Controls & Transformers
Questions and Answers
The direct hot-pressing technique of powder materials is referred to as field assisted sintering. In this case resistance heating through the green pressed graphite part would generate the heat. The combination of heat and pressure would generate the desired final properties in the finished graphite product.
The heat is generated per Joules Law:
Yes, MFDC can be used to generate the heat for hot pressing.
Controls in the marketplace control percent heat. Others control current. Many offer the option of controlling heat or current. Constant current is a common option on many current based controls. The question posed is should we base a weld schedule and ongoing process control on percent heat or current. The answer is simple. Joules Law shows us that current is the most important factor.
In the formula (I)current is squared. It has the most affect upon the heat.
Therefor we should monitor and control this factor CURRENT throughout the process.
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
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