Remove Shunts In Microwave Oven Transformer

Thread Starter

MrAl

Joined Jun 17, 2014
6,599
Hello there,

I was wondering if anyone ever did this before, but i am sure some did, and that is the best way to remove the shunts.

I got as far as hacking off the 2kv secondary and removing all the turns, tested, and now i want to remove the two shunts for further testing.

When i took off the secondary i was very careful not to damage the primary because i dont want to have to rewind that. That's teh most important
point about doing this for me...dont hurt the primary.

What i see though is that the two shunts are mounted right on top of the secondary, and they are probably varnished in place. I am worried that if i knock out the two shunts (tapping them with a drift punch and hammer to push them out of the window) that the bottom edge of the shunt will pull the insulation off the primary and maybe also try to pull up the turns of wire too, possibly damaging the wire or the insulation on the wire itself.

So the big question is, did anyone here do this yet? If so, did you determine the best way to get them out without damaging the primary?
 

Thread Starter

MrAl

Joined Jun 17, 2014
6,599
There are lots of pages on www about doing this.
Hello,

And is that helpful?

I was hoping someone here would have done this too, and found some interesting way to remove them without hurting the primary. Not much on the web is said about being careful about the primary. Maybe it's not that big of a deal, but i want to be sure because i dont want to damage that winding.
 

Thread Starter

MrAl

Joined Jun 17, 2014
6,599
Below is a link to re-purposing MOTs. This suggests heating it up to soften the varnish.
http://www.mattsawesomestuff.com/Tutorials/MOT_Salvage.php

[EDIT] Even if you do damage the winding, getting defunct microwave ovens is fairly easy so you could have another go using the experience of your first attempt.

Hi,

Thanks. Well, i was hoping to get this one going as i like the size of the core. It's not as big as some of them. It's only around 800 watts worth while most of them ar at 1200 watts or above.
I read about heating it up, but was afraid that would hurt the primary too.
Eventually i'll have to do something because i cant use it with the shunts, at least not for everything i'd like to use it for.

These things might make a nice isolation transformer too, if i can wind enough turns. Not sure about that though as that's a lot of turns.
 

AlbertHall

Joined Jun 4, 2014
8,486
Hi,

Thanks. Well, i was hoping to get this one going as i like the size of the core. It's not as big as some of them. It's only around 800 watts worth while most of them ar at 1200 watts or above.
I read about heating it up, but was afraid that would hurt the primary too.
Eventually i'll have to do something because i cant use it with the shunts, at least not for everything i'd like to use it for.

These things might make a nice isolation transformer too, if i can wind enough turns. Not sure about that though as that's a lot of turns.
Yes, you can wind for low voltage without separating the parts of the core by feeding the wire through the slots but if you want a lot of turns then you would need to separate to core so you can remove the bobbin and then perhaps mount it on a spindle with a handle (or maybe a cordless drill which will run slow enough for coil winding).
 

MisterBill2

Joined Jan 23, 2018
3,962
Hello there,

I was wondering if anyone ever did this before, but i am sure some did, and that is the best way to remove the shunts.

I got as far as hacking off the 2kv secondary and removing all the turns, tested, and now i want to remove the two shunts for further testing.

When i took off the secondary i was very careful not to damage the primary because i dont want to have to rewind that. That's teh most important
point about doing this for me...dont hurt the primary.

What i see though is that the two shunts are mounted right on top of the secondary, and they are probably varnished in place. I am worried that if i knock out the two shunts (tapping them with a drift punch and hammer to push them out of the window) that the bottom edge of the shunt will pull the insulation off the primary and maybe also try to pull up the turns of wire too, possibly damaging the wire or the insulation on the wire itself.

