Motor behavior and driveline concepts - (a roundtable discussion?)

[Rasputen]

I wonder if you need a jig or shims to keep the armature centered while you assemble the motor to the chassis, and perhaps another shim to hold the worm near center too.  Otherwise, when you push on the motor frame to install the motor, you will be biasing the armature towards the rear, and the worm towards the front.  This might explain why the current draw moves around versus time.

[PAL_Houston]

Folks,
..
2. So, what's with the higher current?
Usually, motors wear in, smooth out, and the current drops.  Perhaps
the brushes and comm have worn in a little, are making better contact, the motor is making more power, but it's drawing
more current to do it?  If so, I have no problem with that.

...

So there it is, motor nuts and mechanism fools.    I welcome all advice, commentary, and suggestions.   Just
stencil them on the back of  a Steinway Model "O" grand piano and mail to....

Max:

Here is a stray thought for you.   Normally as a motor warms up under a load, the resistance of the windings decreases.   This is purely an effect of temperature on winding resistance, and therefore on the current draw at constant voltage.   If the applied voltage and motor load remain constant, this results in a somewhat higher current, and you will typically observe that a motor speeds up somewhat as this happens.  I would not expect the resistance to drop so much or the current to change so much as you have observed, owing to this effect, however.  So, I think the others have correctly been wondering what mechanical factors could be causing the symptoms you observed. 

(There are other thermal effects, on the mechanical side, such as reduced lubricant viscosity in the bearings, or perhaps reduced bearing friction owing to differential thermal expansion of bearing and shaft.  But one might expect them to result in reduced current draw at constant voltage and load, owing to reduced friction effects in the motor itself.)

[peteski]

Good question.   It must have been an open somehow, because yes, it was cool as a cucumber, and the ammeter
read zero.   Maybe just a flakey thing where a brush lost contact?  I guess it could happen.  But it's odd.

Well, since you had the ammeter attached you now know that the problem was not caused by the mechanism.  The problem was isolated to the motor and the electric circuit around it.  If you at that time tried to move the armature with your finger (and the motor started running again), you could have found out whether the open circuit was caused by debris on the commutator.  Odd? Yes. But this is a hobby motor, not some high quality mission critical equipment. 

EDIT: I read the later posts and you did nudge the armature which caused the motor to start turning again. So the problem was caused by the loss of electrical contact between the brushes and the commutator.  Probably caused by debris.  I would ignore that event as non-repeatable random event.

[SkipGear]

Max,
 You may be over analyzing this. If the motor was new and not broken in yet, it had flat end brushes and contact area was very small across a very small timing segment of the comutator. As the brushes wear in and seat, the contact surface is larger and they stay in contact with each pole of the motor longer, causing more amp draw. There is also more friction from the brushes, because of the increased surface area. The motor also makes more torque because of the length of time each pole is energized. All things that will cause the ampdraw to increase and the performance to increase.

As the bruses wear, they get shorter. If there is dirt, debris in the brush hood, as the brush wears shorter and moves in the hood, it can get hung on the debris. A stuck brush. The brush springs on these motors are so light, it doesn't take much to hang a brush. It could be as simple as a rough spot in a brush hood (holder) from corrosion due to age or a short brush getting cocked slightly in the hood.

I chalk it up to a hung brush and call it a day. You may want to "snap the brushes" as we used to say in RC car races. Compress the brush and release it if possible to help clear any debris.

[victor miranda]

over analyzing....

I'm hoping this opens up contribution of good opinion and knowledge on this stuff.

1. Why did it stop?

so far...
dirt in the brushes
loss of power supplied
motor temp causing changes in resistance
tuning the length of silicon tubing
increase in friction
dirty track
 
2. So, what's with the higher current?

so far...
motor temp causing changes in resistance
increase in friction

3. What about that rubber tube drive?

so far...
might want to replace it with the motor to worm joint from the kato mike
tuning the length of silicon tubing


thems the answers to examine... good luck.


I think I will repeat something I said a little more carefully.

when a loco rolls slowly any electrical stall will stop the loco.
It is an electrical stall. no current flow.
any nudge will send the loco back into motion.
so I was stating it is a slow speed electrical stall. 

[peteski]

Victor, I think you keep missing the point that the loco was being tested (and stalled) while propped up and the electric leads were hooked up to the motor. It was *NOT* running on the track.

[victor miranda]

Victor, I think you keep missing the point that the loco was being tested (and stalled) while propped up and the electric leads were hooked up to the motor. It was *NOT* running on the track.

ok....  I find it funny that you are the one telling me I am missing the point.

other than that, is there something you wanted to say?

[mmagliaro]

Agreed folks.  We've taken as far as it makes any sense to spend time on it (maybe a little more, ha ha)

PAL...  yes,  I am familiar with the phenomena you describe (warming up, speeding up, drawing more current).
But as you point out, that wouldn't explain a jump from 50-75, and even so, remember that I was running this
thing for quite a while at 50ma, so it was as warm and fast as it was going to get.

Victor: I did think about that Kato Mikado driveline coupling (very clever, rubber, yet not firmly connected to either shaft....
them boys at Kato are pretty smart).   But unless this thing quits in my next torture test on a track, that will
wait for another day.

