Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 53)

DESCRIPTION OF OPERATION 5-35
1557-UM050F-EN-P –June 2013
Thyristor Diagnostics Bulletin 1557M Mini Drive
There are two differences between the Bulletin 1557M mini drive and the larger 1557 drives which affect
the way thyristor failures can be detected. First, there are no thyristors connected in series, and second,
there is no feedback from the gate drivers. The consequences of these differences are that the mini drive
must use other feedback signals, specifically the input current and motor current, to detect open or shorted
thyristors, and that failed thyristors can only be detected when the drive is started and not while it is running.
Each time the drive is started, tests are performed to detect the following thyristor failures: line converter
shorted thyristor, line converter open thyristor, machine converter shorted thyristor, machine converter
open thyristor. The tests are performed in the order listed, and if any failed devices are detected the
remaining tests are cancelled.
Line Converter Shorted Thyristor
With the machine converter gating off, one of the six thyristors in the line converter is gated for 20 degrees
at retard limit. If no devices are shorted, no current should flow. If any current is detected it indicates that
the thyristor in one of the other two phases on the same side of the bridge is shorted. The test is
repeated for each of the six thyristors in the line converter, and is terminated if a shorted device is found.
Line Converter Open Thyristor
For the open thyristor test, two GTOs (3 & 6) in the same phase of the machine converter are gated
continuously to provide a path for dc link current which bypasses the motor. The line converter is then
run briefly with the dc current below the discontinuous level. When the dc link current is discontinuous,
each pair of thyristors shuts off before the next pair is gated. If no dc current is detected in two
consecutive 60 degree intervals, it indicates that whichever device was gated in both those intervals did
not conduct. When the test is completed, the line converter is phased back to turn off the current.
Machine Converter Shorted Thyristor
With the line converter phased back to retard limit, one of the six thyristors in the machine converter is
gated. The line converter is then phased up to try to drive some current through the dc link. If no devices
are shorted, no current should flow. If any current is detected, it indicates that one of the devices in the
other side of the bridge is shorted. If the current flows through the motor then the shorted device must be
in one of the other phases and cannot be uniquely identified. If the current does not flow through the
motor then the failed device must be in the same phase as the device that is being gated. The line
converter is then phased back to turn off the current. The test is repeated for each of the six thyristors in
the machine converter.
The voltage across the machine side of the dc link is
also measured during this test and a “No Vdc fdbk”
fault is issued if no dc link voltage is detected. This usually means that the dc link voltage feedback is not
connected.
Machine Converter Open Thyristor
With the line converter phased back to retard limit, both thyristors in one phase of the machine converter
are gated. The line converter is then phased up to try to drive some current through the dc link. If no
current is detected, it indicates that one or both devices have failed to conduct. The fault messages
indicate that both devices are open, even though only one of then may be defective. This test is repeated
for each of the three phases in the machine converter.