So the big question is, did anyone here do this yet? If so, did you determine the best way to get them out without damaging the primary?
To avoid damaging that primary there are two things that you can do, but the first one takes a very steady hand. You have some access to the ends of those shunts, the trick is to use a drill bit to put a bit of a notch into the end of the shunt. Do that on both ends of the shunt, where you would be tapping it with a pin punch, not a drift punch. BUT before punching away, there should be a small amount of clearance between the shunt and the primary, and you can slice a thin bit of steel, possibly cut from a can, or something like that, as a shield for the winding. Then use the punch with the end set in the small drill impression, and very gently tap at a bit of an angle to push the shunt out and away from the primary. Just a bit at a time, alternating sides, with the idea always being to drive the shunt away from the primary. Slow and patient is the plan.
 

Thread Starter

MrAl

Joined Jun 17, 2014
6,599
Yes, you can wind for low voltage without separating the parts of the core by feeding the wire through the slots but if you want a lot of turns then you would need to separate to core so you can remove the bobbin and then perhaps mount it on a spindle with a handle (or maybe a cordless drill which will run slow enough for coil winding).
Hi again,

Did you ever try winding small segments, then connecting them later end to end?
I was thinking one way around it might be to wind several windings, then connect them end to end using a good procedure for that like twist and solder or even weld. That would also give me several 'taps'.
 

Thread Starter

MrAl

Joined Jun 17, 2014
6,599
To avoid damaging that primary there are two things that you can do, but the first one takes a very steady hand. You have some access to the ends of those shunts, the trick is to use a drill bit to put a bit of a notch into the end of the shunt. Do that on both ends of the shunt, where you would be tapping it with a pin punch, not a drift punch. BUT before punching away, there should be a small amount of clearance between the shunt and the primary, and you can slice a thin bit of steel, possibly cut from a can, or something like that, as a shield for the winding. Then use the punch with the end set in the small drill impression, and very gently tap at a bit of an angle to push the shunt out and away from the primary. Just a bit at a time, alternating sides, with the idea always being to drive the shunt away from the primary. Slow and patient is the plan.
Hi,

Unfortunately there is no room between the shunt and the primary, that is what worried me. If there was a distance there then i would not worry about removing it, but it is 'glued' to the insulation that is around it, and the insulation around it is glued to the insulation around the primary, and so it may mess up the primary.
What i wonder now that i've read your post though is if the shunts would 'peel' up, like an orange skin, one lamination at a time. I might be able to start with the top lamination and peel it, then the next layer. etc.

I took an old transformer apart way back in the 1970's but that did not have shunts, and i actually peeled the laminations apart. Made a big mess though and the laminations where bent up a bit so i ended up using several transformers to make up the one i had to replace. It was for an oscilloscope. That was the last time i had to take one apart :)

I am too used to using fresh new laminations i guess :)
 

MisterBill2

Joined Jan 23, 2018
3,962
Hi,

Unfortunately there is no room between the shunt and the primary, that is what worried me. If there was a distance there then i would not worry about removing it, but it is 'glued' to the insulation that is around it, and the insulation around it is glued to the insulation around the primary, and so it may mess up the primary.
What i wonder now that i've read your post though is if the shunts would 'peel' up, like an orange skin, one lamination at a time. I might be able to start with the top lamination and peel it, then the next layer. etc.

I took an old transformer apart way back in the 1970's but that did not have shunts, and i actually peeled the laminations apart. Made a big mess though and the laminations where bent up a bit so i ended up using several transformers to make up the one i had to replace. It was for an oscilloscope. That was the last time i had to take one apart :)

I am too used to using fresh new laminations i guess :)
I just examined my present stock of microwave oven transformers, and, as you said, the shunts are right against both the primary and the secondary. BUT the shunts in each are wrapped with the paper insulation, the same as the windings. So it seems that with a thin screwdriver that had a flat edge like a new screwdriver, I could drive out a lamination or two and then slip in a thin pin punch to drive them out the rest of the way. I can leave the last laminations of the shunt until all oft the others are out so that it will protect the windings. Removing the very last shunt may be a challenge, but in your case, with the secondary gone, leaving only one shunt lamination should not cause much problem. Also, on my transformers the primary is quite heavy wire, looks like about #14, or possibly #16, so it would not be super-delicate.
 

