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MOT dissapointment

In the thread "Need linear supply 3.3V/15A or 5V/25A" that Joerg started on 12/2 I posted the idea of rewinding a MOT for ~6V. I've never done it, but decided to try it. If anything good comes from it, Joerg gets the credit because his question planted the seed.

Things went so well with the initial experiments that I decided to actually make a useful transformer for a

13.8V ~25A (peak) supply.

To do that, I need about 100' of #14 enamelled wire. (Gonna add 24-30 turns on the primary to knock the

5.62 magnetizing current down to 1.4A or lower. 24 turns knocked it down to 1.4. The secondary will be 3 or 4 18 turn windings in parallel.)

The dissapointment: I found another oven yesterday. Today I ground off the welds and pried off the primary. Now, get this: it looks like #14 copper, but weighs only 6 ounces! It has about 132 feet of wire (based on turns count x average perimeter of the winding) The damn stuff must be aluminum.

So I'll have to keep on scrounging until I find one that has real copper. The rewinding will be a bitch, so I'm going to wait until I can get the real stuff.

If anyone has any pointers to offer - about the winding, getting the wire, whatever, I'd love to hear them. By the way, the ~ 25A is not a requirement. I just want to get the maximum current I can from it, reaonably. There's no point in going through the effort for say 10 amps, and my main experimental interest was in finding out what it would take to get an MOT into "re-windable condition" . Now it is sort of a challenge to try to get the most out of it reasonably. I already know that Joerg could get his 15A at 3.3 or 25A at 5V from this xformer. It will easily take 50 feet of #14 enamelled wire, with room to spare.

Ed

Reply to
ehsjr
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Richard the Dreaded Librarian ;-) has written about MOT rewinding here in sed:

MOT hacking

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Boy, do I feel stupid!

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Cheers, James Arthur

Reply to
James Arthur

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You might consider using an old Powerstat or Variac (which are really toroidal transformers), and then add whatever number of turns you need through the donut hole. I have made transformers that put out 1000 amps continuous and over 10kA pulses by using one or two or four turns of bus bar as the secondary. You can also use copper pipe and run water through it for cooling. Even a burned up Powerstat can be repaired or even totally rewound, or you can get a complete kit from about 80 VA to 1.4 kVA from

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Toroids are notoriously quiet, small, and efficient. It's not too much hassle to wind a low voltage secondary. They are generally 0.2 to 0.8 volts per turn.

Paul

Reply to
Paul E. Schoen

A microwave oven? Considering that they often retail for $49.95 they will cut every corner they can. If non-copper windings save only one penny they'll do it.

What I usually try to get away with is snip some windings out of a transformer to reduce the dissipation in the linear regulator, not wind a new one. Ok, I have wound some transformers but I do not enjoy that. Good transformers will have their laminations in with alternating orientation. So the last 4-5 must be tapped into the pack with a wodden mallet. Often the last one or two just don't want to go in and then it's always knocking wood that it'll be enough and won't hum.

By the way 14AWG for 25 amps is, ahm, quite brazen ... watch for any "amperage stench" when drawing lots of current for more than a few minutes. When it ticks like a wood stove better turn it off ;-)

--
Regards, Joerg

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Reply to
Joerg

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Thanks! Good stuff. I had a vague recollection that someone here had done it - you nailed it. :-)

Ed

Reply to
ehsjr

These MOT transformers don't fall into the "good" category. The core is all E's in one direction with the I's across the open E's.

Naw - I want to run the #14 windings in parallel to share the current. I'm hoping for 4 windings, 18 turns each, at a bit over 6 amps each - about 72 feet of #14 to fit on the core. Otherwise, I'll have to come to you to ask for sound effects of a melting transformer. :-)

I'm also looking to wind another ~ 25 turns in series with the primary, so I need about 100 feet of #14. As I struggle to get all that wire wound into the existing core openings, I will _not_ need any sound effects. I'll supply those myself.

Ed

Reply to
ehsjr

Be careful. Many of those things are economized down to bare minimums. Just enough to survive 3-4 Redenbacher popcorn bags in a row. Like old Muntz used to do. "Still no amperage smell after 10 minutes? Let's take the primary down another couple of turns and run that test again."

You probably mean the sound of sirens and fire engine horns, wheel chocks being thrown onto the asphalt, people yelling, hoses being dragged up the driveway ... ;-)

That might even be needed. I've seen transformers of that ilk that drew an impressive current without any load on the secondary.

--
Regards, Joerg

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Reply to
Joerg

s

ok this is tricky...If you are talking about delivering 25 Amps DC at the output, then the RMS current in the secondary may have to carry can be as high as 2x that. It depends on the type of input filter on the rectifier. If you use a big C, the xformer and rectifer will deliver the current in pulses, and to deliver 25 Amps average current, the instantaneous value of the pulses is a lot higher than 25 Amps. When you figure out the RMS value of the pulses (which determines how hot the wire will get) you will see its can be much higher than 25 Amps. This is the detailed part of designing a high current supply.

The situation is actually better if you use a smaller filter C so the current pulses are longer in time and lower in magnitude, but then you have more ripple on the output. This is not a simple problem.

Mark

Mark

Reply to
makolber

Yes I understand that. It won't be a problem *if* I can get four secondary windings on there, plus the extra 25 turn primary winding. The secondary resistance will be low - about 0.0113625 ohms if the wire tables are right. That's 4 windings at 18 feet of #14 copper all in parallel. Even at 45 amps, that's only ~ .54 volt drop, and the peak computes to 20.29, so ~ 17.29 after the rectifiers. So ripple needs to be less than 3.49 - 80,000 uF should do.

But that's all on paper. I don't know how the damn thing will perform, and it's really an experiment to get the most I can from a scrounged MOT. All I've been able to do so far is reduce the no load, no secondary winding current draw from

5.62 to 1.4 amps by temporarily winding 24 turns and putting them in series with the primary.

Ed

Reply to
ehsjr

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