Lincoln Electric V250-S service manual Figure E.3 Main Transformer

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E-4E-4

THEORY OF OPERATION

FIGURE E.3 MAIN TRANSFORMER

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INPUT

INPUTRECTIFIER LINE

SWITCH

"A"

L

E

A

D

FAN

MOTORS

R

E

C

O

N

N

E

C

T

S

W

I

T

C

H

18VAC

POWER BOARD		MAIN
		TRANSFORMER
CR1
RELAY
IGBT
			POSITIVE
CAPACITOR			OUTPUT
			TERMINAL
IGBT
IGBT		SHUNT
	CURRENT
	TRANSFORMER		NEGATIVE
CAPACITOR		F	OUTPUT
			TERMINAL
		E	F
IGBT		E	E
		D	E
		B	D
		A	B
		C	A
O		K	C
V
E		THERMOSTATS	K
R
V
O
L
T	IGBT GATE SIGNALS		THERMAL
A
G
E	PROTECTION SIGNAL	CONTROL BOARD	LIGHT
CR1 RELAY DRIVE SIGNAL

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AUXILIARY

TRANSFORMER

REMOTE

RECEPTACLE

OUTPUT STRIKE ARC			LOCAL/
		MODE
			REMOTE
CONTROL CONTROL FORCE	SWITCH
CONTROL			SWITCH

MAIN TRANSFORMER

Each IGBT pair acts as a switch assembly. Each assembly feeds a separate, oppositely wound primary winding of the main transformer. The reverse direc- tions of current flow through the main transformer pri- maries and the offset timing of the IGBT pairs induce an AC square wave output signal at the secondary of the main transformer.

The DC current flow through each primary winding is redirected or “clamped” back to each respective filter capacitor when the IGBTs are turned off. This is needed due to the inductance of the transformer pri- mary winding.

The primary currents also pass through the current transformer which sends a signal to the control board. If the primary currents are not equal the control board compensates by adjusting the IGBT gate signals.

The firing of both IGBT pairs occurs during halves of the 50 microsecond intervals, creating a constant 20 KHZ output.

NOTE: Unshaded areas of block logic diagrams are the subject of discussion.

INVERTEC V250-S