I need a 60hz power transformer for a project of mine (resistance soldering unit), and it must have custom windings. I want to have 6 to 12 primary windings, and a single secondary (or multiple in parallel for amperage) with no more than five turns. I could go for either EI-core (shell-type) or a core-type transformer but toroidal is too expensive.

The question is, would it be better/easier to modify an existing transformer or buy the parts to build one from scratch?
I'll probably want to remove the bobbin from the core before winding it, won't I? Would I be able to simply cut the primary winding at points where I want to add taps, or are they typically wound in layers?
I have read some things on the internet about heating a transformer in the oven and then pulling apart the laminations while they're still hot. Will the quality of the transformer diminish if I do this? How do I put them back together properly?

If I built a transformer from scratch, then I don't know where I would buy the laminations. I haven't found any websites that list the prices for them. I am in Canada, by the way.

Using a recycled transformer or laminations would be fine, although I don't know where to obtain one.

 

Anyone know? There must be someone with experience in winding transformers.

Should I disassemble an existing transformer and change the windings, or should I build a new transformer from scratch?

 

It can be difficult to get the laminations out without damaging them and even more difficult to get them all back in again. If any of the windings are high voltage, say over 50V, then I don't recommend this as a first attempt. You need to keep the winding neat with the correct tension and without kinking the wire (it is awfully easy to kink) and you need to make sure the insulation is sound. There used to be transformer kits available with the mains primary already wound and this makes custom transformers fairly easy and safe. From my experience of doing this, it is a lot harder than you think.

 

It can be difficult to get the laminations out without damaging them and even more difficult to get them all back in again. If any of the windings are high voltage, say over 50V, then I don't recommend this as a first attempt.

I'm sorry. You don't recommend what as a first attempt. You didn't say which method you're referring to.

 

I'm sorry. You don't recommend what as a first attempt. You didn't say which method you're referring to.

I don't recommend any of them for windings over 50V as a first attempt. If this proposed transformer has a mains voltage primary then you need that prewound and pre-insulated and if dismantling an existing secondary winding take great care not to damage the primary or its insulation.

 

The problem is if you need to change the primary, which usually entails dismantling the transformer as long as the lamination's do not have a weld bead across them.
Then remove the existing and rewind the bobbin on a coil (hand made?) winder.
I did this many,many moons ago to make a transformer for a Valve tester that had multiple cathode heater voltages.
It requires patience and is not for the faint at heart.
A scrap yard for old transformers is a possibility.
To buy on line the shipping is usually too expensive.
The Toroidal kind is way harder to rewind, these are OK if just adding or subtracting a winding, in this case they are usually easier than the EI type.
In purchasing a possible candidate, you need to know the (K)va requirements.
Max.

 

The question is, would it be better/easier to modify an existing transformer or buy the parts to build one from scratch?
I'll probably want to remove the bobbin from the core before winding it, won't I? Would I be able to simply cut the primary winding at points where I want to add taps, or are they typically wound in layers?
I have read some things on the internet about heating a transformer in the oven and then pulling apart the laminations while they're still hot. Will the quality of the transformer diminish if I do this? How do I put them back together properly?

Yeah, if you want to rewind a transformer you need to take it apart. The real issue dismantling an E-I transformer is taking out the first couple of laminations which are usually very tight, the rest are a piece of cake. I actually prefer when the core is welded together with all the Es stacked together, since you cant just cut through the weld and weld it back when you're done.

As far as the winding goes, in my opinion, there's nothing really critical about it, you can do it by hand. Ideally it should be done as neat as possible but in reality it doesn't even matter how messy it ends up looking or how the wire overlaps etc. Just avoid scratching the wire's insulation. I've rewinded a lot of trafos and none have failed so far.

This site I'm gonna link has great info on designing and building transformers, but it's not very easy in the eyes:
Coils and transformers.
Practical transformer winding.

 

Can you start by saying what is the Primary voltage range and what Secondary voltage output and current you need, this will determine the Va size?

