuit diagram of a welding current control according to the invention for a multispot resistance welding machine operated with alternating current, Fig. 1b is the circuit diagram of the welding current control according to the invention of Fig. 1a for a multispot resistance welding machine operated with direct current and Fig. 2 shows the chronological course of the primary welding current, the mains current and the intermediate circuit voltage.

The arrangement according to the invention illustrated in Fig. 1 serves to produce a single-phase, pulse-like welding current of high power density at a simultaneously essentially constant sinusoidal mains current. Such a welding current is used by a multispot resistance welding machine 1 for manufacturing gratings or wire mesh mats consisting of longitudinal elements L and transverse elements Q arranged perpendicularly to the former. In the multispot resistance welding machine 1 the longitudinal elements L and the transverse elements Q are welded together at their crossing points with the aid of several top welding electrodes 2 arranged in line and corresponding bottom welding electrodes 3. The top and bottom welding electrodes 2 and 3 respectively that are positioned opposite one another in each instance form a pair of welding electrodes. The manufacturing of the grating or wire mesh mat takes place in fixed-cycle operation, wherein the arrangement must make the welding current available with full power in the corresponding cycle times.

_

The arrangement illustrated in Fig. 1 essentially comprises an input computer 4, a machine control unit 5, a three-phase mains connection L1, L2, L3, an input bridge 6, several intermediate circuit condensers 7 as stores for the energy required for the welding and an inverter 8 to supply at least one welding transformer 9 and 29' respectively, a control computer 10 for the input bridge 6 and a control computer 11 for the inverter 8.

The input bridge 6 comprises for every phase of the mains connection L1, L2, L3 a pair of power transistors, preferably IGBTs (Insulated Gate Bipolar Transistor) 12, which are controlled in pairs by a driver 13 each, as well as a diode 14 arranged parallel to the IGBTs and several intermediate circuit attenuation condensers 15. Instead of IGBTs it is also possible within the framework of the invention to use other suitable power semiconductors, e.g. GTO-thyristors of

IGCTs; in the case of a lower voltage and lesser power also MOSFET transistors can be used. After switching on the multispot resistance welding machine 1 with the aid of a main switch 16, the intermediate circuit condensers 7 and the intermediate circuit attenuation condensers 15 of the input bridge 6 are first charged via the charging resistances 17 and the diodes 14 of the input bridge 6, to the maximum value of the mains voltage Un. The charging resistances 17 serve to limit the charging current for the intermediate circuit condensers 7 in such a way that during the charging operation the diodes 14 will not be overloaded. By means of a mains voltage measuring device 18, the size of the mains voltage Un is ascertained and the measured values determined in this way are fed via a data line 19 to the control computer 10. A voltage measuring device determines the intermediate circuit voltage Uz, the measured value of which is also passed on to the control computer 10 via a data line 21. When the intermediate circuit voltage Uz has nearly reached the maximum value of the mains voltage Un, the switch-on charging operation is ended and the charging resistances 17 are bridged by closing switches 22, which to this end receive a corresponding control signal 23 from the control computer 10.

®

The welding data required for welding the to be manufactured product, e.g. size, course as well as duration of the primary welding current Ip, and other parameters required for the control of the multispot resistance welding machine, as well as the limit values for the maximum mains current In and for the maximum and minimum intermediate circuit voltage Uz are entered into the input computer 4. The input computer 4 is connected to the machine control unit 5 via a data line 24, so as to be able to pass the input values required for the control of the multispot resistance welding machine on to the machine control unit 5. The desired nominal values of the intermediate circuit voltage Uz are transmitted by the input computer 4 via a data line 25 to the control computer 10 for the input bridge 6. Vice-versa the measured actual values of the intermediate circuit voltage

Uz are sent back via the data line 25 to the input computer 4.

So that the arrangement according to the invention can carry out its function as mains current regulation, after ending the switch-on operation the intermediate circuit voltage Uz must continue to be increased further via the mains voltage Un; at a mains voltage of 400V AC customary in Europe, for example, the intermediate circuit voltage Uz is increased from approx. 560V DC to 750V DC.

