ESAB LAW 400, Service Manual

Page: 27 / 66

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13 PROCESSOR
In addition to the features already described, the processor performs the following
functions :
S
Frequency measurement
Measures the mains frequency when the unit is turned on. If the frequency is
either 50
¦
4 Hz or 60
¦
4 Hz, the processor starts the main program. If the
frequency is not within the tolerances, the main program that controls the
machine will not start.
S
Voltage monitoring
By sensing that the power supply, +12 V, to the main electronics board is
present, the processor monitors the power supply to the board. If the supply is
lost, the main program stops. When the supply returns, the program starts. See
’Frequency measurement’, above.
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Overload protection
The LAW 400/410 and LAW 500/510 have average current limits, known as
slow overload trips, at 412 A and 516 A respectively.
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Short--circuit protection
Both machines incorporate short--circuit protection (fast current trip),
set at 750 A.
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Firing pulses
The processor generates firing pulses as required by the set welding voltage.
Pulses are duplicated in order to prevent any misfiring of the thyristors.
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Display management
The processor supplies information on the welding voltage and welding current
to the digital display instrument.
S
Crater filling
Reduces the arc voltage when the wire feed unit calls for crater filling.
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Test routine
Controls the test routines, described on page 31.
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Elevated starting voltage (Hot Start)
Applies to machines with serial numbers 512 . . . . . . . and above:
These machines are supplied with controlled thyristor firing angle in operation.
With this arrangement, the starting voltage in the LAW 400/410 is 30% higher
than the set voltage, and in the LAW 500/510 is 20% higher than the set
voltage. Arc voltage control comes into operation 300 ms (50 ms when
pulsing) after the arc strikes, which reduces the arc voltage to the set value. If
the machine is operating with a fixed thyristor firing angle, the hot start feature
is inactive.
Machines with serial numbers 434 . . . . . . . and circuit board 0486 220 880:
These machines can be operated only with a fixed firing angle.
Starting voltages are as follows:
If the set value of arc voltage is less than 21 V, starting voltage will be 20%
higher. If the set value of arc voltage is over 21 V, starting voltage will be 50%
higher. The elevated starting voltage ceases 100 ms after starting.
For these machines there is an exchange circuit board available, E486 220 882,
which has controlled thyristor firing angle and the same starting voltages as
circuit board 0486 291 880.