Universal motor rewinding, help needed.

[Fflint]

I have this universal motor I need to fix by rewinding the armature, but I have doubts I'm not making a mistake with the winding pattern. I read a number of articles and book fragments which describe simplex lap winding design with a certain coil pitch etc.

Specifically the motor is a 2 pole universal motor with 16 slots in the iron core and 32 sections in the commutator. The typical way to wind it would be double flier lap design with a pitch of 8. However, in the limited tracing I could do during disassembly I found it currently uses pitch of 9. So coils go through slot 1 and 9, then 2 and 10... Then 9 and 1 on the other side.

Here is how the motor looks:

And these are my 2 drawings (each coil has a connection in the middle too, these are not shown for clarity, therefore empty commutator sections)



Could someone with experience have a look and see if there are no obvious errors, please?

Also brushes (as can be seen below) are 90 degree turned (see below)

So I'm thinking I'll have to move commutator connections by 90 degrees too as in my drawing slots 1 to 9.5 have wires with current going down from the commutator, 9.5 to 16 have current going up towards the commutator.) So there will be magnetic N between 12-4 and S 4-12. If brushes are at locations 1 and opposite of the commutator connected directly to coil 1 the motor will not start as the field coil magnetic field will be exactly the same as the rotor or 100% opposite while it should be turned by 90 degrees. Therefore my drawing has to be modified by rotating the commutator by quarter turn right. Does it make sense to anyone? Can anyone confirm I'm on the right/wrong track?

[Neomys Sapiens]

Can you read and understand German? I could scan you some excerpts of an old German classic: 'Der Katecismus fuer die Ankerwickelei'.

[andy3055]

Don't complicate the issue by trying to move the brush positions and other stuff you are planning. The easiest way is to start from the top most coil. Note down the slot pitch and which commutator bars are the ends are connected to.  Then keep going back one winding at a time till you get the pattern. Go all the way to the last coil and start back from there. If there are balancing weights, keep a note where they are. If not, you may have places where the laminations are ground off to balance it. So, it is paramount that you start from that bottom coil and keep reversing the process so that the armature is mechanically balanced. This is important.

[Fflint]

Can you read and understand German? I could scan you some excerpts of an old German classic: 'Der Katecismus fuer die Ankerwickelei'.

I can auto-translate it. Please do send it :-) (I have OCR-ed and auto-translated multi-hundred page manual for my German milling machine before).

Don't complicate the issue by trying to move the brush positions and other stuff you are planning. The easiest way is to start from the top most coil. Note down the slot pitch and which commutator bars are the ends are connected to.  Then keep going back one winding at a time till you get the pattern. Go all the way to the last coil and start back from there. If there are balancing weights, keep a note where they are. If not, you may have places where the laminations are ground off to balance it. So, it is paramount that you start from that bottom coil and keep reversing the process so that the armature is mechanically balanced. This is important.

I've received advice to "unwind it one by one" and similar before, but the success of following it depends very much on the type of epoxy used to glue the motor together and the wire thickness. In this motor there was no way it could be done. The best one can do is to see the pitch of the topmost coils. That's it.

Now, thinking in retrospect, I could've energised individual coils with dc and maybe use a nail to find which commutator bar leads to which coil? That's an idea to try for future.

Before I removed the windings I spent good couple of hours trying to do just that, but it wasn't possible to find which commutator bars coils are connected to because it was impossible to locate end wires from the coil side and from the commutator side all wires dissappeared into a mass of windings made by putting an extra turn or two around the shaft before it goes to the coil. Pulling on any wire (0.35mm in diameter) after few mm it would try to dissappear beneath another winding where no heating or pulling would separate them.

Regarding balance, I believe the rotor has to be balanced after gluing by removing small bits with a grinder. It seems it is impossible to perfectly balance coils by winding them. I do balance my grinding wheels. I hope this will be similar and not that difficult.

