CEBORA S.p.A.
5
NOTE
In the shut-off board (48) there are two low-powered thyristors, inserted into two similar
circuits, one of which is always connected and available, while the other is parallel connected to
the first through a relay, when the welding current exceeds approximately 50 Aac. Both are
controlled by the same driver signal and thus work simultaneously.
Because of this characteristic, in this manual these two thyristors shall be considered as a
single one, known as SCR3, “the shut-off thyristor”.
SCR3 and the circuits of the shut-off board (48) serve to force SCR2 (47) to shut off during
AC operation. This function, simultaneous with the engagement of SCR1 (47), makes it possible
to generate the high voltage pulse necessary to perforate the oxidation that forms when welding
certain materials (aluminium and similar) and thus makes it possible to maintain the arc when the
polarity of the welding current changes. This function repeats during each period of the welding
current.
Since the value reached by the high voltage depends on the type of oxidation that the material
presents, a special circuit of the control board (40) makes sure that this voltage does not reach
levels hazardous for operation of the converter. If necessary, it prematurely engages SCR2 (47).
In this case welding is poor quality in AC (see par. 3.3.9).
At the converter output, on the connection before the “-”output terminal (AB) of the power
source, is connected to the HF transformer (28) which, appropriately driven by the HF board
(42), generates high voltage and high frequency to strike the arc in TIG welding.
Operation of the HF board (42) is dependent upon the presence of 27 Vac alternating voltage
on the connector J9, terminals 5 and 6, of control board (40) (HF board (42) power stage power
supply) and the HF start command on the connector J9, terminals 7 and 8, of control board (40)
(HF relay start command).
The connection before the “+”(AC) output terminal of the power source hosts the Hall-effect
current transducer (5), which sends to the control board (40) the output current feedback signal,
used to regulate the welding current.
Near the output terminals of the power source is the HF-filter board (41), which is of primary
importance to TIG operation with HF, since it prevents the HF pulse from climbing through the
internal circuits of the power source, damaging other parts. Thus, during various maintenance
operations, make sure that this board is always firmly connected to the original terminals before
activating HF start-up.
The mains voltage, again at 230 Vac, to power the service transformer (4) combined with the
fuse board (4), is drawn from the terminal board (52). This transformer provides both the supply
voltages needed by the various sections of the control board (40) and the synchronization signal
of the pulse generators on control board (40). This is why it is important to match the polarity of
the connections for both the primary circuit of the service transformer (4) and the secondary
synchronization circuit leading to connector J4.
The control board (40) is the heart of the power source. It supervises management of the other
boards with more specialized functions, regulates the welding current by generating the pulses to
send to the thyristors (47). It contains the microprocessor that manages all of the functions of the
power source, including those that interface with the user, obtained via the control panel built
into the board itself.
The signal of the power source output voltage, is drawn from the output terminals of the
converter. Appropriately filtered by the snubber board (6), this signal continues toward the
control board (42), where it is used to check for any presence of “hazardous voltage” at the
power source output (see par. 3.4.1).
3.302.221
15/01/07
Содержание TIG 250 AC-DC
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