To achieve this increase, the drivers 13 of the IGBTs 12 of the input bridge 6 receive from the control computer 10 pulse-width modulated control signais 26 with a frequency of between 2 kHz and 10 kHz, which ensure that in the known manner in interaction with power chokes 27 and the intermediate circuit condensers 7, the intermediate circuit voltage Uz is increased. On reaching the preset maximum intermediate circuit voltage Uz, the control computer 10 for the input bridge 6 via a data line 28 sends a release signal to the control computer 11 for the inverter 8. Before releasing the welding, the input computer 4 also transmits the required welding data via a data line 29 to the control computer 11 for the inverter 8. Via the data lines 25 and 29 the input computer 4 furthermore receives answer-back signals from the control computers 10 and 11 regarding the actual measured values of the welding current Ip, the mains current In and the

® intermediate circuit voltage Uz, as well as signals regarding possible errors that occur during the welding, e.g. an exceeding of the limit value by the measured values, excessive temperatures due to coolant failure etc. Error signals that require a stopping of the machine are transmitted via the data lines 34 and 39 by the control computers 10 and 11 directly to the machine control unit 5.

The inverter 8 has two pairs of power transistors, preferably IGBTs 30, which are controlled in pairs by a driver 31 each, as well as a diode 32 each arranged parallel to the IGBTs and at least one intermediate circuit attenuation condenser 33. During the switch-on operation described in the foregoing, the intermediate circuit attenuation condensers 33 are also charged.

To start the welding, the machine control unit 5 via a data line 34 sends a welding release signal to the control computer 11 for the inverter 8, which in turns transmits corresponding control signals 35 to the drivers 31 of the IGBTs of the inverter 8, as a result of which a primary welding current Ip now flows through the primary winding of the alternating current welding transformer 9. For the welding carried out with alternating current, preferably a sinusoidal primary welding current Ip with a frequency of 40 - 200 Hz is chosen. The multispot resistance welding machine for direct current welding indicated diagrammatically in Fig. 1b operates with a medium frequency technique with a rectangular primary welding current Ip with a frequency of approx. 1000 Hz. In this case high-current diodes 36 arranged on the secondary side rectify the voltage supplied by the welding transformer 9'. Within the framework of the invention several welding groups, i.e. several inverters 8 and welding transformers 9 or 9' can be connected to the intermediate circuit consisting of an input bridge 6 with control computer 10 and the intermediate circuit condensers 7, as is customary especially with multispot resistance welding machines for the manufacture of gratings or wire mesh mats.

®

The size of the primary welding current Ip is measured with a current sensor 37 and the measured value is fed via the data line 38 to the control computer 11 for the inverter 8. In the control computer 11 a current regulation ensures that the preselected size of the primary welding current Ip is maintained. This is important seeing that the welding transformer 9; 9' takes the energy required for the welding essentially from the intermediate circuit condensers 7, as a result of which the intermediate circuit voltage Uz drops. Without this current regulation the primary welding current lp, because of the dropping intermediate circuit voltage Uz, would also drop during the welding operation. To be able to provide the required energy, the capacity of the intermediate circuit condensers 7 is dependent on the required power of the multispot resistance welding machine 1.

The capacity of the intermediate circuit condensers 7 is considerably greater compared to a customary arrangement, the customary arrangement being determined by the technical data of the IGBTs, especially by the nominal current.

