Scientific journal
European Journal of Natural History
ISSN 2073-4972

TRANSISTOR CHOPPERS FOR ELECTRIC-ARC WELDING

Magazinnik L.T.
A disadvantage of native and foreign secondary power sources (SPS) designed for electric welding is the fact that current amplitude in the invert circuit power transistors of the specified SPS designed for electric welding units is great and makes about one third of the load current output, i.e. in the electric arc. The specified disadvantage is explained by the fact that in order to stabilize the electric arc "combustion" and its easy "ignition" the denoted devices designers have to make the matching transformer secondary voltage greatly overstated compared to the arc voltage and equal to about 100V.

At the same time it is known that in practice the arc voltage does not exceed 25V when welding in the air, and when welding in СО2 and argon the arc voltage is considerably lower. The described disadvantage, i.e. voltage uprating, leads, first - to cost and power excursion of the invert circuit transistor; second - to their losses increase.

For this essential fault elimination a new SPS network [3] is offered, in which the matching transformer is provided with a supplementary secondary winding with an in-series capacitor, that has allowed imposing the "ignition" function and that of electric arc stabilization in the field of low currents on a supplementary power source and reducing the voltage of the matching transformer main secondary winding up to 35V and, in such a way, reducing the invert circuit transistor current amplitude almost thrice.

The device performed according the offered network is characterized by the following voltage regulation characteristics:

  1. The main power source voltage regulation characteristic ( chart 1) - is typical of the system closed in current. The characteristic is depicted perfectly stiff. Really, it has a little inclination conditioned by the supply main impedance and internal impedance of the device itself.
  2. The supplementary power supply voltage regulation characteristic ( chart 1) at load currents linear and hard, as the capacitor doesn´t manage to charge during the time of the τ half cycle to auxiliary secondary winding voltage excursion and that is why (if neglecting significant losses in the capacitor) doesn´t influence the amount of half- period average voltage . At > (chart 1) the capacitor charges up to the voltage excursion during the time t1 < τ (chart 2). The rectified voltage amplitude in the capacitor at

 ,          (1)

as during the time τ the load current is reputed to be invariable (τ ≈ 10-5 с).


 


By analogy, the voltage excursion in the mode >

                                                 (2)

As the voltage excursion  in the compared modes is identical, the expressions (1) and (2) can be equated:

.                                           (3)

But in concordance with the chart 1 , where  is the half-period average voltage. Thus, , from which at any t < τ.

So, in the field  <  < the supplementary power supply has got a hyperbolical voltage regulation characteristic with load constant power: , that is ideally suited for the electric arc in the air.

At  > the supplementary supply is locked as its voltage is lower than that of the main source, and, finally, at the voltage of both sources verges towards the null tight under the influence of the current feedback within the system of the invert circuit control.

From the quoted diagrams one can see that in the field of operating load currents (welding) the supplementary source, i.e. diode bridge, is locked. At the same time relatively high open circuit and small current voltage provides an easy "ignition" and the arc maintenance at small currents.

In the known secondary power sources the secondary winding voltage of the matching transformer, as it is shown above, ≈ 100 V.

In the offered device voltage is lowered up to 35 V, it means that at the load current equal to those of the specified devices, the voltage excursion in the invert circuit transistors will be 100/35 ≈ 2,85 times as little, that is rather vital as the power transistors or solid-state compound modules, first, make up an essential part of the total cost of the device; second, the reduction  in current reduces losses. Notice, that the losses in the secondary power source are ignorable as it is switched on only for short time. Thus, the offered device doesn´t trail the known one in the sense of generality, and, at the same time, possesses essential advantages in the mode of welding in the air: the 2 invert circuit current of the transistors is lowered almost three-fold compared to the current (voltage excursion) in the known devices, and the conditions of the electric arc "ignition" are even somehow improved as the electrode contact with a weldment is attended by a forced energy discharge accumulated in the filter capacitor.

References:

  1. "DS-200 AL - Technotron" - Russia, catalog, 1995.
  2. "Invertec V-130-S" - Linkoln-USA (1997).
  3. Russian patent № 2242835. Secondary power source / Magazinnik L.T., Magazinnik A.G. published 20.12.2004. BI № 35.