A DC transformer in this case refers to the traditional 50/60 Hz three phase transformer used for many years for high power requirement. These low frequency transformers are high power machines capable of long duty cycles. These tend to be large transformers due to their power ratings and applications.
DC LOW FREQUENCY TRANSFORMER
Burrs inside a tube could be formed by several resistance welding operations. It could be butt welding longitudinally to form the tube. It could be spot or projection welded to attach some lead or bracket to the tube. It could be an end closure weld to seal the end of the tube. Let’s look at these one at a time.
An inquiry came in asking why is MID FREQUENCY WELDING called MID FREQUENCY? That is a very logical question which goes back to the frequency of the power being generated in MID FREQUENCY resistance welding. In much of the world this is 1000 Hz.
In the world electricity frequency is divided into ranges of:
LINE FREQUENCY: This is the normal power grid in the world which run as 50-60 Hz
MEDIUM: 600Hz – 4000 Hz
High: 10 KHz – 50 KHz
The resistance welding inverters produce a 1000 Hz Dc output which is in the medium range which is why it is called a Mid or Medium Frequency welding.
If one looks beyond the electrical world we find that :
Speech and Sound have a similar break down and description of frequencies.
Lowest Sound Ear can detect - 20 Hz – 250 Hz
Mid Frequency (Human Voice) – 250 Hz – 4000 Hz
High Frequency (Cymbals) – 4 KHZ-20 KHz
Radio Waves have a similar but different range:
Low frequency is 30kHz to 300 kHz
Medium Frequency 300kHz to 3 MHz.
High Frequency 3 MHz to 30 MHz
VHF (Very High Frequency) 30 to 300 MHz.
Manuals are supplied with a new resistance welder for the major components of the machine. If this is an old in house machine or a used machine manuals may not be present. In that case a thorough search of the machine to find the machine builder label with model number and serial number (if present) is in order for all of the components.
A weld gun would have the same information on it's components.
You stated that all runs well but once in a while you have a bad push out on your nut welding. Apparently your weld schedule works and produces good welds but some variable is causing discrepancies occasionally. Let’s address the important factors in projection welding.
It is important that you have good equipment alignment so that you apply force to all projections on the nut at the same time and with equal force. Next the force system of your welder or gun must be able to move quickly as the projections collapse in order that full force is maintained. This is called fast follow up. A large press welder not properly equipped or a sticking cylinder or a slow servo could cause a problem.
FAST FOLLOW UP HOLDER
Projection consistency is important. Any variation in the projections on the nut will be an issue leading to inconsistencies. Nut feed consistency is imperative. If the feeder misplaces the nut it can lead to a momentary misalignment that squeeze time does not correct. Some projection welds are very fast with little or virtually no squeeze time. Though short some squeeze time may be necessary.
Projection Weld Nuts and Studs
If there an insulated locator pin. Is the insulation worn? Are there signs of arcing? The current could be shunting through the pin and bypassing the projections. Additionally it may be damaging the threads.
Is there a buildup of flash/residue that is interfering with alignment or causing a current/shunting path?
The fact that you have a working process most of the time indicates a variable is present that needs to be controlled. Other than a tweak the schedule is probably pretty close.
Additional reference schedules for nut and projection welding can be found in:
AWS J1.1:2012 Recommended Practices for Resistance Welding
Ref: CMW Inc. Catalog
RWMA: Resistance Welding Manual, Section 3
AWS: J1.1:2012 Recommended Practices for Resistance Welding
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