Output Mode And Control, Rectification And Feedback - Lincoln Electric IDEALARC SVM 122-A Service Manual

E-3
FIGURE E.3 – OUTPUT MODE AND CONTROL, RECTIFICATION AND FEEDBACK
CONTROL
TRANSFORMER
POWER
SWITCH
TRANSFORMER
R
E
C
O
N
N
E
C
T
INPUT
CONTACTOR
REMOTE
TO
CONTROL
CONTROL
BOARD
AMPHENOL
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.
NOTE: Unshaded areas of Block Logic Diagram are the subject of discussion.
THEORY OF OPERATION
OUTPUT
CONTROL
CONTROL BOARD
MAIN
115VAC
T
E
S
R
T
M
R
I
I
N
P
A
L
FAN
42VAC
14 PIN
IDEALARC DC-400
LINCOLN
ARC FORCE
CONTROL
PART OF
MODE
SWITCH
G S
I
A
T G
N
E
A
L
S
F
PART E
OF E
MODE D
SWITCH B
A
C
K
OUTPUT
CAPACITORS
SHUNT
SCR DIODE /
HYBRID BRIDGE
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.
established, the reed switch closes and the start board
becomes inactive.
®
ELECTRIC
E-3
FEEDBACK
NEGATIVE
OUTPUT
TERMINAL
PART
OF
MODE
SWITCH
OUTPUT
CHOKE
START
BOARD
POSITIVE
OUTPUT
TERMINAL
REED
SWITCH
When weld current is