 

If you need to make a reliable custom transformer find a used HID lighting ballast from a Mercury Vapor, Metal Halide, or High Pressure sodium lamp and redesign it.

They are some of the best built, H or H+ temperature rated windings and conservative VA/wattage ratings, yet easiest to take apart and redesign being their cores are modular rather than interwoven E & I laminations and their primary and secondary windings are completely independent of each other as well.

All attributes that make them very easy to customize and still have a good solid quiet cool running custom unit in the end.

 

I'm sorry that I've been away so long, but I'll explain some things now.

... Primary voltage range and ... Secondary voltage output and current ... Kva size?

Actually, it's the secondary voltage that is a range, while the primary will remain at 100-120V AC. The reason for the multiple primary windings is that I would use a rotary switch to change the length of the primary coil to give different secondary voltage outputs. The reason for being on the primary side is that being a resistance soldering unit, I must have as little resistance as possible on the secondary side. The secondary side should range from roughly 1/4V to 5V.
Not Kva, or even kVA (gotta use correct capitalization), but it should be something from 40VA to 80VA. It's really hard to predict what current this will be, being a resistance soldering unit. It's purpose is to be short-circuited through various objects (but for short bits of time). But probably from 15A to 50A, from what I've heard. No rectification required.

... Just avoid scratching the wire's insulation.

This site I'm gonna link has great info on designing and building transformers, but it's not very easy in the eyes:
Coils and transformers.
Practical transformer winding.

Good to know that the enamel coating is scratchable. I'll be careful.
Did you heat your transformers to disassemble them?

I already knew about the second page you linked to, which has some good stuff but skips many details. I'll look into the first one though.

If you need to make a reliable custom transformer find a used HID lighting ballast from a Mercury Vapor, Metal Halide, or High Pressure sodium lamp and redesign it.

They are some of the best built, H or H+ temperature rated windings and conservative VA/wattage ratings, yet easiest to take apart and redesign being their cores are modular rather than interwoven E & I laminations and their primary and secondary windings are completely independent of each other as well.

Thank you. I will look into these. Although it looks like I'm going to completely replace all windings anyway.

 

Did you heat your transformers to disassemble them?

Only for smps high frequency transformers I must heat the core up with a heat gun or in boiling water to disassemble, so far I've never needed it with a laminated trafo.

 

I need a 60hz power transformer for a project of mine (resistance soldering unit), and it must have custom windings. I want to have 6 to 12 primary windings, and a single secondary (or multiple in parallel for amperage) with no more than five turns. I could go for either EI-core (shell-type) or a core-type transformer but toroidal is too expensive.

The question is, would it be better/easier to modify an existing transformer or buy the parts to build one from scratch?
I'll probably want to remove the bobbin from the core before winding it, won't I? Would I be able to simply cut the primary winding at points where I want to add taps, or are they typically wound in layers?
I have read some things on the internet about heating a transformer in the oven and then pulling apart the laminations while they're still hot. Will the quality of the transformer diminish if I do this? How do I put them back together properly?

If I built a transformer from scratch, then I don't know where I would buy the laminations. I haven't found any websites that list the prices for them. I am in Canada, by the way.

Using a recycled transformer or laminations would be fine, although I don't know where to obtain one.

RS components used to sell transformer kits that you could assemble your own, pretty sure they had empty bobbins for custom windings. There was almost certainly an application note with winding tables.

Kits like that are hard to find nowadays and manufactured transformers are usually vacuum impregnated - its hard work unpicking the laminations, and even harder putting them all back without bits left over.

The easiest route is if you don't need many turns on the secondary - cut away the old secondary and feed in the new windings inside the side limbs.

Or you might get lucky and find a transformer with non-interleaved E/I laminations.

 

I think the most important thing to be considered is the rating.
You can modify an existing transformer but once you put it in oven for disassembling it, the lamination between the stamping will be damaged and so eddy current losses will increase. For low rating transformer its not a problem but for higher rating it will cause too much heating of core.
One more thing that you can do is just without removing the stamping and disassembling the whole transformer u try to take out some turns if possible so that u could get adequate turn ratio.