The control computer 11 for the inverter 8 via the data line 18 receives the values in respect of the size and duration of the primary welding current Ip measured with the current sensor 37 and via the data line 28 passes these on to the control computer 10 for the input bridge 6. The control computer 10 for the input bridge 6, with the aid of the intermediate circuit voltage measuring device 20, determines by which value the intermediate circuit voltage Uz has dropped during the welding and now calculates from the number of cycles of the multispot resistance welding machine 1 preset by the machine control unit 5 and transmitted via a data line 39, with which mains current In the multispot resistance welding machine 1 must be supplied, so that before starting the next welding cycle the intermediate circuit condensers 7 are again fully charged. An increase in the number of cycles of the resistance welding machine 1, for example, automatically brings about an increase of the mains current In. Further- more, via the data line 39 an answer-back signal is transmitted from the control computer 10 for the input bridge 6 to the machine control unit 5 when the

_ intermediate circuit voltage Uz has reached its nominal value and, therefore, a new welding cycle can take place. The mains current In is measured with the aid of a mains current sensor 40 and the value of the mains current [n is transmitted via a data line 41 to the control computer 10 for the input bridge 6. Now the control computer 10, based on the measured values of the mains current In, sends corresponding control signals 26 to the drivers 13 of the input bridge 6, as a result of which the IGBTs 12 switch the intermediate circuit current lz required by the inverter in a pulse-width modulated time with a cycle frequency of between 2 kHz and 10 kHz, so that the mains current In irrespective of the size and course of the welding current will not exceed the preset maximum value, always flows in-phase with the mains voltage Un and assumes a sinusoidal curve shape.

Furthermore, via the data line 34 the measured values of the primary welding current Ip as well as any disturbances or irregularities that occur during the welding are passed on to the machine control unit 5, so that it will be able to control the subsequent welding cycles accordingly.

Fig. 2 shows the chronological course of the mains current In, the intermediate circuit voltage Uz and the primary welding current Ip, wherein the chronological course and the size of the primary welding current Ip are preset by the welding parameters for the to be manufactured product. As shown in Fig. 2, the intermediate circuit current Uz drops during the welding, while the current regulators in the control computer 11 for the inverter 8 ensure that the primary welding current Ip is always kept at a constant value. At a mains voltage Un of 400V AC customary in Europe, the intermediate circuit voltage Uz drops for example from approx. 750V DC to approx. 600V DC.

During the welding intervals the intermediate circuit condensers 7 are recharged with a practically uniform sinusoidal current In from the three-phase mains until

® the intermediate circuit voltage Uz has almost reached its maximum value.

So as to prevent a dropping of the mains current In to zero when this maximum value of the intermediate circuit voltage Uz is exceeded, the next welding cycle starts already before this maximum value is reached.

The mains current In also during the welding cycle continues to flow on in the same size without dropping, seeing that the required welding energy is taken mainly from the intermediate circuit condensers 7 and only partly from the mains current In.

The mains current In therefore flows during the welding cycles and during the intervals between the welding cycles always with a practically identical, optimal current strength.

The optimal mains current In occurs when over the production time of the multispot resistance welding machine 1 it displays a practically uniform size on an as low as possible, constant level.

The control computer 10 determines the optimal value of the mains current In based on the welding data made available to it by the input computer 4 via the data line 25, e.g. machine cycle number, welding current and welding time, which depend on the to be manufactured product and the production conditions of the multispot resistance welding machine 1. Furthermore it is possible within the framework of the invention, especially as correction factor, to determine from the measured actual values of the welding current Ip and the measured minimum and maximum values of the intermediate circuit voltage Uz, i.e. from the efficiency of the multispot welding machine 1, the optimal value of the mains current In.

The power chokes 27 operate during the taking of the welding energy from the mains current In as an additional energy store.

Claims

_ Claims:

1. Method for producing a periodically changing welding current (Ip) of high power density with any curve shape for a fixed-cycle muitispot resistance welding machine operated with single-phase alternating or direct current, wherein the welding current is produced with the aid of a controllable, pulse- width modulated inverter circuit (8) from a three-phase mains voltage source (L1, L2, L3), characterised in that the welding current (Ip) is taken for the greater part from a rechargeable energy store (7) arranged between the mains voltage source (L1, L2, L3) and the user (1) and only a small portion is taken from the mains voltage source (L1, L2, L3), during the welding intervals the energy store (7) is charged from the mains voltage source (L1, L2, L3) with a constant mains current (In) to a selectable, maximum voltage value (Uz) which is greater than the maximum value of the mains voltage (Un), the mains current (In) also during the welding is regulated to a constant value independently of the size and course of the welding current (Ip), wherein the mains current (In) assumes a sinusoidal curve shape and is in-phase with the mains voltage (Un), and with the aid of a current regulation (11) for the inverter circuit (8) the welding current (Ip) is regulated to a preselected value.

