Basic AC Resistance Welding System
Description :
The “line contactor” is basically a controlled switch which connects the ac mains voltage across the primary winding of the “welding transformer” only during the welding interval. It otherwise disconnects the mains from the primary.
The “welding transformer” is a step down transformer which supplies a reduced (steps down) voltage on the secondary side but increases (steps up) the secondary current which is the “welding current”. This current is usually in the range of several hundred to several thousands of amperes, depending on the nature of weld.
Thyrite resistor :
- This resistor has a property that its resistance decreases with increase in the voltage across it. It is connected across the primary winding of the welding transformer in order to protect its insulation against high voltage spikes that are induced due to high rate of change of current in the primary winding when the contactor is suddenly turned on or off.
- The control circuit design depends upon the nature and precision of welding. The control circuit decides the instants at which the line contactor is turned on and off j.e. the welding time. In order to get better quality of weld the control Circuit may control : (a) time for pressure to build up at the electrodes. (b) time for metal to cool down before electrode pressure is released. (c) time interval between successive welding operations.
- The line contactor may use a “controlled switch” like ignitron tubes or thyristors in order to control the magnitude of the welding current.