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microwave oven transformers - welds across the laminations




 
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Author Topic: microwave oven transformers - welds across the laminations  (Read 1401 times)
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Patrick J. / KD5OEI
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« on: March 07, 2023, 11:12:39 AM »

Do the bead welds cause eddy currents an make the transformer less efficient?

Is their purpose just to avoid the use of costly bolts to hold the cores together?

I notice other power transformers don't have them.

When using a microwave oven transformer I take out the magnetic shunts, but the beads can't be removed.



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« Reply #1 on: March 08, 2023, 03:03:18 PM »

several reasons outlined here: https://www.electro-tech-online.com/threads/why-microwave-oven-transformer-is-welded.148678/

I never noticed or knew that all the I and E laminations were stacked in the same direction on a MOT.

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« Reply #2 on: March 09, 2023, 08:21:34 PM »

Wow that's a good discussion on the site - cleared up the facts well, and other stuff.

I had no idea aluminum windings is a thing on the cheapies (according to participants).
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« Reply #3 on: March 18, 2023, 12:24:37 PM »

MOT is a highly specialized / cheaply designed transformer.  To use one as a hv amplifier power supply is an exercise in clever tricks to get a good design.

first note that the secondary usually has little insulation from one end of the winding to ground because that winding end is connected to ground.  bridge rectifier schemes are out unless you are willing to float the transformer case to 1/2 B+ value.  Dangerous!

secondly.  If you can find 2 identical MOT then full wave center tap and voltage multiplying variants become more feasible.  there is more ....

a test needs to be run on a MOT being considered.  with no secondary load remove the magnetic shunts and using a variac excite the primary while monitoring current draw.   a decent MOT will draw a small current until approaching 85 Volts or so and then rapidly draw more current and start humming as we are used to hearing while zapping our burritos, etc.  this shows how unsuitable this design is for continuously generating hV while standing by.  

there is a possible way out.   the second MOT mentioned (identical).   try this:   place the primary windings in series to provide 60 Vac to each MOT.  use a stacked topology for the secondary (hwct or multiplier)
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« Reply #4 on: March 18, 2023, 06:05:35 PM »

secondary close to ground or grounded: A friend ran a dozen of them with series secondaries in a big tub of oil. The primaries were in series parallel and connected to 240V with automotive jumper cables. For a ballast, he used several old toasters in parallel on each side of the 240.
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« Reply #5 on: March 20, 2023, 09:17:27 AM »

secondary close to ground or grounded: A friend ran a dozen of them with series secondaries in a big tub of oil. The primaries were in series parallel and connected to 240V with automotive jumper cables. For a ballast, he used several old toasters in parallel on each side of the 240.

Now that's Texas sized!

I have been considering this a bit more.  by placing the MOT primaries in series will put a constraint on the secondary rectification circuits; that is, both windings need to be loaded during both positive and negative swings.  proof of this can be found by scope monitoring of primary center tap.  the most desirable condition would be:  Vac /2 = 60V with no variation due to changes in secondary current draw. 
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« Reply #6 on: March 20, 2023, 11:24:03 AM »

I would like to find a way to draw skizmatics on here.....

I have preliminary full wave doubler circuit.  it will use 2 MOT secondaries with one end grounded on each - 4 diodes and 3 capacitors (understood that each component may require more parts in series and voltage equalizing resistors to meet spec)

if any interest will provide ...
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« Reply #7 on: March 21, 2023, 07:28:58 AM »

Iíll be watching for your results.  I have 2 identical old copper-wound 10lb MOTís awaiting a peoject.  With all the reading above it seems I may have 20lbs of primo scrap.
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« Reply #8 on: March 21, 2023, 01:23:51 PM »



Knowing that this website is viewed by non hams, wannabees, etc., the following video has a bit of information regarding the microwave oven transformer.

The most deadly project on the Internet

https://www.youtube.com/watch?v=FBeSKL9zVro

I found the body count somewhat interesting.

klc
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« Reply #9 on: March 21, 2023, 02:48:18 PM »

could not agree more ....   SAFETY First
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« Reply #10 on: March 21, 2023, 10:34:44 PM »

I agree on safety.

I would like to see the schematic.

I guessed at it, having to keep currents equal and one end of secondary grounded. C1 might get hot with lots of current. 

The schematic's ugly but I just drew it in Paint program with copy/paste of drawn symbols from an old free bitmap or PNG file that is just a collection of symbols.


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« Reply #11 on: March 22, 2023, 10:06:03 AM »

good job Patrick  Cheesy.  that's the one  

now to properly phase the MOTs

without the rectifiers and capacitors connected but the secondary winding grounds connected- supply a low voltage ac with a means of monitoring current.  with nothing connected to hv end the MOT primaries can be connected either phase in series with no effect. this of course assumes no magnetic coupling between MOTs.    identical MOTs should    
show same output voltage.  to get proper phasing such that the rectifier filter will operate as a full wave ckt the MOT outputs need to be 180 degrees out of phase.  scope monitoring will show when this is right.  be sure to use appropriate hv scope probes!

your warning on C1 are appropriate.  use low ESR filter rated caps.
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« Reply #12 on: March 22, 2023, 11:19:18 AM »

I assume the magnetic shunts need to be removed, and they are acoustically noisey under load. Are the foregoing true?

That being said, I want to model the circuit using LTC spice to look at regulation. I want to examine using caps from defibrullators (very low ESR). Does anyone have a DC resistance reading for a MOT primariy and secondary?

Are there magnetic effects that would affect regulation? That would be hard to model.
TNX, Rod
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« Reply #13 on: March 22, 2023, 03:02:25 PM »

good questions, Rod.   yes the shunts should come out.  I am thinking that the smallish core coupled with lack of EI interleaved laminations vs the power levels the MOT runs at speak to me initially of saturated cores with short thermal duty cycle.  I assume as the load increases that the output voltage will sag beyond a certain point.  this could indicate onset of core saturation.

dc winding resistance is variable according to mfgr.  I recall measuring one maybe 20 yrs ago as 0.8 Ohm primary and 35 Ohms secondary.  the leakage reactance portion of each winding may be the most difficult to model in a lossy MOT.   typicals are in the 50 mH range.

Tron offered to the group a while back an interesting figure for a 'good' EI transformer as 10W per pound weight
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