“Mechanical” losses in the motor from the motor’s bearings, “belt losses” and especially “switching” losses in the controller all have something in common— they are insignificant compared to iron losses.
The iron losses are in turn insignificant compared to copper losses (when the motor is below 9k rpm).
If you can reduce copper losses by 50% by spinning the motor faster with less current for the same useful output (ie higher gear ratio), even if all the the other insignificant losses increased slightly it’s still more efficient.
That’s why higher gear reduction ratios are generally more efficient.
As an analogy, suppose you owe someone $100 dollars.
They tell you if you drive them to the grocery store they’ll shave $50 off your debt.
Driving them to the grocery store costs you $5 in gas.
By driving them to the grocery store you save $45 dollars factoring the cost of gas.
Driving them to the grocery store is like increasing the gear ratio. The $50 savings is the efficiency improvement from reducing the copper loss. The $5 expenditure on gas is the efficiency reduction from iron, belt & mechanical losses with increased gear ratio.
If you want to spend the least amount of money, do you turn down the $50 savings because it’ll cost you $5 in gas? That would be like saying you want the most efficiency, but you won’t increase your gear ratio because of iron, belt and bearing losses.
Assuming you are running the full current through the motor constantly, yes. But in a real ride, you accelerate, then coast / hold speed for a good while before accelerating or braking again on a street board. Which in turn makes the iron losses more like a constantly happening loss, while the copper losses are minimal during coasting and when holding speed. Therefore iron losses have a larger role.
For an offroad / racer board, this statement above doesn’t hold much truth due to the constantly flowing higher currents, therefore a higher KV is more appropriate for those cases, because high copper losses are constant thing, high speeds usually isn’t.
I’d like to see your graph assuming 500w sustained so roughly 12a at 43v. 500w is roughly what a typical person on a bike can sustain for at least a few minutes.
Apologies, copper losses are not just I^2 * R, its roughly 1.5 * I^2 * R in FOC according to Gamer43 (source: Motor parameter wiki thread)
I’ve found an old log I’ve made, back when my board was a single drive. 140KV FS BH 6384, 20T/44T, 115mm wheels, 12S battery. Reached 57 km/h once in the log, and I wasn’t doing hard accelerations. Around 40A, though getting up to speed took a long while, both due to the board being single drive and due to the low current.
In excel, I’ve calculated that I lost around 2351 units of energy to copper losses and 2935 units of energy to iron losses. I’ve simplified the iron loss calculation, calculated it like if it were linearly dependent on the rotational speed. iirc that is not true, it has linear and quadratic components as well, so it’s not 100% accurate but it should still be somewhat representative.
Not sure how comparable this data is to a dual drive though, if someone sends a CSV log I can calculate the losses based off of that. I don’t have any dual drive logs, and VESC tool over BT is really finnicky on my board, I would prefer to not even touch it, I don’t want to accidentally screw something up.
Tomorrow or Monday I can make a graph for that. The file is on my PC and I am not at home today.
Will make graphs for 30km/h, 40km/h, 50km/h, 60km/h constant speed. Estimating the required power to hold that speed by P=v^3*0.35 where v is velocity in m/s.
That simply impacts how much torque is generated, and usually it’s really small motors that have truly trapezoidal waveforms as they have large gaps where the stator-rotor flux coupling is poor.
You may need more A in FOC to get to “one-to-one” torque, but it’s still going to spit out less heat than trapezoidal.
It’s I^2R but the VESC only measures 1/2 the resistance (from one lead to the center of the winding, ie one of the phases).
During use for trapezoidal you double the VESC measured resistance to get the 2 phase resistance, then I^2R for the motor current where R is 2 times the VESC measured one-phase resistance.
FOC will typically produce around 8% less heat for the same torque, but for motors with concentrated windings like typical a outrunner used in ESK8, FOC will have more torque ripple. You need distributed windings for FOC to eliminate the torque ripple.
I tracked with Strava and the voltage was around 3.2-3.4V per Li ion cell. I don’t remember if I even hit the VC. I have it start @ 40V
Sorry that I didn’t update the results for the 12T vs 20T. Might record both rides again this week on a different route (very flat, mostly asphalt + some gravel roads)
