Motor energy use vs RPM @constant load

[A..dam]

Will be ordering a motor for a small folding bike with 20 inch, tire diameter 1.6 m
Have 2 choices for motor rpm, 201 and 328.
Prefer 201 rpm for a max speed of 19 km/h and good hill climbing.
However, at that configuration I will expect to ride a lot at max rpm BUT at relatively low load,
(i.e. flat road, low weight)

My QUESTION: will such ride @high rpm and low load consume more energy battery THAN
equivalent ride using 328 rpm motor and running it 2/3 throttle?
The motor in question is Q75.
https://bmsbattery.com/ebike-kit/593-q75-36v-front-driving-hub-motor-ebike-kit.html

 

[dogman dan]

It should be more efficient to run the low rpm motor at it's fastest rpm, when the load is heavy, than to run a faster motor below it's fastest rpm, when the load is heavy. If nothing else, the low rpm motor will be unable to pull as much watts as the fast rpm motor can.

Cruising on the flat, (when the load is not heavy) I doubt you could really accurately measure the difference.

Besides, in the 20" wheel, the faster motor is not that fast. Only about a medium rpm anyway. It will perform great in a small wheel. Only if you seriously want the very maximum efficiency, and will be riding at a crawl would the slower rpm motor be justified.

 

[d8veh]

These small motors make their climbing torque at much lower rpm than their maximum speed. The 201 rpm Q75 at 36v in a 20" wheel will reach a maximum speed of 19 km/h on the road, but for hill-climbing, it'll be right down to about 10 km/h or less. Cruising speed would be about 15 km/h.

 

[dogman dan]

Same for big motors. At full rpm, lower torque than at half rpm. So at the same rpm, the faster motor would pull harder up that hill. But will it be more efficient? Possibly not. That's what I meant by the faster motor will be able to pull harder up the hill. But it will likely be more efficient to pull less, not more.


But any motor, slow rpm or medium rpm, in that small a wheel will climb fine for a relatively short hill. Either motor will be pretty efficient climber in 20" wheels.

His speed up the hill will depend on grade, weight, and wattage, including any extra torque he provides with pedals. So his actual performance in terms of how fast he gets up the hill will depend mostly on how much watts he throws at the weight and grade in the equation.

My tests of various motor types, various voltages show one thing. My weight, and my hill (6% for two miles), whatever motor I'm using, 12 mph takes about 900w. (26 inch wheel btw) Another motor might take more watts, but go up the hill faster.

In the end, the slow rpm motor gets to the top on the least energy, mostly because it doesn't climb it so fast.

The nicest way to get up that hill is more wattage, and more like 15 mph or more. But this question just asked about efficiency. In general, more efficient is more slow. But it does not require the motor rpm to be more slow, if the wheel size will be 20". In 26", then the lower rpm motor would be my choice, but not in 20".