It is surprising that rewinding small motors appears to quickly approach the status of lost knowledge. My dad used to rewind motors 35+ years ago and he did it based on his notes (long gone) he made back in school (60+ years ago). The only info I can find online all come from two books that contradict each other on some details. Looking at YouTube, it seems only in places like Pakistan or India anyone is doing such tasks anymore...

There is growing interest in cultivation of "historic trades" etc such as manual machining on old lathes and mills. Perhaps time comes soon people will be interested in this subject too?

Coming back to the point. I'm reasonably certain connections to the commutator have to be rotated by 90 degrees. Why? I read in multiple sources - brushes should be placed where magnetic field has lowest intensity to control sparking. It has the lowest intensity where it flips the sign, right? Almost every other universal motor I found pictures online of has them located like that. Except mine (and one on Wikipedia entry about universal motors). So I think the connections have to be rotated to electrically "move" the brushes into the right position.

If anyone thinks I'm making an error, please let me know.

Then there is the issue of pitch. The books I found all talk about forward and back pitch being different and for my number of slots the pitch should be 8 (while in my motor it was 9 - so exactly symmetric). I'm not sure why none of the materials I saw describe this symmetric pattern. This makes me slightly worried.

On the other hand, perhaps those designs that have coils that don't span half of the armature (but always less) are done this way to favour one direction of rotation? While the device this came from has to rotate both ways at same speed so maybe this is why?

I have lots of questions... Perhaps the German book will answer them :-)

In the meantime I started winding it and I'm tempted to short non existent windings and give it a try with lower voltage as is to see if it starts rotating. At the moment it looks like this:

(I did mess one commutator bar with solder, but it will be cleaned up later).

[Kleinstein]

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.

[andy3055]

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.
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!

[Fflint]

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).

[Kleinstein]

There are different reasons why a motor would spark very much at the brushes. One is an open connection in the windings, causing a massive spart at that one position - usually will damage the collector part overtime. Another possibly issue can be too much voltage / to few turns. Not enough force or not well formed carbon brushes could be an issue too. A possible mistake could also be the connection of the stator windings, so that the coil halfs work against each other.

For checking the coil direction one could use the energize method with AC current and than detect the field with a pick up coil. The coil is more sensitive than looking for magnetic force and in addition it is not effected much be remanence.

With the angle between the coil and collectors at 90 degegree there is no real need to measure the direction, just make sure to have a consistent direction so that all coil turn the motor in the same direction. If the direction is opposite of the original motor, one can fix this at the switch.
 It can still be a bit tricky finding the exact position of the coils, as the fields will couple to each other and the core material may not be 100% isotropic.
For identifying the next coil wires one may use the motor as a kind of transformer, looking for the maximum coupling from one coil to the next.

[andy3055]

I have rewound an old IBM electric typewriter motor that has a core which could accommodate my little finger only. So, I know what it is.
You can try to "electrically" move the brush positions or whatever you think is right for you.  When people design motors I don't believe they allocate for modifications like the way you think. Good luck to you.

[Fflint]

There are different reasons why a motor would spark very much at the brushes. One is an open connection in the windings, causing a massive spart at that one position - usually will damage the collector part overtime. Another possibly issue can be too much voltage / to few turns. Not enough force or not well formed carbon brushes could be an issue too. A possible mistake could also be the connection of the stator windings, so that the coil halfs work against each other.

For checking the coil direction one could use the energize method with AC current and than detect the field with a pick up coil. The coil is more sensitive than looking for magnetic force and in addition it is not effected much be remanence.

With the angle between the coil and collectors at 90 degegree there is no real need to measure the direction, just make sure to have a consistent direction so that all coil turn the motor in the same direction. If the direction is opposite of the original motor, one can fix this at the switch.
 It can still be a bit tricky finding the exact position of the coils, as the fields will couple to each other and the core material may not be 100% isotropic.
For identifying the next coil wires one may use the motor as a kind of transformer, looking for the maximum coupling from one coil to the next.

I finished rewinding yesterday. I just have the final coil to connect to its commutator bar.

