Burnt out Variac

General Discussion
1

Can’t seem to find my card reader so no pics atm but…

I got a Calrad variac from a friend who used to make gliders, pretty heavy duty variac. Weighs a lot.

Because of shipping there was a plastic ring inside that was cracked into pieces. When we were cutting foam on Friday the variac started to smoke. At first we assumed it was the plastic ring melting on the coils so we took it apart and removed the ring. No idea what the ring did. I assume it was a spacer of some kind.

After removing the ring we started cutting again and discovered that it was actually the insulation on the copper coils that was smoking. We kept going till it looked fairly black and we were sure something was wrong.

So I guess the question is this just a grounding problem and through shipping, either through the destruction of that spacer or the resin insulation getting rubbed off the coils or whatever, or are we supposed to have something else to lower amperage or something?

What is to keep the coils in the variac from getting as hot as the wire in the bow?

We were using 18 guage stainless steel wire on a 16 inch bow. We were running around 13 volts. No idea about the amps since we had no amp meter set up.

My setup was Outlet > variac > toggle switch > bow

2

I have another one on order but I’m also thinking about recoating the burnt wire with some epoxy. Somehow trying to get it to penetrate the coils. Maybe stop the short if that is the problem.

3

Hi Kai,

I burned out a variac myself by trying to make a hotwire using my copping saw. The saw shorted out and the variac started smoking. I think wire resistance or conductivity is what keeps the copper from heating up as hot as the wire.

Rio

4

I’m not 100% Kai, but I would think that it was your wire setup that did it.

I got myself an old train set transformer, and the first time I used it I had 22 or 24 gage stainless steel wire(thinner than 18 I think), and while using it the transformer heated up and would periodically cut out and then start up after a few seconds (I could actually see the wire flex when it did it). Needless to say this first blank had a lot of ‘wobbles’ in the cut faces to sand out.

I then went out and got some fishing leader wire (thanks to Benny1 for the advice), stripped the nylon coating off of it and gave it a shot. The transformer doesn’t even get very warm, even doing a whole board cut in two straight passes. I also have a piece of NiChrome wire that I got from a hobby shop that I haven’t tried yet, but it is supposed to be THE stuff.

My electrical knowledge is confined to what I learned in high school, but I think what happens is that even though the setting is only at 13 volts, it takes so much current (amperage) to heat up that thick stainless steel wire that the variac can’t handle it.

Hope you can work it all out. It’s pretty fun doing the EPS thing.

5

I was going to switch to thinner wire. But I only bought 18 and 20 guage. I also bought copper but I think that might be worse.

Any idea what guage fishing leader wire is?

6

I’m not really sure what the gauge of the fishing wire is, but it is quite a bit thinner than the 24 gauge stainless steel. The Nichrome wire I bought is even thinner than that.

7

Thanks, I’ll pick some up tomorrow

8

Lets see-

First off, copper wire is maybe the worst thing to use with a variac…or anything else…for cutting foam and such uses. It conducts electricity quite well, so that it’ll only heat up if you put a helluva lot of power through it. It also melts at a relatively low temp and corrodes and burns, so you’d have some real problems there.

But, lets have a look at the basic idea of an electrical hot wire cutter. You have a power source, a switch, a wire that heats up and the various wires bringing mains power ( wall current ) to the power source and from the power source to your hot wire.

Now, on the power source, it’ll have a power rating, probably in watts, of what it can do. Generally it cranks the voltage up and down to give you the variations in power while the watts stays the same. You don’t want to make it run at the high end of what it can do for very long or unpleasant things happen.

How do you avoid that? Select your wire so that it’s gonna heat up to the right temperature ( 400-600°F or around 250-300°C) at a power inside your power source’s ‘comfortable range’ - less than the maximum it’s rated at.

So you don’t want to fall in love with a particular gauge or start thinking that fishing wire ( usually Monel or stainless steel ) will be the answer if it’s a different gauge …cos it is also a different metal, and like copper it’s gonna work very different. You gotta do the math -

watts = amps x volts.

more math here: http://www.wiretron.com/design.html

Watts- what your power source can put out. Amps - what it draws from the wall current. Volts - well, typically a variac will have a dial, and it’ll range from zero to some figure - lets say 32 for an example, yours is almost certainly different.

If, by the way, you only have amps or watts listed on your variac, that equation above will give ya the other one.

So, imagine you have ( making up numbers here for an example ) a 2’ section of wire, a power source that’ll put out 100 watts between zero and 32 volts. What gauge nichrome wire would you want?