2. Method according to claim 1, characterised in that as energy store at least one intermediate circuit condenser (7) with large charging capacity is used.

3. Method according to claim 2, characterised in that the welding is started shortly before the maximum value of the voltage (Uz) is reached.

4. Method according to any one of the claims 1 to 3, characterised in that the intermediate circuit voltage (Uz), the mains current (In) and the welding current (Ip) are measured.

5. Arrangement for the implementation of the method according to any one of the claims 1 to 4, comprising at least one intermediate circuit condenser, a controllable inverter, a welding transformer, a control computer, an input computer and a machine control unit, characterised in that in order to charge the intermediate circuit condensers (7), per mains phase (L1; L2; L3) a power choke (27) is provided as well as an input bridge (6) controllable by a control computer (10) controlled with the aid of the input computer (4) and the machine control unit (5), which input bridge (6) comprises per mains phase (L1; L2; L3) several power transistors (12) connected in pairs together with parallel connected diodes (4), a driver (13) as well as at least one attenuation condenser (15); the primary welding current (lp) can be measured with a current measuring device (37), the measured values (Ip) of which are transmitted at least to the control computer (10) of the input bridge (6) and to a control computer (11) for the inverter (8); the mains current (In) can be measured with a current measuring device (40) and the intermediate circuit voltage (Uz) with a voltage measuring device (20), wherein the measured values (In, Uz) are transmitted at least to the input computer (10) of the input bridge (6); the control computer (10) of the input bridge (6) based on the measured values (Uz, In, Ip) sends corresponding control signals (26) to the drivers (13) of the input bridge (6), as a result of which the power transistors (12) of the input bridge (6) switch an intermediate circuit current (lz) required by the inverter (8) for the making available of the welding current (Ip) in a pulse-width modulated time and the mains current (In) also during the welding cycle is regulated to a constant value independently of the size and course of the welding current (Ip); and the control computer (11) of the inverter (8) comprises a circuit for keeping the primary welding current (Ip) constant.

6. Arrangement according to claim 5, characterised in that the inverter (8) comprises several power transistors (30) connected in pairs together with parallel connected diodes (32), per pair of power transistors (30) a driver (32) as well as at least one attenuation condenser (33).

7. Arrangement according to any one of the claims 5 to 6, characterised in that the power choke (27) serves as additional energy store for the welding current (Ip).

8. Arrangement according to any one of the claims 5 to 7, characterised in that when the installation is switched on, the mains current (In) flows per mains phase via a charging resistance (17) that can be bridged with the aid of a switch (22).

9. Arrangement according to any one of the claims 5 to 8, characterised in that the power transistors (12, 30) are IGBTs.

10. Multispot resistance welding machine for manufacturing gratings or wire mesh mats consisting of longitudinal and transverse elements that cross one another perpendicularly, comprising an arrangement according to any one of the claims 5 to 9, characterised in that to the intermediate circuit consisting of an input bridge (6) with associated control computer (10) and at least one intermediate circuit condenser (7) at least one welding current circuit con- sisting of a controllable inverter (8) and a welding transformer (9; 9') can be connected.

11. Multispot resistance welding machine according to claim 10, characterised in that the welding takes place with alternating current, wherein a preferably sinusoidal, primary welding current (Ip) with a frequency of 40 - 200 Hz is used.

12. Multispot resistance welding machine according to claim 10, characterised in that the welding takes place with direct current, wherein a medium frequency technique with a rectangular primary welding current (ip) with a frequency of 1000 Hz is used, and high-current diodes (36) arranged on the secondary side rectify the voltage supplied by the welding transformer (9').