I found the split core transformer works pretty well as a pickup,  at least until the commutator is fully connected, however it is best to align it "edge on" (so it's windings are parallel to rotor coils).  Then it is possible to not only see coil direction, but it's position as the amplitude picked up changes dramatically when moving it around the rotor. I'm yet to see if this method works with everything fully connected.

Once I connect it I'll be testing it with low voltage DC with a small resistor in line to monitor current flow on an oscilloscope. Hopefully this will allow me to see any issues if they are present.

Regarding field coils I'm not sure if they're not damaged. There is a blackish spot on the enamel in one place. The total resistance of both coils is 20 ohm (the rotor all together is about 10) - can anyone tell if this is about right? Both coils are wound with the same continuous wire so I would rather not have to cut it, but I will if I have to.

I have rewound an old IBM electric typewriter motor that has a core which could accommodate my little finger only. So, I know what it is.
You can try to "electrically" move the brush positions or whatever you think is right for you.  When people design motors I don't believe they allocate for modifications like the way you think. Good luck to you.

Perhaps a really tiny motor like this develops a lot less centrifugal force so the glue doesn't have to be as strong? Either way, it is impressive to manage it. As I said my hat goes off to you then.

I now found a better method (too bad it was too late in the process) so if anyone asks me in future I'll advise them to do that (cut the bottom windings- opposite the commutator - clean and probe ends for continuity and resistance to commutator bars).

About the brushes,it is useful to clarify in case someone in future finds this thread as the only reference they can find how to do this task.

I get an impression you think I'm trying to "modify it". Perhaps, because "moving something" sounds like I'm actually making a change. That's not it at all. Speaking precisely. In your typical AC angle grinder, or a corded electric drill there is a very similar universal motor. My dad rewound some of them in long gone past. He doesn't remember much, but what he does remember is that he always connected the coil start wire to the commutator bar directly above the slot the coil starts in(bar 1), then the next bar was connected to the tap in the middle of the coil, finally the end of the coil was connected to bar 3. Next coil start wire went to bar 3 too which now was conveniently positioned above the next armature slot. This is the "straight" connection.

What did all his universal motors have in common? The brushes were positioned between the field coils, not in their centers(in lowest field strength). It seems very rare to find an ac electric drill or an angle grinder with brushes positioned differently. I checked all of my own. (This is not true to "all universal motors" , just those commonly used in angle grinders, drills that have 2 poles and use  simplex lap winding).

This motor however, has its brushes positioned in the middle of field coils so if one used the same method it would mean brushes short commutator bars at highest field intensity. So to resolve this one has to make his bar 1 a quarter turn forward from the coil start slot. Everything else is the same. If this is a mistake I'll find it out very quickly and I'll correct this thread.

As for motors not being designed for changes... Despite what I said I beg to differ. As long as you stay within the design rules you can make all sorts of changes resulting in different motor parameters. It is important to understand the principles when doing them. I don't claim to know it all, but I've gathered some info.

Edit: few photos added. The commutator will be cleaned up. The blue stuff in the slots is indeed a blue zip tie. This in there just temporarily so I can test it until proper rigid isolation board arrives next week. The yellow strips is a special paper isolation used for this stuff back in the day. It has much better temperature resistance than Pet film(of course kapton would be better).

[Fflint]

It is working with low voltage. I used two separate power supplies for field coils and the rotor to be able to capture current waveforms for both with resistors and an oscilloscope. Now I have to grab another one of those motors and compare the waveforms. It looks good to me. Anyone has a comment regarding the waveforms?

Resistors are 0.36ohm for field and 0.18ohm for the rotor. Left psu is powering the field, right the rotor. Field is on yellow Channel, rotor on blue.

[Kleinstein]

If the stator coil is wound from a sinle wire, there is no need to cut that. In many cases the stator windings are 2 seprate parts that are only connected externally and than it is possible to mix things up.
20 Ohms sounds reasonable for a relatively small motor, but a shorted turn in the stator would normally not show up as a significant change in the resistance. A broken wire would be way higher resistance.
One could do a crude check of the coil for a shorted loop by looking at the phase shift / Q factor of the coil.  A shorted turn reduced the inductance quite a bit and adds loss. I still don't have a good value to compare. I have some startor windings and if needed could measure a few (if in range of my LC meter).

