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User name M. Stevens
Log entry time 15:54:29 on November 27, 2007
Entry number 216521
keyword=Left Q2 overtemp problem
The failure mode was an overtemp condition at 1200 amps, with fans and
the large blower operating. We replaced the thyristor stack with one
that was new from the factory, and has silver thermal/conductive paste
between the thyristors. We ramped the supply to 1560 amps when it
tripped on overtemp. We installed fans on top of the supply, and ramped
it to 1590 amps when it tripped on overtemp. We installed the large
blower fan to direct air on the thyristor stack, and the supply would
ramp up to 1600 amps and stay there for several minutes. The inboard
pair of thyristors is again the pair that is overheating. We do not yet
understand why. We put a scope on the output before the transistor bank
(across C1) and saw a stable 3 phase ripple, with no asymetry.
We noticed evidence that the transformer wires on the high voltage side
had overheated and melted some of the insulation. We looked at those
wires with the thermal imager at full power and saw no overheating at
this time. The damage was probably from an earlier casualty.
A copy of this log entry has been emailed to: fansler@jlab.org, stevens@jlab.org