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.
MFDC stands for MID FREQUENCY DIRECT CURRENT. This is a control and power supply system that takes an AC power input and converts it into an inverted higher frequency power output. Three phase 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.
In mid frequency (MFDC) resistance welding the AC current is transformed from 60 cycles/second (hertz) to 400-4000 hertz and the negative half cycle is inverted to positive creating a DC current. The current does not have zero cross overs therefore it is conducting power continuously and can heat the part much faster than traditional AC welding. The weld control which controls this current amplitude and weld time must be very responsive to maintain control. It measures time in milliseconds. There are 16.66 milliseconds per each AC cycle. This is equivalent to 1000 milliseconds per 60 AC cycles. Welds are made at less than 10 milliseconds and up. They can also be run at modestly longer times and reduce the current level to compensate.
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