JPH02263579A - Inverter type spot welding machine - Google Patents

Abstract

PURPOSE:To supply a high electric current by making a base drive circuit to operate continuously a semiconductor device by a switching current value more than a rated current value and the semiconductor device into noncontinuity forcibly when these are made in a continued state for more than the prescribed time. CONSTITUTION:The subject inverter type welding machine is provided with an inverter part 24 which allows the semiconductor device 36a, etc., to execute switching operation to converts DC into AC. The welding machine is provided with the base drive circuit 30 to operate continuously the semiconductor device by the switching current value more than the rated current value and a timer circuit 40 to measure the continuity time of the semiconductor device. The semiconductor device can be used continuously by the current more than the rated current and the high current can be supplied to materials to be welded by monitoring the time when the semiconductor device to constitute the inverter circuit 24 is in the continued state. By this method, the semiconductor device to constitute an inverter is used in the switching rated region and the high current of several times as compared with the continuous use by the rated value can be supplied.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an inverter type spot resistance welding machine for spot resistance welding workpieces, and more particularly, to a power semiconductor element constituting an inverter circuit of the welding machine. The present invention relates to an inverter type spot resistance welding machine that prevents a power semiconductor element from being destroyed by monitoring the time during which the power semiconductor element is in a conductive state.

[Background of the Invention] A resistance welding machine, for example, holds a set of workpieces between a pair of electrodes, generates Joule heat by passing a welding current between the electrodes, and presses the electrodes relatively. This is a device that joins workpieces. The resistance welding machine is a joining method with excellent work efficiency because it does not require a welding rod or the like during joining.

In this case, the joining method by resistance welding requires a much larger welding current than, for example, the arc welding joining method, and therefore the welding transformer tends to be large and heavy. Therefore, this has been pointed out as a difficulty when attaching it to the arm of a welding robot or the like.

Therefore, recently, in order to reduce the size of this welding transformer, direct current is first converted into high-frequency alternating current under the action of a power semiconductor element, and this high-frequency alternating current is supplied to the welding transformer to step down the voltage. Inverter-type spot resistance welding equipment is beginning to be adopted, which supplies DC directly or by using a rectifier to the welding gun arm. The reason for converting to high frequency AC is that the welding transformer can be made relatively small and lightweight by utilizing the fact that the cross-sectional area of the transformer core constituting the output transformer is inversely proportional to the frequency of the high frequency AC.

In the inverter type spot resistance welding machine configured as described above, there is a constant demand for a larger supply current and a smaller size of the apparatus.

This is because a large current can be supplied even when welding steel plates with materials with different melting points such as plated steel plates or materials with high thermal conductivity such as aluminum plates.

Incidentally, in an inverter circuit used in this type of device, a protection means is provided to prevent power semiconductor elements such as power transistors constituting the circuit from being destroyed due to heat generation or the like. Conventional protection means include, for example, those in which an inverter circuit is provided with an overcurrent detection means, a temperature sensing element, or a thermal resistance element, as disclosed in JP-A-57-113776 and JP-A-62-064274. Therefore, it has been common practice to detect abnormal heat generation in semiconductor elements that make up the circuit.

However, with measures using these protection measures, the detection speed is slow, so the actual power consumption value relative to the rated power of the semiconductor element must be set small, in other words, it is necessary to set a large margin. is exposed. Furthermore, in conventional inverter circuits, the transistors are operated within their continuous rated values (DC rated values), and even if the base circuit operates abnormally and the transistors remain conductive, the If so, the inverter circuit is designed to avoid destruction. For this reason, it is necessary to set the operating value lower than the continuous rated value in advance, which is ultimately inconvenient for applications such as resistance welding machines that supply large currents.

[Object of the Invention] The present invention has been made in order to overcome the above-mentioned disadvantages, and the present invention has been made in order to overcome the above-mentioned disadvantages, and by monitoring the time during which the semiconductor elements constituting the inverter circuit are in a conductive state, the semiconductor elements constituting the inverter circuit are kept at a continuous rated value or higher. An object of the present invention is to provide an inverter-type spot resistance welding machine that can be used with a current of 100 kW and can supply a large current to a workpiece to be welded.

