We have disassembled a transformer from a microvawe oven by cutting the core, put a secondary winding suitable for our purposes (so transformer outputs 16VAC rms) and then tig-welded the core back. Now the core is heating up while the transformer sits with no load on the secondary. By heating up i mean that core gets too hot to touch in about an hour. The primary and secondary do not heat by themselves, i.e. they are cooler than the core.

What might be causing this? Are there any voodoo to fix it?

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    \$\begingroup\$ "Are there any voodoo to fix it?" Not if it's caused by voodoo in the first place. You can't use other voodoo to un-voodoo. It's one of the Laws of Thermodynamics, I think. \$\endgroup\$
    – stevenvh
    Aug 20, 2012 at 9:18
  • \$\begingroup\$ Measuring magnetisation current before and after would be interesting. In an ideal transformer the current will be almost 90 degrees out of phase with the voltage so what you get at rest is reactive magnetisation current with minimal power component. \$\endgroup\$
    – Russell McMahon
    Aug 20, 2012 at 9:46
  • \$\begingroup\$ As your output was presumably previously many hundreds of volts and your core probably gave you somewhere around a turn per volt- may be less but probably not vastly more worst case, it should be [tm] easy enough [tm] to cut off the secondary and wind the new secondary back through the available winding window. eg even at say 5 turns per volt the secondary would only be 80 turns. "Thread the needle" This is not so effective if the primary is buried under the secondary :-). \$\endgroup\$
    – Russell McMahon
    Aug 20, 2012 at 9:49
  • \$\begingroup\$ @Russell - Agreed, that would have been better than cutting the core. But if they want to use a microwave oven transformer it's probably because of its high power. Like 1000 VA or so. At 16 V that's good for a 60 A secondary. Even winding just 80 turns of a wire for that can be a chore :-(. \$\endgroup\$
    – stevenvh
    Aug 20, 2012 at 10:10
  • \$\begingroup\$ we needed 20A at 16V or so from it. I had to wind 19 windings to get that, so i made 3 parallel windings. But alas, the transformer probably was flawed by design as it's core already was welded before we cut it. \$\endgroup\$
    – miceuz
    Aug 20, 2012 at 13:04

3 Answers 3


Wait, you cut the core?

Well, congratulations, you have ruined/severely damaged it.

Transformers are made of lots of sheets of steel, with very thin insulating layers between them. This is to keep eddy-current losses from causing lots of heating, as you have discovered.

From wikipedia:

Ferromagnetic materials are also good conductors and a core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness.[53] Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies use laminated or similar cores.

Microwave transformers are normally somewhat lossy, as they are not operated for a significant period of time. A stock microwave transformer will get noticeably warm if sitting unloaded for a while. You have just increased the losses by many times, by shorting out the laminations.

There is nothing you can do with the transformer you have. You need to get another transformer, and not cut the core to remove the secondary. You have to remove the secondary without damaging or dinging the core significantly, and then wind your new secondary in place. by threading the wire through the core.

For what it's worth, microwave transformers run pretty warm without any load. Have you compared this transformer to another, without the core damage?

I'd be interested in some measurements of no-load power draw on the hacked-up transformer vs a stock one. That would let you measure the increase in losses due to eddy currents.

  • \$\begingroup\$ i'm aware of transformer construction using thin plates to avoid eddy currents, but originaly transformer was welded in exactly the same places we've cut it. Maybe it was welded using electrotechnical steel and we used simple steel rod. We will compare with another untouched microwave transformer. BTW, ampermeter shows somewat 2 amps going into the primary. \$\endgroup\$
    – miceuz
    Aug 20, 2012 at 9:21
  • \$\begingroup\$ @miceuz - It's less the weld, and more the mechanical cutting that I would expect to cause the problems. In addition to likely shorting across the entire surface of the core (the insulation is REALLY thin. Mechanical perturbations (such as sawing) likely would break it), you have introduced something of an air-gap. However, this does not seem to be the origin of your problems, as any increase in the core's reluctance would cause more primary magnetising current, which would make the primary get hot, not the core. \$\endgroup\$ Aug 20, 2012 at 9:49
  • \$\begingroup\$ As an aside, did you remove the shunts that are normally between the primary and secondary? They look like two insulation-wrapped little rectangles of lams. (At least for US microwaves. I don't know how 240V transformers would be different). \$\endgroup\$ Aug 20, 2012 at 9:52
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    \$\begingroup\$ Basically, what I think happened is that when you cut the core, the insulation on the lams on the surface where you cut likely got disturbed. The lam insulation is literally as thin as they can get away with, as thicker insulation means bigger windings, higher costs, etc. As such, you have effectively shorted across the entire bulk of the magnetic circuit. This is different then just a small seam-weld along the edge of the magnetic circuit, WRT eddy currents. \$\endgroup\$ Aug 20, 2012 at 9:55
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    \$\begingroup\$ @Fake - You're adding much here in comment, I think it would be interesting to add as an edit to your answer. (Not all users take the trouble to read a lot of comments.) \$\endgroup\$
    – stevenvh
    Aug 20, 2012 at 10:14

Microwave Oven Transformers (MOT) are generally poor candidates for other applications for a number of reasons:

  • They are designed to give high power output per cost so "cur corners" or push limits in design.

    • They "use their copper well" - ie they have higher than usual copper losses.

    • They use their iron well - ie they run the core "iron" well up its saturation curve and so have high core losses.

    • They think they come from Mote prime - They are designed to drive a capacitive load so they purposefully add a magnetic shunt between primary and secondary to provide purposeful leakage inductance to compensate for driving the target load.

