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Ben Solon

Sat Mar 31 2012, 05:05PM

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

hey, does anyone know off the top of their head how many primary turns an average mot has? if not, someone with a working mot can you put 1 turn through it and measure the voltage per turn?

i did something similar a while ago and measured the output/turn, and i recall something like 1.7v or 1.7v per turn this would mean roughly 100 primary turns to 1 secondary turns.

reason being that i have a core with no primary or secondary, and its about time i have a decent ac/dc power supply. tapped with a couple step downs and a couple step ups.

i need to wind my own primary that is similar to the original. so if anyone knows what awg is typically used can you also post that? i am going to try to use a couple awg sizes lower.

thanks

Steve Conner

Sat Mar 31 2012, 06:43PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

1.7v per turn is about right for MOT service, but if you want the transformer to run continuously without overheating, you'll need to settle for less.

wikipedia wrote ...

If the flux in the core is purely sinusoidal, the relationship for either winding between its rms voltage Erms of the winding, and the supply frequency f, number of turns N, core cross-sectional area a and peak magnetic flux density B is given by the universal EMF equation

Erms = 4.44*f*N*a*B

As I grew up with SI units, I'd use a in square meters and B in Teslas. (about 1.2 for typical transformer iron)

Dr. Dark Current

Sat Mar 31 2012, 10:43PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I think 1.7V per turn is a bit extreme... Our MOTs (50 Hz) have usually around 1.0 V per turn, in the 60 Hz lands I imagine this could be around 1.2, maybe some really large ones have 1.7 but idk... The induction in a typical MOT is around 1.8-2.1T.

Ben Solon

Sun Apr 01 2012, 01:02AM

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

It probably was 1.2 or similar. I may have had my scope set with p-p reading rather than rms. it was a long time ago and I was more concerned with melting the turn than measuring it at the time

for now I will assume 1.2. Its not really very important what the exact value is as long as I know how many turns I should put on the primary anyways.

klugesmith

Sun Apr 01 2012, 04:59PM

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

Hi Ben.
As others have said, the maximum volts per turn is determined by the core area
( Care to tell us that value? Is it from a high or low power MOT? ),
and by how far you want to push the flux swing = core heating.
A MOT with its original primary pushes it pretty far, and needs forced-air cooling for continuous operation even with no load. Can be measured on its 2 or 3 turn filament winding.

The other critical core dimension is the window area, which determines the maximum winding ampere-turns. That's invariant as you trade wire gauge for number of turns. So the area product Ac*Aw gives the designer a volt*amp limit.

Do you already have a low-voltage, high-current transformer? Then without much toil, you could run your MOT core with a temporary primary and no secondary. Measure primary current at your target volts per turn value. If your low voltage transformer is big enough to sustain the load, check the MOT core temperature after 1/2 hour. To keep copper loss down for the experiment, you can use the whole window area. Perhaps use thick stranded wire, or a bundle of stranded wires.

[edit]
This post on another forum shows measurements on a similar-sized core.

A chart shows the primary current and core heat rising sharply as voltage approaches saturation. The design point is 0.69 V RMS per turn at 60 Hz, from 2.2 in^2 (14.2 cm^2) core area

Ben Solon

Sun Apr 01 2012, 11:32PM

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

as i don't have the core with me right now, i cant say what the Ae or window area is. if the window isn't large enough to have a primary and secondary with enough turns and current rating, i can always make an autotransformer. i would rather not though... within the next few days i will get the core(its at my school being used as a ballast). i know its a bit smaller than the other two i have, which is why i am using it as a ballast anyways. if i took a guess(which is probably WAY off) it is about 2-3"*2-3". if anyone has actual core Ae measurements so that i can do rough calculations, it would be appreciated.

Forty

Mon Apr 02 2012, 05:38PM

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

I've got a bare MOT core right here. I don't see a power rating on it anymore, but it was a 120v 60hz pretty average size one.
cross section of center leg is 2.4"x1.4" and it is 2.1" long.
side legs (and the size of the winding window as it turns out) are half that, at 2.4"x0.7"x2.1"

Patric

Mon Apr 02 2012, 07:09PM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 255

I have a mot, 220V in, 2100V out, 1100Watt.
0.85V/winding.

klugesmith

Mon Apr 02 2012, 07:10PM

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

Forty's numbers match the "golden ratio" for transformer EI cores.
Perhaps not optimal for VA per pound, but the steel shapes can be punched out without wasting material.

Ben Solon

Mon Apr 02 2012, 07:47PM

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

hmm. good, just picked up that transformer, and it looks like forty has the exact core that i do.

forty: if that core still has a primary on it, could you give a guess by counting the length and width in units of wire?

using steve c's formula: Erms = 4.44*f*N*a*B(f=60Hz; n=1; a=.0021m^2; B=1.2T)
i get .67v per winding while staying out of saturation. meaning i need 186.1981028647695 windings to stay out of saturation with primary Erms of 125v. i am on 120v mains(178.7501787501788 primary turns), and when measuring it i typically get 118vrms.

any questions/comments before i go on to wind this thing?

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