JP2507295C - - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to resistance welding. More particularly, the present invention relates to an improvement of a resistance welding machine suitable for an automatic pressure welding machine using a rohot welding machine. Resistance welding is a well-known method for joining two conductors. In this method, the current is applied in such an amount that the conductor is locally heated and melted to join the conductors. This is usually done by sandwiching two conductors that join a pair of electrodes, and applying pressure and voltage between the electrodes to reduce the heating time generated by I ² R so that the members are sufficiently melted and not melted too much. Adjust. This method can be implemented using either DC or AC voltage. However, considering the contact resistance of the two conductors to be joined, the voltage is reduced from a normal power supply voltage to several volts or several tens of volts using a step-down transformer, and the current is reduced to a value of thousands to tens of thousands of amps. It is desirable to do. The simplest resistance welder comprises a step-down transformer, a switch controlling the supply of voltage to the step-down transformer, and a pair of electrodes electrically connected to the secondary winding of the step-down transformer. It is composed of When the electrodes are arranged so as to sandwich the members to be joined, a voltage is applied to the joints of the conductors by closing the switch, and the resulting electric heat melts the points in contact with the electrodes and welds the conductors to each other I do. In considering the actual welding operation, various improvements have been made to the above process. In order to minimize the cost of the welding current supply transformer, it is desirable that the peak value of the voltage supplied from the power supply to the transformer reaches the saturation value of the transformer core or the bend of the BH curve of the core. Is to do so. For this purpose, it is necessary to always know the condition of the core of the transformer when applying voltage to the transformer. If the last voltage applied to the transformer magnetizes the transformer in one particular direction and the next applied voltage causes current to flow in the same direction, the magnetizing current required by the transformer will be Added to the current
Overload current will flow through the transformer during the first cycle. This is an undesirable condition, which has been prevented in the following way: the control circuit of the welding transformer always completes welding when it has supplied the full cycle of input voltage, Is controlled so that the input voltage waveform always points in the same direction. This ensures that the first cycle of the supply voltage always closes the hysteresis curve of the transformer core, prevents supersaturation current and ensures that the peak voltage value remains constant during each welding cycle. Therefore, all that remains is a method of supplying a predetermined number of cycles of the input voltage as the welding voltage to complete the welding. This predetermined number of cycles is determined empirically and is usually a very small value and should be controlled electrically, because the required supply time is reliable by the operator This is because it is too short to operate with. The resistance welding apparatus described above has significant disadvantages when it is used for resistance welding in a production line. When welding two sheet steels, a current of the order of 10,000 to 30,000 amps is usually required, regardless of whether they are galvanized. Transformers wound to supply such current to the secondary circuit typically weigh 200 to 600 pounds and need to be cooled with water or external cooling. The wires carrying the above currents are actually quite large. In order to deal with these problems in a manufacturing line of the automobile industry or other industries, a transformer is suspended from an upper retainer, an insulated conductor is wired to a weld having a water-cooled electrode, and an operator welds the electrode. During the welding operation, a force is applied from the outside to hold the electrode. The device described above has several obstacles when it is used in robots or automatic welding machines. Robots generally have weight restrictions, and reducing weight improves operability. In automatic welding machines, the minimum spacing between adjacent welding points is limited by the size of the transformer. Robotic welding machines are also severely hindered by being connected to thick power cables designed to carry thousands of amps of current, and the life of such thick cables is affected by the movement of the robot. Is shortened. It is an object of the present invention to provide an electric circuit for a resistance welder with reduced weight near the weld. Yet another object of the present invention is to provide an electric circuit of a resistance welding machine suitable for use in a robotic welding device. Yet another object of the present invention is to provide a compact and lightweight transformer, which is small and lightweight so that it can be mounted close to the welding work area and uses a lightweight primary conductor. And the secondary conductor can be shortened. Yet another object of the present invention is to provide an electric circuit of a resistance welding machine suitable for use in automatic welding equipment. Other objects will become apparent in the detailed description of the invention. SUMMARY OF THE INVENTION A method and apparatus for DC resistance welding having a rectifier connected to an AC power source for rectifying an AC voltage and generating a rectified voltage having both DC and AC components. The rectified voltage is supplied to the inverter, and the inverter generates a rectangular wave having a frequency higher than the frequency of the AC power supply. This square wave is fed to a step-down transformer having a secondary winding with a center tap. This secondary winding is connected through a full-wave rectifier to a welding contact, which conducts welding current through the member. Using a frequency higher than the power supply frequency as the inverter output makes it possible to use a step-down transformer that is lighter than a transformer that operates at the input AC voltage frequency.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a circuit for realizing the present invention. FIG. 2 is a more detailed block diagram of a circuit for realizing the present invention. FIG. 3 is a circuit diagram showing the operation of the inverter devices 44 and 46 in FIG. 2 in detail. FIG. 4 is a detailed circuit diagram of the timer 48 of FIG. FIG. 5 is a detailed circuit diagram of the drive circuit 52 of FIG. FIG. 6 is a cutaway perspective view of a step-down transformer manufactured and used to realize the present invention. FIG. 7 is a time chart of a voltage waveform in the circuit of FIG. DETAILED DESCRIPTION Figure 1 of the invention is a block diagram of a circuit for implementing the present invention. AC voltage power supply 1 in FIG.