[Means for Achieving the Object] In order to achieve the above object, the present invention provides an inverter-type spot resistance welding machine having an inverter section that converts direct current into alternating current by switching semiconductor elements.
The present invention is characterized by comprising a base drive circuit that operates the semiconductor element at a switching current value greater than or equal to a continuous rated current value, and a timer circuit that measures the conduction time of the semiconductor element.

[Embodiments] Next, preferred embodiments of the inverter type spot resistance welding machine according to the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a diagram showing a schematic configuration of an inverter type spot resistance welding machine according to this embodiment. The inverter-type spot resistance welding machine basically includes a converter section 22 that converts three-phase AC output from a three-phase AC power source 20 into DC, and a converter section 22 that converts the DC output from this converter section 22 into a predetermined high-frequency AC. Inverter section 24 and this inverter section 2
Pulse width modulation is applied to the output transformer 28, which is a welding transformer that transforms the voltage output from the inverter section 24, and the semiconductor elements connected in a full bridge configuration that constitute the inverter section 24, such as power transistors 36a to 36d. a base drive circuit 30 that supplies a base current of
It is composed of a control circuit 32 and a timer circuit 40.

In this case, the value of the base current supplied from the base drive circuit 30 is such that the collector current is the continuous rated value (DC rated value).
A value of 1 is set which is sufficient to obtain the above current.

The timer circuit 40 includes an AND gate 41, an oscillator 42,
It is composed of a counter 44 and a setting device 46.

This timer circuit 40 calculates the conduction time by operating a counter 44 based on a timing gate signal T synchronized with the conduction time of the transistors, which is supplied from the control circuit 32 to the transistors 36a to 36d via the base drive circuit 30. If the conduction time is longer than a predetermined time preset in the setting device 46, an output signal Toff is output, forcing the base current to be supplied from the base drive circuit 30 to each transistor 36a to 36d. It operates so that it becomes non-conductive.

Furthermore, the converter section 22 is composed of a rectifier diode stack 34a, an axle 34b, and a capacitor 34c, and the output transformer 28 is composed of a primary coil 52, a transformer core 54, a secondary coil 56'J6, and a center tap 57. . Furthermore, the secondary coil 56
is connected to one end side of the rectifiers 31a, 31b, and the common connection terminal on the other end side of the rectifiers 31a, 31b and the center tap 57 of the output transformer 28 are connected to the workpieces W-, Wb.
It is connected to welding electrodes 58a and 58b which sandwich the .

The inverter type spot resistance welding machine according to this embodiment is basically constructed as described above, and its operation and effects will be explained next.

First, the workpieces W, , W, are connected to the welding electrodes 58a and 58.
While being clamped by b, initial pressure is applied. In this state, the three-phase AC output from the three-phase AC power supply 20 is transmitted through the rectifying diode stack 34a, the reactor 34b, and the capacitor 34c that constitute the converter section 22.
The direct current is converted into a direct current by the inverter section 24, and this direct current is introduced into the inverter section 24.

On the other hand, the base drive circuit 30 receives a timing gate signal T, (see FIG. 2a) from the control circuit 32, and generates a base current sufficient to drive the full-bridge transistors 36a to 36d constituting the inverter section 24. The signal is amplified and supplied to each of the transistors 36a to 36d, and the transistors 36a to 36d are sequentially controlled to be conductive or nonconductive at a predetermined timing. Thereby, the direct current introduced into the inverter section 24 is converted into alternating current.

The output alternating current of the inverter section 24 is introduced into the primary coil 52 of the output transformer 28, where it is transformed and induces a predetermined alternating current in the secondary coil 56. The alternating current induced in the secondary coil 56 is rectified by the rectifiers 31a and 31b, and then connected to the common connection point and the center tap 57 of the output transformer 28.
The workpiece W, , through the welding electrode 58a158b from
Supplied to Wb.

By the way, as mentioned above, the transistors constituting the inverter section according to the prior art are designed to operate within the continuous rated value range, and if the output signal from the base drive circuit corresponds to the conduction state. As a result, even if the transistor remains conductive, the inverter circuit will not be destroyed within thermal limits.

For this reason, the actual power used must be designed to be smaller than the continuous rated value, making it unsuitable for applications that supply large currents such as welding machines.

