My gut feeling is that there is a voltage transient at turn off causing the
problem. Maybe a Transzorb or MOV would help. Personally, I prefer good old
fashioned contactors, although the buzz you sometimes get when operating on AC
can be annoying.
I worked some 20+ years ago with a guy whose first job after graduation was
assisting in commissioning railway locos running off 25kV 50 Hz. They had just
moved from steel tank mercury arc rectifiers to silicon, and his stories of the
problems due to unexpected transients killing them was interesting, to say the
least - including having blocked the main line with a failed loco, they had to
get an old steam loco out to haul the test train out of the way! So transients
and semiconductor problems aren't new...
73
Peter G3RZP
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