Tranformer Tear Down, To do it or not to do it?

[MrAl]

Hi,

I have a transformer made for 120vac 60Hz input, and it is made for a microwave oven.  It would be rated for about 1000 watts or maybe even 1500 watts, so it's a pretty decent size.

At 120vac it would put out something like 1500vac or higher.  I think that would be great for high pot testing.

But here's the dilemma.  Originally i was going to remove the high voltage secondary and replace it with a low voltage high current secondary, like 15v at 30 or more amps, although 30 would probably do it.  Then i started to think about the high pot testing.  If i remove the secondary i ruin it forever for high pot testing.  But then again the high pot testing doesnt require anything near 1000 watts, if anything, 1 watt probably, so it would sort of be a waste for that.  If i rewind it for 15v output (or near that) then i can use it for a very nice high current low voltage power supply for all kinds of applications i normally do, and a transformer like this would cost around 100 dollars (USD) or more.

So what are the opinions here, tear off the secondary and build the transformer of my dreams, or keep it as is and use it for only high pot (and possibly leakage current) testing?

I think it will be interesting to hear other peoples ideas about this, thanks.

[malch]

Good one! 

[Richard Crowley]

There are dozens (or maybe hundreds?) of videos on YouTube about re-purposing or re-winding Microwave Oven Transformers (MOT).

[malch]

There are dozens (or maybe hundreds?) of videos on YouTube about re-purposing or re-winding Microwave Oven Transformers (MOT).

For high pot (and possibly leakage current) testing?

[Smokey]

HiPot is essentially measuring insulation.  You want as little current as possible.  If things pass HiPot then the current meter should read damn near 0A.  You want to be able to check the insulation withstand voltage without frying everything.  If you want to measure HiPot, get a HiPot meter with proper current limit and measurement capabilities.  To use an unregulated secondary from a microwave transformer designed to put out 1000W for HiPot is a little crazy.

http://en.wikipedia.org/wiki/Hipot
http://electrical-engineering-portal.com/what-is-hipot-testing-dielectric-strength-test
http://www.megger.com/us/products/ProductDetailsBySubGroup.php?PSGC=I106&BS=I

[helius]

There are some nice webpages/videos out there that use repurposed MOT transformers for sputtering thin films inside a vacuum chamber.

[Chris Jones]

I would not use a microwave oven transformer for hipot testing. Those are one of the few household devices that really are quite likely to kill someone. It is unnecessarily dangerous and would cause unnecessary damage to items that fail the test.

It would be better to choose a HV source that is not capable of supplying enough current to be lethal, (or at least not enough to be probably lethal to a healthy person). If you can current limit it to the permitted maximum leakage of consumer electronics (2mA? - better look it up) then that would be nice. If it has to be something reclaimed from your junk pile, I would suggest the EHT transformer (LOPT) from a CRT television or monitor, under-run with power limiting on the input side and using the focus divider to allow you to regulate the output voltage.

[Zero999]

If you want to use the microwave oven transformer for high pot testing then for goodness sake add a suitable current limiting resistor which is rated for the voltage, plus a safety factor.

Why are you worried about ruining it? A used transformer can be salvaged from a scrap microwave for a fraction of the cost of a new one.

[MrAl]

Hello again,


I guess i did not expect people to actually believe i would use the output DIRECTLY for high pot testing or similar high voltage testing, but would expect that i would know enough to use at least a resistor of some appropriate value to lower the maximum current.  And that is where part of the dilemma comes into play, because using a 1 meg ohm 10 watt resistor for example would be limiting the current to around 1.5ma, when the secondary is capable of around 1 amp at 1500 volts.  So you see how the waste factor comes in.  Such a nice big transformer but used at 1.5ma seems like such a waste.  That's why i mentioned the 1 watt in my first post.  Maybe 2 watts, but using a 1500 watt transformer to supply 2 watts at most seems like such a huge waste.  It's too bad that the secondary window is almost full already or i could have made it multipurpose.  Of course rewinding for lower current and still high voltage and also a winding for high current and low voltage is an option, but i really didnt feel like winding that many turns on the secondary either.

Maybe the better idea would be to try to acquire another smaller transformer for high pot testing.  But i still hate having to ruin the original transformer.  But otherwise it just sits there doing nothing :-)

[nickds1]

Please note that the HV secondary on a MOT tends to have one end of the secondary connected to the core - it is NOT safe to simply disconnect this from the core and then earth the core - the insulation between the inner layer of the secondary & the primary is not designed to take a full 2000VAC PD. Please just don't go there...

MOTs are designed and built to a price - they are NOT general purpose transformers and will kill you in an instant. They are also NOT designed for continuous running - typically only a few minutes at a time - they are very inefficient.

Having said that, many people remove the secondary & shunts, put on a heavy new secondary, and make something fun like a spot-welder with it

Modern microwave ovens use electronic transformers - not nearly so much fun, but far far cheaper to make, so stock up on the old MOTs whilst you can...

[Smokey]

....They are also NOT designed for continuous running - typically only a few minutes at a time - they are very inefficient....

I'm not sure about that.  People cook turkeys in the microwave, running it for a really long time like hours. 
http://mashable.com/2014/11/25/cook-a-turkey-in-the-microwave/

[nickds1]

....They are also NOT designed for continuous running - typically only a few minutes at a time - they are very inefficient....

I'm not sure about that.  People cook turkeys in the microwave, running it for a really long time like hours. 
http://mashable.com/2014/11/25/cook-a-turkey-in-the-microwave/

Read the article - the turkey is cooked at only 50% power....

