Sunday, August 21, 2011

Welding Transformers

       Most of the power supplies available now are a.c. Therefore a transformer for welding is most commonly used as compared to motor-generator set. Moreover motor-generator set has to be kept in the running position continuously during the weld is made.
Fig.  Use of welding transformer

        Welding transformer is a transformer having thin primary winding with large number of turns. While the secondary is having more area of cross-section and with less number of turns. This ensures very high current and less voltage in the secondary.
       One end of the secondary is connected to welding electrode and another end is connected to the pieces to be welded. Due to the contact resistance between the electrode and pieces to be welded, when a very high current flows, I2R heat is produced. This heat is very large. Due to this heat, a tip of the electrode melts and fills the gap between the two pieces.
       A winding used for the welding transformer is highly reactive or a separate reactor may be added in series with the secondary winding.
1.1 V-I Characteristics
       Volt ampere characteristics for a welding transformer is as shown in the Fig. 2.
Fig. 2  Volt ampere characteristics for a welding transformer

1.2 Reactors used with with Welding Transformer
       The welding transformer can be used with various reactors for control of arc. The various methods of such control are :
i) Tapped Reactor : In this, output current is regulated by taps on the reactor. This has limited number of current settings.
Fig. 3  Tapped reactor

ii) Moving Coil Reactor :
       In this method, the relative distance between primary and the secondary is adjusted.
       When the distance between the coils is large the current obtained is less.
Fig. 4 Moving coil reactor

iii) Magnetic Shunt Reactor : In this method, position of central magnetic shunt can be adjusted. This adjusts the shunted flux and hence output current gets changed.
Fig. 5  Magnetic shunt reactor

iv) Continuously Variable Reactor : The height of the reactor is continuously varied in this method. Greater the core insertion greater is the reactance and less is the output current.
Fig. 6  continuously variable reactor

v) Saturable Reactor :
       The reactance of the reactor is adjusted by changing the value of d.c. excitation obtained from d.c. controlled transducer. More the d.c. currents, reactor approaches to saturation. This changes the reactance of reactor and hence changing the current.
Fig. 7  Saturation reactor

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