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MOT being repurposed, need help related to heating.

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Patrick

Wed Apr 10 2013, 11:40PM

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

I need some help, I feel foolish, I bought a 120V (MOT 1kW-ish ) I cut the primary off and counted the primary turns (but forgot that number) any way this was done months ago.

ive read the typical articles and googled much, everbody reminds me of the 0.9 to 1V/T common rule.

but heres where I need your opinion, I want to re-wind the whole thing, using it for a 120 pimary to a 45V, 15V, and 12 volt secondary, at high current. and I want it to run cooler than it was originally meant, but hotter than a normal transformer. (id like the temperature to be about mid-point) then ill over cool it with fins and a fan. it will power my CNC mill...

so can I get reccomendations/educated guesses on the primary turns count, volts per turn, and cross sectional copper area?

I can tolerate quite a bit of temp deviation, I just don't want to cook eggs and bacon on it...

Ive already removed the mag shunt pieces, and will never need this transformer to run in its "conventional mode" ever again, it will be fused for safety of course.

[the cross section of the center limb is 1.4 x 2.4 inches]

Thank You...

EDIT: what I want :
120 VAC, 9 amp input.

45 Vdc, at 12 amps
15 Vdc at 12 amps
12 Vdc at 5 amps
5 Vdc at 5 amps

So..
45 / 1.4 = 32 Vac sec output
15 / 1.4 = 11Vac sec output
12 / 1.4 = 9 Vac sec output
5 / 1.4 = 4 Vac sec output

Ash Small

Wed Apr 10 2013, 11:56PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I think this is quite a complicated question, and I suspect the answer is related to how much power you want out of it.

ie, when a 'standard' MOT, for example, is connected to a smaller doubler capacitor it doesn't get as hot as when you are putting 1kW through it, so first question is 'how much power do you want out of it?'.

The 'volts per turn' question is a separate issue, I think. If you use more turns to lower the volts/turn you need smaller diameter wire, and you then get copper losses, or melted wire.

So I think it's two separate issues here.

What I'd do is work out how much power (eg amps) you want out of each winding, then work out diameter of wires required, then work out how low you can get the volts per turn while still fitting all the wire on the core, then you have the optimum transformer for your project using that core.

Edit: you've added some more details to your post while I typed the above (power requirements, etc), but the process will still be the same. Choose your wire diameter, then see how much you can fit onto the core.

Patrick

Thu Apr 11 2013, 12:17AM

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

yeah Ash, I was thinking of 120x 9 = 1080 watts, and that would be peak, with all four steppers moving and not just "holding" position... but at normal long-term load 800 watts would be expected...

that's 28% less power and heating just by ifself... I was also estimating 0.71 to 0.75 volts per turn, less again than the 0.9 typical V/T.

Steve Conner

Thu Apr 11 2013, 09:44AM

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

Simple answer, buy a nice new transformer from Antek, they specialise in transformers for audio amps and CNC machines.

A MOT is only good for about 300VA continuous.

Ash Small

Thu Apr 11 2013, 08:26PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'm also working on a stepper motor circuit at the moment, but I don't need anywhere near as much power as you do.

I might use your idea for my project, as I'll need 12v @~1A for the two steppers, plus maybe 24V (not sure exactly yet) for driving the MOSFETs. (I may use a bit more power for the steppers as I develop the circuit, but to start with I won't be driving them in chopper mode or anywhere near their rated power)

Dr. Dark Current

Thu Apr 11 2013, 08:32PM

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

I think the MOTs usually run with a Bmax of ~2T, so winding it for 1.6T should be OK when fan cooled. From these input parameters (and assuming 60 Hz supply), it comes to 0.92 V/t. For the current density, try something like 3.5A/mm2 and calculate how much % of the winding window you fill with copper. For hand winding and no layer insulation, copper fill factor up to about 40% is achievable, not much more...
Then, like Steve said, you will want to buy a better transformer

Patrick

Thu Apr 11 2013, 09:19PM

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

Dam,. I was afraid some people would say these things.... "300VA, cheaper to buy a new one..." and so on... I still need a 45Vdc, hich current transformer for another app, so ill try anyway...

Steve Conner

Fri Apr 12 2013, 10:00AM

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

If you want lots of power intermittently out of a small core, the compromise will be between regulation and core losses. If you wind on enough turns to stop it overheating from core losses while sitting unloaded, then you'll need to use thin wire to squeeze them all in. That means the voltage will sag a lot under load due to IR drop. (or conversely, if you design for the correct voltage under load, it'll soar sky high when unloaded)

Marko

Fri Apr 12 2013, 11:18AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi Patrick?

Is it really necessary to use massive iron transformers in the world of tomorrow we live in? SMPS are really cheap today, and I severely doubt the value of effort and copper required to rewind a MOT will beat it - not counting the extreme heating and regulation problems you'll probably run into.

Marko

Patrick

Fri Apr 12 2013, 05:24PM

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

well if i were rich, id buy an smps for each of the voltages i need, but as a poor college student i have most of my dollars tied up in my academic bicopter multirotor. ( ive even had to be sober more often ) so i must make due with what i can find, and the ability to specify odd voltages like 37 or 43 at 15 amps probly means a variable smps...

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