Theory Of Operation; General Description; Input Voltage - Lincoln Electric INVERTEC STT Series Service Manual

E-2
S
W
T
C
H
LINE INPUT
SWITCH RECTIFIER
CR1
"A"
LEAD
CR2
115 VAC
18
FAN
TRANS VAC P B
T1 O O
W A
115
E R
VAC
2
R D
4
V
A
TRANS
C
T4
T
H
E
R
PWM SIGNAL
M
O
S
15VDC
T
A
T
GUN TRIGGER
36VAC
PEAK
CURRENT
METER

GENERAL DESCRIPTION

The Invertec STT is a 225 ampere, inverter based, arc
welding power supply specifically designed for the
Surface Tension Transfer (STT) welding process. It
cannot be classified as either a constant current (CC)
or a constant voltage (CV) machine. The STT pro-
duces current of a desired waveform to reduce spatter
and fumes. The STT process is optimized for short -
circuit GMAW welding only.
NOTE: Unshaded areas of Block Logic Diagram are the subject of discussion.

THEORY OF OPERATION

FIGURE E.2 – INPUT VOLTAGE
B
FET
O
I
A
CAP
R
FET
D
PRE -
CHARGE
R
P D
B
E
R
B
R
C O O
CURRENT
T O I
O A
TRANS
E
A V T3
N
C R
N T R E
D
I
E
D R
O
C
N
T
PRE -
CHARGE
S B
FET
W
O
I
A
CAP
T
R
C
FET
H D
LESS THAN 1VDC
PULSE TRANSFORMER SIGNAL
CONTROL BOARD
BACK- WIRE WIRE
BACK-
PEAK HOT
GROUND SIZE TYPE
CURRENT START GROUND
METER SWITCH SWITCH
CONTROL CONTROL
CONTROL
INVERTEC STT
MAIN
TRANSFORMER
T2
CURRENT
SENSOR
CHOKE
CURRENT FEEDBACK
DARLINGTON
DRIVER
BOARD
VOLTAGE FEEDBACK
10VAC AND 6VAC
42VAC

INPUT VOLTAGE

The Invertec STT can be connected for a variety of
three-phase voltages. The initial input power is applied
to the STT through a line switch located on the front of
the machine. The AC input voltage is applied to the
input rectifier and the T1 auxiliary transformer. The T1
transformer develops the appropriate AC voltages to
operate the cooling fan, the power and control boards.
The T1 transformer also supplies primary voltage to the
T4 auxiliary transformer as well as 42 VAC to an exter-
nal wire feeder. The T4 transformer supplies power to
the Darlington drive board and the control board.
E-2
POSITIVE
OUTPUT
TERMINAL
1 OHM
DARLINGTON
NEGATIVE
MODULE OUTPUT
TERMINAL
R VOLTAGE
SENSING
E
RECEPTACLE
M
O
D
T
R
REMOTE
I E
B CONTROL
V
RECEPTACLE
E P
O
R
A
S
I O
R
G
T
N
D
E
A
C
L
T
WIRE
I FEEDER
RECEPTACLE
O
N