Well, look at http://www.wiretron.com/nicrdat.html , a table of wire data. now you can start designing and calculating.

You’re shooting for the right numbers at around 16 volts. 100 watts/16 volts = 6.25 amps. From the table at http://www.wiretron.com/nicrdat.html , you see that 18 gauge wire needs 6.5 amps to get it to 600F… should you use 18 gauge?

No - you have two feet of wire there and those numbers are per foot. Divide by two - you want something that’ll get you up between 400 and 600 F at a little over 3 amps, so ( looking at the table again ) your choices are between 22 and 23 gauge Nichrome wire.

And note I said Nichrome wire - copper, or stainless or Monel wires, will behave differently, so ya gotta have different ( much thinner) wire gauges for those to behave properly. And I don’t have tables for those easily available. The resistance changes at different temperatures, so it’s not especially easy to figure.

Now, I was just making up the numbers above - you want to have a look at the numbers on your own power source to see what wire gauge is right for you. I’ll note that when I put together my setup I had the 23 gauge wire so I had to go transformer shopping to get the right one for the wire length and gauge I had planned. Wound up with the biggest doorbell transformer I could fine, which works okay but not great for a 12" cutting length, but if I go longer I will have to go to lighter wire…

hope that’s of use

doc…

9

That is a good information source to have Doc. …aaahhhh Ohm’s law. I haven’t looked at that formula in a while. I’ll have to plug my transformer and wire numbers into it and see what I get.

Thanks.

10

Hi Johan,

Their downloadable resistance ( and other things too) calculator isn’t bad either - http://www.wiretron.com/free.html . Handy when you want to plug in values for other types of wire like copper,stainless or monels - the latter two being typical fishing leader wires -

For instance, resistance ( in ohms) per foot for #23 wire,

Monel: .567781

Nichrome A: 1.32156

Pure copper: .020305

Stainless type 304: .857546

In order to get the same resistance/ft in stainless wire, you’d need to go down to 25 gauge, in monel 27 gauge, in copper… .0028 diameter 41 gauge stuff, and we’re talking about wire that’s around the thickness of a human hair and would probably melt on you.

always happy to be of help

doc…

11

Hey Johan:

Doc is right on the ball with his info. If you are really going to be getting into cutting a lot of EPS you really need to get away from the guitar strings and other stuff and go with the stuff that has the right properties for the job.

A couple of comments:

What you were doing was smoking your variac’s insulation by overheating the copper coils which burns off the shellac/varnish coating. If you do enough of it, you will create a dead short in the coil and it will be useless at that setting as it will be way hotter on the “up” side of the coil than you could use. (I have done quite a bit of this in the past) You may have noticed that your wire was colder/hotter than before and you had to make adjustments.

Try to stick with a proven product like Nichrome A wire. That is the industry standard for cutting EPS in all of the CAD machines and in all handcutting operations. Make sure the table you are using to calculate is for Nichrome A and not Nichrome C as they have quite a bit of difference in resistance.

Some of the quick cutters I have seen are starting to use Kanthal but I think it might be too brittle to use in surfboard hand operations. There is also a lot of Nichrome around as it is used in so many other industries that you can find it even on EBay in most gauges.

If you take the time to do the math, you will end up with a better cutter and a better cut in the process. Don’t forget to figure out a hand cutter gauge for cutting rail bands as this will save you a lot of time vrs. using a planer.

I hope this helps!

Surfteach

12

I ended up using a car battery charger at 6 volts. I regulated the amps by attaching the contact to my spring instead of the wire itself. By moving the contact up and down the spring I could control the amps very accuratley.

I found that 28 guage stainless breaks around 4 amps. Ran fine at 3.5.

24 guage stainless breaks around 6 amps. Ran fine around 5-5.5.

Those breaks were in the foam with 2 inches sticking out either side. Might do better with less gap.

So to get hotter I either need a wire with more resistance like NiCr or a variac that can handle more amps right? Also thicker wire can get hotter without breaking correct? Thicker wire needing more amps to get hot though.

I’m pretty sure there was shipping damage in my variac because we tested it with the 18 guage wire and I think it was only pulling 8 amps which, though I do not know since I cannot find what it’s rated for since the company does not make it anymore, is probably less than it can handle.

Also it pulls 1 amp with out anything attached to the out terminals so there is a short in it now. And probably before as well since 8 amps doesn’t seem like enough to cause heating like that.

It was dropped hard enough during shipping to break some of the thermoplastic off the inside bottom, bend the steel case some, and break off part of the dial.