ESAB LAW 400, Service Manual

Page: 21 / 66

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5 SYNCHRONISING
Firing is synchronised to the mains by three zero crossing detectors, IC11;1, IC11;2
and IC11;13. The voltage between the phases at inputs E1, E2 and E3 is about 19 V
in the LAW 400/410 and about 25 V in the LAW 500/510.
6
THERMAL OVERLOAD CUTOUT
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LED V7 lights if the cutout operates. The current through the LED is about 16 mA.
When the cutout operates the thyristor ignition pulses will be blocked.
LAW 400/500 with serial numbers of 434 . . . . . . .
The thermal overload cutout, ST1, is fitted to the thyristor heat sink (= cooling fins).
In the LAW 400, the cutout operates at a temperature of 60
_
C, while in the LAW
500 it operates at 70
_
C. It resets at 50
_
C in both machines. See page 33 for fitting
instructions.
LAW 400/410 with serial numbers of 512 . . . . . . . and above
The thermal overload cutout, ST1, is fitted in the interphase transformer winding,
L1. It opens at a temperature of 140
_
C, and closes at 110
_
C.
LAW 500/510 with serial numbers of 512 . . . . . . . and above
The thermal overload cutout, ST1, is fitted in the inductor winding, L2. It opens at
a temperature of 160
_
C, and closes at 130
_
C.