JP2507295B2 - Resistance welding machine - Google Patents

Description

BACKGROUND OF THE INVENTION The present invention relates to resistance welding. More specifically, the present invention relates to an improvement of a resistance welding machine suitable for an automatic pressure welding machine using a robot welding machine.

Resistance welding is a well-known method for joining two conductors. In this method, the electric current is applied in such an amount that the conductor is locally heated and melted. This is usually done by sandwiching two conductors that connect a pair of electrodes, and applying pressure and voltage between the electrodes so that the heating time generated by I ² R is sufficiently melted and the members do not melt too much. Adjust. This method can be implemented using either DC or AC voltage. Moreover, considering the contact resistance value of the two conductors to be joined,
It is desirable to reduce the voltage from a normal power supply voltage to several volts or tens of volts by using a step-down transformer and set the current to a value of thousands to tens of thousands of amps. The simplest resistance welder is a step-down transformer, a switcher controlling the voltage supply 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 placed so as to sandwich the members to be joined, a voltage is applied to the joint of the conductor by closing the switch, and the resulting electrical heat melts the point where it is in contact with the conductor, Weld each other.

When considering the actual welding operation, various improvements have been made to the above-mentioned processing steps. To minimize the price of the transformer for welding current supply, it is desirable to make the peak value of the voltage supplied from the power source to the transformer reach the saturation value of the transformer core or the bend of the BH curve of the core. It is to be.
For this reason, the condition of the transformer core must always be known when applying voltage to the transformer. If the last voltage applied to the transformer magnetizes the transformer in a certain direction and the next applied voltage drives current in the same direction, the magnetizing current required by the transformer will The current will be added to drive an overload current through the transformer during the first cycle. This is an undesired situation and has so far been prevented as follows: the control circuit of the welding transformer always completes the welding at the time when it has supplied the entire cycle of the input voltage and at the beginning of welding. Controls so that the input voltage waveform always points in the same direction. As a result, the first
The cycle always closes the hysteresis curve of the transformer core to prevent over-saturation current and to ensure that the peak voltage value is constant during each welding cycle. Therefore, what remains is only the 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 empirically determined and is usually a very small value and should be electrically controlled, because the required supply time depends on the operator. It is too short to operate reliably.

The resistance welding device described above has major drawbacks when it is used for resistance welding in the production line. The welding of two sheet steels, whether or not they are zinc plated, typically requires currents of the order of 10,000 to 30,000 amps. A transformer wound to supply such current to the secondary circuit typically weighs 200 to 600 pounds and needs to be cooled with water or an external chiller. The electric wire that carries the above current is actually quite large. A device for treating these problems in the manufacturing line of the automobile industry or other industries is to hang a transformer from an upper retainer and wire an insulated conductor to a weld having a water-cooled electrode, and an operator welds the electrode. The electrode is held by applying force from the outside during the welding operation at the point. The device described above has several obstacles when it is used in robots or automatic welders. Robots are generally weight limited, and reducing weight improves maneuverability.
In automatic welders, the minimum spacing between adjacent weld points is limited by the size of the transformer. The robot welder is also greatly impaired in its operability by being connected to a thick power cable designed to carry thousands of amps of current, and the movement of the robot causes The life of thick cables is shortened.

It is an object of the invention to provide an electric circuit of a resistance welder with reduced weight near the weld.

Yet another object of the present invention is to provide an electrical circuit of a resistance welder suitable for use in a robotic welding device.

Still another object of the present invention is to provide a small and lightweight transformer, which is small and lightweight, so that it can be mounted near the welding work area and has a lightweight primary conductor. It can be used and the secondary conductor can be shortened.

Yet another object of the present invention is to provide an electrical circuit of a resistance welder 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 to rectify an AC voltage and generate 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 that of the AC power supply. This square wave is fed to a step-down transformer having a secondary winding with a central tap.
The secondary winding is connected to a welding contact through a full wave rectifier which conducts the welding current through the member. Using a higher frequency than the power supply frequency for the inverter output allows the use of a step-down transformer that is lighter than a transformer operating at the input AC voltage frequency.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a circuit which realizes the present invention. FIG. 2 is a more detailed block diagram of a circuit which realizes the present invention. FIG. 3 shows the operation of the inverter devices 44 and 46 of FIG. The circuit diagram shown in detail.

