Rewinding armatures - questions

[Maletrain]

Now that Max has said he isn't going to do it, this might be beating a dead horse on the dangers of stalled motor current testing. But, I am wondering if the 1.28 amp "fusing current" for #42 wire is the appropriate parameter to think about for safety of a wound coil.  Isn't the fusing current the current that will just about immediately part the wire, like a fuse blowing?  Even a "slow blow" fuse blows in mere seconds.

I would think the real issue for an armature winding is how much current does it take to raise the temperature to the insulation failure point inside the winding coil, where heat cannot be easily dissipated, for a period of time needed to conduct the test? 

[peteski]

Permanent motors are fairly complex electical devices (as far as their operation is concerned).  I just have small inkling about how all this works.

These motors have maximum torque at stall (and at maximum operating voltage), but I don't think that is really good test.  We would be much more interested in the motors dynamic parameters (like the RPMs, and torque at various speeds).  That is where the motor performance is important.  That's why motor manufacturers provide graphs showing various motor parameters at the full range of their operating speeds.

[mmagliaro]

The fusing current is the point where the actual copper wire itself will physically melt.   Yes, we cannot use that as our actual current limit because as I theorized earlier, the insulation or the plastic parts of the armature will melt or otherwise self-destruct from the heat, most likely, long before the fusing current is ever reached. 
I am going to conduct an experiement with a piece of this wire, pulled through a slit in a piece of styrene, and gently increase the current through the wire until it starts pulling through the plastic (because the plastic is melting).  That's probably a reasonable guess at the current limit.

Meanwhile, there's been a setback.  In the course of preparing one of these armatures to go in the Rivarossi can, one of the things I have to do is put it between the jaws of a jeweler's vise with a piece of tubing slipped over one shaft, and push on it to "ooch" the commutator a little closer to the coil bundle.  I need just that extra bit of reduction in length to make it all fit and line up under the brush holders.  This is no big deal on the stock armature.  But my rewound armature is so packed with wire that there is no way to do this without putting pressure on the coils.  I apparently broke a wire somewhere in one of the coils so that coil is now bad and there's no way to repair it.

I will have to wind another armature.  And this time, I'll squeeze it down BEFORE I wind it.   It will be a few days before I complete this.  I think the second one will go faster since I've been through this once and I have my handling techniques down.

[mmagliaro]

I finished rewinding a second armature.  As I hoped, it went a lot faster than the first time, just because I had some practice.  I did put 460 turns on each coil again, just as before.  They fit better this time, again,  because I've had some practice and could do a neater job.

This one properly fits back inside the Rivarossi can, and early tests on the bench look encouraging.  It starts and runs at the same voltage, or a little LOWER than the China motor armature, and runs at a lower starting RPM.  That's all really good, because it does imply the motor is making enough extra torque to at least run itself at lower speeds than the original.  The question is, still, is it making enough torque at the lower speeds to move the engine any slower?  We'll know in a few more hours when I can get this into my test engine and try it out.

Here are some pics.

Here are the basic pieces of how this whole Rivarossi rebuild works.  One thing you cannot see is that I drilled out the RR brush holders to a slightly larger diameter inside so that I could put the large brushes and springs in from the donor motor.  That makes it MUCH more durable under load and stall.
I am very happy with how nice this armature turned out.  I got the windings much neater and less like a "ball of yarn" than I did on my first try.

[peteski]

That should be a huge improvement over the vinyl magnet, and the crappy armature/commutator in the original motor!  Supercharged! 

One thing that has me worried is how secure is the front bearing, now that you removed all that metal up front?

[mmagliaro]

That should be a huge improvement over the vinyl magnet, and the crappy armature/commutator in the original motor!  Supercharged! 

One thing that has me worried is how secure is the front bearing, now that you removed all that metal up front?

That was one of the things I had to experimentally determine by destroying a number of these cans.  I still have a good .030" thickness in that bearing area, so it is quite strong, even being drilled out like that.    In model train world, there is really not that much load on that nose.  And I have a repeatable process.  I use a 1/4" cobalt screw-machine bit (short, sharp, rigid) to bore .025" for the "dished" portion, and then follow that up with a 5/32" end mill to drill .040" for the inner flat portion.  Since I'm doing this on a lathe with handwheels, they come out exactly the same every time (well, within a thou or two)

That magnet is massively powerful.  In fact, in some ways too powerful for my own good.  On a 4-6-2, it is so strong that it lifts the trailing truck right off the track (it sits directly under the motor).  I was forced to remove the steel strip on the top of the RR trailing truck and replace it with a brass one to fix that problem.

And of course, any bit of metal dust ANYWHERE gets stuck to that magnet.  But a piece of masking tape can be stuck and pulled to remove metal dust and debris if that happens.

