Controls & Transformers
Questions and Answers
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.
In resistance welding there are three main functions performed during the weld operation. They are pressure application, and current flow for a length of time “PCT”. All of these functions are initiated, controlled or monitored by the weld control. The weld current amplitude and time are controlled by the weld control. The weld time in an AC control is measured in cycles. There are 60 cycles/second in North America. Most resistance welds are 5 – 30 cycles long making them very fast at less than a second long.
Duty cycle is used to determine the actual work that the power system of a resistance welder is performing. Machines by design are not meant to exceed 50% duty cycle. The cables, shunts, transformer, controls and all conductors are designed with this value. If exceeded the life of the machine components will be shortened dramatically.
Knowing at what level you operate at is important in setting your weld schedules. The formula shown below is based upon standard AC - 60 cycle welding with a one minute integration time period.
IGBT’S are the switching devices in inverter controls. They handle large amounts of power during resistance welding and can heat up. Yes, they must be cooled. They are normally mounted on a water cooled plate to remove the heat. IGBT's are indirectly cooled with this mounting system.
For AC welding it is important to operate at a relatively high percent heat. This gives you the maximum efficiency and minimum off time during each weld cycle of the sine wave. So your weld schedule should be designed to optimize this situation as opposed to a high tap and a low setting where a large percentage of the time (each cycle) there is no current flow and the weld is actually cooling down.
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