Tony, thanks for your run-down about brushes and increased power, friction, and load.  Yes, this motor is
new old stock.  So it has been sitting around for at least 10 years, maybe more, without being run, and the
brushes were new (not run in).   When I started this thread, I postulated about what you describe: that
the brushes would wear in, conform to the commutator, make more contact, increasing friction but also
conductivity, thus increasing power and current draw.    I guess I have always thought about this
backwards - that the current draw would go down as the motor broke in, but that makes no sense, physically,
unless the thing has really shoddy bearings and lack of oil.

One final thought about the stall:
The little power supply I use to run these has a pretty low-current wal-wart providing its input power.   So its output voltage
sags quite a bit as the current draw goes up.   Initially, the motor spun quite fast at 6v, drawing only 50ma,
so there's no way it would come to a stop because it's running slowly.
However, if the current draw increased steadily overnight as it ran, the output voltage of my little supply would plummet.
It could easly drop from 6 to 3 with the current rise from 50 to 75.   This is where I dropped the ball in collecting data.
I saw the current was zero, but I didn't look at the voltage    This motor cannot turn at 3v.  It needs about 3.5 in
this mechanism to turn reliably.   So that would explain how it slowed down and stopped, and why it might start turning
again when I nudged it.
HOWEVER... it still doesn't explain why the ammeter read zero.  No motor draws zero, even if it's not turning, unless there is
an open somewhere.

Thank you, friends.   Physics class should end now, I think.   I now return you to your regularly scheduled fun with model trains.

[peteski]

ok....  I find it funny that you are the one telling me I am missing the point.

other than that, is there something you wanted to say?

Maybe then you could explain how your statement:

I think I will repeat something I said a little more carefully.
when a loco rolls slowly any electrical stall will stop the loco.
It is an electrical stall. no current flow.
any nudge will send the loco back into motion.
so I was stating it is a slow speed electrical stall. 


applies to a loco with 6 volts applied to its motor, spinning its wheels in the air?  BTW, 6 volts IMO is not slow speed. It is about half the operating voltage (medium speed).  I guess then I just don't get it.

[victor miranda]

Maybe then you could explain how your statement:

I think I will repeat something I said a little more carefully.
when a loco rolls slowly any electrical stall will stop the loco.
It is an electrical stall. no current flow.
any nudge will send the loco back into motion.
so I was stating it is a slow speed electrical stall. 


applies to a loco with 6 volts applied to its motor, spinning its wheels in the air?  BTW, 6 volts IMO is not slow speed. It is about half the operating voltage (medium speed).  I guess then I just don't get it.

did I mention track?

[SkipGear]

did I mention track?

Yes....

1. Why did it stop?

so far...
dirt in the brushes
loss of power supplied
motor temp causing changes in resistance
tuning the length of silicon tubing
increase in friction
dirty track

[LV LOU]

Max,
 You may be over analyzing this. If the motor was new and not broken in yet, it had flat end brushes and contact area was very small across a very small timing segment of the comutator. As the brushes wear in and seat, the contact surface is larger and they stay in contact with each pole of the motor longer, causing more amp draw. There is also more friction from the brushes, because of the increased surface area. The motor also makes more torque because of the length of time each pole is energized. All things that will cause the ampdraw to increase and the performance to increase.

As the bruses wear, they get shorter. If there is dirt, debris in the brush hood, as the brush wears shorter and moves in the hood, it can get hung on the debris. A stuck brush. The brush springs on these motors are so light, it doesn't take much to hang a brush. It could be as simple as a rough spot in a brush hood (holder) from corrosion due to age or a short brush getting cocked slightly in the hood.

I chalk it up to a hung brush and call it a day. You may want to "snap the brushes" as we used to say in RC car races. Compress the brush and release it if possible to help clear any debris.

Tony,I was just getting ready to post almost word for word what you said,I just started reading this thread.Only one though to add,if the motor was fairly new,as the brush wore,it deposited an unusually high amount of carbon gunk in a short period of time,and just gummed up the brushes to the point where one of them just lost contact.If it were broken in while running it like you would normally run a train,going back and forward with it,it would have cleaned itself a little,an maybe kept going.If it stopped under normal running,a bump may have gotten it going,Max,and you would have more than likely blamed it on dirty track..[Well,maybe not now..Inside joke..LOL!!]
 Tony's description of the brush breaking in and drawing more current is exactly how it should have acted.When we build RC car electric motors,we break them in under water to let them keep cool,keep the comm clean,and draw more amps because the water loads the armature,which makes them break in faster,and with no arcing..

[peteski]

did I mention track?

Not in that particular post, but you have before (as Tony has pointed out).  Still, in the statement I critiqued, in the following sentences:
when a loco rolls slowly any electrical stall will stop the loco.
It is an electrical stall. no current flow.
how can a (or "the") loco be rolling slowly if its wheels are freely spinning in the air?!  At least that is how Max described his test. Your statement strongly implied that the loco rolls slowly, on the track.  How else could it be rolling? That is the reason I was confused as to why you are discussing a "rolling" loco when the problem was with a "free floating" loco. 

[Lemosteam]

Ahhhhh,  Classic.  Thanks for the Atlas memories! 

[victor miranda]

Ahhhhh,  Classic.  Thanks for the Atlas memories! 

Dear moderators,

the last time I made a post to attempt to clear up what is obvious to me about
a lack of ablity to read and comprehend,
I was told to show some respect to the fine folks who add value to this forum.

so, am I allowed to explain my thinking or will I once again be told to mind my Ps and Qs?

victor