MOT rewinding

Discussion in 'General Electronics Chat' started by R!f@@, Oct 2, 2013.

  1. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
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    I have two MOT's. :D
    I believe one is from a 600W and the other is 900W Mag's.

    I have read a lot but still need a confirmation.

    Is it true that MOT core is saturated in normal operation ?

    I know I can cut the core apart but question is, can I rewind the secondary for my needs. Like 2 separate winding at 15-0-15 VAC at around 10 Amps each.

    If I can what will be the wire gauge for 10 Amps and how can I guess the turns ratio ?
     
  2. PackratKing

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    Jul 13, 2008
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    I take it, your acronym is Microwave Oven Transformer ?? Is it on a welded, interlaced " E and bar " frame ???

    If so, which approach do you intend to use to cut it open...

    Just slicing off one end , say on a bandsaw, raises cain with hysterisis and eddy currents...

    You have to do it the hard way, grind off the welds, loosen the first few leaves, doing as little damage as possible, but you can afford to lose maybe 2 sets...

    After you apply your new coil, putting the whole thing back together, is a real sloppy proposition, as you have to dip the leaves in thinned varnish to immobilize them... and clamp it squarely in a jig till it dries...

    All told, a load of fun :D done a few... 18 Ga. wire is good for 15 amps...
     
  3. R!f@@

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    I believe it is not interlaced. I can see the welding on both sides.

    Will it work, or do I need to rewind the primary too.
     
  4. PackratKing

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    By interlaced, I mean the " E's " are facing in opposite directions... look closer... Some do have all E's facing in one direction, with a laminate bar across the end... those are rare as snake feet...:D

    HELL no.... leave your primary intact if you can !!!

    Afterthought : What are you winding your new coil on ? It has to fit as tight to the frame as you can without damaging the magwire...

    Measure the cross-section of the center of the frame before cutting it open, and leave room for a
    bobbin of some sort, to make it easier to reassemble the frame.
     
    Last edited: Oct 2, 2013
  5. tinkerman

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    Jul 22, 2012
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    I've never looked at one if it's microwave you are asking about but I've done several from street light ballasts and they're fairly easy to come by. They were easy to rewind to whatever voltage output you need. They should be adequate for your 10 amp needs. The E cores all faced the same direction with a welded top. Grind through the welds with a disc grinder and you're set to go. The primary stays intact. When you're done weld the core back together. I've built growlers (for testing armatures) with these things too.
     
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  6. PackratKing

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    Thank you for that info... I never considered that source, :)
     
  7. R!f@@

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    Can u show me ?
    Any pictures so I know what u are talking about.
     
  8. shortbus

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    Usually they only open up the core when a new primary is needed. The secondary is usually the outside winding and can be cut off with a hacksaw or band saw. Then the new secondary is wound through the openings in the core.

    R!f@@, to figure out the number of windings needed for the new secondary, put 10 wraps on and power up the transformer. Measure the voltage you get, and do the math to figure how many more or less you need to get what you want.

    tinkerman, never thought of those for a base to wind on, thanks.
     
  9. gerty

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  10. MaxHeadRoom

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    MOT's are often fitted with a magnetic shunt bar through the core, if removing the windings it is usually recommended that this be punched out.
    Max.
     
  11. gerty

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    Here's a bad picture of one that we use. It's out of a real old microwave, the secondary has been cut off with a bandsaw. I tell the students what voltage I want on the secondary, they then calculate the turns ratio and rewind it. It's hard to see in the pic, but this student wanted to add a tap to the secondary. I told him I needed 18 volts, with a 12 volt tap. Voltage measured 19 with a 14 volt tap.
     
  12. R!f@@

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    Will the rewound MOT work like an ordinary power transformer ?
     
  13. gerty

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    Ours does, although we only load it to approximately 2 amps. Did you watch the video in post #9..
     
  14. tinkerman

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    Jul 22, 2012
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    Soon as I get a chance I'll load some pics. I'll see what I can find for old ballasts too. Not that it's worth shipping anywhere. Just for pics.
     
  15. pilko

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  16. tinkerman

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    I believe that should be written volts per turn not turns per volt. So that would be 1.2 volts/turn. It's also very easy to find what that number is. Just wrap a few turns around the core, apply primary voltage and read the output. Two turns would give you 2.4 volts.

    If the secondary will require quite a number of turns it's much easier to take a small disc grinder and cut through the welds as can be seen in the first picture. Then the coil form can be placed on a spindle and turns easily wound. If you wish to get fancier and make the job much neater and easier build a winding jig with a mechanical counter.
     
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  17. R!f@@

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    So a MOT wastes a lot of energy due to more iron in the core.! Eh!

    Would it need to be forced cooled if I plan to make a PSU out of it.

    I am thinking of rewinding it to make a dual 0-30V 10A PSU.
     
  18. doug08

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  19. Alec_t

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    You will almost certainly need forced cooling if you are trying to pull 600W out of it.
     
  20. richard.cs

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    Mar 3, 2012
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    All the ones I've ever taken apart (about a dozen) are not interleved. The best way is to carefully make a shallow hacksaw cut most of the way through the weld at both ends of the I, then clamp the core firmly in a vice and use a wide cold-chisel to separate the I from the E. Usually the I remains together but sometimes the laminations break into a couple of chunks. After rewinding I clamp back together for testing and when I'm happy I reweld, the weld is always really messy but they work just fine.

    With the E and I sections separated the windings and the magnetic shunts can be removed. I often end up removing all the windings then tapping the primary back down onto the E so it is the first winding (normally it has to be removed to get the secondary off). The best way to remove the primary is often to drive wedges into the magnetic shunts between the laminations. Rewinding is best done by making up a stiff cardboard bobbin.

    These transformers are intended to be run at full load all the time and therefore run with a very high flux density, the magnetising current and core losses are both large but it allows the transformer to be smaller for a given power output. The efficiency will be very low if run with a small load (unless extra turns are added to the primary). Note that the primary is nearly always wound with aluminium wire (with copper-coloured varnish).
     
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