For electric drills (e.g. Bosch and Metabo) one finds some that move the mechanical position of the brushes by some +-30 to +- 45 deg. depending on the direction. The position of the stator windings make is tricky to move more. So some motors try to improve on the position.
Machines like a saw or angle grinders that run in only 1 direction can have a different angle at the electrical connections. So they can do the twist at the collector. These motors would not run well in the other direction. The motor still turns but with more sparks and less efficient, especially if fast (some machines (older Bosch rotary hammers) reduce the maximim voltage when running in reverse).

With fixed brush position and reversible direction one is kind of fixed to the in between position to get the same for both directions. There would be nothing gained by having the brushes at a different fixed angle, as the collector connections would need to compensate.

The centrifugal forces depends a lot on the speed / RPMs.  Motors made to get a high specific power density often run quite fast (e.g. 30000 RPM, faster for small ones).  As smaller motors also tend to run faster,  a small motor is not per se less critical.

p.S: The current waveform looks rather good and flat. It is not perfect, but this could be time needed to smooth out / clean the collector. So it does not look like there is a shorted winding or one that is significant too short.

[Fflint]

Thanks :-)

The gluing setup will be a drill powered by a dc PSU so it is running about 1 rev per second. I'm planning to use a two part epoxy and to warm it up before mixing and continue warming it as it sets with a heating bulb.

[Kleinstein]

There is usually no need to warm up the epoxy before mixing. Especially with fast setting versions this can give a little more time.  A main point maybe to have the motor warm before applying the glue. Depending on the speed, just the heat capacity can keep it warm for long enough.

[Fflint]

There is usually no need to warm up the epoxy before mixing. Especially with fast setting versions this can give a little more time.  A main point maybe to have the motor warm before applying the glue. Depending on the speed, just the heat capacity can keep it warm for long enough.

It's very cold around here. My workshop "room temperature" is about 2 degrees C. This is why I'm planning to use some gentle heat.

However, I'm still thinking about possible reasons why that motor has failed in the first place (if not due to a manufacturing fault). Based on the spec this is supposed to be a 90W 240V motor. I wonder if there is a chance they put in a 110V rotor by mistake. I guess there isn't really any way to check other than running it and measuring power consumption.

Once it is done and put together again I plan to try it with an auto transformer (my dc psu only goes up to 60V) and to slowly increase the voltage while measuring current.

Also I'm thinking about adding a fuse. I read somewhere one is supposed to use a "time delay" fuse rated at 175% of motor current. This would mean a 0.6A~0.7A time delay fuse. I never saw such a tiny time delay fuse, do these exist? If not, is there some other method I can use to protect the motor? The control circuit is dead simple. It is just an SCR with a potentiometer to set the firing voltage.

[Kleinstein]

With such a small motor a normal fuse may be to slow. Usually the time constant for the motor gets shorter the smaller the motor gets.  90 W is a relatively small motor. I am afraid the fuse would be more preventing excessive smoke/ fire once the motor is damaged. A fuse would not hurt, but I would not have too much hope saving the motor.
It would be more like a thermal protection at the stator that could work.
There are motor protectors even for small motors, starting at some 20 W. They are still relatively expensive (e.g. 30-50 EUR).
There are also thermal fuses that are cheaper, like these:
https://www.tme.eu/en/details/t11-814-0.6a/circuit-breakers/schurter/4400-0221/


When running on DC the motor can run with a considerably lower voltage. So a motor for 230 V AC may be only 100 V or so DC rating. Part if this seems to be voltage lost from the inductance of the stator.  If the motor does not run well (to much sparcs) with AC, one could still give it a try with DC and reduced voltage. Ideally I would expect it to be a bit more powerful with DC, though there can also be a change in the characteristic (e.g. more load dependent RPMs).

A poorly working SCR circuit could stress the motor and at least operate in an unusual way (e.g. skipping on half the cycles and thus running on pulsed DC). Another point could be voltage spikes, that may damage a weak isolation.

