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Old 04-24-2015, 04:07 AM #1
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Default Repairing or reusing a heat-damaged Variac.

A while ago, I obtained a Superior Electric Type 136BU. This is rated at 22-amps and about 3.1kVA. I knew something was wrong with it when I got it, but the other day I finally took it apart and found that the damage was much worse than expected.

The coil originally had 180 turns, but on both ends of it, multiple turns had melted down due to severe over-current. All told, 46 turns were breached. Collateral damage appears limited and is either cosmetic in nature, or can be repaired by filling-in with Bondo or similar epoxy.

The rest of the coil appears intact and still works. I've checked it with a mega-ohm meter and there is no short to the iron core. I've tested it with 3 volts AC (supplied via another variac and a 24-volt isolating transformer) I see no evidence of shorted turns. The insulating varnish doesn't appear darkened, except where smoke from the meltdown got to it.

The question I have is... what should I do with this thing? Options I've considered:

1) Rewind the missing coil ends and solder/splice onto remaining coil. The variac wiper runs on a flattened or sanded area on the winding and would be easy enough to duplicate.

2) Assume remaining coil is bad, cut it all off, and rewind the whole thing for use as a variac.

3) Same as #2, but rewind it as a normal toroidal transformer (not auto-transformer) possibly with taps to select different voltages. I could definitely use this in like a custom power supply. It would be a hell of a power transformer; the primary winding looks like 14 gauge which is thicker than even the secondary on large-ish low-voltage supply transformers.

4) Reuse remaining coil, varnish off the wiper contact, and wind a few turns of jumper cable wire around the toroid and melt some stuff. I don't think it would work; with 134 turns on the coil it's almost down to 1 turn per volt and would probably just burn up what's left.

Hate to just throw it out, it's a nice toroidal core but I'm not sure putting mains voltage on the remaining windings would be smart, given the type of failure and damage this unit sustained. What would you guys do with it?


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Old 04-24-2015, 05:02 AM #2
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Default Re: Repairing or reusing a heat-damaged Variac.

I've done both #1 and #2 before, and #2 almost always turns out as a more permanent solution. It really depends on how much of the coil is bad. With so much bad, you might as well rewind it entirely.

Likewise you can up the wire gauge this way and insulate it from the core better. The cores are usually such that you can overload them significantly for short periods as it is the wire and insulation that usually fails. You can make it even better if you rewind it.
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