Transformer recycling

[NMNeil]

I have been recycling some old circuit boards and many have small transformers on them, about 20mm square. No idea what they were intended for so I'm stripping them to get the bobbin and ferrite core out so that I can rewind them. Does anyone have any suggestions on how to remove the ferrite E pieces without them breaking?, had about a 75% breakage rate so far. The other thing is that on some of the winding's are made up of 3 or 4 thin wires wound together rather than just 1 wire, is there a reason for this?

[schmitt trigger]

The other thing is that on some of the winding's are made up of 3 or 4 thin wires wound together rather than just 1 wire, is there a reason for this?

High frequency transformers, in an effort to mitigate the "skin effect" use two, three or more wires in parallel.
See: https://en.wikipedia.org/wiki/Skin_effect

Some transformers go as far as to use Litz wire: https://en.wikipedia.org/wiki/Litz_wire

[SeanB]

As these typically are glued together you can only use something like MEK or acetone to dissolve the epoxy, which might also damage the coil former as well before the join pops. Otherwise heating up the joins with a hot air stream from a hot air station till the epoxy gets past it's glass transition temperature and releases will work, though you will also cook the yellow tape off in some nasty smoke as well.

[Zero999]

I've found heating the core to destroy the glue works but a lot of care is needed to avoid destroying the coil former.

I don't think it's worth it. Just buy known core and former from a reputable supplier. It will come with a data sheet with all the characteristics specified, which will take all of the guess work out of the design.

[Cliff Matthews]

I put on gloves and throw the cores into a pot on the stove (when the wife's not home to freak-out). Boil and grab some BBQ tongs to fish them out one at a time. All the tape and glue comes off like butter  on 95% of them, but be gentle with the small ones, they'll snap in a heartbeat.

[Cliff Matthews]

Since no one answered your other question (*edit: at least more on multiple conductors), being a dumb, dumb,  I'll try...
A good example of using multiple enameled conductors can be found in Litz-wire. Its use is growing since switching frequencies for SMPS are trending upward in recent years in the quest for smaller size and better efficiency. According to this, the functional skin depth of copper wire is about 0.01 inches (0.25 mm) at 60Khz, so with newer designs reaching >300Khz down the road, expect to see more neat stuff! (with lots of physics to explain the why's).
When the wires are wound in 2's, 3's, quads or more (as you mentioned), it also yields a higher current capacity and/or makes it easier to make split's for center-tapped winding's. In my mind, I picture an assembler with 4 spools where he/she solders them all together and inserts the first end into a transformer bobbin mounted on a winding machine. The machine counts while turning and when it stops, the worker splits and solders 2 and 2 and terminates those ends back in the bobbin and Presto! a center-tapped winding. You will also find much more, like winding's separated at intervals by plastic and end spacer-strips to stop arc-overs and moisture entry. Some winding's can be made from 2 or 3 turns of flat insulated (or taped) copper foil and double as both shielding and feedback winding's with bobbin leads soldered to the ends. Tearing stuff apart, eventually once you see enough you sort of figure things out. Hope that helps..

[NMNeil]

Thanks for the info about the high frequency reason for the multiple winding's, and I'll try Cliff's boiling water suggestion the moment the wife goes shopping