Last night I was running my BR 85 that is equipped with a LokPilot decoder.

It was running very slowly in reverse over some track that is typically not used much and was somewhat dirty. The loco stopped and I went to it to clean the track a bit ahead of it. I suddenly noticed some smoke coming from the cab, which was not good as there is no smoke unit in this loco. I yanked it off the layout right away and opened it up on my workbench.

I could not see anything burnt, but I could smell some smoke on the motor.

I put the loco on the track without its cover and gave it current and sure enough smoke came out the motor again. I removed it quickly again.

I took the rotor out and looked at it and it seemed fine. I tried to move the gear train (which is very long in this loco) and there was a sticky part of the movement. I looked and looked for something caught in the gears, and cleaned them with a pipe cleaner. I did not find anything but after the cleaning they moved freely again.

I oiled them and reassembled the motor. The loco then ran fine again.

So, what I suspect happened is that some tiny thing (like a grain of ballast?) got caught up in the gears, preventing them from turning. With the motor then stalled, the decoder presumably increased the power to the motor based on the back EMF from the motor. This ran power through the single winding of the 3 pole rotor, and that caused it to overheat and start smoking.

Clearly I was lucky in seeing the smoke and stopping it quickly, saving the rotor.

Does anyone know if the load control of the LokPilots will keep increasing the power even if the motor is not turning at all?
If so, this can be a real issue and lead to burnt out rotor windings. I would hope that if the motor is not turning at all, the decoder would only apply current for short durations, but perhaps that would lead to less smooth starts... thoughts?

Hi Dale, usually when this happens you will hear the coil buzzing as it draws more current.

If the stickiness is just temporary then the loco will jerk into motion after the voltage increases enough. A hard obstruction as you suspect will make it just draw maximum current until the coil overheats.

the coil was removed and replaced with a permanent magnet when the decoder was installed. Back EMF is only possible with a permanent magnet motor.

The winding that were overheating was one of the three windings on the rotor, technically also a coil of sorts.

Hi Dale,

Interesting observations and a good question?

Hopefully someone knows the answer.

I believe with Lokpilot decoder you can adjust the amount of load regulation. I have noticed a tendency, among some US modeller, to limit the load regulation to mimic a more prototypically run where the loco slows down on grades unless the driver apply more throttle.

Brgds Lasse

I doubt it will turn off totally, but it should limit the current to some defined value.

Now wether that is a low enough current to stop the armature burning out when stalled is another question.