4 is a detailed circuit diagram of the timer 48 shown in FIG. 2. FIG. 5 is a detailed circuit diagram of the drive circuit 52 shown in FIG. 2. FIG. 6 is a broken down step-down transformer manufactured and used for realizing the present invention. Perspective view FIG. 7 is a time chart of voltage waveforms in the circuit of FIG. 4. Detailed description of the invention FIG. 1 is a block diagram of a circuit implementing the invention.
The AC voltage power supply 10 of FIG. 1 is connected to a rectifier 12.
The connection is shown here with three wires, which is the 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 containing 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 controlled inverter. The control circuit 14 is used to convert the output of the rectifier 12 into an AC voltage having a frequency higher than the input frequency, and the effective value of the converted AC signal can be controlled.

Transformer 16 is shown in dotted lines, but this is a step-down transformer.
It is useful for changing the voltage supplied to wires 18 and 19 as the input of 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 central tap. The secondary conductors of step-down transformer 20 are connected to rectifiers 22 and 24, forming a full wave rectifier. The common connection of the rectifiers 22 and 24 is led to the welding electrode 26. Step-down transformer 20
The central tap 28 is connected to a welding electrode 30. The welding electrodes 26 and 30 are arranged so as to sandwich the members to be welded, and when the control circuit 14 is operated to supply an electric current through the welding electrodes 26 and 30, resistance welding is performed between the members to be joined.
When the frequency of the output voltage of the control circuit 14 is higher than the frequency of the power source 10, the step-down transformer 20 can be made smaller and lighter than it would otherwise be. This not only eases the operation of the operator,
It allows the transformer to be placed in the arm of the robot welder, thus allowing free movement of the arm over a wide range.
In addition, smaller transformers allow for closer welds to be achieved in an automatic welder.

FIG. 2 is a detailed block diagram of a circuit for implementing the present invention. In FIG. 2, the rectifier 12 is connected to the power supply 10 and supplies a voltage between the buses 40 and 42. Inverter device
44 and 46 are connected between buses 40 and 42, the midpoints of which are connected to conductors 18 and 19 and step-down transformer 20.
In the block diagram of the circuit of FIG. 2, the transformer 16 shown in FIG.
Are excluded. As mentioned earlier, this is a design issue. In the circuit shown in FIG. 2, the rectifiers 22 and 24 are connected to the secondary winding of the step-down transformer 20, and the full-wave rectified output is passed through the welding electrodes 26 and 30, a member not shown in the figure, to the central tap 28. It causes it to return. The control of the circuit shown in FIG. 2 is started by a timer 48, which operates the operating periods of the inverter devices 44 and 46 and the welding electrodes 26 and 30.
And the relative time for adjusting the intensity of the current flowing through. The current detection 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 bar 40 exceeds a predetermined value.

The timer 48 outputs two outputs, which are drive circuits 52.
This drive circuit is input to the inverter devices 44 and 46.
It is connected to drive the switch elements such as power transistors inside the inverter device.
It is controlled so as to flow to the bus bar 42 through the upper part of 44, the primary winding of the step-down transformer 20, the primary winding of the step-down transformer 20 and the lower part of the inverter device 46. The current of the second half cycle passes through the upper half of the inverter device 46, the lead wire 19, and the step-down transformer 20.
Flows in the opposite direction to the primary winding. The current flows through the conductor 18 to the inverter device 44, and then to the bus bar 42 via the lower half of the inverter device 44. Details of this control will become apparent by examining the more detailed schematics. Thyristor, SC can be used as a switch element of inverter device 44.
R, gate turn-off element, etc.

FIG. 3 is a circuit diagram for explaining more detailed operation of the inverter devices 44 and 46 shown in FIG. In FIG. 3, the AC power from the power source 10 is input to the rectifier 12 via the three fuses 60, the three limiting resistors 62 and the three contacts 64. The power supply 10 is usually three-phase 60 Hz and has a voltage of 480 volts or the like at hand. Contact 64 is shown here to be energized by contactor 63 which is controlled by pushbutton 65.
This assumes that the push button 65 is operated as part of the opening and closing operation of the welding electrodes 26 and 30 before the welding current is supplied and after the welding is completed. Alternatively, the contactor 63 can be controlled by the timer 14 of FIG.

