While working with conventional two phase AC welding machines, we experience difficulty in connected power and phase balance. To overcome is problem, three phase balance input is an answer.
In AC machine KVA demand is dependant of geometry of secondary circuit, size & material of conducting components, presence of magnetic material fixtures in secondary loop. Impedance of this loop need to be minimized for minimum KVA demand for delivering welding current at welding point. AC machines need to be selected for minimum throat depth. The reactive power is consumed to maintain magnetic field around conductors, work piece and fixture. These induced current losses adds opposition to welding current . One method of eliminating this effect is DC secondary current.
In DC type machines current is function of resistance of secondary path. Being a DC system, impedance is not playing role in this case. Power is saved as lower power loss is experienced by the machine to flow current upto application point.
Technological Marvel
Here the three phase AC line power is first rectified by full wave bridge rectifier. Three phase input voltage 440 V generates 600 V DC. This DC bus is now single phase. This bus is connected to H bridge of IGBTs. The bridge inverts at 1000 Hz nominal frequency and this wave form is again converted to high power DC generating currents at lower voltages from 2500 amps. to 12000 amps.
As the controller is operating at higher frequency, it is now possible to control the cycle to precision of 1 millisecond. This eliminates interruption of welding current which is inherent in AC systems. Thyristor controlled AC system need to wait till next half cycle making heating and cooling during welding and nugget formation. In present system the weld current is continuous, this facilitate to attain melting temperatures quickly and avoid thermal shocks during welding giving more perfect weld, also saves energy as the heating is continuous. This is more important in welding of ‘high conductive’ metals like copper and aluminium. These metals need quick melting temperatures as the heat carried away by them is more with increased weld times. |