0 is connected to the rectifier 12. Here, the connections are shown by three wires, which are common when power supply 10 is a three-phase AC power supply. However, any number of phases of electrical energy can be used. The rectifier 12 is connected so as to output a substantially DC voltage including an AC component as its output. The output of the rectifier 12 is supplied to a transformer 16 via a control circuit 14 which is a control type inverter. The control circuit 14 includes the rectifier 12
Is converted into an AC voltage having a frequency higher than the input frequency, and the effective value of the converted AC signal is controllable. Transformer 16 is shown in dashed lines because it is useful to modify the voltage supplied to wires 18 and 19 as an input to step-down transformer 20. However, it will be appreciated that under certain conditions it is preferable to connect the wires 18 and 19 directly to the control circuit 14. This is a matter of design choice. The step-down transformer 20 has a secondary winding with a center tap. Step-down transformer 2
The zero secondary wire is connected to rectifiers 22 and 24 to form a full-wave rectifier. The common connection of the rectifiers 22 and 24 is led to a welding electrode 26. Step-down transformer 20
Center tap 28 is connected to a welding electrode 30. The welding electrodes 26 and 30 are arranged so as to sandwich the member to be welded, and the control circuit 14 operates to supply a current to the welding electrodes 26.
And 30 provide resistance welding between the members to be joined. If the frequency of the output voltage of the control circuit 14 is higher than the frequency of the power supply 10, the step-down transformer 20
Can be smaller and lighter than otherwise. This not only eases the operation of the operator, but also allows the transformer to be placed in the arm of the robotic welder, thus allowing the arm to move freely over a wide range. In addition, smaller transformers allow welding to be achieved closer together in an automatic welding machine. FIG. 2 is a detailed block diagram of a circuit for realizing the present invention. In FIG. 2, rectifier 12 is connected to power supply 10 and supplies a voltage between buses 40 and 42. Inverter devices 44 and 46 are connected between buses 40 and 42, and their midpoints are connected to conductors 18 and 19 and to step-down transformer 20. In the block diagram of the circuit of FIG. 2, the transformer 16 shown in FIG. 1 is omitted. As mentioned earlier, this is a design issue. The rectifiers 22 and 24 in the circuit of FIG.
