Lincoln Electric IM649 manual STT Surface Tension Transfer

B-4 STT WAVE SHAPING PRINCIPLES Appendix B

B.3 STT (SURFACE TENSION TRANSFER)

The Invertec STT power source is a high-frequency, current-controlled machine that delivers power to the arc based on immediate arc requirements. The Invertec STT produces large electrode current changes within microseconds. It operates in the short-circuiting welding mode. The electrode current supplied by the power source is guided by the arc voltage state. An ordered sequence of events controls the current throughout the droplet transfer process. The electrode current and voltage waveforms for a typical welding cycle are shown in Figure B-3. Following Figure B-3 are detailed descriptions of the STT welding cycle events.

FIGURE B-3. ELECTRODE CURRENT AND VOLTAGE WAVEFORMS

FOR A TYPICAL STT WELDING CYCLE.

Surface Tension Transfer™

Electrode

Electrode to work volts Electrode Amperes

	T0	T1 T2 T3 T4 T5 T6 T7	Time
			27850063
T0 – T1	A background current between 50 and 100 Amps is delivered to the arc prior
	to shorting of the electrode to the weld puddle. Background current is a steady-
	state current level that serves two purposes. First, the background current
	supplies power to the arc to maintain the fluidity of the molten drop at the end of
	the electrode. (Failure to supply the minimum required current causes the upper
	portion of the molten ball to freeze. As more of the ball solidifies, arc instability
	and finally stubbing occurs.) Secondly, the background current level greatly
	effects plate heating.
T1 – T2	Ball time occurs when the electrode initially shorts at the background current
	level. When the Invertec’s arc voltage detector circuit signals that the “arc” is

shorting out, the background current is immediately reduced for approximately

0.75milliseconds. The rapid current drop promotes good electrical contact and “wetting” of the ball to the weld puddle. Wetting prevents an “incipient short”, wherein the ball is violently repelled from the weld puddle, breaking apart and causing spatter.

WAVE DESIGNER