VARIAC Issues -- Need Help

[w9bea]

Good Evening to the list:

I come to you for some advice/help regarding VARIACS.

BACKGROUND: I am rebuilding my shack.  Put in a subpanel for the radio area with plenty of 220/120, well split up panel.  All fed with conduit.  Well grounded, per code.  Real FB.

I have 4 VARIACS. Each variac is fed by my subpanel on it's own 20 AMP breaker.  The variacs are 15-20 ampere rating each.  The output of each variac is fused with a 20 AMP cartridge fuse, and is subsequently routed to it's outlets that is serves.  The intent is to throttle the AC voltage back to 114 VAC or so to keep the BA stuff happy.

My problem is, that without a load, I flip on the variac and the circuit breaker immediately flips to red.  I flip it on again, and the breaker holds OK. 

Checked my wiring, no problems.

Checked the variacs, no zorches, shorts, etc.  2 are NOS, the other two were taken out of equipment working and checked on the bench prior to use.  The two NOS ones were used exactly the same way in my old upstairs shack, never a hint of trouble.

Tried to measure the current draw with a clamp on ammeter.  I saw about 1-2 amps draw without a load, but I am not sure that this is correct. (I will wire in an Ammeter later tonight to see the real deal.)

I lifted the ground wire off of the variac cases.  Didnt make a bit of difference.

I suspect that there is a temporary high load upon powering up (a phase issue?) that surfaces on occaison and doesnt surface on other occaisons.

So, I am lost.  Why does this happen?  Any ideas?

73--Wally W9BEA

[WA1GFZ]

Wally,
I have a Variac that does the same thing once in a while. I moved the input to the top of the winding to reduce surge current. This means you won't be able to boost above line any more. More turns on the core limits the mag current and surge current.
The only other thing you could do is use a bigger breaker or one with a longer time delay similar to a motor start. My variac is rated for 20 amps and I had it on a 20 amp breaker. I took it all apart and found no problems or FOD. The only other thing I consider would be if the brush was warn and shorting two turns.

[k4kyv]

I have had the same thing happen with plate transformers.  You need a safety margin at the breaker.  Use a 25-amp or 30-amp breaker for a 20-amp transformer, or else a breaker designed to hold a momentary surge.  Some of my plate transformers, even when completely unloaded,  make a "bongggg" sound and pull down the voltage for a split second to the entire house, if I happen to hit the switch right at the peak of the a.c. cycle.  If I happen to hit it just as the voltage is crossing baseline (zero volts), it won't make a sound or even blink the  lights.

[W2VW]

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