RE: Variac turn-on surge solution
Terry Fritz <twftesla-at-uswest-dot-net> wrote:
>I note that the breaker probably only trips when the variac is energized
>near the peaks of the AC waveform. You could install a big 25 amp solid
>state relay that is designed to switch at the zero voltage crossings. Thus
>the circuit would only be completed near the low voltage points in the AC
>cycle. That "should" fix the problem. Omron's G3NA series as shown on
>page 456 of the current Digi-Key catalog shows them for about $30. Big 25
>amp 240VAC version should be bullet proof but hard to say for sure given
>our system's surges ;-)
Not necessarily true. Depends how the core is polarized before the relay
turns on. If the core was left polarized by a positive half cycle, then if
the relay turns on - the core may get yet ANOTHER positive half cycle -
which will saturate it and draw a large current.
Our failure mode analysis indicated that using the random phase solid state
relays would at least minimize this problem. We had issues with popping
fuses on a dual voltage power supply upon turnon.
So far I think the slow turn on technique sounds the most promising!
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