Let’s first define Reactance. Reactance is an induced magnetic field around a conductor which resists change.
It is associated with AC circuits. The constant changing of the sine wave causes the induced magnetic field to build and become an energy drain. Reactance is a component of the impedance of the total circuit. This magnetic field is also affected by the throat size and any magnetic materials in or near the throat.
LARGER K & L'S INCREASE THE REACTANCE
Material in the throat comes into play when as a series of welds are made and the welded section progresses into the throat of the machine. If this is a magnetic material, reactance will be increasing and progressively reduce the current flow.
MFDC and DC are considered to have zero reactance. This means there are no throat effects when running magnetic materials into the throat of the machine.
The voltages will vary depending upon the machine components and design but commonly at the open electrodes ready to weld the voltage will read about 2 volts. Depending upon the input transformer voltage and the machine this range could be 2 – 5 volts. Many MFDC transformers in the field have 9 or 13 volt outputs. Due to inherent machine resistances this voltage will drop and is frequently measured at 2 volts at the electrodes.
The Resistance Welding Manufacturing Alliance “RWMA” has defined duty cycle in the Resistance Welding Manual, 4th Edition, Section 19-3. It reads “Since resistance welding equipment is used intermittently, the rating is based on a 50 percent duty cycle over a one minute (60 sec) integrating period”. This value was determined long ago and has served the industry well. ISO 5826 also specifies the same 50% duty cycle one minute integration period.
Transformer and machine designers design to a 50% duty cycle. Since most resistance welders operate in the 20% range, equipment has proven to be very robust. Duty cycle is actually a heat rating to protect the insulation in the transformer.
In other industries and specific applications transformers are manufactured at various other duty cycles ranging from 20 – 100%.
Reference: RWMA – Resistance Welding Manual Section 19.3
ISO - ISO 5826
Resistance welding transformers are manufactured with very little space between windings and between the primary and secondary coils. To provide electrical integrity they must be separated by insulation. This must be maintained throughout the life of the transformer. This insulation is very thin but can last for a very long time if not abused by heat, moisture or physical damage.
TRANSFORMER COILS ASSEMBLED SHOWING PRIMARY & SECONDARY COILS
As transformers age or a machine is being recommissioned the condition of the transformer should be considered. Is it in good shape and ready for another long model run? The question can be evaluated and tested. A transformer manufacturer has the proper test equipment to do this and can evaluate and potentially repair, if necessary.
To calrify this issue attached is a previous article which shows the sequence of changes from the initial three phase 60 Hz AC input through several combinations of frequency and waveform changes to the final MFDC 1000 Hz output.
The ripple in the MFDC plot represents the 1000Hz frequency. The entire MFDC curve is on the positive side of the sine wave. MFDC does have a rise and decay of 3-8 milliseconds before and after full current is reached. This appears in the plot as an upslope or downslope. This should clear up the any questions. AN ARTICLE ON MFDC RISE AND DECAY IS ALSO AVAILABLE.
WHY or HOW DO MFDC CONTROLS CHANGE CURRENT FROM AC TO DC TO AC TO DC?
Schematic of Power Conversion In Control and Transformer of Mid Frequency Inverter
The advantage of MFDC is that the weld current has no zero cross overs so it heats the part quickly. Also, it is DC so there are no inductive power losses or problems with magnetic material in the machine throat.
Generally the plant power requirement is reduced substantially with the 3 phase input. Another advantage is robot payload. Higher frequency in the transformer allows the iron core to be smaller. This is a weight reduction which is significant on the end of a robot arm.
Reference: ENTRON Controls, LLC
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