[Fflint]

With such a small motor a normal fuse may be to slow. Usually the time constant for the motor gets shorter the smaller the motor gets.  90 W is a relatively small motor. I am afraid the fuse would be more preventing excessive smoke/ fire once the motor is damaged. A fuse would not hurt, but I would not have too much hope saving the motor.
It would be more like a thermal protection at the stator that could work.
There are motor protectors even for small motors, starting at some 20 W. They are still relatively expensive (e.g. 30-50 EUR).
There are also thermal fuses that are cheaper, like these:
https://www.tme.eu/en/details/t11-814-0.6a/circuit-breakers/schurter/4400-0221/


When running on DC the motor can run with a considerably lower voltage. So a motor for 230 V AC may be only 100 V or so DC rating. Part if this seems to be voltage lost from the inductance of the stator.  If the motor does not run well (to much sparcs) with AC, one could still give it a try with DC and reduced voltage. Ideally I would expect it to be a bit more powerful with DC, though there can also be a change in the characteristic (e.g. more load dependent RPMs).

A poorly working SCR circuit could stress the motor and at least operate in an unusual way (e.g. skipping on half the cycles and thus running on pulsed DC). Another point could be voltage spikes, that may damage a weak isolation.

Thanks for pointing out that thermal protection. I will look into getting one. As for what you said about the feasibility of a fuse protecting such small motor perhaps that's why they haven't installed one from the factory? Interestingly the company that makes those table feed devices (AL-310S ALSGS) has a de-facto monopoly on such devices. I never saw one made by anyone else (other than the OEM in their custom machine). They are everywhere on YouTube in machining channels, but there are no mention of armatures burning out on any machining forums etc. So perhaps I was just unlucky. On the other hand if you go to Aliexpress and put the machine type in search a replacement rotor has higher popularity than the device itself... So I'm not sure...

The scr circuit is very simple, it has just an Scr, two surge protectors, few resistors (including a 10W one and the speed control pot), few caps, a zener diode and a normal diode plus a neon bulb. If I see anything suspicious in the drive waveform I might need to look for some replacement.

Coming back to the motor. When putting in those zip ties it took quite a bit of persuasion to get them in. I doubt I could get fr4 in without ripping wire insulation. So I did a temperature test by heating those zip ties with a heat gun. I got to 120C and they still remained strong enough not to elongate when pulled by hand. The spec for nylon66 gives a range from 80C to 140C for "working temp". So I decided to leave them in. They're covered by epoxy anyway.

So having decided that I glued the motor yesterday in the setup shown previously. I left the heat lamp on the slowly rotating motor on overnight. Its temperature was about 35C. The process is shown on photo 1 below.

Then today I put the motor into a mini lathe with as small runout as possible and I tried to clean up the commutator with the smallest possible cuts(epoxy got into the commutator partially too despite the protective tape). Following that I used auto-body-paint sandpaper (2000 grit) and polishing paste to polish it further and it was then cleaned very well to avoid abrasive grit staying it.

Then I balanced it using the commonly known method. I used a granite surface plate. If I didn't have it I would use a kitchen stone counter top. That's photo 2.You can see the top bearing on the motor there I've balanced it without the bearing, but I forgot to take the photo before putting the bearing back on. To balance one needs small diameter. Using a bearing for this doesn't work.

Finally I reassembled everything and I gave the motor some DC first. I kept it running at about 30V. The oscilloscope tells me it run at 72hz so a bit over 4k rpm. 45V results in 6k rpm. I didn't go higher as I wasn't sure what the proper max rpm wirh AC is (and my epoxy hasn't achieved its full strength yet).

It is important to note, despite the commutator being mirror polished there was a bit of whooshing noise at first, like white noise almost. This went away after the first minute of running the motor and it did become silent. I suspect it had to break in the new commutator surface. Now I came in to get some food, and I left the motor running at 35V and overcurrent protection set to 400mA. If there is any more of a break in that it needs it should suffice. Then I plan to try it on AC slowly increasing voltage.

