Q. Define Current for electrical resistance welding?
- With other parameters kept constant, the temperature in resistance welding is regulated by controlling the magnitude and timing of the the welding current
-Enough welding current is required to heat the metal pieces being welded to their plastic state
-The current is obtained from a step-down transformer. The magnitude of current may be controlled through taps on the primary of the transformer or by an autotransformer. That varies of the primary voltage supplied to the main transformer.
-Low welding current does not provide proper fusion whereas if welding current is too high, the entire thickness of the work metal between the electrodes is heated to the plastic state by the time the weld zone reaches that fusion temperature, and the electrodes embed themselves deeply into the metal.
-As the current / current density* is increased, the weld time can be decreased sufficiently to produce a weld without overheating the electrode contact surfaces.
-As the welding current increases, the nugget diameter, breaking load of welded joint and the electrode indentation into the work pieces, all, increase.
-In resistance welding, three types of current supply systems generally are used
(i) AC systems.
(ii) DC systems.
(iii) Stored-energy current systems.
By far the majority of resistance welding machines operate on single-phase alternating current of the power lion frequency, usually 50 cycles / second. A single phase transformer converts the power line voltage to a low voltage and provides the high currents needed for welding.
High frequency resistance welding is used for applications of continuous seam or butt seam welding. The welding current frequencies are of the order of 450,000 cycles per second.
In DC systems, energy is delivered directly from the power line and rectifier to direct current on the secondary side of the welding transformer.
Stored energy systems are ; storage batteries, electromagnetic type, the homo polar generator and capacitor type. Capacitor stored-energy type involves charging a group of capacitors from a high-voltage rectifier unit and subsequent discharge of the energy from the capacitors through a welding transformer.