For the rotation of the coils relative to the comutator it depends on the use of the motor. For a motor that is used with both directions the 90 deg. shift should be correct, as a kind of compromise to get the same angle for both directions.
Some machines use exrta effort to change the position of the brushes depending on the direction. So there seems to be an advantage if there is a different angle. Similar a motor only to be used in one direction could use a different angle to get the best performance. Some such motors with a different angle limit the speed in the lesser used reverse direction - no need to do this if 90 deg would be best.
Thank you :-) this motor indeed has to turn both ways (ideally at same speed). It is used for a mini milling machine table feed and it uses a simple tyrystor control board for speed. It uses a set of switches to reverse the field coils for the direction change.
Now I'm sort of half way there with winding new coils and I wanted to ensure I didn't make any mistakes (yet) with coil direction etc. Unfortunately the idea to energise individual coils with dc and "see" the magnetic field with a nail doesn't work(neither with a magnet). I can send 4 amps of current via the wire before it starts heating up, but I can't detect any difference when reversing polarity.
If anyone has a method to check coil's direction(without seeing the ends) , please do let me know.
Also I tried to use a little "wire finder" device that sends a signal and uses an rf probe tied to a speaker. Sending it via one set of coils while grounding another again there was no difference at all...
Perhaps some clever idea can be done with what I have (oscilloscope, signal generator, split core current transformers). Today I'll try sending some sine signals via the coils and observing them picked up by the split core transformer on the oscilloscope to see if the direction can be verified. If I made a mistake in winding I would much rather detect it now, and not in the end.
You don't need to unwind turn by turn. All you need is to find the top end of the top coil first. Then cut the winding and start pulling out the copper till you get to the bottom of that winding. At that point you will be able to find the bottom end of that winding.
Believe me I gave it my best effort for multiple hours (half a day at least) and I didn't get anywhere. Photo 1 shows the last coil wound cut for example.
Perhaps I didn't describe how small that motor is, how many windings there are and how strong epoxy it uses. Also those motors have a piece of isolator inserted into the haps in the core above the windings and the slot themselves are epoxied too. With slightly bigger motors it is possible to push the windings out with a punch for example. I would've destroyed it sooner than made it move. I had to hacksaw each and every slot half way and then use a sharp knife to pry windings out. Some would go out easier, most would tear. And this is after top and bottom of those coils were removed clean cut. You can see this on photo 2.
This motor is 30mm (an inch and a quarter roughly) in diameter, uses 0.35mm wire (29 gauge), has 16 slots in which there are 32 coils (or 16 with a middle connection depending how you count).
If you managed to deconstruct a well epoxied, thin wire motor like this mechanically my hat goes off to you...
Now that I think about it there is a different method I could've used I think. I could've cut bottom wiring clean off and now all coils being disconnected from the bottom I should've used a multimeter to probe ends of wires in each slot with the commutator bars. I could've "reverse engineered" it that way. Unfortunately this idea comes too late now. Perhaps it helps anyone else doing the same so let me put it in bold.
What makes you think that you need to move the brush positions from where they are? That will take you down a rabbit hole and you will destroy everything. If you can't figure out the stuff that is already applicable in this case, how are you going to redesign the whole thing? Just work with what you have and forget trying to re-engineer the thing!
I think you are thinking of something completely different. I' m not moving the brushes mechanically (by hacking on the casing, drilling etc). I'm moving the coil connections on the commutator block by 90 degrees effectively "electrically rotating" them by 90 deg. At this stage I'm pretty certain this is the correct thing to do.
Also, I'm not redesigning that motor. I'm rewinding it as it was by discovering most likely initiall design (based on known rules and what I observed). If I redesigned it, it would be much simpler. I would just pull a winding design from a book for the corect number of poles, slots and commutator bars, then rotate the brushes electrically to account for their physical position and call it a day. But I prefer to try restoring it to close to original.
However, I keep thinking why did this motor spark like hell from the start and died so soon (it was bought in 2018, since then it was used maybe once per month, so almost new). So maybe following the initial design is not the smartest thing to do, but I'm hoping if the initial design was erroneous it was due to straight connection of brushes located at field maximums. This would essentially short them at worst possible time. That will be corrected if it was different.
Edit: I found a cool way to check coils are wound and connected correctly. I set the signal generator to 200hz (seems to work well with my coil size). Then I put the spit core transformer flat on top of the rotor (rotor lying on its side). And connecting to coils in sequence it is clear if the cooks go in the right direction (the phase is advanced or moved back).