A PWM throttle might also have negative effect on coreless motors.
This post goes beyond the simple issue with this locomotive but might be found helpful for with the interest to follow this issue. Helpful for those fooling with locos, re-motoring, and small motors devices in general.
My fairly extensive tests with small coreless motors don't show any problems at all with PWM throttles, even very low frequency, IF... the PWM is used just for low speed throttle control and NOT used as a voltage reducing device. If the voltage supplied to the PWM controller is at or below the motor's rated voltage I have seen NO brush life problems. IF the voltage is higher and PWM is used to average it to a lower voltage it apparently still spikes the voltage across the brush/ commutator contact and the little precious metal finger brushes don't like that and will fail/ burn prematurely. In my early quest for tiny motors and mechanisms one motor supplier suggested that PWM was OK for voltage reduction from 12v to 6v
if the frequency was high enough, 20k or higher, but I found that while the brush life was better than lower frequency, it still took a toll on the brushes and reduced life
at least 1/3, with the higher the pulsed supply voltage, the shorter the brush life.
An extra confirmation of this that I stumbled on in my own tests while dissecting some motors with various voltage ratings was that pretty much any of the motors rated around 4.5 volts or less (maybe up to 6V) had the precious metal brushes running metal to metal on the commutator and with surprisingly (to me) good life. In fact, I found that the motor failure mode in those motors was most likely to be shaft bearing wear and not the brushes when run at their rated voltage or less. That was a very welcome surprise to me.
Around six volt rating and certainly the higher rated motors, a dissection showed that the brushes were similar in construction
except that the higher rated motors had the same type finger brushes BUT... with a square block of what appears to be regular carbon brush material attached to the brush finger (soldered on?) where it engages the commutator. That follows logically with my observations and confirms for me that the finger brushes can't take higher voltages, PWM or not, and they need to have a more robust brush material for the higher voltages.
Another note on the smaller motors, seen time and again, is that they have VERY little torque but very HIGH RPM's. In the standard horsepower equation the high RPM's is the source of the power that they do have, not brute torque as a larger motor may have. Either one gets you power but the high RPM's of the tiny motors require gear reduction to make it
usable. A LOT of gear reduction. Without that reduction they either literally won't turn at all or are sitting at the cusp of stalled. This makes them uncontrollable and will cook them as they are basically operating,
quite poorly, at stall current draws. Get the gear reduction high enough to allow the motor to run in its "power band" and it will work (at least 100:1+?) but good luck fitting that reduction into a tiny device. Typical loco gear reductions are in the 35:1 range and not at all likely to be higher than 50:1 which is also why they need bigger motors (higher torque/ lower RPM's) and because of the gearing are often too fast for these purposes. If you have enough torque (big motor) it can work because it has the brute force but the speed and often the low speed control are not ideal.
Those who have had success with remotoring and keeping control like
@randgust and
@Chris333 have accomplished it with planetary gear heads attached, typically 4:1, which plugs in perfectly with what I just described. 36:1 x 4:1 = 144:1. Low speed, good power, and good low speed control. Even a 2:1 gear head (72:1 final) goes a long way to getting a tolerable speed and with a not super tiny motor can go a long way to making a workable package. But space for these things is
always an issue.
Hope this adds to some understanding of these applications and saves folks from experiments with tiny motors that don't address the gearing/ torque vs: RPM facts of the matter. Those efforts are denying physics and will not work today, tomorrow, metric or English measurements, and regardless of your longitude and latitude.
