Controls & Transformers
Questions and Answers
In many facilities it is beneficial to insure that the multiple welders in a station not fire at the same time to prevent a sudden large draw on the plant power supply. To spread this out cascade controls can be employed where one control will sequence through several contactors in sequence. The sequence and separation between the welds can be controlled to give an appropriate modest draw on the power supply.
In North America the power grid is 60 hertz. All schedules and operations are based upon that power. In other parts of the world power is generated at other power levels. Many use 50 hertz. If one has a good weld schedule in a 60 hertz area and a new operation were set up in a 50 hertz region a conversion would be needed for weld time. The formula is:
Weld Time @60 Hz X (50/60) = Weld Time @ 50 Hz
OR
Weld Time @50 Hz X (60/50) = Weld Time @60 Hz
Pressure, current and time (PCT) are the basic functions performed by the resistance welder. These functions are controlled or initiated by the weld controller. It initiates each step when told to start the weld process by input from the foot switch or automation PLC. The controller allows time for each step to operate and controls the current amplitude. In simplified form this is Squeeze, Weld and Hold. The squeeze sequence allows the pressure system to build up the force to contain the weld. The weld function is the actual current flow and is totally controlled by the weld controller. It regulates the amplitude and time of current flow. Hold is the period which allows the weld nugget to cool down and solidify under force. The controller regulates this time.
Most published weld schedules are published in terms of AC welding. If you acquire a new control and it is an inverter/mid frequency control you probably will program in milliseconds rather than the traditional AC Cycles. The mathematical conversion is:
1 AC CYCLE = 16.7 MILLISECONDS
MID FREQUENCY WELDING is when inverted DC power is used for the welding. This is performed using a control and power supply system that takes and AC power input and converts it into an inverted higher frequency power output. AC 60 hertz goes in. It is inverted and converted a combination of times in the control and transformer to end up with a 400 – 4000 hertz inverted DC output.