Last week I was involved in troubleshooting a motor failure that was running a supply air fan. Initially it was thought that the motor failed but it turned out to be the controls transformer - burnt up bad. The transformer was 600V:120V @ 60VA. The starter equipment is possibly 1980's vintage. Besides the transformer burning-up there was a blown fuse on the controls side that was rated for 3A/250V. We didn't have a 60VA trans. so we used a 100VA. That's when it occurred to me - why was the 60VA trans. secondary fused @ 3A? I went back the next day and measured the current on the conductors at the load side of the fuse. There are two conductors soldered on the load side of the fuse - one going to the controller for the supply air fan motor and another going to another controller (? haven't traced it out yet) that is interlocked with the first. Current to the controller on the first = 0.4A. Current on the second conductor that goes to another controller - 1.8A. Am I possibly misreading something here or was the 60VA transformer being overworked which might explain why it burnt up? I've been told that it's been like that for years so it was simply an old transformer that finally quit. Something doesn't seem right with that explanation.
