Author Topic: Rewinding transformer  (Read 946 times)

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Offline icharters

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Rewinding transformer
« on: June 24, 2019, 03:47:50 am »
I accidentally bought a 220V hot air station with an EU plug and live in NA.  I've pulled the transformer out and I want to attempt to rewind it, but have a few questions.

The transformer is 220v in, and 31v and 11v out.

Can I just half the number of primary winds as I'm going from 220v to 110v and maintain the same ratios and output?
Can I use electrical tape, or do I have to use Kapton or "margin tape"?  Also, what kind of compound should I use to "glue" the block back together?
Can I just cut the tape and attempt to unwind it without dismantling the block, and then re-tape it?

Thanks.

« Last Edit: June 24, 2019, 03:56:22 am by icharters »
 

Offline bob91343

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Re: Rewinding transformer
« Reply #1 on: June 24, 2019, 04:12:42 am »
The answer is a resounding 'it depends'.

First you need to decide if you want to connect such an item to your power line.  In case of an insurance claim, a homemade transformer just might invalidate the chance for collecting.

Having said that, the technical answer is that you can indeed rewind the primary with one half the original number of turns provided you increase the wire size due to the fact that the current will double.  Most transformers have the primary wound closest to the core so you may end up stripping all of it.

A better solution is to step up the 120 that you have available into 240 and run the machine from that.  Any isolation transformer of sufficient rating will work; you add the output to the input and, if you get the phase correct, you have your 240 volts.  This is, I believe, the best solution.  It involves minimal effort and risk but of course the downside is you will have this extra transformer dangling.

There are other ways but this last one is the cleanest and easiest.
 

Online bdunham7

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Re: Rewinding transformer
« Reply #2 on: June 24, 2019, 04:20:14 am »
What hot air station and how many watts is this?  Are you saying it uses a linear power supply with a 220 volt 60 Hz standard e-core or toroid transformer?  Or am I misunderstanding something?  Most hot-air stations use a SMPS and the transformer is fairly small.
 

Offline icharters

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Re: Rewinding transformer
« Reply #3 on: June 24, 2019, 04:28:29 am »
The answer is a resounding 'it depends'.

First you need to decide if you want to connect such an item to your power line.  In case of an insurance claim, a homemade transformer just might invalidate the chance for collecting.

Having said that, the technical answer is that you can indeed rewind the primary with one half the original number of turns provided you increase the wire size due to the fact that the current will double.  Most transformers have the primary wound closest to the core so you may end up stripping all of it.

A better solution is to step up the 120 that you have available into 240 and run the machine from that.  Any isolation transformer of sufficient rating will work; you add the output to the input and, if you get the phase correct, you have your 240 volts.  This is, I believe, the best solution.  It involves minimal effort and risk but of course the downside is you will have this extra transformer dangling.

There are other ways but this last one is the cleanest and easiest.

I would prefer to convert this unit rather than adding a hack to get the desired input.  This is more of a learning exercise than anything at this point.  If I have to increase the gauge of the primary winding, maybe I should just buy a transformer kit or something and replace the entire thing.

What hot air station and how many watts is this?  Are you saying it uses a linear power supply with a 220 volt 60 Hz standard e-core or toroid transformer?  Or am I misunderstanding something?  Most hot-air stations use a SMPS and the transformer is fairly small.

It's a Chinese 858D clone.  220v input to an e-core transformer.
 

Online bdunham7

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Re: Rewinding transformer
« Reply #4 on: June 24, 2019, 04:58:40 am »
OK, to answer your original question in general, you would rewind the primary with wire 40% larger in diameter and use 55% as many windings, give or take a few.  I don't think regular electrical tape would be appropriate, something stiffer and higher temp such as you mentioned.  I've never glued a transformer back together and I'm not sure how you will get it apart, but I would try a potting epoxy.  Rewinding in place sounds difficult, but I'm still not sure what your transformer looks at.

I have an 858D clone (SumSour to be exact) and I only see one very small transformer inside it.  Taking a close look for the first time, I see no voltage or power spec labels anywhere on the unit!  And it isn't clear to me that the little transformer is a mains unit.  I can't imagine rewinding it unless you are a jeweler or something.  Could you post a photo to see if your unit is the same inside?  And tell us how you got those voltage specs you listed. 
 

Offline icharters

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Re: Rewinding transformer
« Reply #5 on: June 24, 2019, 05:13:02 am »
Label


Where I desoldered from the board is also marked
 

Online bdunham7

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Re: Rewinding transformer
« Reply #6 on: June 24, 2019, 05:21:10 am »
How big is it?  Does it supply all the power for the unit or is there another mains connection to the circuit board?  Your unit is entirely different than mine!
 

