Lincoln Electric SVM 122-A service manual Output Mode and CONTROL, Rectification and Feedback

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E-3 E-3

THEORY OF OPERATION

FIGURE E.3 – OUTPUT MODE AND CONTROL, RECTIFICATION AND FEEDBACK

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CONTROL

TRANSFORMER

POWER
SWITCH			MAIN
		TRANSFORMER
		R
		E
		C
		O
		N
		N
		E
		C
		T
INPUT
CONTACTOR
		T	115VAC
	REMOTE	E	S
TO	CONTROL	R	T
CONTROL		M	R
BOARD		I	I
		N
			P
		A
		L
			FAN
			42VAC
		14 PIN
		AMPHENOL

OUTPUT	ARC FORCE
CONTROL	CONTROL
	PART OF
	MODE
	SWITCH
			FEEDBACK
CONTROL BOARD				NEGATIVE
				OUTPUT
G	S		PART	TERMINAL
A	I		OF
T	G		MODE
			SWITCH
E	N
	A
	L
	S			OUTPUT
				CHOKE
		F
	PART	E
	OF	E
	MODE	D
	SWITCH	B	START
		A	BOARD
		C
	OUTPUT	K
	CAPACITORS
				POSITIVE
				OUTPUT
				TERMINAL
SCR / DIODE		SHUNT	REED
			SWITCH
HYBRID BRIDGE

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OUTPUT MODE AND CONTROL, RECTIFICATION AND FEEDBACK

The three-phase AC output from the main transformer secondary is rectified and controlled through the SCR/ diode bridge. Output current and voltage is sensed at the shunt and output terminals. This feedback infor- mation is processed in the control board. The control board compares the commands of the Mode Switch, the Arc Force Control (constant current mode only), and the Output Control (or remote control) with the feedback information and sends the appropriate gate firing signals to the SCR/diode bridge.

A “dry closure” of leads #2 and #4, either at the termi- nal strip or the 14 pin amphenol, signals the control board to apply gate firing signals to the SCR/ Diode Bridge, which creates a DC voltage at the output of the bridge assembly. If the Mode Switch is in the constant voltage (FCAW/GMAW) mode, this DC voltage is fil- tered by the Output Capacitors.

The heavy current carrying portion of the Mode Switch is connected between the output choke, which stores energy and provides current filtering, and the negative output terminal. Depending upon the mode selected, different portions of the choke are needed. In the constant current mode (CC), the entire choke is connected into the welding output cir- cuit. When constant voltage (CV) is required, only part of the choke is utilized.

The starting circuit board function is to momentarily change the inductance effect that the choke has on the welding circuit. This is accomplished by changing the amount of “pinch” or arc control windings that are active in the output choke. When weld current is established, the reed switch closes and the start board becomes inactive.

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NOTE: Unshaded areas of Block Logic Diagram are the subject of discussion.

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