, And a full-wave rectified output is generated so as to return to the center tap 28 through the welding electrodes 26 and 30 and members not shown. The control of the circuit shown in FIG. 2 is started by a timer 48, which controls the operation cycle of the inverter devices 44 and 46 and the relative time for adjusting the intensity of the current flowing through the welding electrodes 26 and 30. I do. The current detecting element 50 is connected to supply an input to the timer 48, and terminates the operation of the inverter devices 44 and 46 when the current in the bus 40 exceeds a predetermined value. The timer 48 outputs two outputs, which are input to a drive circuit 52, which is connected to drive switch elements such as power transistors in the inverter devices 44 and 46, and which outputs current. The first half cycle is controlled so as to flow through the upper part of the inverter device 44, the conductor 18, the primary winding of the step-down transformer 20, the conductor 19, the lower half of the inverter device 46 and the bus 42. The current of the second half cycle flows in the opposite direction to the primary winding of the step-down transformer 20 via the conductor 19 through the upper half of the inverter device 46. The current flows through the conductor 18 to the inverter device 44, and then to the bus 42 via the lower half of the inverter device 44. The details of this control will become apparent from a study of a more detailed circuit diagram. Thyristors, SCRs, gate / turn-off elements, and the like can be used as switch elements of the inverter device 44. FIG. 3 is a circuit diagram illustrating more detailed operation of the inverter devices 44 and 46 shown in FIG. In FIG. 3, AC power from the power supply 10 is input to the rectifier 12 via three fuses 60, three limiting resistors 62, and three contacts 64. The power supply 10 is usually of three-phase 60 Hz and has a nearby voltage of 480 volts or the like. The contact 64 is shown here energized by a contactor 63 controlled by a push button 65. This assumes that the push button 65 is operated as part of the opening and closing operations of the welding electrodes 26 and 30 before the welding current is supplied or after the welding is completed. Alternatively, the contactor 63 can be controlled by the timer 14 in FIG. In FIG. 3, rectifier 12 comprises a suitable number of diodes 66 connected to form a full-wave rectifier bridge rectifier. Here, six diodes 66 are shown, but it will be apparent that this number will vary for different currents, voltages applied to the diodes, and a different number of phases than the three phases. These are design issues. The output of the rectifier 12 is a full-wave rectified voltage, and the bus 40 is positive with respect to the bus 42. Inverter devices 44 and 46 are connected to buses 40 and 42
Connected between Since the inverter devices 44 and 46 are the same, only the inverter device 44 is shown. In the inverter device 44, the power transistor 6
8 is connected to the negative bus 42 via a series connection with another power transistor 70. Power transistor 68 is driven by Darlington transistor 72, which is disturbed by drive circuit 52. Power transistor 70
Is similarly driven by a Darlington transistor 74, which is driven by a drive circuit 52. Inverter device 46 includes power transistors 76 and 7
8 which are also driven by Darlington transistors, which are not shown here. The power transistor 68 is bypassed by a diode 80 and a resistor 82 and a capacitor 84 connected in series, which suppresses a sudden rise in the voltage across the power transistor 68. This may not be necessary depending on what power transistor 68 is selected. Power transistors 70, 76 and 78 are similarly bypassed.
The common junction 86 of power transistors 68 and 70 is connected via lead 18 to one end of the primary winding of step-down transformer 20 and to common junction 8 of power transistors 76 and 78.
8 is connected via a conductor 19 to the other end of the primary winding of the step-down transformer 20. A stabilizing capacitor 90 is connected to a positive bus 40 and a negative bus 42 through a resistor 92.
Connected between The galvanic element 50 has a current transformer 94, which detects a current flowing through the positive bus 40 and the capacitor 90. This combination of connections prevents the circuit from being accidentally tripped by the current charged in capacitor 90 when the circuit is first started. The current transformer 94 is connected to the current detector 96 and outputs a signal proportional to the measured current. This signal is input to a comparator 98 and compared with a predetermined voltage. When a signal whose current value exceeds a predetermined reference value is generated, this signal is input to the timer 48 of FIG. 2 to control the operation of the timer 48. When the circuit shown in FIG. 3 is constructed and tested, the input voltage from power supply 10 has a frequency of 60 Hz and the timing circuit of timer 48 shown in FIG.
had been operated to generate the input of step-down transformer 20 operating at z. Under such conditions, voltage fluctuations between the buses 40 and 42 in each cycle can be ignored. However, the output of a normal full-wave three-phase bridge rectifier has an AC voltage component of 360 Hz and its harmonic frequency. It is known to have The operation of the circuit may be considered that a DC voltage is applied between the buses 40 and 42. The AC voltage is first provided to step-down transformer 20 by conducting power transistors 68 and 78, while power transistors 70 and 76 are non-conductive. Thus, a half cycle of the AC voltage supplied to the step-down transformer 20 is generated.
