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Registered Member #1484
Joined: Wed May 14 2008, 03:24PM
Location: Cary, NC, USA
Posts: 27
Neat, I think that's a stack of the exact same Variac I have, W50H. There's an irritating-to-access fuse behind the panel there, if you haven't found it. It's a slow-blow type and supposedly it will allow around 150% current for a minute. There's also a whole lot of documentation about them on IET Labs website, including the uprating curves and limiting curves of the fuse in pulsed operation. When the fuse it blows the replacement part is pretty hard to order and quite overpriced -- I spent like $40 on a pack of 3 fuses for mine When I run out of fuses I will be fusing it with some bus bar
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi christopher -
That is one admirable transformer - at school we have a 3 phase variac with about the same 'segment' size but is only rated 10A per toroid. I wonder if that is because each of them uses only a single brush?
I have some other questions as well:
...Have you considered the heavy 'inrush' current the large variac can draw when plugged in without load? It was impossible for us to plug in even a smaller 5A variac at school without blowing 16A fuses instantly. The way we got around that was to use a beefy 3 phase potentiometer, connect the variac over it first and tehn slide the resistance to zero.
The classic way of startup, though, would be to use few hundred ohm resistors which are shorted over by a relay after a 500ms or so delay.
I don't know whether that will matter for your massive 100A+ feeds but I'd try to avoid massive current surges when possible.
Also, what is the black material you're using for your output chokes? I thought it has to be strip wound iron, so is it suspended in some sort of resin?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I'd consider 2000 watt resistors pretty oversized for startup - a 500 resistor would continuously dissipate less than 150W in your case if relay was malfunctioning and the output was shorted... and in operation the resistors aren't going to see dissipation but for a second or two before they're shorted over by a relay. So you could probably even go with small 10W resistors as long as you don't try to run a load over them without the realy acting!
..Otherwise I'm curious, what is the trip current of your breakers at home right now?
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The Variac
When I run out of fuses I will be fusing it with some bus bar 
