Lincoln LT-7, Troubleshooting Manual

Page: 8 / 43

THEORY OF OPERATION
A-3 A-3
LT-7 TRACTOR
Arc voltage is monitored by the voltmeter and variable
voltage board. When the LT7 tractor is being operated
in the constant current mode the variable voltage
board is essential in the control of the wire feed speed.
As the arc length changes the arc voltage will also
change. The variable voltage board recognizes this
change and signals the logic board to either increase or
decrease the wire feed speed. This function is neces-
sary to maintain a constant electrode arc length and a
stable and high quality weld.
The variable voltage board also generates a low volt-
age which is applied to the electrode during the inch down mode. When the electrode makes contact with
the work piece this low voltage is “loaded down” thus
signaling the control circuitry to stop the wire feed
motor. This feature allows the operator to utilize “work
touch sensing”.
FIGURE E.3 - VARIABLE VOLTAGE BOARD
VARIABLE VOLTAGE BOARD
GLP WELD CURRENT
SENSING REED
2CR
TRAVEL
RELAY
TRAVEL
SWITCH
TRAVEL
MOTOR
TRAVEL BOARD
TRAVEL
SPEED
CONTROL
POWER SOURCE
CONTACT RELAY
R1
CIRCUIT
BREAKER
115VAC
FROM
WELDING
POWER
SOURCE
INCH UP
SWITCH
WIRE
DRIVE
MOTOR
START
SWITCH
STOP
SWITCH
INCH
DOWN
SWITCH
WIRE
SPEED
CONTROL
LOGIC
BOARD
CONTROL
BOARD
VARIABLE VOLTAGE
BOARD
ARC
VOLTAGE
METER
115VAC
115VAC
115VAC
115VAC
115VAC 115VAC
115VAC
115VAC
LOGIC CONTROL SIGNALS
ARC VOLTAGE SENSING SIGNALS
24VDC
ELECTRODE
WORK
VOLTAGE
SENSING
LEADS
REED
SWITCH 4CR SWITCH 3CR
TRAVEL
DIRECTION
SWITCH
CONTROL
1CR
#2
#4
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NOTE: Unshaded areas of the Block Logic Diagram are the subject of discussion