Hi folks! For background, please reference the following threads:
https://ogrforum.com/...5#182044000640598695
https://ogrforum.com/...c/175286893012707946
To recap: Some have observed that traditional series-wound universal motors (aka "Pullmor" motors) tend to speed up once they've been running for a while. And when this happens, they lose much of their slow-speed performance.
-You can slow down a universal motor by reducing armature current relative to its field.
-Some folks have reported better performance from Lionel-type motors when operated on DC instead of AC. (Obviously if you put DC on the rails, you would have to disconnect whistles and horns, etc. However it's possible to convert AC to DC inside of the locomotive by installing a rectifier.)
In my research about motors I've come across terms like "eddy currents," "thermistors," etc. I'm not 100% sure what these things mean and I would love to have an explanation in laymans' terms. But it seems plausible that adding a properly-sized Positive Temperature Coefficient (PTC) thermistor across the brushes would increase the armature resistance as the motor warms up. This would reduce armature current relative to the field, and offset the tendency of the motor to speed up when it gets hot.
This leads to several questions: Are thermistors safe to use, and do they work on AC? Must the thermistor be placed close to the brushes, or could it reside a few inches away in the tender? If my theory is plausible, what size(s) of thermistor would be appropriate for experimentation (ohms, watts)?
I'm intrigued by the idea of installing a variable resistor (would that be a rheostat, or potentiometer?) in parallel with the brushes to really explore how decreasing armature current affects motor speed. I'm familiar with online electronics outlets like Mouser and Digikey. Would someone please recommend a compact component part suitable for on-board installation, that would facilitate experimenting on the fly? (I've read that suggested values might be 4-10 ohms and 25-50 watts.)
Thanks to all in advance for sharing your expertise!