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Fault Tracing Thyristors
3ADW000803R0101 DCS880 H4T (Tosa) manual en a
Fault Tracing Thyristors
Units size H4T
These units require semiconductor fuses in the 3 AC lines.
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The unit must be disconnected from the mains.
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One load cable should be disconnected from the unit.
Blown fuses
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Make sure, that the connection to the load is open (e.g., disconnect load cables).
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Using the OHM function of a normal multimeter, measurements must be made from each AC terminal
to each DC terminal (U1 to C1, V1 to C1, W1 to C1, U1 to D1, V1 to D1 and W1 to D1:
Bridge configuration H4T
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Normally, each measurement should show high resistance (> 1 kOhm).
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Target: Find a short circuit, indicated by low resistance (< 1 Ohm) (destroyed thyristor).
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If the unit is designed with thyristor modules, then a module consists of two thyristors. In this case it
is sufficient to know which thyristor module has a defective thyristor because the complete module
must be replaced.
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After a thyristor module is replaced, the above mentioned measurement should be done another time
to make sure that all faulty thyristors have been detected!
Note:
The RC/snubber circuit could also cause 0 Ohm results for a short time.
The measurement, showing less resistance than 1 Ohm should be made a second time with test leads
applied to the terminals with opposite polarity; if this measurement shows the same result, one or two
thyristors located in that path are faulty; they need to be replaced.
