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Fault Tracing Thyristors
3ADW000488R0101 DCS880 Service manual e a
Converters size H1 … H4 (20 ... 1000 A)
These converters require semiconductor fuses in the 3 AC lines.
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The converter must be disconnected from the mains.
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One motor armature cable should be disconnected from the converter.
Blown fuses
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Make sure, that the connection to the motor is open (e.g. disconnect motor 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 H1 … H4
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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 converter 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.