[Kleinstein]

It is rare to have protectors with universal motors. Some of the Festool machines have a temperature sensor in / at the stator winding that is coupled to the control electronics. So Ideally it would turn off before really overheating. It is much more common with induction motors.

In the pictures I don't see a fan. Normally the universal motors com with a fan directly at the rotor to have quite some active cooling.

I have not seen a good scale to judge the size of the motor, but it does not look that small for only 95 W. So chances are it would not run super fast, so maybe 10000 RPM. In theory the gear ratio and data sheet on the output RPMs should give a value for the expected speed.

From the cut windings it looks like the number of turns is not very high. So it could be a lower voltage one and relying the electronics to reduce the voltage.

[Fflint]

There is a plastic fan I've taken off. It is on the bottom just above the bearing. The length of the entire shaft is 150mm. The number of turns was 76 per slot (19 turns x2 x2).

I think I've found the original cause. When I came from lunch the motor sounded a lot louder. I removed the brushes and there was still resistance to rotation felt. I disassembled it, I oiled the bearings and it became quiet again for entire 2 minutes... After which it started making a noise. So I think bearings are shot.

The motor has very high heating ratio. Now thinking about it I can say It has a 5 tooth helix and the other gear is around 120mm in diameter with a tooth every 3.5mm or so. So that would be 1:100 or so. At full speed it would move the entire table in under 15s. The table is 600mm and one turn is 2mm so that's 300 turns in 15s or 2 turns per second, giving 200 turns/s on the motor which is 12k rpm. You were pretty close with your estimate of 10k rpm. :-)
Edit:no, I forgot about another set of gears under the table, those are just spur gears, one about 60mm in diameter, the other 20,so probably the motor is closer to 30k rpm than 10k rpm.

However, I have not noticed it by turning the bearings in my hand. I couldn't feel any drag, but spinning the bearings they woukd stop immediately. Could such a small amount of drag kill the motor?

[Fflint]

Well, I swapped bearings, I tested it on AC and it seems to run fine now, but perhaps a bit fast... (as it did before) on rapid traverse. I checked it through a 110v transformer. Rapids sounded much more sane (at 240v it sounds like it's reaching 35k rpm+), but the pot stops moving the motor at 3 out of 10. On 240V it stops at 1.5.

I have to measure the current on 240V during rapid traverse and see if it amounts to no more than 90W.if it does, job done :-)

[max.wwwang]

This is a feat. My hat goes off to you sir.

I have a broken rotor from a universal motor made by Miele. It appears some of the windings are faulty. I'm now inspired by you.

[Neomys Sapiens]

That excerpt will be delayed, because of workload and another problem. At home, I don't have a scanner and on my  day in office I did not find time yet. Sorry.

[Fflint]

Finally I can consider it done :-)

I was bugged by what sounded like the motor running in the region of over 30-few k rpm during rapid table move. I tried running it on a US transformer (120V) as a test, but then lower 40% or so of the pot was unusable.

The device has the pot and the scr for "normal feed" and a momentary button that shorts the scr for rapid feed. The result is even at the fastest normal setting the motor gets half wave AC (as an scr always chops half of the wave), while when the button is pushed it gets full 240V AC. This "rapid" setting is used rarely.

Then I looked up SKFs max bearing speed formula and I found for greased, deep groove ball bearings (these are actually deep groove 627Z bearings) they say 22k rpm is max. And I realised it is quite likely they designed everything for operation with half wave voltage mostly allowing it occasionally to be used outside specs.

So in the end I decided to add a large rectifier diode in line with the rapid switch. The rapid move is now slowed down to the highest "normal" speed. It means moving entire table takes 20~30s instead of 10~15, but I expect much better longevity. So that's it :-)

That excerpt will be delayed, because of workload and another problem. At home, I don't have a scanner and on my  day in office I did not find time yet. Sorry.

No worries :-)

This is a feat. My hat goes off to you sir.

I have a broken rotor from a universal motor made by Miele. It appears some of the windings are faulty. I'm now inspired by you.

Thanks :-)