Tell me more ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.
up vote 3 down vote favorite

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?

share|improve this question
7  
"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. – stevenvh Aug 20 '12 at 9:18
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. – Russell McMahon Aug 20 '12 at 9:46
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 :-). – Russell McMahon Aug 20 '12 at 9:49
@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 :-(. – stevenvh Aug 20 '12 at 10:10
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. – miceuz Aug 20 '12 at 13:04

2 Answers

up vote 10 down vote accepted

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.

share|improve this answer
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. – miceuz Aug 20 '12 at 9:21
@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. – Connor Wolf Aug 20 '12 at 9:49
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). – Connor Wolf Aug 20 '12 at 9:52
1  
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. – Connor Wolf Aug 20 '12 at 9:55
1  
@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.) – stevenvh Aug 20 '12 at 10:14
show 6 more comments
up vote 3 down vote

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:

NB!!!:

  • 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

And

  • ... 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

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.