Next, by changing the conditions, the power transistors 76 and 70 are turned on, and the power transistors 68 and 78 are turned off. This provides the reverse voltage to the step-down transformer 20 and another half cycle of the step-down transformer traffic voltage. The voltage supplied to the primary winding of the step-down transformer 20 is a 1200 Hz rectangular wave. Step-down transformer 20-2
The secondary winding responds to the 1200 Hz square wave to output a 1200 Hz substantially square wave full wave rectified signal with a slight ripple to the welding electrodes 26 and 30. FIG. 4 is a detailed circuit diagram of the timer 48 circuit shown in FIG. In FIG.
The monopulse generator 110 generates a rectangular monopulse having a width of 1.6 milliseconds. This pulse is input to the welding storage circuit 112. The pulse generator 114 outputs a variable width pulse having a predetermined frequency. These pulses are also input to the welding storage circuit 112. The welding timer circuit 116 generates a variable width rectangular pulse which is connected to the welding memory circuit 112 to enable welding for a predetermined time. The welding storage circuit 112 is also invalidated by a signal indicating an overcurrent from the current detecting element 50 shown in FIGS. 2 and 3. The output signal from the welding storage circuit 112 is input to the delay ignition circuit 118. This is a flip flop, delaying the signal. The output of the delay ignition circuit 118, the output of the pulse generator 114, and the output of the welding storage circuit 112 are the NOR gate 12
0 is input to the flip-flop 122. Flip flop 122
Produces two outputs, which are square wave, opposite sign signals. One of these is input to the driving transistor 124 and the other is input to the driving transistor 126.
The outputs of drive transistors 124 and 126 represent the output of timer 48, which is provided to two inputs of the drive circuit shown in FIG. Considering the circuit of FIG. 4 in more detail, the monopulse generator 110 has a switch 128, which changes the input state of a pair of NAND gates 130 and 132. These are connected to each other to form a flip-flop, and the output is input to a single pulse generator 134. This is an anti-bounce circuit. Here, switch 128 is shown as a push button because it was used in this manner in the example circuit. The initial trigger of the monopulse generator 110 can be triggered by an electrical signal from another circuit or a signal from a microprocessor used in the welding controller. This depends on design choices or convenience of use. The pulse generator 114 shown in FIG. 4 has a retriggerable, resettable monostable circuit, which is designated as pulse generator 136 for simplicity. A network consisting of a capacitor 138 and resistors 140 and 142, a variable resistor 144, diodes 146 and 148 is connected through a resistor 150 to a positive power supply. The common point of the diodes 146 and 148 is connected to the pulse generator 136, and one of the diodes 146 or 148 is turned on according to the sign of the voltage applied to the common point. When the diode 148 is conducting, the sum of the resistance values of the left half of the resistor 142 and the variable resistor 144 is connected in series to the capacitor 138,
One pulse time is determined. When the diode 146 is conducting, the resistance that determines the opposite half cycle of the pulse is the sum of the resistor 140 and the rest of the variable resistor 144. That is, when the setting of the variable resistor 144 is changed, only the relative length can be changed without changing the sum of the pulse widths. As a result, a rectangular wave of a constant frequency, here 1200 Hz, is output from the output terminal 152, and the conduction period of each code can be changed by the variable resistor 144. The welding time timer 116 uses a single pulse generator 154. Capacitor 1
56 is connected in series with a resistor 158, both of which are connected to a single pulse generator 154. The welding time is determined by the sum of the resistance values of the resistor 158, the resistor 160, and the variable resistor 162 and the value of the capacitor 156. The ON time length of the welding machine is adjusted by setting and adjusting the variable resistor 162 and setting the resistance value. It will be apparent that this function can adjust the resistance in an analog manner and control the RC time in the monopulse generator. Alternatively, a microprocessor may be used to control the welding time in response to predetermined scheduled times and other various information, such as measurements of the quality of the weld. These are design issues and depend on what is expected of the circuit. In the illustrated welding time timer 116, the rectangular pulse output from output terminal 164 has a pulse width equal to the required welding time, typically on the order of seconds or fractions of a second. Signals from the single pulse generator 110, the pulse generator 114, and the welding time timer 116 are all input to the welding storage circuit 112. The output of monopulse generator 110 is provided as one input of NOR gate 166, and the signal from output terminal 164 of welding time timer 116 is provided as the other input of NOR gate 166. N