Therefore, in the spot resistance welding machine, the inventor of the present application
Compared to the operating time of the device, the time (usage rate) for passing electricity between welding electrodes is considerably small. For example, when welding aluminum materials, the time for passing electricity to the electrodes is 15 cycles (for example, 0 cycles). , 3 sec), and it was noted that the ratio of the current application time to the operating time of the welding apparatus itself, including the movement of the electrode and workpiece, was about 10%. Further, assuming that the inverter circuit performs a switching operation at 800 Hz, the time during which the transistor on one side of the inverter is in a conductive state is at most 0.625 msec, and the transistor is in a non-conductive state for a period longer than this. Moreover, it is known that in semiconductor devices such as power transistors, when used as a switching device such as an inverter circuit for continuous rated use, it is possible to increase the current flowing through the device. In inverter circuits used in welding machines, semiconductor devices can be used with switching current values greater than the continuous rated value.However,
In this case, if the switching operation stops and the semiconductor element remains in a conductive state, there is a risk that the semiconductor element will be destroyed, and in order to avoid this inconvenience, a protection circuit for the inverter is required.

The timer circuit 40 is provided for this purpose, and among the current signals (base drive signals) supplied to the timer circuit 40 from the control circuit 32 to the transistors 36a to 36d via the base drive circuit 30, the transistors 36a to 36d When the timing gate signal T synchronized with the conduction time of 36d is input, the AND gate 41 is activated for a high level period corresponding to the conduction time, and the oscillator 4
2 output pulse S, (see Figure 2) is the AND gate 4.
1 and is introduced into the counter 44 as a signal S (see FIG. 2C), and the counter 44 starts counting. Then, the conduction time of the transistors 36a to 36d is measured under the action of the counter 44, and when the measured conduction time exceeds the time preset in the setting device 46 (time t
(see point 1), the timer circuit 40 outputs an output signal Toff (see FIG. 2C). Here, the output signal Toff is input to the base drive circuit 30, and the transistor 36a
The base current supplied to 36d to 36d is forcibly cut off. Therefore, when the conduction time of the transistors 36a to 36d exceeds the set time, the transistors 36a to 36d
is forced into a non-conducting state and does not lead to destruction.

The counter 44 starts counting at the rising edge of the timing gate signal T, and is reset when the counting ends at the falling edge.

In the above embodiment, the time corresponding to the conduction state of the transistors 36a to 36d is detected based on the base drive signal output from the base drive circuit 30, but the present invention is not limited to this, and as shown in FIG. Current detection means 6 consisting of a Hall element etc. on the output side of the inverter section 24
0, and the waveform shaping circuit 62 shapes the output signal of the current detection means 60 to a level at which the AND gate 41 can operate, and supplies it to the AND gate 41 and the RESET terminal of the counter 44. For example, the actual conduction time of transistors 36a to 36d can be monitored.

In addition, as another embodiment, as shown in FIG.
Two current detection means 60 shown in the figure are interposed at the input terminal of the inverter section 240, and the output signal of the waveform shaping circuit 62 is inputted to a timing gate signal T by an electronic switch 64 such as a multiplier.
It is also possible to operate in a similar manner even if the circuit is configured to switch at the rising edge of the signal and measure the conduction time using the timer circuit 40.

[Effects of the Invention] As described above, according to the present invention, a timer circuit is provided that monitors the conduction time of a semiconductor element constituting an inverter circuit, and when the semiconductor element is in a conduction state for a predetermined time or more, The transistor is configured to be forcibly rendered non-conductive. For this reason, it is now possible to use the semiconductor elements that make up the inverter section in the switching rated range, and the welding machine can supply two to three times as much current as conventional continuous rated values. can get.

Although the present invention has been described above with reference to preferred embodiments, the present invention is not limited to these embodiments.
Of course, various improvements and changes in design are possible without departing from the gist of the present invention.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an embodiment of an inverter-type spot resistance welding machine according to the present invention, FIG. 2 is an explanatory diagram of the operation of the welding machine, and FIGS. 3 and 4 are inverter-type spot resistance welding machines according to the present invention. It is a block diagram which shows another embodiment of a welding machine. 22... Converter section 24... Inverter
Part 9 28... Dekadransu

Claims (1)

[Claims] (1) In an inverter-type spot resistance welding machine that has an inverter section that converts direct current to alternating current by switching semiconductor elements, a base drive circuit that operates the semiconductor elements at a switching current value that is higher than the continuous rated current value; An inverter-type spot resistance welding machine characterized by comprising a timer circuit for measuring conduction time of an element.