In FIG. 3, rectifier 12 comprises a suitable number of diodes 66 connected to form a full wave rectifying bridge rectifier. Six diodes 66 are shown here, but it will be clear that this number may be varied in the case of currents to be handled, voltages applied to the diodes and a number of phases other than three. 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 between buses 40 and 42. Inverter device 44
Since and 46 are the same, only the inverter device 44 is shown. In the inverter device 44, the power transistor 68 is connected to the negative bus 42 through a series connection with another power transistor 70. The power transistor 68 is driven by the Darlington transistor 72, and the transistor 72 is driven by the drive circuit 52. The power transistor 70 is likewise driven by the Darlington transistor 74, which is driven by the drive circuit 52. Inverter device 46 includes power transistors 76 and 78, which are likewise driven by Darlington transistors, which are not shown here.
The power transistor 68 is bypassed by a diode 80, a series-connected resistor 82 and a capacitor 84, which suppresses the sudden rise of the voltage across the power transistor 68. This may not be necessary depending on what you choose for power transistor 68. Power transistors 70, 76 and 78 are also bypassed. The common connection point 86 of the power transistors 68 and 70 is connected via the conductor 18 to one end of the primary winding of the step-down transformer 20, and the common connection point 88 of the power transistors 76 and 78 is converted via the conductor 19. It is connected to the other end of the primary winding of the step-down transformer 20. A stabilizing capacitor 90 is connected between the positive electrode bus 40 and the negative electrode bus 42 through a resistor 92.

The galvanic element 50 has a current transformer 94, which detects the current flowing in the positive electrode bus 40 and the capacitor 90. This combination of connections prevents accidental tripping due to the current charging the capacitor 90 when the circuit is first activated. 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 the 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 and controls the operation of the timer 48.

When the circuit shown in FIG. 3 is constructed and tested, the power supply
The input voltage from 10 has a frequency of 60Hz, and the timing circuit of the timer 48 shown in Fig. 2 is a step-down transformer operating at 1200Hz.
Was working to generate 20 inputs. Under such conditions, the voltage fluctuations between the buses 40 and 42 in each cycle may be ignored, but the output of a normal full-wave three-phase bridge rectifier has an AC voltage component of 360 Hz and its harmonic frequencies. Is known to have. The operation of the circuit may be regarded as a DC voltage being applied between the buses 40 and 42. The AC voltage is initially supplied to step-down transformer 20 by conducting power transistors 68 and 78, while power transistors 70 and 76 are non-conductive. As a result, a half cycle of the AC voltage supplied to the step-down transformer 20 is generated. Next, the conditions are changed so that the power transistors 76 and 70 are made conductive and the power transistors 68 and 78 are made non-conductive. This provides the reverse voltage to the step-down transformer 20 and the other half cycle of the step-down transformer AC voltage. The voltage supplied to the primary winding of the step-down transformer 20 is a 1200 Hz square wave. The secondary winding of the step-down transformer 20 responds to the 1200 Hz square wave to a 1200 Hz almost square wave,
Welding electrode 26 and full-wave rectified signal with a little ripple
Output to supply to 30.

FIG. 4 is a detailed circuit diagram of the timer 48 circuit shown in FIG. In FIG. 4, the monopulse generator 110 produces a rectangular monopulse having a width of 1.6 milliseconds. This pulse is input to the welding memory circuit 112. The pulse generator 114 outputs a variable width pulse having a predetermined frequency. These pulses are also inputs to the weld memory circuit 112. Weld timer circuit 116 produces a variable width rectangular pulse which is connected to weld memory circuit 112 to enable welding for a predetermined time. The welding memory circuit 112 is also shown in FIGS.
It is invalidated by a signal indicating an overcurrent from the current detection element 50 shown in the figure.

The output signal from the welding memory circuit 112 is input to the delayed ignition circuit 118. This is a flip-flop and delays the signal. Output of delayed ignition circuit 118, pulse generator 1
The output of 14 and the output of the welding memory circuit 112 are input to the flip-flop 122 through the NOR gate 120. The flip-flop 122 produces two outputs, which are square wave and opposite sign signals. One of these is input to the drive transistor 124 and the other is input to the drive transistor 126. The outputs of drive transistors 124 and 126 represent the output of timer 48, which is provided to the 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 modifies the input state of a pair of NAND gates 130 and 132. These are connected to each other so as to form a flip-flop, and the output thereof is input to the monopulse generator 134. This is an anti-bounce circuit. Switch 128 is shown here as a push button because it was used in this manner in the example circuit. It is also possible to initially trigger the monopulse generator 110 with an electrical signal from another circuit or with a signal from a microprocessor used in the welding controller. This is due to design choice or convenience in use.