The output of OR gate 166 is applied to the input of the D terminal of flip flop 168, the clock of which is applied by the signal from terminal 152. When an overcurrent occurs, the flip-flop 168 is reset by a signal from the current detecting element 50 shown in FIG. The non-Q output of flip flop 168 is provided as a reset signal for single pulse generator 154 and as an input signal for delay ignition circuit 118, where it is input to single pulse generator 170. Single pulse generator 1
The output of 70 is provided as one input of NOR gate 120. NOR gate 12
The other inputs of 0 are the output of pulse generator 114 supplied from terminal 152 and the Q output of flip-flop 168. The output of NOR gate 120 is the input of flip flop 122, where inverter 172 or the respective NAND
It is provided as one input of gates 174 and 176. The output of inverter 172 becomes the clock input for flip-flop 178, which outputs signals that are inverted from each other, which are provided to the inputs of NAND gates 174 and 176, respectively. The result is two equally oppositely-signed square waves, which are inputs for driving transistors 124 and 126. Referring again to the three inputs of NOR gate 120, the input from delay ignition circuit 118 causes the first pulse to be shorter than the subsequent pulses in any welding interval. This prevents the core of the step-down transformer shown in FIG. 1 from being saturated at the start of welding. The input signal from the welding storage device 112 to the NOR gate 120 is N
The total time during which a rectangular pulse can be output is determined at the output of the OR gate 120. This is the length determined for a single weld. The input from the terminal 152 to the NOR gate 120 generates all the remaining rectangular pulses except the first pulse at each welding. These pulses are signals having a fixed frequency and a pulse width determined by the variable resistor 144. FIG. 5 is a detailed circuit diagram of the drive circuit shown in FIG. In FIG. 5, a timer 184 inputs a signal from the drive transistor 124 of FIG. Timers 186 of the same specification receive an equivalent signal from drive transistor 126. Since the circuits used as the timers 184 and 186 are the same, only the timer 184 will be described in detail. Timer 184 generates a pulse which is supplied to power amplifier 188 shown in FIG. 5 and used to drive inverting amplifier 90. The input pulse from the driving transistor 124 is also supplied to the non-inverting amplifier 192. Both the inverting amplifier 190 and the non-inverting amplifier 192 are connected to the primary winding 194 of the transformer 196, and a part of each cycle current is supplied from each of these amplifiers. The transformer 196 has a secondary winding 198 and a secondary winding 200. Secondary winding 198 is connected to shaping circuit 202 and secondary winding 200 is connected to shaping circuit 204. The shaping circuit 202 is connected to the circuit shown in FIG. 3 and triggers the conduction of the power transistor 68. Shaping circuit 204 is connected to the circuit shown in FIG. 3 and triggers conduction of power transistor 78. The corresponding shaping circuits of the same part shown in FIG. 5 are similarly connected as shown, one to trigger the conduction of the power transistor 76 shown in FIG. 3 and the other shown in FIG. Connected to trigger conduction of power transistor 70. In the circuit shown in FIG. 4, the inputs to timers 184 and 186 are shown in a sense reversed. Accordingly, the voltage of the transformer 196 and the voltage at the circuit portion shown in FIG. 5 which are symmetrical thereto have opposite phases. Considering the outputs of shaping circuits 202 and 204 in relation to the circuit shown in FIG. 3, when pulse shaping circuits 202 and 204 generate current,
It will be seen that transistors 68 and 78 are conducting. The conduction of the step-down transformer 20 shown in FIG. 3 is from left to right. The converse is also true, and when the input polarity in FIG. 3 is reversed, power transistors 70 and 76 are turned on, causing current to flow from right to left of the step-down transformer shown in FIG. As a result of this operation, the step-down transformer 20 shown in FIG.
