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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