While I agree thats true, I think unless someone measures the losses we won’t have a better number.
Actually probably a higher number might be better because of this… 8600 RPM means 60k ERPM on a 14 pole motor… which is the limit of HW 4.x
2 Likes
1 Like
Agreed. Lots of dynos being built, hopefully this year will get in to the nitty gritty a bit more with all the various escs popping up and wanting to tweak FW more.
2 Likes
If the motors are the same size, same copper fill, same no load speed at the wheel, you should get almost identical efficiency, regardless of the kv.
1 Like
This is what I’ve seen explained as well.
1 Like
Spin both motors to top speed with no load (same voltage) and likely even with the higher kv having more losses at that full speed it’s going to be a small increase compared to how much better it will do with the extra gearing. The 190kv w more gearing will likely have better range and keep cooler.
6 Likes
I am also sure that the 190kv setup is more efficient. But the question is by how much? Do you have any idea?
of course will depend on how you use the motors and how much torque youre trying to get out of them and how fast you go. do you have the motors? spin them on the bldc tool and tell us the no-load current draw of each and we’d know more.
1 Like
Unfortunatly I do not have them at the moment. I will keep you updated.
We can discuss all we want, but it turns out that the riding style is what matter most to answers that question
This will determine what losses are the bigger contribution to the losses, copper or core
If you build one setup and log your rides, I have a program that will calculate what would be the new efficiency for any other motor/gearing combo, as long as I have both motors modeled
3 Likes
Thank you for your reply! It is very nice to know that you have such program! In what situation does the 90kv setup or lower kv would be more efficient?
In a setup and riding style that your board requires little torque to maintain speed, you don’t accelerate and brake aggressively often, the lower Kv would do better. This is due to the resistive losses being low since the torque is low, but you are spinning the motor fast all the time, so the core losses are high and add up over the entire ride, spinning the motor slower would lower them
Here is an example of nearly 400 km of logs for my board, top graph is resistive losses and bottom core, resistive losses are 1.5% of the total energy used by the board and core losses 3.28%, so for my riding style it’s not optimal, you have the highest possible efficiency when both resistive and core losses are equal.
For my setup, this means that lowering the gear ratio would make it more efficient, in fact I just did that, from 14/80 to 14/72, and increase the motor current by the same ratio, will log a few hundred kilometers and run this analysis again
5 Likes
this is seen in terms of Amps to the motor over time?
you could assume you would draw power less of the time because when you hit the throttle you go 99mph in 5 sec
so whenever you don’t ride like that you will waste heat into the motor is the point maybe.
Yes, the higher the current the higher the losses to a square factor (1.5*I²R)
This would be a interesting test to do and would highly depend on the setup, if someone has a regular comute with lots of stops, it would be the perfect scenario
In theory you would want to accelerate keeping the motor in the sweet spot of efficiency, that would following my nicely drawn black line. As speed increase you increase the current
Would be cool to have a mode that automatically does that with VESC, maybe when you press and hold a button the max current can’t go above this max efficiency line, this way you still have full power just by releasing the button
1 Like
this would require a speed input or something no?
haha I hope it waits for you to unpress throttle first
2 Likes
Don’t think so. The way I imagine is, if this secondary button is pressed, it just restricts the maximum throttle to the maximum efficiency line, so by going full throttle you are at the maximum efficiency point, just ease on it to maintain the speed as always
Some more logic is needed so that when you release the secondary button it doesn’t suddenly goes full throttle
1 Like
I think 90kv with 1:5.5 gear ratio and twice the voltage would have higher efficiency and a bit more acceleration than either option.
24s is a little too much for me 
!!! But yeah the acceleration would be insane!
1 Like
Very nice info Pedrodemio! You seem to have very great knowledge! So lowering kv is nice when you ride fast on streets without braking and accelerating. So it would not be great for a mountainboard that goes up and down hills, gets stuck in mud, etc.
You say that resistive losses is 1.5% total energy used and core losses is 3.28%. To me that is pretty small. Do you have an idea how worst it could be for a mountain board running the setup I mentionned? 12s, 6374 90kv, 1:2.6 ratio, 200mm wheels. Could it get to let’s say 15% total energy lost because of the low kv and low gear ratio? Or do you think it will always stay under 15% with good motors like the maytechs?
Also do you think going 16s or 18s instead of 12s with also an higher ratio like 1:4 would reduce the losses?
Thank you!
2 Likes