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windings on an mot

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Forty

Mon Apr 02 2012, 08:38PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

Sorry the primary from that core was removed and reused so I can't count it.
I do however, have 2 slightly smaller mots (one is about .3" thinner and the other .5" thinner) and their primaries are approximately 12x10 turns and 12x11 turns, so 120 to 132 turns of what i think is 16awg. Again, both are for 120v 60hz.

Ben Solon

Wed Apr 04 2012, 12:29AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

So with 200 turns of 16 awg magnet wire assuming each winding to act as a square, the 200 turn primary theoretically should take up 18mm by 18mm square through the window(or equiv). but I am not going to be the one to wind a prefect primary without a bobbin, so I am going to assume double

Patrick

Wed Apr 04 2012, 12:37AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ben123324 wrote ...

So with 200 turns of 16 awg magnet wire assuming each winding to act as a square, the 200 turn primary theoretically should take up 18mm by 18mm square through the window(or equiv). but I am not going to be the one to wind a prefect primary without a bobbin, so I am going to assume double

what freq will the 16 awg be seeing? (60Hz or 20kHz)?

Ben Solon

Wed Apr 04 2012, 01:43AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

ua mains: 60hz. i tried to keep my threads seperate, but i guess it cant be helped when the same people are posting in both threads.

Patrick

Wed Apr 04 2012, 03:08AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ben123324 wrote ...

...ua mains: 60hz. ...

yeah your 16 awg is fine then, (i would have said different if skin and/or proximity effect came into play).

Forty

Wed Apr 04 2012, 04:00AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

haha I think skin and proximity effects would be the least of your concern as your inductively heated mot melts all your windings together at 20khz. (but again, it's not going to be used that way)

Don't forget to account for the insulation size. It's very small, but if you made a bobbin based off of the conductor size, you might not be able to fit your windings on it as you intended. I'd wrap the desired number of turns per layer onto a dowel or something, and then measure off of that to make your bobbin.

Also, unless you think you can manage a perfectly square packed coil, every even number layer will rest in the grooves of the previous layer and will have 1 less turn. If your bobbin is tight fitting enough, then you shouldn't have to worry about the end turns of the odd layers falling down into the previous layer, but inter layer insulation tape, a dense piece of string, or generous amounts of superglue will help solve the problem if they do. I'd probably scrap the superglue idea for epoxy, since this transformer might be heating up a bit.

16awg can get a bit strenuous to work with (just wound a coilgun coil out of mot primary wire the other day) so I'd recommend a simple jig that can at least hold the bobbin and have a padded clamp nearby so you can secure the wire should you need a break.

Also recently read about winding large wire rectangular coils (for a mot core actually) that suggested bending the wire slightly outward before you go about winding the long segment of the rectangle so that the wire lays flat.

hope any of that helps.

klugesmith

Wed Apr 04 2012, 06:24AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Have fun, Ben!
FWIW, here are some pictures of my last transformer winding project, in September 2010.
18 AWG, 5 layers of 14 turns on a rectangular bobbin.

I put the bobbin on a hollow square shaft and turned that with my fingers. Put a heavy c-clamp on the shaft at break time. Wire tension came from the friction of full spool rotating in a cardboard box.

Tapped after 2nd and 4th complete layers.

Bobbin was long enough for about 14.5 turns, and it wasn't hard to get the same number of turns in each layer (though I did break a bobbin flange). New wires do lie in the groove between wires on previous layer, except for a cross-over once per turn (at a place of your choosing, preferably in a quadrant that will -not- face the inside of a core window).

1 sheet of clear tape for inter-layer insulation.
Wish I had paid more attention to flattening the layers on the sides facing the core. Forty's suggestion sounds like a good one. I had to file some of my core E-sections to install them without scraping the magnet wire.

This was for a compact, low-voltage autotransformer. The cool HV-isolated transformer can't handle more than about 60 V in, 1.2 V out, which was insufficient to power the intended filament.

A more semiconductor-intensive solution was to hack a cheap 12V:120V inverter to put out 120 V at 150 Hz. I learned that at 60 Hz, one can squeeze out a bit more RMS voltage for a given RMS magnetizing current, using the distorted voltage waveform that comes with dropping primary voltage through a resistor. These are all stories for another thread!

[edit]

ben123324 wrote ...
So with 200 turns of 16 awg magnet wire assuming each winding to act as a square, the 200 turn primary theoretically should take up 18mm by 18mm square through the window(or equiv). but I am not going to be the one to wind a prefect primary without a bobbin, so I am going to assume double

If you have removed the core shunts, why can't your primary use the entire length of the center leg? Say, 54 x 6 mm instead of 18 x 18? The your secondary -also- can have fewer layers, and will be better coupled (less leakage inductance).
From

, it might be a squeeze to get 40 turns per layer. Then go for 5 layers of 39, or 38 , or 35 turns. There's hardly a brick wall just above 1.2 teslas!

Ben Solon

Wed Apr 04 2012, 02:43PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

I am probably going to use as few layers as possible anyways. Thats why i said: or equiv. again rough calculations, so insulation does play a significant role in volume. And unfortunately I do not have a mot sized bobbin, or one of those cores with an I section that comes off. Not do I want to cut it. When I wind this I will use maybe wax coated paper as a former/interlayer insulation.

But thanks guys! I will let you know how it turns out!

Forty

Wed Apr 04 2012, 04:31PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I'd just cut the transformer I off then. You'll go crazy trying to thread a few hundred turns back and forth through the core, not to mention you risk damaging the wire every time. Afterwards you can just clamp it back together or weld it if you have the means to.

(part 3 of here

is where I saw the wire bend tip)

I don't think MOTs ever come with bobbins. You have to make one yourself. That video also has some info on that process (He's using an existing primary though, so your bobbin will be the full length of the core leg.)

That source also helped me realize that if you have two rather identical mot cores you could butt the E's together to form a larger transformer.

Anyways, good luck with your transformer and be careful not to damage the wire in the process.

Ben Solon

Wed Apr 04 2012, 10:01PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

didn't know that every core was an ei, i thought that many had that weld for keeping the laminations together. i guess i will hacksaw it off, then weld it back together this long weekend sometime with my neighbors welder. thanks for the bending tip, i learned a few more things with that video.

and for those who are interested, i plan on putting this project up on my website sometime in the next few days for those who need to wind one of these from scratch like me. website here:

and it will be under high power, as it can theoreticaly put out a few kw without overheating(1-3 depending on awg).

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