, A rectangular wave current having a frequency determined by the pulse generator 114 shown in FIG. 4 flows. The effective value of the current flowing through the step-down transformer 20 shown in FIG.
44 settings. The number of these pulses indicating the welding time is determined by the setting of the variable resistor 162 shown in FIG. FIG. 6 is a cutaway perspective view of the step-down transformer 20 manufactured and used in the embodiment of the present invention. In FIG. 6, a ferromagnetic core 210 includes a primary winding 212 and another primary winding 2.
14 and these windings are connected to each other. Secondary winding 216
Is the terminal 218. The secondary winding 216 is a single water-cooled conductor,
It is arranged so as to surround the next winding 212 and the associated iron core 210. Secondary winding 216 continues to a central terminal 220, which forms the center tap of secondary winding 216. Secondary winding 216 then surrounds primary winding 214 in the direction from corner 222 to corner 224 to maintain the correct orientation as the secondary
26 to the terminal 228 to complete the secondary winding 216. When a steel sheet having a thickness generally used by the Japan Automobile Manufacturers Association is resistance-welded, a current of about 10,000 to 20,000 amperes is usually required. In FIG. 3, 2
Although two rectifiers 22 and 24 are shown, four are shown in the transformer of FIG. 6 which implements this circuit, and this number is necessary to carry the required current. Sixth
In the figure, the diode 230 is used in parallel with the diode 232 to supply a required amount of current. These two diodes are connected in parallel to make them equivalent to the diode shown in FIG. Similarly, in FIG. 6, the diode 234 is arranged in parallel with the diode 236 and has a function equivalent to the rectifier 24 shown in FIG. The common connection of the diodes 230, 232, 234 and 236 can be realized by sandwiching the conductor between gaps 238 which are not shown in FIG.
The plurality of inlets 240 and outlets 242 are for flowing cooling water to the grooves 244 in the secondary winding 216. The transformer shown in FIG. 6 has been manufactured and tested for use in the circuit shown in FIG. It is shown here to characterize the transformer rather than the method required to assemble it. One of these features is a two-turn secondary with a center tap that can be used as a connection. One of the features is that the rectifying semiconductor is attached to the water-cooled terminal mounted on the transformer so that it can be securely fastened to the common terminal. However, one of the distinguishing features of the present invention is that it supplies a higher frequency than the commercial frequency to the primary side of the transformer shown in FIG. Can be less than required when the frequency is low. If the amount of iron is reduced, the amount of copper required is also reduced, and the weight of the transformer shown in FIG. 6 can be reduced, and the transformer shown in FIG. Easier to place. FIG. 7 is a time chart of the voltage waveform in the circuit shown in FIG. Each waveform is distinguished by giving an element number in FIG. 4 at an appropriate place on the horizontal axis, and these waveforms show respective output signals. In FIG. 7, the waveform marked “114” is a rectangular wave generated by the self-excited oscillator 114. This waveform is repeated at 417 microsecond rise at the time it was marked as T _1. The time at which this square wave falls is indicated by an arrow as being variable, and this time is determined by the variable resistor 14 shown in FIG.
4 can be adjusted and set. The second waveform in FIG. 7 is the output of the single pulse generator 110 marked "110" and is a rectangular wave with a pulse width of 1.6 milliseconds. Although the rectangular pulse is shown to start at time T ₀ in FIG. 7, this time is determined by the operation switch 128 shown in FIG. 4, and as described above, other signals are used. Alternatively, it can be similarly determined by a program. After time T ₀ , T ₁ is determined as the time when the rectangular wave of the pulse generator 114 first rises. At this point, a rectangular pulse marked "112" is set, which is the output of the welding storage circuit 112. This will start to time T ₁ a rectangular single pulse,
This time, which lasts until the end of welding, is between ten and several seconds to several seconds. Time T ₁ also,
It is also the starting point of the waveform marked "118". This is a rectangular single pulse that starts at time T ₁ and ends after 208 milliseconds. This signal is an output of the delay ignition circuit 118 and makes the width of the first pulse after the start of welding shorter than the remaining pulse widths. In the waveform shown in FIG. 7, the signal marked "120" is the NOR gate 1
20 output. This corresponds to the waveforms "112", "114" and "118" in FIG.