MisterBill2

Joined Jan 23, 2018
3,962
I just examined my present stock of microwave oven transformers, and, as you said, the shunts are right against both the primary and the secondary. BUT the shunts in each are wrapped with the paper insulation, the same as the windings. So it seems that with a thin screwdriver that had a flat edge like a new screwdriver, I could drive out a lamination or two and then slip in a thin pin punch to drive them out the rest of the way. I can leave the last laminations of the shunt until all oft the others are out so that it will protect the windings. Removing the very last shunt may be a challenge, but in your case, with the secondary gone, leaving only one shunt lamination should not cause much problem. Also, on my transformers the primary is quite heavy wire, looks like about #14, or possibly #16, so it would not be super-delicate.
 

Thread Starter

MrAl

Joined Jun 17, 2014
6,599
I just examined my present stock of microwave oven transformers, and, as you said, the shunts are right against both the primary and the secondary. BUT the shunts in each are wrapped with the paper insulation, the same as the windings. So it seems that with a thin screwdriver that had a flat edge like a new screwdriver, I could drive out a lamination or two and then slip in a thin pin punch to drive them out the rest of the way. I can leave the last laminations of the shunt until all oft the others are out so that it will protect the windings. Removing the very last shunt may be a challenge, but in your case, with the secondary gone, leaving only one shunt lamination should not cause much problem. Also, on my transformers the primary is quite heavy wire, looks like about #14, or possibly #16, so it would not be super-delicate.
Hi,

That sounds very good. I was hoping for this kind of reply because it's better to get a first hand explanation then one where we just get some talk about it where there isnt enough detail to be sure. I think i can be sure now, thanks, and i think i am almost ready to try this now.
This was somewhat critical, as this was the last stage of disassembly and next comes winding some turns and such. I had a heck of a time getting the secondary off so i was hoping not to waste this core.

Also thanks to Albert for his quick understanding once i explained the problem a little more clearly. It's also a little more 'fun' to talk about it here then just read some bland site that doesnt go into as much detail. This transformer thing is a hobby for me right now so i look for some fun out of it really :)
 

MisterBill2

Joined Jan 23, 2018
3,962
Hi,

That sounds very good. I was hoping for this kind of reply because it's better to get a first hand explanation then one where we just get some talk about it where there isnt enough detail to be sure. I think i can be sure now, thanks, and i think i am almost ready to try this now.
This was somewhat critical, as this was the last stage of disassembly and next comes winding some turns and such. I had a heck of a time getting the secondary off so i was hoping not to waste this core.

Also thanks to Albert for his quick understanding once i explained the problem a little more clearly. It's also a little more 'fun' to talk about it here then just read some bland site that doesnt go into as much detail. This transformer thing is a hobby for me right now so i look for some fun out of it really :)
Before you put on any new windings be sure to put some tape over the sharp edges to prevent damaging the insulation on the new wire. You can peel the tape off after the winding is done, but it is quite important to not damage that insulation. One other caution is that these oven transformers are not always rated for constant use, and so heat might possibly be an issue.
I have used them but not rewound them because the laminations are welded and not removable. I have added windings to other transformers to get just a few volts for battery replacements in analog meters. That was always an interesting exercise.
 

Thread Starter

MrAl

Joined Jun 17, 2014
6,599
Before you put on any new windings be sure to put some tape over the sharp edges to prevent damaging the insulation on the new wire. You can peel the tape off after the winding is done, but it is quite important to not damage that insulation. One other caution is that these oven transformers are not always rated for constant use, and so heat might possibly be an issue.
I have used them but not rewound them because the laminations are welded and not removable. I have added windings to other transformers to get just a few volts for battery replacements in analog meters. That was always an interesting exercise.
Hi,

Some good points, thanks. I like the one about the sharp edges best because i wasnt thinking about that really, just about the place where the wire sits. I have fish paper and mylar tape so i think that should help.
Believe me, when it comes to rewinding, just as when disassembling, i am one of the most persnickity people on the planet :)

The constant use vs designed use for intermittent operation...
I will derate the stack for constant use, even though i probably wont actually be using it for constant use. This will come in the form of adding more turns to the primary which reduces flux density which reduces core heating.
This will reduce the power output also, but i dont need the full 800 watts no where near that much anyway, and also require a few more secondary turns for a given voltage like 12vac, but i accept the extra work and a little more wire. The inner leg is 1.375 and the stack is 2.0625 so even with two layers the extra wire length will only be less than 8 inches. So another foot or two will do it.