[peteski]

That was one of the things I had to experimentally determine by destroying a number of these cans.  I still have a good .030" thickness in that bearing area, so it is quite strong, even being drilled out like that.    In model train world, there is really not that much load on that nose.  And I have a repeatable process.  I use a 1/4" cobalt screw-machine bit (short, sharp, rigid) to bore .025" for the "dished" portion, and then follow that up with a 5/32" end mill to drill .040" for the inner flat portion.  Since I'm doing this on a lathe with handwheels, they come out exactly the same every time (well, within a thou or two)

That magnet is massively powerful.  In fact, in some ways too powerful for my own good.  On a 4-6-2, it is so strong that it lifts the trailing truck right off the track (it sits directly under the motor).  I was forced to remove the steel strip on the top of the RR trailing truck and replace it with a brass one to fix that problem.

And of course, any bit of metal dust ANYWHERE gets stuck to that magnet.  But a piece of masking tape can be stuck and pulled to remove metal dust and debris if that happens.

LOL about the magnet's strength.  Should provide lots of torque!  I also use masking tape to remove metal shaving from magnets.  Works quite well.

But going back to the bearing, isn't it normally crimped or flared out on the inside to lock it in place?  After machining, that locking feature is no longer there.

[MK]

I can see your magnet being so strong that you derail locos on the parallel track as you pass by.  "Oh no!!!!!  Here comes Max again!!".   

[mmagliaro]

LOL about the magnet's strength.  Should provide lots of torque!  I also use masking tape to remove metal shaving from magnets.  Works quite well.

But going back to the bearing, isn't it normally crimped or flared out on the inside to lock it in place?  After machining, that locking feature is no longer there.

Actually, SOME of the Rivarossi bearings are like that and some are just straight tubes that push into the housing.  I have yet to find any pattern to when one type or the other is used.  Be that as it may, I put my bearing in with red Loctite, precisely to avoid the concern you raised.  It's a bronze bearing in a steel case.  Good luck getting THAT thing out of there.  Also, I machine it after I remove the bearing and press it in from the outside, so if it's going to let go, I'd know it when I start trying to cut into it.  (Fun fact... it NEVER lets go).

[peteski]

Actually, SOME of the Rivarossi bearings are like that and some are just straight tubes that push into the housing.  I have yet to find any pattern to when one type or the other is used.  Be that as it may, I put my bearing in with red Loctite, precisely to avoid the concern you raised.  It's a bronze bearing in a steel case.  Good luck getting THAT thing out of there.  Also, I machine it after I remove the bearing and press it in from the outside, so if it's going to let go, I'd know it when I start trying to cut into it.  (Fun fact... it NEVER lets go).

Good to know Max. Thanks!

[mmagliaro]

Okay, folks!  Here's the video.
The rewound armature is awesome.   It does just what the science predicts it should do.  Lower speed, more torque, better-running engine at low speed.  My only regret is that I made another improvement - I replaced the idlers with Delrin gears.  I don't have any independent video showing just the improvement that makes.  You'll have to trust me on this.
Stock engine: min speed 15, but motion jerky
New armature, ring magnet, brushes.  Min speed still about 15, but motion is better (and you have a motor that won't blow up).
Add Delrin idlers - min speed 11 mph
Add rewound armature - min speed 6 mph and the start-up and overall running are dramatically better.

[wazzou]

That's really, really fantastic Max.
I'm glad you got the laundry out of the dryer before the money shot of the 3rd part.   

[mmagliaro]

That's really, really fantastic Max.
I'm glad you got the laundry out of the dryer before the money shot of the 3rd part.   

Heh.. yes, I know you can hear the dryer going in the background.  Just one of those things.  I wasn't super-concerned
with "production quality" on this one.  I was too excited that the principle worked....
Now.. if only I could find a company that can do custom armature winding.  I've done some poking around the internet and
on Alibaba, but found nothing yet.

[peteski]

Excellent results Max!
As far as the video goes, why are you hiding the starts and stops "off stage".  You only show drive-bys.  I would have much rather seen starting and stopping (maybe along with drive-bys) performed on "stage".  Starts and stops are where smoothness really counts.

I'm also curious about what type of improvement you would notice if you only replace the magnet, and keep all the other parts of the motor and loco original.

[mmagliaro]

Excellent results Max!
As far as the video goes, why are you hiding the starts and stops "off stage".  You only show drive-bys.  I would have much rather seen starting and stopping (maybe along with drive-bys) performed on "stage".  Starts and stops are where smoothness really counts.

I'm also curious about what type of improvement you would notice if you only replace the magnet, and keep all the other parts of the motor and loco original.

I'll take another video to show the starts and stops.  The white poster board wasn't very wide and I just tried to show as much moving as I could.  Actually, in some of the beginning clips, you can see the starts and stops of the unmodified engine, and the engine with the new motor, and they are flipping lousy - very "jackrabbit".  Look at 0:21 and 0:54.  Only with the rewound armature and the Delrin gears did I get rid of that so that it starts up nice and smooth.  That was the nicest improvement, so I should take some video of it.  I think the jackrabbit starts are even more offensive than running too fast.