Fast Vs Slow RPM motors
Dear Deem,
Quote:
What are the advantages of the low rpms versus high rpms? My goal is to get the smoothest starts and crawl I can get, as I rarely go fast and never pull a long train on my branch line.
Sounds as if a quick review of the "motor replacement guide" section of the NWSL Catalog might be time-well-spent.
https://web.archive.org/web/20180325051500/http://www.nwsl.com/tutorials.html
https://web.archive.org/web/20180321124804/http://www.nwsl.com/nwsl-online-catalog.html
Now, when it comes to electro-mechanical things,
there are some basic physics in play which can be easily understood in generalities,
but the real power comes when you start plugging in some "known application-specific" values,
because once a few numbers are in-place, the rest of the values (or the appropriate range of possible options)
fall into place reasonably quickly...
To wit:
(I apologise in advance to our man Bernd for the massacre-ing of loco mechanism wisdom below... 
).
- There are a few general "performance characteristics" which we need to consider
1- Overall target Speed of model
(Motor RPM x gearbox ratio x wheel circumference/"distance travelled per wheel revolution")
2- Overall target Torque of model
(Does the mechanism provide enough force at the wheels to start-moving the weight of the loco + desired train against expected grade and curve resistance)
3- Max Current draw
(the harder you ask the electric motor to work, the more current it will demand from the decoder/throttle,
and the more stress it will have to handle)
4- Acoustic Noise
(the faster the collective parts of the mechanism are spinning,
the louder and higher-pitch/annoying acoustic noise is created)
5 - Mechanical sympathy and wear/"operational lifespan"
Now, how does this relate to "High or Low RPM motors"?
For the "FAST RPM motor" side:
- A Fast RPM motor requires more gearbox-reduction (higher ratio) in order to slow the wheel/axle rotation to achieve the "slow speed operation" you're looking for (could be good or bad, keep reading).
- A Fast RPM motor + associated high-speed-rotating gearbox elements will likely generate more noise in operation (generally bad, and particularly-annoying esp at "high pitched whine" frequencies).
- The same "high-speed rotating-elements" will likely require more-frequent lubing just to stay operational.
(problematic)
- In order to operate at "higher speed", the gearbox will need to be more-precision-engineered and constructed,
(IE more $$$)
but this may also allow use of smaller-pitch teeth and elements.
- However, we may be able to get away with a smaller motor
(smaller motor = higher mechanical RPM envelope)
- We may be able to get away with a weaker motor
(the motor does not to do the "heavy lifting",
the mechanical advantage of the gear-reduction makes the motor's life real easy...)
- The motor will be drawing measurably less-current, as it's not having to work tremendously-hard.
- ...and, it's arguable that a faster-rotating motor armature imparts it's own mechanical "flywheel" effect,
(higher-rotational speed = more rotational inertia), which may be beneficial.
For the "SLOW RPM motor" side:
- A Slow RPM motor can get away with a lower-ratio gearbox,
while still achieving the desired "slow speed operation"
- A Slow RPM motor and associated slow-spinning gearbox elements should generate less "annoyingly audible" noise. (some noise, maybe some "growl", but not a "whine").
- The resulting gearbox can maybe be a little more "agricultural" in it's design,
using larger-tooth/easier-to-manufacture elements...
(this will become important later-on, keep reading)
- However, the motor will be physically larger
(larger electric motor = lower RPM)
- Without the gear-reduction assistance, it's all on the motor to do the "heavy lifting" against the weight + grade + curve-resistance load of starting the loco and train moving. We need a motor with _TORQUE_...
...and with a strong motor applying a lot of Torque, the gearbox will need to be suitably "heavy duty" to transfer the torque to the axles. (If the gearbox is "delicate/weak", then it won't handle the fight-forces between the motor and the load, and simply break....)
- The motor will likely demand higher-current, esp as the Load increases and the Motor-rotation is having to put in a lot of effort to keep the whole system spinning...
Make sense?
As a practical pair of HO/OO examples,
- Some gorgeous little UK/Euro small-steam mechs run tiny (8-12mm dia) motors,
thru ultra-precision gearboxes with 100-200:1 ratios
achieving ultra-crawly performance from an 0-4-0 with enough torque to pull entire road-length trains.
- On the flipside, Athearn and similar "centre-motor --> gear-tower" diesel mechs get away with 14:1 ratio gearboxes,
with comparatively agricultural big/strong teeth and gear-elements,
powered by relatively-brawny 18-24mm dia motors...
Now, in your specific case, you have the loco in question, which means you have:
- the wheel diameter (allows calc of circumference, and thus "desired trackspeed < --- required axle/wheel RPM")
- existing gearbox ratio/reduction
- existing gearbox gear/tooth size/strength
(gear ratio and gear-tooth-size are inter-related, and will determine "how much motor torque" the gearbox could handle without breaking)
- the available space for the new/replacement motor
- ...and hopefully, you have a bead on the typical curve + grade + train-load the loco
(new motor + gearbox combo) will need to handle.
If all you're looking to do is replace the motor,
but you're keeping the rest of the mechanical system/gearbox,
then the Replacement Motor performance envelope (RPM, Torque)
can't really deviate beyond what the stock mech can handle...
I hope this helps...
Happy Modelling,
Aim to Improve,
Prof Klyzlr
PS I can't seem to find it right now, but IIRC NWSL used to have a great "motor selector" primer,
and the key takeaway was
"a Torquey Low-reving motor + Low-Reduction gearbox
is generally preferrable
to a High-Reving motor + High-Ratio gearbox".
