How would it impact range on that specific eskate case? Is the efficiency loss minimal or enough to say we would need to add another p group on the 12s battery of the 90kv setup so it has the same range of the 190kv one?
The motors would certainly heat up more with the lower KV, and that will likely be your biggest bottleneck so I’d recommend not going that route. Efficiency wise direct drive seems to be a bit better than belts, but if both options here involve a gear ratio, it’s probably going to be very similar.
I think I would worry more about overheating the motor than a loss in range. Higher RPM means more iron losses and lower Kv (usually) means more copper losses. You would ideally want to balance the two out, and according to Vedder you should keep the motor RPM around 8600 for 50-60mm outrunners (so for the motors we use). That’s probably the point where the iron losses start to get higher than the copper losses. With a voltage of 12S (assuming full charge for these numbers and full duty cycle) 90 Kv would be around 4500 RPM which means that copper losses would likely be more significant than iron losses. With 190 Kv the motor would rotate at about 9600RPM. 170 Kv would mean 8600 RPM.
These numbers above are not realistic numbers though, it doesn’t account for things like max 95% dutycycle with VESCs, most of the riding is not at the full voltage of the battery, and that you are not always riding full speed, etc. 190 Kv should be your best bet to minimize copper losses, without iron losses overtaking.
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.
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.
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.
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
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
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