Is the negation of the logical unit of. Time T ₂ is shown as the falling time of the square wave representing the output of pulse generator 114, while time T ₃ is defined as the time at which the square wave output from delay ignition circuit 118 falls. If time T ₂ occurs before time T ₃ as shown in the figure, the waveform marked “120” will start at time T ₃ and thereafter be the inverse of waveform “114”. If time T ₂ is selected after T ₃ , waveform “120” is the inverse of waveform “114”. Waveform "120
Is the source of the waveforms marked "174" and "176."
The waveforms labeled "174" and "176" respectively correspond to the NANDs shown in FIG.
Outputs of gates 174 and 176. As is clear from the figure, the waveform "174"
Is a combination of every other pulse of the waveform “120”, and the waveform “176”
Is composed of the remaining pulses of waveform "120", which alternately switch inverter devices 44 and 46 to generate the number of rectangles as required.

Claims (1)

[Claims] 1. A step of rectifying an AC voltage of a power supply frequency to obtain a rectified voltage, inverting the rectified voltage at an operation frequency higher than the power supply frequency at the start of a welding cycle, and ending the operation frequency at one complete cycle after the end of the welding cycle; Generating an AC voltage, transforming the AC voltage to obtain a step-down voltage, applying the step-down voltage to a rectifier through a center tap of a transformer to rectify the voltage, and obtaining a DC voltage; Applying a DC voltage to the workpiece through the welding contacts, wherein when the AC voltage is reapplied in the next welding cycle, the current from the first pulse is applied to the AC at the operating frequency of the previous welding cycle. Resistance welding of a conductive workpiece adapted to flow in said transformer in a direction opposite to the direction of the current flowing during the last cycle of voltage . 2. First rectifying means for rectifying an AC voltage at a power supply frequency to obtain a rectified voltage, means for starting a welding cycle, inverting the rectified voltage at an operating frequency higher than the power supply frequency at the start of the welding cycle, and Means for generating an AC voltage of the operating frequency ending in one complete cycle of the
A voltage converting means for converting a voltage C to obtain a step-down voltage; rectifying the step-down voltage of the operating frequency applied from a center tap of the voltage converting means to obtain
Second rectifying means for obtaining a C voltage, means for applying the DC voltage to the workpiece via the welding contacts, wherein when the AC voltage is applied again in the next welding cycle, the current of the first pulse is reduced. A resistance welding apparatus for welding a conductive workpiece adapted to flow through said transforming means in a direction opposite to a direction of a current flowing during a last cycle of said operating frequency AC voltage of a previous welding cycle. 3. 3. The apparatus of claim 2, wherein the transformer includes a step-down transformer for generating the step-down voltage at a pair of secondary terminals, the welding contacts include a pair of welding electrodes, Each of the secondary terminals of the step-down transformer is
The resistance welding apparatus is connected to one of the pair of welding electrodes, and a center tap of the step-down transformer is connected to the other one of the pair of welding electrodes. 4. 4. The apparatus according to claim 3, wherein the second rectifier is a full-wave rectifier connected to a secondary terminal of the step-down transformer, the center tap, and the welding electrode. A resistance welding apparatus comprising: rectifying an output voltage of a step-down transformer; and applying the rectified output voltage to the welding electrode. 5. 5. The apparatus according to claim 4, wherein said inverting means monitors a primary circuit and generates a square wave pulse of said operating frequency with a controllable pulse width, said timer circuit and said timer circuit. A plurality of semiconductor elements connected to the step-down transformer and alternately switching a direction of a current flowing through the step-down transformer at the operating frequency. 6. 5. The apparatus of claim 4, wherein the step-down transformer is wound in two turns in the same direction around the core, a primary winding surrounding the core, and a primary winding surrounding the core. And a secondary winding made of a single water-cooled conductor. 7. 7. The resistance welding apparatus according to claim 6, wherein a center tap of the step-down transformer is the connection point of the two turns.