Bob Engelhardt wrote:
JR North wrote:

Haven't seen your design, but my .02 anyway-if you are switching the
primary with a SS relay, be careful of letting it cook as you
described. The first indication the relay is overloading is when it
pukes.


An SSR may be in its future, but now I was just closing a contactor.
About the cooking though - if I was well below the current rating of the
SSR, would it be OK? Say drawing 16A on a rated 25A SSR?

Also about the SSR: I've read that they should be derated 30-70% for
inductive loads. They were probably referring to motor loads, but the
spot welder is certainly inductive. As much as a motor, though? Did
you derate yours?
Right. I have one on my air compressor, and I am now using a 50
A, 400 V SSR on a motor that draws 11 A nominal. But, it has to
start the motor. It was running the motor almost continuously
all day today, with lots of starts, and no trouble (it has done
the same service before, too.) But, I did have problems with a
25 A SSR on that motor - well, just one problem, it shorted out.
The welder may not have the same sort of starting surge as a
motor, though, so the current derating may not be such a
problem. 16 A at 230 V is about 3700 W, that should be able to
melt a tiny button of metal.

Jon