Timer for spot welding machine. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Timer for spot welding machine. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / welding equipment

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The case is the final element of any large electrical or electronic design. Its manufacture in amateur conditions often takes no less time than the assembly and adjustment of the device for which it is intended.

Typically, amateur radio and industrial equipment enclosures are made of sheet steel to ensure high mechanical strength. In addition, such a housing is particularly preferred in cases where the device being designed must be shielded from external electric or magnetic fields.

In the manufacture of housings, rivet or threaded connections are often used. It is possible to greatly facilitate the manufacture of cases, boxes, as well as the connection of individual structural elements, by using spot electric welding.

The device described below is one of the practical variants of the electric spot welding apparatus. The "Electric Welding Machine" described in the article by E. Godyna ("Radio", 1974, No. 12, pp. 39-41), which allows welding various parts from sheet steel, as well as steel wire, is taken as a basis. Mechanically and kinematically, our apparatus almost does not differ from it. The difference lies in the significantly improved electronic dosing device for the duration of the welding current pulse.

As is known, in accordance with the Joule-Lenz law, the amount of heat W released at the contact point of the welded parts depends on the duration t of the current pulse I and the electrical resistance R to the current through the contact:

W=R*t*I^2

When calculating the welding current and pulse duration, the resistance is considered the initial parameter, since it can be determined in the first approximation, knowing the material of the parts to be welded, their thickness and the required welding temperature.

According to the Joule-Lenz law, an increase in resistance should increase the amount of heat released. But according to Ohm's law

I=U^2/Z,

where U2 is the voltage on the secondary winding of the welding transformer; Z is the impedance of the secondary circuit, which includes the contact resistance R.

Therefore, with an increase in R, I will decrease, and it is included in the formula of the Joule-Lenz law squared. The amount of heat released during welding depends on the ratio R and the impedance Z of the secondary circuit.

The smaller Z, the more welding current can be provided with the same U2. At the same time, the smaller R compared to Z, the less useless power loss for heating the secondary winding of the transformer

Welding with a low resistance of the secondary circuit is accompanied by unsteady heating and, as a result, instability of the quality of the joints. This shortcoming can be minimized by reliably compressing the parts and cleaning their surface, which will ensure the constancy of R.

It is most convenient to optimize the welding mode at a constant value of voltage U2 by adjusting the duration t of the welding current pulse.

The scheme of the electronic unit of the welding machine is shown in fig. one.

Timer for spot welding machine

In the initial state, the welding transformer T1 is de-energized, since the contacts K1.1-K1.3 of the relay K1 are open. The winding of the relay K1 AC, included in the input diagonal of the diode bridge VD2, is also de-energized.

Despite the fact that a rectified mains voltage is applied to the trinistor, the current bridge does not conduct, since the trinistor VS1, which closes the output diagonal of the diode bridge, is closed. Capacitor C1 is shunted by resistor R1 and is therefore discharged.

The switch SF1 is installed on the frame of the welding machine and is connected to the pedal that controls the compression of the parts to be welded by the electrodes, so that the switch occurs at the end of the pedal stroke. At the moment of switching, the capacitor C1 begins to charge, the charging current opens the trinistor VS1, which closes the output diagonal of the diode bridge VD2, and it connects the winding of relay K1 to the network. At the same time, the EL1 lamp flashes.

The relay is activated, and the closed contacts K1.1 -K1.3 connect the primary winding of the welding transformer T1 to the network. A powerful alternating current pulse that occurs in the secondary circuit heats the metal of the parts to be welded at the point of compression by the electrodes to the melting point.

After some time, the charging current of the capacitor C1 drops so much that it can no longer open the trinistor VS1 at the next half-cycle of the mains voltage. Therefore, the trinistor remains closed. The winding of relay K1 is now de-energized. Contacts K1.1 - K1.3 of the relay open and disconnect the welding transformer from the network. This completes the process of welding the next point.

The pedal of the apparatus is released and it is prepared for welding the next point. When the pedal is released, the contacts SF1 return to their original position and the capacitor C1 is discharged through the resistor R1.

The time during which the trinistor opens in each half-cycle of the mains voltage, with the values \u1b\u1bof the capacitor C0,1 and resistor RXNUMX indicated on the diagram, can be changed in the range from XNUMX s to several seconds. Thus, the electronic unit of the welding machine is a combination of a powerful current pulse shaper and a time relay that determines the duration of this pulse.

The welding current in a pulse can reach 1500...2000 A depending on the material and thickness of the parts to be welded. The current consumed from the network does not exceed 8 A.

The R3C2 circuit is designed to extinguish sparks between contacts K1.1-K1.3 and reduce the generated interference. An EL1 incandescent lamp with a power of 60 or 75 W for a voltage of 220 V is used to ensure more stable operation of the trinistor with a significant inductance of the relay winding K1. Diode VD1 prevents the possibility of a negative voltage at the control transition of the trinistor.

As a relay in the block, a magnetic starter PME-071 MVUHLZ AC3 with a winding for an alternating voltage of 220 V and three pairs of working contacts was used. The trinistor is mounted on a copper heat-removing mounting bracket with a usable surface area of about 8 cm2. Capacitors C1, C2 - any type, and C2 should be selected for a rated voltage of at least 630 V. Variable resistor R2 - any, with a linear characteristic

Welding transformer T1 is converted from laboratory control LATR-9 (RNSH). Its winding contains 266 turns of wire with a diameter of 1 mm. The engine and the contact roller are dismantled, the contact path on the winding, which is free from insulation, is cleaned of dust, varnished, after which the winding is insulated with varnished cloth. The conclusions from the winding, which will serve as the primary, are made with a flexible insulated wire with a cross section of 1,5 ... 2 mm2.

The secondary winding is wound with stranded copper wire with a copper cross section of at least 80 mm2 in heat-resistant external insulation. The number of turns - 3.

The electronic unit is located in the lower compartment of the body of the welding machine (Fig. 2). The control knob for the duration of the current pulse, calibrated in seconds, is displayed on the side panel.

Timer for spot welding machine

Information about many design aspects missing in the article, about the operation and operation of welding machines can be found in the book by V. T. Gevorkyan "Fundamentals of Welding" (M .: Higher School, 1991).

A properly assembled apparatus, as a rule, does not require adjustment, it is only necessary to calibrate the scale of the time delay regulator R2. Here, however, it is appropriate to note that the time limits of this scale strongly depend on the parameters of the VS1 trinistor used in the device. Therefore, in some cases it may be appropriate to select a more suitable instance of the trinistor and capacitor C1.

Before starting welding of prepared parts, it is necessary to determine in advance the optimal duration of the welding pulse for each combination of their thickness and material. If the pulse is too short, the connection will be fragile, and if it is too long, through-burning of parts is not ruled out.

The device allows you to weld steel and stainless steel wires with a diameter of up to 3 mm, tinned copper - up to 2 mm, steel sheets - up to 1,1 mm thick.

The front-top view of the apparatus is shown in Fig. 3.

Timer for spot welding machine

It should be borne in mind that welding is often accompanied by sparks from the point of contact of metals, so it is necessary to familiarize yourself with the safety regulations and strictly follow them. It is possible to work with the device only in non-combustible clothes, in mittens and with a protective mask on the face.

Authors: G.Chiketaev, B.Karimov, Bishkek, Kyrgyzstan

See other articles Section welding equipment.

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