Lincoln Electric IM649 manual STT Wave Shaping Principles

Appendix B	STT WAVE SHAPING PRINCIPLES	B-5
T2 – T3	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.
T3 – T5	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 T4 raises the dV/dt value to a designated maximum. The
	power source immediately reduces the arc current to 50 Amps. The current drop
	occurs before the shorted electrode separates. The T4 event indicates that fuse
	separation has occurred, but at a very low current.
T5 – T6	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.
T6 – T7	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