G
A couple of weeks ago we had a fuse blow in a mainswitch which was supplying a 3 phase 575volt distribution trough. This occurred during the weekend when there was no one around to notice the effects. All the VSD's (Variable speed drives rated at 575v) connected to this trough blew 2 of their protective input fuses which were very conservatively rated as compared to the manufacturers specs. The drives were however, all short circuited on their input ! I tried simulating the condition (without the drives connected) by removing a fuse and taking some voltage measurements with a scope. I could not see anything out of what I would have expected to see....reduced voltages and one phase missing. There is a 3phase delta transformer connected to the same trough so I tried the test with and without it connected but the results were the same. Back EMF from the transformer??? Closer examination of the situation revealed that the original blown fuse was carring an excessive load...95A on a 100A fuse. We reduced the load to the trough so the fuse will no longer be a problem.....and I have isolated the transformer loads in case of phase loss, but the question still remains.....why would a missing phase blow out the drives????? at reduced voltage????? This one cost us $6000 worth of low hpdrives.....I'm sure glad it wasnt the 75hp units we have elsewhere in the plant. Any comments would be appreciated Gerald Beaudoin