Motor burn out question

[mecgp7]

This may seem like a dumb question especially since I have been in the hobby for forty plus years, but when a motor burns up (actually smokes) what is it that actually happens. I sort of understand if a motor is seized and you put the power to it, but I was recently playing with an older Arnold steamer and it runs great. The motor gets hot and smokes, but I runs. I immediately stop it when I see smoke. It does not smoke instantly. Maybe 30 seconds after supplying power. What is actually generating the heat? Brushes? Armature? I am sure there is more than one reason.

[peteski]

Hobby Permanent-magnet DC motors are very simple electrical devices.  They consist of enameled copper wires wound on the poles, and a commutator which consists of copper segments with air gaps between them, and spring-loaded carbon-based brushes.

So the only things that can "smoke" are the actual windings, or something related to the commutator.

The wire can overheat if excessive current flows through it. While the wire itself is metal and does not burn, if it gets hot enough, the enamel coating the wire (the insulation) will burn up.  That will likely short out the adjacent windings, shorting the entire pole.  That is a permanent type of damage. This almost never happens in any of the contemporary motors.

But in the past there were few infamous motors which had windings that could burn out.  One of them was the Mehano/Model Power/Life0Like motor. It actually had 5-pile rotor, and well made commutator, but had a very weak magnet with a large gap between magnet poles and the rotor.  The windings were wound using very thin wire.  Because of that design, it had very little torque. That, combined with the high-friction mechanism in the locos, it required lots of voltage (and current) to run the model.  The current was excessive for the thin wire winding, so if it was running for extended period of time the wire would overheat, and smoke. 

Much mode common cause of "smoking" motors is the commutator. If some oil accumulates in the gaps between commutator segments, the powdered brush material (it is normal for brushes to wear down) sticks to the oil, eventually filling the gap with conductive brush powder.  it is not a zero-ohm short, but the resistance is low enough for the current between the segments flow through the conductive "muck",  causing the oil/carbon mixture to heat up and smoke. That in most cases is not a permanent damage. The motor can be cleaned/degreased (especially the gaps between commutator segment), then re-lubricated, and it can be used again.

[dem34]

And of course, windings are fixable, but not really worth doing in any way unless a rare model can only accept a very specific model of out of production motor.

[Doug G.]

A couple of things about the video. Although it's OK to wind and solder each coil to its associated commutator segments as each coil is wound, it's actually better to leave all the coil leads long and position and solder at the end, after all the coils have been wound. This allows plenty of wire length and you can twist leads together that go to the same segments, resulting in a neater assembly.

He used super glue to keep windings in place but super glue is not particularly tolerant to heat. I have used it, in the past, but have found that a clear lacquer or, even better, a clear varnish to be better.

One last thing. If a motor was assembled with magnets that don't respond to a magnetizer, it won't make any difference if you try to magnetize or remagnetize it. The magnetic strength of the assembly will remain the same. Mehanotehnica N scale motors (the early five pole motors) are this way. If a motor was assembled and then the motor was magnetized and subsequently, the motor magnet and pole pieces are disassembled, it will severely weaken the magnet/pole piece assembly and it will have to be remagnetized to avoid excess current draw. You can tell if the magnet/pole piece assembly has been weakened by the motor running faster than originally, drawing more current, and having much less torque.

Doug

[mmagliaro]

In the case of the OP's Arnold steam loco, which heats up and smokes after about 30 seconds of running, since it continues to be able to run after cooling, I'd bet that SO FAR, it is clogged commutator slots.   As Peteski pointed out, that stuff is creating partial shorts in the commutator and can be burning off.  The high temperature created by partially shorted commutator slots will eventually destroy the motor.
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The brushes are involved in this equation too.  As the commutator segments clog up, causing the motor to draw more current and run very hot, the brushes will start breaking down faster, causing more commutator clogging, and more heat.   Eventually, it will draw so much current and get so hot that the armature will warp or melt, or the coil wire insulation will burn.  All of these things make "smoke".  But in your case, you might be able to save that motor if you clean it now.  It's a death spiral if you don't clean the commutator.
 

[peteski]

I sharpen a flat wooden toothpick keeping the end still flat but much thinner, I then use that toothpick to pick the crud from the slots between commutator segments.   I use a wooden toothpick because the segments are made from soft copper, and would likely get scratched if using some metal tool.  Scratched commutator will cause premature brush wear.  Some motor designs allow access to the commutator without taking the motor apart. That makes the cleaning process easier. 

Since I have the motor out of the loco, I usually give it a through cleaning. I dunk it in ultrasonic cleaner filled with Naphtha.  Naphtha is flammable so I'm very careful.  Then I blow the Naphtha out using compressed air, and let the motor sit for few hours to make sure any remaining Naphtha evaporates before I lubricate the bearings with light oil, and test-run it.  Motor brushes create arc when running, so I want to make sure no trace of the Naphtha vapors remain.

[Doug G.]

I sharpen a flat wooden toothpick keeping the end still flat but much thinner, I then use that toothpick to pick the crud from the slots between commutator segments.   I use a wooden toothpick because the segments are made from soft copper, and would likely get scratched if using some metal tool.  Scratched commutator will cause premature brush wear.  Some motor designs allow access to the commutator without taking the motor apart. That makes the cleaning process easier. 

Since I have the motor out of the loco, I usually give it a through cleaning. I dunk it in ultrasonic cleaner filled with Naphtha.  Naphtha is flammable so I'm very careful.  Then I blow the Naphtha out using compressed air, and let the motor sit for few hours to make sure any remaining Naphtha evaporates before I lubricate the bearings with light oil, and test-run it.  Motor brushes create arc when running, so I want to make sure no trace of the Naphtha vapors remain.

Unless you want to simulate a GE Techno-Toaster.