Understanding gear reduction on a mid drive vs rpm/v

[pwd]

I'm having a bit of a brain block and could use some help understanding this concept.

On a mid-drive motor such as the Cyclone 3000W or pretty much any other mid-drive aimed for a bicycle; there is some sort of reduction from the main motor shaft to the chainring output. On the Cyclone; there is a planetary gear reduction (and another from the motor to chainring but I'll ignore that for now), on the BBSXX there is an internal geared reduction.

Why not just wind the motor for a lower RPM per volt / lower kv and do away with the internal reduction part? I understand that electric motors will be more efficient at higher rpm, but doesn't lowering the kv have a similar effect?

 

[Hummina Shadeeba]

Can get a lot more power out of the motor with gearing as you can convert speed to torque.

A motor has a maximum torque it can produce based on the airgap diameter and stator iron. You’ll need gearing to produce more or potentially a much bigger heavier motor.

Motors run more efficiently at higher speed. There’s two heat producing phenomena that limit efficiency when producing power out of a motor: your wire gets hot from flowing current or your iron gets hot from hysteresis. If u use gearing u can trade the copper losses for iron where with just a motor without gearing (running less rpm) your copper will likely be ur larger heat producer and your missing out on some cooler power production

Maybe more importantly if it’s not a hub motor it possibly can be cooled with airflow.

Showing off my progression from hubs to mid-drive:

Much lighter. Much quicker. More efficient.

 

[pwd]

Thanks for the explanation, that gives me something to chew on. Those are some cool looking builds you've got!

 

[pwd]

Can get a lot more power out of the motor with gearing as you can convert speed to torque.

A motor has a maximum torque it can produce based on the airgap diameter and stator iron. You’ll need gearing to produce more or potentially a much bigger heavier motor.

Motors run more efficiently at higher speed. There’s two heat producing phenomena that limit efficiency when producing power out of a motor: your wire gets hot from flowing current or your iron gets hot from hysteresis. If u use gearing u can trade the copper losses for iron where with just a motor without gearing (running less rpm) your copper will likely be ur larger heat producer and your missing out on some cooler power production

Maybe more importantly if it’s not a hub motor it possibly can be cooled with airflow.

Showing off my progression from hubs to mid-drive:

Much lighter. Much quicker. More efficient.

Does this illustrate your explanation?

Motor Simulator - Tools

Our ebike motor simulator allows you to easily simulate the different performance characteristics of different ebike setups - with a wide selection of hub motors modeled, and the ability to add custom batteries and controllers and set a wide variety of vehicle parameters you'll be able to see...

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I chose the same motor in the simulator but System B's motor is half the rpm/v of System A's motor and the motor gear ratio is also half (to try and keep the math simple):




With that simulation I can see that the motor with the higher gear ratio and higher rpm/v is running cooler during that simulated hill climb.

 

[Hummina Shadeeba]

If u can run that simulator u can figure everything yourself. That’s a surprising extreme end result with those two motors with their relatively small variation

 

[pwd]

Thanks for your input. Yes the efficiency difference is less than I was picturing but one ends up cooked and the other tops out at 79 C in that scenario.

 

[thepronghorn]

The wheel torque is the same in both setups, but the half Kv motor with half gearing has to put out twice the torque. As @Hummina Shadeeba said, motor torque is constrained by its size and design - the way it is wound doesn't affect the torque capabilities very much. To put out twice the torque, the motor is generating twice the heat, and that is why it is burning up in 5 min. The reason it is generating twice the heat is because the rewind for half Kv doubled the motor's resistance. You can put in the same current and get double the torque, but that comes at the expense of double the heat.

 

[pwd]

Thanks for that explanation too, that helps clear things up.