The pulse generator 114 shown in FIG. 4 has a retriggerable and resettable monostable circuit, which is designated as pulse generator 136 for simplicity. The network consisting of capacitor 138 and resistors 140 and 142, variable resistor 144, diodes 146 and 148 is connected through resistor 150 to a positive power supply. The common connection point of the diodes 146 and 148 is connected to the pulse generator 136, and the diode 146 or 14 is connected.
One of the eight is turned on according to the sign of the voltage applied to its common connection point. When diode 148 conducts, resistor 1
The sum resistance of the resistance values of the left half of 42 and the variable resistor 144 is connected to the capacitor 138 in series to define one pulse time. When the diode 146 is conducting, the resistance value that determines the period of the opposite half 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 pulse widths. As a result, a rectangular wave having a constant frequency, here, 1200 Hz is output from the output terminal 152, and the conduction period of each code can be set and changed by the variable resistor 144.

The welding time timer 116 uses a single pulse generator 154. Capacitor 156 is connected in series with resistor 158, which are both connected to monopulse generator 154. Resistor 15
8, the welding time is determined by the sum of the resistance values of the resistor 160 and the variable resistor 162 and the value of the capacitor 156. By adjusting and adjusting the variable resistor 162 and setting the resistance value, the welding machine O
Adjust N time length. It will be clear that this function can adjust the resistance value 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 accordance with a predetermined scheduled time or other various information such as measured values such as the quality of welding. These are design issues and will vary depending 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 a few seconds or a fraction of a second.

The signals from the single pulse generator 110, the pulse generator 114, and the welding time timer 116 are all input to the welding memory circuit 112. The output of the single pulse generator 110 is supplied as one input of the NOR gate 166, and the signal from the output terminal 164 of the welding time timer 116 is supplied as the other input of the NOR gate 166. The output of NOR gate 166 is a flip-flop 168 D
This flip-flop clock applied to the input of the terminal is multiplied by the signal from terminal 152. The flip-flop 168 is reset by a signal from the current detecting element 50 shown in FIG. 2 when an overcurrent occurs. Flip
The non-Q output of the flop 168 is supplied as a reset signal to the single pulse generator 154 and as an input signal to the delayed ignition circuit 118, where it is input to the single pulse generator 170. The output of the monopulse generator 170 is provided as one input of the NOR gate 120.

The other inputs of NOR gate 120 are the output of pulse generator 114 supplied from terminal 152 and the Q of flip-flop 168.
And the output. The output of NOR gate 120 is flip-flop
It becomes the input of 122, where it is provided as one input of the inverter 172 or of the respective NAND gates 174 and 176. Inverter 172
Is the clock input to 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. As a result, two equal and opposite signs square waves are generated which serve as inputs for driving transistors 124 and 126. Referring again to the three inputs of NOR gate 120, the input from the delayed ignition circuit 118 causes the first pulse to be shorter than the subsequent pulses in any weld interval. This prevents the magnetic core of the step-down transformer shown in FIG. 1 from being saturated at the start of welding. The input signal from welding memory 112 to NOR gate 120 defines the total time that a rectangular pulse can be output at the output of NOR gate 120. This is the defined length for a single weld. The input from terminal 152 to NOR gate 120 produces all the remaining rectangular pulses except the first pulse during each weld. 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, the timer 184 inputs a signal from the driving transistor 124 of FIG. The timers 186 having the same specifications input the same signal from the drive transistor 126. Timer 184
Since the circuits used as and 186 are identical, only timer 184 will be described in detail.