Offline icharters

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Re: Rewinding transformer
« Reply #7 on: June 24, 2019, 05:31:39 am »
It's quite small.  The mains come in and wire in series through a fuse and front panel switch and then onto the on a header on the PCB.  Those red leads actually hooked into that header.  I'm not sure why it feeds into the header and back out.  I've tested the transformer with a 16.5v ac wall adapter (actual output was 17.65v) feeding into the input and got 2.465v on the 31v wires and 0.875v on the 11v wires, so that header has to be passing 220v straight back into the transformer to be getting 11v and 31v unless I'm missing something.

« Last Edit: June 24, 2019, 05:33:24 am by icharters »
 

Online james_s

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Re: Rewinding transformer
« Reply #8 on: June 24, 2019, 05:38:38 am »
I've never had much luck getting a transormer like that apart without destroying it but you can try. You will need to unwind the whole primary and wind a new one using thicker wire since the primary current will be double to get the same power with half the voltage. When you put it back together the correct thing to glue it with is transformer varnish. When transformers are made, they are typically dipped in varnish, the better ones do this in a vacuum chamber then feed them current limited DC to warm it up and cure the impregnated varnish. Cheap transformers often skip this step.
 

Offline icharters

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Re: Rewinding transformer
« Reply #9 on: June 24, 2019, 05:51:24 am »
I've never had much luck getting a transormer like that apart without destroying it but you can try. You will need to unwind the whole primary and wind a new one using thicker wire since the primary current will be double to get the same power with half the voltage. When you put it back together the correct thing to glue it with is transformer varnish. When transformers are made, they are typically dipped in varnish, the better ones do this in a vacuum chamber then feed them current limited DC to warm it up and cure the impregnated varnish. Cheap transformers often skip this step.

It seems like it would be very difficult to disassemble without mangling - that's why I was thinking if I can cut the tape and rewrap it and retape it while it's still in the block that would be ideal.
 

Offline techman-001

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Re: Rewinding transformer
« Reply #10 on: June 24, 2019, 06:20:59 am »
I accidentally bought a 220V hot air station with an EU plug and live in NA.  I've pulled the transformer out and I want to attempt to rewind it, but have a few questions.

The transformer is 220v in, and 31v and 11v out.

Can I just half the number of primary winds as I'm going from 220v to 110v and maintain the same ratios and output?
Can I use electrical tape, or do I have to use Kapton or "margin tape"?  Also, what kind of compound should I use to "glue" the block back together?
Can I just cut the tape and attempt to unwind it without dismantling the block, and then re-tape it?

Thanks.

This is what I'd do:
1) heat the transformer up until whatever is gluing the laminations together softens enough to pull them out. You can heat it up by passing dc thru a winding, just be careful not to melt the plastic former or you'll need to make a new one, cardboard etc will do.
2) Remove the Kapton tape and  unwind the primary. The windings look to be split, primary and secondary so it should be easy. What won't be so easy is collecting the primary wire so you can reuse it by making two windings with the same wire to provide half the previous turns and twice the wire diameter. Both windings go on together and are in the same direction. One way is to run out the primary then find the middle and make that the start of the winding.
3) They used to drill each corner of the laminations and bolt them so no glue was then needed. This made them very easy to take apart. Bolting them may hold it all together nicely if the laminations are a bit ratty after being pulled out of the transformer.

Once it's finished and tested, I'd normally heat each winding and apply two part high temperature epoxy (90C) to seal the windings and make them super robust but a few layers of Kapton tape will probably be enough.

 
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Offline icharters

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Re: Rewinding transformer
« Reply #11 on: June 24, 2019, 02:33:15 pm »
I accidentally bought a 220V hot air station with an EU plug and live in NA.  I've pulled the transformer out and I want to attempt to rewind it, but have a few questions.

The transformer is 220v in, and 31v and 11v out.

Can I just half the number of primary winds as I'm going from 220v to 110v and maintain the same ratios and output?
Can I use electrical tape, or do I have to use Kapton or "margin tape"?  Also, what kind of compound should I use to "glue" the block back together?
Can I just cut the tape and attempt to unwind it without dismantling the block, and then re-tape it?

Thanks.

This is what I'd do:
1) heat the transformer up until whatever is gluing the laminations together softens enough to pull them out. You can heat it up by passing dc thru a winding, just be careful not to melt the plastic former or you'll need to make a new one, cardboard etc will do.
2) Remove the Kapton tape and  unwind the primary. The windings look to be split, primary and secondary so it should be easy. What won't be so easy is collecting the primary wire so you can reuse it by making two windings with the same wire to provide half the previous turns and twice the wire diameter. Both windings go on together and are in the same direction. One way is to run out the primary then find the middle and make that the start of the winding.
3) They used to drill each corner of the laminations and bolt them so no glue was then needed. This made them very easy to take apart. Bolting them may hold it all together nicely if the laminations are a bit ratty after being pulled out of the transformer.

Once it's finished and tested, I'd normally heat each winding and apply two part high temperature epoxy (90C) to seal the windings and make them super robust but a few layers of Kapton tape will probably be enough.



Awesome.  Thanks for the advice.  I'm going to do this when I get back from work today. :-+  I'll let you know how it goes.
 