One remaining issue is I have not found any data on the distance between the secondary and primary windings yet though. The leakage inductance increases as the secondary moves farther from the primary (after all they are not one on top of the other in these designs) but i cant remember the calculation for this. Looked on the web, cant find anything yet. I have a feeling i will just have to put up with whatever i get. Maybe 1/8 inch separation distance, that's as close as i want to get for isolation purposes.

Thanks again.
 

MisterBill2

Joined Jan 23, 2018
3,962
Hi,

Some good points, thanks. I like the one about the sharp edges best because i wasnt thinking about that really, just about the place where the wire sits. I have fish paper and mylar tape so i think that should help.
Believe me, when it comes to rewinding, just as when disassembling, i am one of the most persnickity people on the planet :)

The constant use vs designed use for intermittent operation...
I will derate the stack for constant use, even though i probably wont actually be using it for constant use. This will come in the form of adding more turns to the primary which reduces flux density which reduces core heating.
This will reduce the power output also, but i dont need the full 800 watts no where near that much anyway, and also require a few more secondary turns for a given voltage like 12vac, but i accept the extra work and a little more wire. The inner leg is 1.375 and the stack is 2.0625 so even with two layers the extra wire length will only be less than 8 inches. So another foot or two will do it.

One remaining issue is I have not found any data on the distance between the secondary and primary windings yet though. The leakage inductance increases as the secondary moves farther from the primary (after all they are not one on top of the other in these designs) but i cant remember the calculation for this. Looked on the web, cant find anything yet. I have a feeling i will just have to put up with whatever i get. Maybe 1/8 inch separation distance, that's as close as i want to get for isolation purposes.

Thanks again.
While there is some separation and therefore increased leakage inductance the trade-off is separation and less chance of primary to secondary short circuiting. Also, there is not a lot that can be changed about the transformer core construction. Both of the ones that I examined have the cores welded in stripes so that it would be very challenging to change anything. But you may be able to crowd the secondary right up against the primary winding which would reduce the leakage inductance some.
 

Thread Starter

MrAl

Joined Jun 17, 2014
6,599
While there is some separation and therefore increased leakage inductance the trade-off is separation and less chance of primary to secondary short circuiting. Also, there is not a lot that can be changed about the transformer core construction. Both of the ones that I examined have the cores welded in stripes so that it would be very challenging to change anything. But you may be able to crowd the secondary right up against the primary winding which would reduce the leakage inductance some.
Hi,

Not sure what you mean about changing the core because it is welded. Why would i change that?
Do you mean in an attempt to interleave the core laminations? I would never attempt that.
 

Thread Starter

MrAl

Joined Jun 17, 2014
6,599
The link in post #4 describes ways to remove that core weld.
Hi,

Thanks but i dont need to change the core weld or anything else about the core itself.
I see them doing it on the web and if you dont have a welder then you have to bolt it back together. Not into that myself.
 

Thread Starter

MrAl

Joined Jun 17, 2014
6,599
Hi again,

Oh i think that maybe by "derate" the core you may have thought that i wanted to alter the core stack itself, such as add laminations.
All i was planning on doing though was increasing the number of primary turns, which effectively reduces the maximum power handling capability.
The last calculation i did with the estimated number of turns the core now was about 700 watts, so i will reduce the projected max to 600 or 650 watts. I'll never need that much anyway. I have two DC power supplies that go up to 10 amps (300 watts each), and i've only needed both of them two times in the past 5 years and only at 12v or less, so 600 watts would be more than enough.
 
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