The timer 184 generates a pulse which is fed to the power amplifier 188 shown in FIG. 5 and used to drive the inverting amplifier 190. The input pulse from drive transistor 124 is also provided to non-inverting amplifier 192. The inverting amplifier 190 and the non-inverting amplifier 192 are both connected to the primary winding 194 of the transformer 196, and a part of each periodic current is supplied from each of these amplifiers. The transformer 196 has a secondary winding 198 and a secondary winding.
Has 200. The secondary winding 198 is connected to the shaping circuit 202, and the secondary winding 200 is connected to the shaping circuit 204. Shaping circuit 202
Is connected to the circuit shown in FIG. 3 and triggers the conduction of the power transistor 68. The shaping circuit 204 is connected to the circuit shown in FIG. 3 and triggers the conduction of the power transistor 78. 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 the conduction of the power transistor 70.

In the circuit shown in FIG. 4, the inputs to timers 184 and 186 are in some sense reversed. Therefore, the voltage of the transformer 196 and the voltage at the circuit portion shown in FIG. Considering the outputs of shaping circuits 202 and 204 in connection with the circuit shown in FIG. 3, it will be seen that when pulse shaping circuits 202 and 204 generate a current, they cause transistors 68 and 78 to conduct. The conduction of the step-down transformer 20 shown in FIG. 3 is from left to right. The reverse is also true, and when the input polarity in FIG. 3 is reversed, the power transistors 70 and 76 are turned on, causing current to flow from right to left in the step-down transformer shown in FIG. As a result of this operation, a rectangular wave current having a frequency determined by the pulse generator 114 shown in FIG. 4 flows through the step-down transformer 20 shown in FIG. The effective value of the current flowing through the step-down transformer 20 shown in FIG. 3 is determined by the setting of the variable resistor 144 shown in FIG. 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 step-down transformer manufactured and used in the embodiment of the present invention.
FIG. 20 is a cutaway perspective view of 20. In FIG. 6, a ferromagnetic iron core 210 is surrounded by a primary winding 212 and another primary winding 214, and these windings are connected to each other. Secondary winding
The beginning of winding 216 is terminal 218. Secondary winding 216 is a sheet of water-cooled conductor and is arranged to surround primary winding 212 and associated iron core 210. Secondary winding 216 is center terminal 2
Continuing to 20, this central terminal 220 forms the central tap of the secondary winding 216. Secondary winding 216 then surrounds primary winding 214 in the direction of corner 222 to corner 224 and is connected to terminal 228 through opening 226 to maintain the correct orientation for the secondary winding. 216 has been completed.

Resistance welding of steel sheets of the thickness commonly used in the Automobile Manufacturers Association usually requires currents of the order of 10,000 to 20,000 amps. Although FIG. 3 shows two rectifiers 22 and 24, four transformers shown in FIG. 6 which realize this circuit are shown with four rectifiers, and this number is necessary for passing the required current. . In FIG. 6, the diode 230 is used in parallel with the diode 232 to carry the required amount of current. These two diodes are connected in parallel to be 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 that of the rectifier 24 shown in FIG. diode
The common connection of 230, 232, 234 and 236, which is not shown in FIG. 6, can be realized by sandwiching a conductor between the now separated gaps 238. Multiple entrances 240 and exits 242
Is for flowing cooling water into the groove 244 in the secondary winding 216.

The transformer shown in FIG. 6 was manufactured and tested for use in the circuit shown in FIG. It is shown here to clarify its characteristics rather than the method required to assemble it. One of these features is a two-turn secondary winding with a central tap available as a connection. Attach the rectifying semiconductor to the water-cooled terminal attached to the transformer,
One of the features is that the common terminal can be tightened tightly. However, one of the outstanding features of the present invention is that it supplies a frequency higher than the commercial frequency to the primary side of the transformer shown in FIG. 6, and as a result, it is used in the iron core 210. The iron content can be less than that required at low frequencies. 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. 6 can be installed in a robot arm or in an automatic welding machine. It is easy to place

FIG. 7 is a time chart of voltage waveforms in the circuit shown in FIG. The respective waveforms are distinguished by giving the element numbers in FIG. 4 at appropriate positions on the horizontal axis, and these waveforms show respective output signals. In FIG. 7, "11
The waveform marked 4 "is a square wave generated by the free-running oscillator 114. This waveform repeats with a rise of 417 microseconds at the time marked T _{1. The} time when this square wave falls is variable with the arrow. It is shown that this time is the 4th
The variable resistor 144 shown can be adjusted and set. The second waveform in FIG. 7 is the output of the monopulse generator 110 marked "110" and is a square wave with a pulse width of 1.6 milliseconds. The rectangular pulse is shown to start at time T ₀ in FIG. 7, which is the operating switch shown in FIG.
It is determined by 128, and it can be similarly determined by other signals or programs as described above.