[malch]

So you use a hipot tester or any other tester continuously?

[Smokey]

....They are also NOT designed for continuous running - typically only a few minutes at a time - they are very inefficient....

I'm not sure about that.  People cook turkeys in the microwave, running it for a really long time like hours. 
http://mashable.com/2014/11/25/cook-a-turkey-in-the-microwave/

Read the article - the turkey is cooked at only 50% power....

So 500W continuously for 2 hours?  Is that low power to you?

[DanielS]

Read the article - the turkey is cooked at only 50% power....

Microwave energy only heats the surface, so the main reason to limit power to 50% is likely to let heat soak in. Otherwise, the surface would be over-cooked while the middle would still be close to raw at 100%.

I rarely set my microwave to more than 70% for (re-)heating food because of that. If I re-heat lasagna at 100%, the outside is scalding hot while the middle is barely warmer than fridge temperature.

[DanielS]

So 500W continuously for 2 hours?  Is that low power to you?

Most magnetrons do not work with variable power. They are either on or off and you get "50% power" by running them at 100% power 50% of the time. Duty-cycle modulation.

If you use an iron-core MOT, you can hear the MOT relay click on and off periodically every few seconds. For inverter-based MOTs, you might hear the fan noise's pitch and interior light brightness change periodically, just as you should with the iron-core one.

[calexanian]

HI POT testing is a thing all to itself. What are you really wanting to test. Breakdown testing? Leakage testing? Second hand HI POT testers are all over ebay for very reasonable amounts. So reasonable it is not worth risking your life for a couple hundred bucks. If you cant afford that then you have no business doing anything that will require a HI POT test.

[MrAl]

Hi,

Some interesting replies here.

I liked #9 because it talks about the insulation between primary and secondary breaking down.  that's something i would worry about too if the transformer was built with the secondary wound right on top of the primary, but this transformer has physically separated windings each in a different place on the center leg.  This is typical for safety in other transformers too where the physical separation is almost a guarantee that they wont ever short together.  But yes, the good point of post #9 is that we might be concerned about the voltage between the secondary and the core metal.  We dont want to disconnect the wire from the core and then connect the other wire for example.

In #16 i see the author relates cost to utility:  if you cant afford something then you have no business using it.  Sorry, but i cant agree with that as the two are not that  tightly related.

When i worked in the field, we designed high powered equipment that makes this look like frying pancakes.  We went up to 30000 watts three phase 240v and even 480v systems, 240 being much more common though.  1500 watts would be an afternoon walk in the park.  I've seen 10 inch high electrolytic caps blow up like bombs, high power transistors blow their tops after melting the metal cases spewing hot molten metal all about the lab.
With my 40 or more years experience in the field, i either know what safety is already or im already dead and just dont know it yet.

Still, it's good to be reminded so thanks for that.

My main concern is what is the best use for this product.  I intend to use it safely no matter how i do it and i know very well how to be safe with it.  So i think we can get past the safety rants now and maybe concentrate on the uses.

I've read about the welder too and that sounds interesting.  I dont think i need one right now though so im not sure about that yet.  I can use a high current low voltage power supply so maybe im better off with that and then maybe all the grannies here will be happy too (just kidding grannies) :-)

Seriously though, a 30 or 40 amp power supply would really be nice.  I dont need if for long operating periods either, as one person brought up the continuous operation vs non continuous issue, which was good.  I could also use a variac on the input side to lower the input power also, because i did notice that the design of this thing is such that the core is operating near saturation, or what we might call underdesigned.  A lower input voltage will take care of that problem too.

Thanks for all the interesting ideas and comments.  Safety is always a concern and anyone else reading this should be very careful too.

[nickds1]

I liked #9 because it talks about the insulation between primary and secondary breaking down.  that's something i would worry about too if the transformer was built with the secondary wound right on top of the primary, but this transformer has physically separated windings each in a different place on the center leg.  This is typical for safety in other transformers too where the physical separation is almost a guarantee that they wont ever short together.  But yes, the good point of post #9 is that we might be concerned about the voltage between the secondary and the core metal.  We dont want to disconnect the wire from the core and then connect the other wire for example.

Even if the primary & secondary are separate (there are two secondaries, BTW - heater & HT), the secondary is still wound close to a lump of metal at 0V/GND and its insulation is not designed to take 2000VAC on the inner windings, hence the inner tap being taken to the core...

[justin66]

if you need both of thiese find an other MO in scrap and simply rewind one and keep an other for your pusposes

[SeanB]

If you are going to use these transformers for long periods of operation ( over 2 minutes) then you do need to have active cooling, using a fan to blow cool air over the windings and core, and use a chassis as added heat sink area. The microwave always has a fan blowing over the transformer, along with an air duct cooling the magnetron. Half the power wasted as heat is in the magnetron, the other half is the transformer iron and copper losses. The transformers are designed to run hot, with high temperature insulation, to cut cost and mass.

You can run a 220V transformer on 110V with absolutely no cooling other than natural convection, at a load that draws 5A on the primary winding, just from the lower losses in the core.

[MrAl]

Hi,

Some good ideas.

Yes, the voltage from the winding to the core is always something to think about.  If the closest winding connected to the core already then leave it that way.  Good idea.

And yes, finding another small transformer would cover the high pot tester if i decided to do that too.

And also yes, a fan is a good idea if the unit will be getting hot.  Luckily i dont need the full 1500 watts, maybe up to 500 tops for the low voltage high current power supply, and for short time periods for testing other things.

Thanks again.