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Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039
Okay, so I was at the geek group a few months back, and a friend there let me have a choice of variacs he bought from a school... I took one home, and wired it up. They were rather cool, and are made in Chicago. Really old. So, I replaced the wire, fixed the plug, etc. I did a test on my variac, doing a variact -> Variac setup. I was able to get 0-140VAC with the switch set to 140. With it set to 280, i was able to get 288VAC from 120V mains. Works right?
Well, after i turned it off, and flip it to 240, BANG. smoke. i found a possibly issue was due to a wrong screw in the core, going too deep. it WORKS now, but my 120V 8A variac fuse was destroyed to hell.
So......I need a soft start. The issue is i never made one, and need help knowing WHAT parts to use. I dont want to manually turn a switch that engages sa resistor. then switch to full power. Because one day, ill forget to switch it, and thing will go to hell....Is there anything, thta istn far too expensive, that will automatically engage itself? Turn on with the resistor, wait a sec, then switch over to full power? Something that wont require a 120 -> 12V transformer possibly..?
Rated 240V in, 9.5A (2.2KW) Datasheet though shows alternative wiring for 120 in, 240 out.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546
If you are "hot switching" it, the following may be happening:
When you flip the switch, the inductive arc from the break is shorting over to the closing arc from the remake contacts. Yes, it makes a bang and blackens fuses.
Some multiple pole AC switches are labeled 'same polarity only' to show you cant always get away with doing voltage transitions. Electricians sometimes try and use the wrong switches to switch a 240 volt heater over to 120 volt operation and the same thing happens. (Why do I know that).
Figure out a way to de-energize the circuiit during transition.
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
No, its a 3 position switch. 120-off-240. Thta switch just directs 120V mains to 2 different taps. Center, is fully off. The issue is, 120V in, to the mid-point to allow 240 out, is a HUGE inrush of current to energize this core. There's a large surge to charge it to power. My 8A 120V variac, when flipped on, makes my lights go off for a fraction of a second due to inrush. This things inrush is SO high at mid-tap, that it kills my 8A INSTANTLY. So, i need a soft start. But instead of manual, i need relay thats auto. You start it with a 100ohm resistor, then flip it to full power. No massive inrush current.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
You're not applying 240V to the midpoint, right?
Variacs tend to draw a huge inrush current, and a soft start should solve your problem, yeah. I like this circuit, it's one of the simplest ones that I know of. The only critical part is to size C1 for your relay. I guess the reactance of C1 at 60Hz should be Mains voltage minus (Relay voltage divided by the square root of two) divided by the nominal relay current. This equation ignores the voltage drop across the diodes, but that shouldn't make much of a difference.
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
yeah I'm giving it 120. but because the 120 to 240 tap is in the center, there's half the windings used on the core, and the inrush is huge...that's why i need soft start :) ill give that circuit a try and see what happens,thanks!
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Step-Up variac help