After time T ₀ , T ₁ is defined as the time when the rectangular wave of pulse generator 114 first rises. At this point, a rectangular pulse marked "112" is set, which is the output of the welding memory circuit 112. This is a rectangular monopulse, starting at time T ₁ and continuing until the end of welding, this time being between a few tens of seconds and a few seconds. Time T ₁ is also the starting point of the waveform marked “118”. This is a rectangular monopulse that begins at time T ₁ and ends 208 milliseconds later. This signal is delayed ignition circuit 118
The width of the first pulse after the start of welding is made shorter than the remaining pulse width.

In the waveform shown in FIG. 7, the signal marked "120" is the output of NOR gate 120. This is the negation of the logical units of the waveforms "112", "114" and "118" in FIG. Time T ₂ is designated as the fall time of the square wave representing the output of pulse generator 114, while time T ₃ is defined as the time the square wave output of the delayed ignition circuit 118 falls. When time T _2, the provisional occurs before time T ₃ as illustrated in FIG, then starts to "120" and marked waveform the time T ₃ becomes an inversion waveform "114". If time T ₂ was selected after T ₃ , waveform “120” would be the inverse of waveform “114”. Waveform "120" is the source of the waveforms marked "174" and "176". These "174" and "176"
The waveforms marked with are the outputs of NAND gates 174 and 176, respectively, shown in FIG. As is apparent 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 the waveform "120". These alternately switch the inverter devices 44 and 46 to generate the number of rectangles as required.

Claims (7)

(57) [Claims] 1. A step of rectifying an AC voltage of a power supply frequency to obtain a rectified voltage, and at the start of a welding cycle, the rectified voltage is inverted at an operating frequency higher than the frequency for power supply, and one complete cycle after the welding cycle is completed. A step of generating an AC voltage of the operating frequency ending with a step of transforming the AC voltage to obtain a stepped-down voltage, applying the stepped-down voltage from a center tap of a transformer to a rectifying means to rectify DC A method of resistance welding a conductive work piece comprising the steps of obtaining a voltage and applying the DC voltage to the work piece through a welding contact. 2. A first rectifying means for rectifying an AC voltage of a power supply frequency to obtain a rectified voltage, a means for starting a welding cycle, and inverting the rectified voltage at an operating frequency higher than the power supply frequency at the start of the welding cycle. , AC of the above operating frequency, which ends in one complete cycle after the end of the welding cycle
A means for generating a voltage, a transformer for transforming the AC voltage of the operating frequency to obtain a stepped-down voltage, a step-down voltage of the operating frequency applied from the center tap of the transformer, and rectifying a DC voltage. A resistance welding apparatus for welding a conductive work piece, which includes a second rectifying means for obtaining the DC voltage and a means for applying the DC voltage to the work piece through a welding contact. 3. The apparatus according to claim 2, wherein the transformer means includes a step-down transformer for generating the stepped-down voltage at a pair of secondary terminals, and the welding contact is a pair. A welding electrode, wherein each of the secondary terminals of the step-down transformer is connected to one of the pair of welding electrodes, and the center tap of the step-down transformer is the other one of the pair of welding electrodes. Resistance welding device characterized by being connected to. 4. The apparatus according to claim 3, wherein the second
Is a full-wave rectifier connected to the secondary terminal of the step-down transformer, the center tap, and the welding electrode, and rectifies and rectifies the output voltage of the step-down transformer. A resistance welding apparatus characterized in that an output voltage is applied to the welding electrode. 5. The apparatus according to claim 4, wherein the inverting means monitors a primary circuit and generates a square wave pulse having the pulse width controllable operation frequency. A resistance welding apparatus, comprising: a plurality of semiconductor devices connected to the timer circuit and the step-down transformer, and alternately switching a direction of a current flowing through the step-down transformer at the operating frequency. 6. The apparatus according to claim 4, wherein the step-down transformer has a ferromagnetic core, a primary winding surrounding the core, and two windings in the same direction around the core. And a secondary winding formed of a single water-cooled conductor wound in the above state. 7. The resistance welding apparatus according to claim 6, wherein a center tap of the step-down transformer is a connection point of the two turns.