Online james_s

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Re: Rewinding transformer
« Reply #12 on: June 24, 2019, 04:24:17 pm »
Kapton tape is not ideal because it isn't stretchy. The tape I used is I think polyester, it is slightly stretchy so you can wrap it tightly and it is tolerant of the temperatures involved. Transformers do not typically get hot enough to need kapton.
 

Offline icharters

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Re: Rewinding transformer
« Reply #13 on: June 25, 2019, 02:59:56 am »
This is not coming apart very nicely and suspect it's going to go back together even worse.  I think I'm going to try to find some sort of kit to make my own.  I can't find anything in 110v close to 31v/11v.
 

Offline tautech

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Re: Rewinding transformer
« Reply #14 on: June 25, 2019, 03:23:27 am »
Avid Rabid Hobbyist
 
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Offline techman-001

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Re: Rewinding transformer
« Reply #15 on: June 25, 2019, 03:43:03 am »
Here's some good guidance:
https://ludens.cl/Electron/trafos/trafos.html

That is one awesome transformer winding link!

Offline icharters

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Re: Rewinding transformer
« Reply #16 on: June 25, 2019, 04:01:53 am »
Wow, what a wealth of information.  Thank you for the link.
 

Offline techman-001

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Re: Rewinding transformer
« Reply #17 on: June 25, 2019, 04:22:15 am »
This is not coming apart very nicely and suspect it's going to go back together even worse.  I think I'm going to try to find some sort of kit to make my own.  I can't find anything in 110v close to 31v/11v.

Oh dear!
They can be pretty nasty depending on the glue used, but once the first few laminations are removed the rest are much easier due to the extra room.

Try to resist screwdrivers (which only deform them) etc, you need a really thin blade, the kind that slices onto fingers and hands easily, so utmost care is also needed. A jig to push the 'E cores" out rather than a blade to separate them would also be handy and less life threatening.

A hammer and anvil may be useful to panel beat bent laminations.

Above all, try not to rush, and be patient:)


Offline lordvader88

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Re: Rewinding transformer
« Reply #18 on: June 25, 2019, 05:58:47 am »
Well it uses 240V u say, well what all would have to change run the thing off 120V ? Maybe it would be easier than taking apart a transformer.
 

Offline ggchab

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Re: Rewinding transformer
« Reply #19 on: June 25, 2019, 06:48:43 am »
The transformer is probably not the only problem: 110V might be too low for the heating element  :-\
 

Online james_s

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Re: Rewinding transformer
« Reply #20 on: June 25, 2019, 02:47:12 pm »
Isn't the element low voltage, powered by the transformer?

I've bought cores and bobbins from Bridgeport Magnetics on a few occasions, they offer some that have prewound primaries with the turn count listed so I just copied that with good success.

You might also find something off the shelf or surplus that is close enough to work.
 

Online bdunham7

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Re: Rewinding transformer
« Reply #21 on: June 25, 2019, 03:10:33 pm »
I'm not 100% sure this unit will run on 120VAC even with a proper transformer, but as for the transformer replacement, it seems that if a single transformer with the proper voltages can't be found, there is always the option of just using two transformers.  10 or 12 VAC and 28-32 VAC should suffice.
 

Offline icharters

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Re: Rewinding transformer
« Reply #22 on: June 26, 2019, 02:33:06 am »
This unit isn't heated from 110v.  It has two output voltages from the transformer, meaning the board gets 11v and 31v.  It should draw twice the current as I'm halving the input voltage, but maintaining the output voltages.  I'm going to just buy the individual parts and attempt to wind a new transformer, as I can't find any with 11 and 31v outputs.  Having a bit of trouble finding a kit at the moment.
« Last Edit: June 26, 2019, 02:44:38 am by icharters »
 

Online bdunham7

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Re: Rewinding transformer
« Reply #23 on: June 26, 2019, 02:54:16 am »
I thought the transformer was small.  You think they are running the entire 700 watts thru the 31V winding of the transformer?  If you post a photo of the guts of the unit, including both sides of the PCB, I'm sure we can point out how it actually works.  It probably has a triac, which is what I imagine mine has--I've never really checked.
 

Offline icharters

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Re: Rewinding transformer
« Reply #24 on: June 26, 2019, 03:02:58 am »
I thought the transformer was small.  You think they are running the entire 700 watts thru the 31V winding of the transformer?  If you post a photo of the guts of the unit, including both sides of the PCB, I'm sure we can point out how it actually works.  It probably has a triac, which is what I imagine mine has--I've never really checked.

You know what?  I think you might be onto something. Mains run series through a fuse and front panel switch, then into a PCB header.  It then returns 220v on 2 different wires from that header into the transformer which gives 11v and 31v which then run back to the board.  They could very well be using the 220v, which would make converting this to 110v not pan out the way I want to in the end.
« Last Edit: June 26, 2019, 03:42:35 am by icharters »
 


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