Controls for resistance welding machines
The main controls required for a resistance welding machine are:
i. Initiation and termination of the current.
ii. Controlling the magnitude of the current.
iii. Timing and controlling the mechanical operations of the welding machine.
Three main categories of controls are:
a. Welding contactors
b. Timing and sequence controllers and
c. Other controls and regulators.
Welding control is lifted in the primary circuit of the welding transformer. The contactors may be mechanical (foot pedal), magnetic (electromagnetic) or electronic of which electronic contacts are the most useful and reliable. The electronic contacts may be ignitrons, thyratrons or SCR's. We have to go in for ignitron tubes it the primary current exceeds 40 A.
Time and sequence controls can be pneumatic, R - C controlled, motorised or digital. Synchronous and non synchronous controls are two main possibilities. Non - synchronous controls provide variations of timing and current input to the machine from closing and opening of the welding contractor at random points on the wave form. On the other hand, synchronous controls provide switching at the same electrical angle each time. Non - synchronous controls are accurate for weld times of 20 cycles or longer.
Heat control: The major adjustment of the welding current is done by tap changing. For fine intermediate current control, electronic heat control is used. Ignitron tubes with SCR or thyratron firing is employed.
Upslope control: Upslope permits the welding current to be increased over several cycles from a low value to that needed for welding, instead of having the full welding current applied instantly. This eliminates the expulsion of metal (splashing). This is used at high current values and for welding scally stock and for almost all plate metals.
Downslope control: This permits the welding current to decay gradually to a low value, instead of ending suddenly, and it helps to produce good welds in some types of heat treatable metals by lengthening the cooling time of steels (less than 0.15 % carbon).
Current and voltage regulators: Electronic current regulators are used to maintain a constant welding current under severe conditions by compensating for welding current variations or load changes resulting from insertion of magnetic materials into the throat of the welding machine.
Preheat and postheat control: These controls are important for flash butt welding. The joint is first heated up in one or more stages by applying pressure to the joint and then separating it before the weld temperature is reached. This may be repeated several times, depending on the type of work and the capacity of the machine in relation to the size of the joint. By adopting this interrupted preheating process, it is possible to increase the maximum capacity of the machine, as the preheating operations reduce the final kVA needed to raise the joint to weld temperature. In sophisticated flash butt welding machines, the preheating is digitally controlled and ignitrons in the circuit, for each preheat cycle, the time for which the jobs are in contact, the time taken for retraction of one job, the time for which the jobs are allowed to cool and the time for bringing the jobs together can be independently set with the thumb wheel switches. The number of preheat cycles can also be selected.