They typically have about 1 turn per volt, maybe less. So a 16 VAC winding would probably be about 12 to 16 turns. If winding this in the space available is difficult (Copper crowbars are annoying to wind with) you may be able to build a winding or single or a few turns at a time and spot or other wise weld the windings together ! :-)

MOT video rebuild have only skimmed page and not watched video BUT it looks competent.

Excellent discussion, guidelines, limitations

They note:


  • Remove the shunts, by knocking them out carefully with a pin-punch. This improves the leakage inductance for "normal" transformer operation. In the space vacated by the shunts, wind a few extra primary turns, to reduce the primary turns per volt and hence core flux, and take the transformer out of saturation. This improves the magnetising current.

See shunts shown on photo below:

enter image description here


  • ... steps up wall voltage to around 2 kVAC, at power usually between 900 W and 1700 W. Be careful- these are not current limited!

    This is a non-ideal transformer whose purpose is to generate typically 1 kW of pulsed 5 kV DC into a magnetron, by driving a half-wave doubler.

    The turns ratio is designed to give about 2 kV AC to the main seconddary winding, one end of which is bonded to the grounded core. An additional secondary provides an isolated supply of typically 3 V at 15 A for the magnetron heater.

    As it is intended to drive a capacitive load, the leakage inductance of the tranformer is deliberately increased by adding a small magnetic shunt between the primary and secondary coils. The inductance is roughly equal and opposite to the doubler capacitance, and so reduces the output impedance of the doubler. This specified leakage inductance classifies the transformer as non-ideal.

    The transformer is designed to be as cheap to manufacture as possible, with no regard for efficiency. ... Thus the iron area is minimised which results in the core being taken well into saturation with result high core losses.

    The copper area is also minimised, resulting in high copper losses.
    The heat that these generate is handled by forced air cooling, usually by the same fan that is required to cool the magnetron. The core saturation is not part of the non-ideal classification, it is merely as a result of the economics of manufacture.

Found it walks funny but does not know why


I am looking for online answers for the same question. Because an MOT is built as cheaply as possible and forced air cooled, it may mean that all overheat if you just disassemble them, take out the secondary, then hook it up to a wall socket. You have to find a way to "push it to its design limits as a cost saving measure" less.

One way is a variac, which drops wall socket voltage from 120VAC to 80VAC or 60. But unless they are built for a high power, they may overheat too, moreover some modern electronic variacs may output a lot of high frequency harmonics which also cause overheating.

My first idea was just using a capacitor in series to limit the current, and roughly 300uF/160V motor start capacitors give you an 8 ohm reactance at 60Hz that would draw ~15A/120V from a wall socket, the max allowed by UL. But I don't have one handy, and the capacitor that comes inside the microwave is like 0.8uF.

So then I thought all you really need is extra reactance. One idea that naturally comes to mind like a lot of online responders answer is to wind more primary turns but that gives you oversaturation issues as mentioned above (because they are saving on iron too).

Note: at saturation the change in magnetic flux with increased current is zero, and there is no "reactance" generating opposing voltage past the saturation limit, the only thing holding back current flow is the resistivity of the copper in the primary winding, say you hit saturation at 110V by adding too many primary turns, then the leftover 10V to a 120V will generate current as if you applied DC 10V to the bare primary copper, which could be in the tens of amps, depending on the primary DC resistance.

So the best idea I'm coming up with as I write this, is to use inductance, but one separate from the iron core of the microwave transformer. So you basically just get a high power rated coil (maybe a motor or another transformer) which would act like a variac, and power your transformer at say 60V/60Hz, or 80V/60Hz. Also using a 2nd inductor in series is much better than a capacitor which risks creating a 60Hz resonant tank circuit with enormous currents, if you happen on the wrong L and C values, and there is no such risk with an inductor.

Obviously you could drop the voltage with an external nichrome wire from a hair dryer, but resistance wastes power, while reactance limits ac current flow without consuming power (other than having power factor issues, and large back and forth copper current due to poor power factor, for which the power company may or may not charge you for (industrial customers often pay a penalty for poor power factor, and they apply power factor correcting capacitor banks, or pfc motor/generators driven at the right speed and slip to make their inductance look like capacitance).

A flow of current +90 or -90 degrees out of phase with the voltage (capacitive or inductive load) consumes no power I.V.cos(phi), the generator motor at the power station would feel no extra load, if you had superconductors bring you the power from the power plant, and not aluminum and copper.)

But yeah, build your own custom "variac" power limiter with a single setting, usually this means find a suitable inductor such as a motor or transformer, and your whole rig would look like a step-down buck autotransformer. Now I gotta go hunt for such a thing too.

PS. I just measured the primary DC resistance on mine, and it was less than 000.4 ohms, which is below my meters accurate range, but yeah, it's down there, if you drive the core past saturation, it will gush a lot of current through the almost zero DC resistance copper.

10V DC through 0.4 ohms is 25 amps for the portion of the AC cycle past saturation (rms 110V to 120V, btw, actual voltage (sqrt2)/2=0.707 factor greater, 155V peak to 169V actual, meaning a single diode rectified capacitor will charge to the 169 DC peak voltage on a 120V AC rms (root mean square) power socket, not 120V, lot of people don't realize that and try to use a 150V DC rated one on 120VAC, in case you try to use capacitors), and may trip your 20A circuit breakers or fast blow fuses in the basement, depending on how fast they react.

So it's best not to wind more primary turns onto the same core, but limit the power input externally. (PWM motor speed controls might be another way, if you have a 120V PWM unit, other than harmonics heating issues, if they are issues, I haven't read up on that.)

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    \$\begingroup\$ This is very hard to read in its current form but seems to be more of running commentary with a few other questions thrown in. Maybe you could edit this to be more of a direct answer and ask a new question about the parts you're unsure of? \$\endgroup\$
    – PeterJ
    May 1, 2016 at 11:28

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