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Appendix B
T
– T
2
3
T
– T
3
5
T
– T
5
6
T
– T
6
7
STT WAVE SHAPING PRINCIPLES
The pinch mode applies an increasing, dual-slope ramp of current to the
shorted electrode. The dual slope ramp of current accelerates the transfer of
molten metal by the electric pinch forces. The pinch force is directly proportional
to the square of the pinch current.
dV/dt is the rate of shorted-electrode voltage change per unit time. On-going
dV/dt calculations are performed electronically during the pinch mode. Due to the
high resistance of molten iron, the electrode-to-work voltage never reaches zero.
Near fuse separation at T
power source immediately reduces the arc current to 50 Amps. The current drop
occurs before the shorted electrode separates. The T
separation has occurred, but at a very low current.
Immediately following separation of the electrode from the weld puddle, peak
current occurs. Peak current is a period of high arc current. The electrode is
quickly saturated by this current and melts back. The jet forces acting upon the
weld puddle (cathode) depress the molten surface, increasing the arc length and
thereby minimizing the possibility of the electrode shorting prematurely. Peak
current is maintained for approximately 1 to 2 milliseconds. Excessive power
would melt too much electrode and cause weld spatter. Even at low power, the
shape of the melted electrode at this point is very irregular.
Plasma is the final period when the arc current is reduced exponentially from the
peak current level to the background Ievel. The exponential current drop
mechanically dampens the weld pool agitation produced during peak current.
WAVE DESIGNER
raises the dV/dt value to a designated maximum. The
4
B-5
event indicates that fuse
4
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