Exactly!
I will point out that while the motors may produce the same torque at a given voltage, the watts are much higher on higher KV motors (if you bump up your VESC motor amps appropriately). Torque to the wheel is a combination of motor watts and gear ratio, so if they are both geared for the same top speed, wheel torque will be higher with a higher wattage motor.
Yes, and to add on what the other two folks said, a higher Kv motor geared down more will have less problems with zero rpm HFI, sensors, or VSS if you use any of that
Higher Kv geared lower is a lot better usually. Use the biggest wheel pulley/cog that you can.
A guy in my local community has pretty much the same Mountainboard as me but instead of Maytech 170KV 6374 he has Flipsky 140Kv 6374 and With the same settings he gets significantly lower torque then me. He also seems to have problems pulling the same 90A motor current as me. We are not sure what’s causing this.
The only other difference is that he uses a maker X 6.6 instead of my Trampa6MK4
Could this be a result of the lower KV or more likely a problem with the VESC ?
It’s nice to see a real world example of KV selection affecting performance!
Lower KV motors use thinner wires and can carry less amps. Take a look at the chart above for that Reacher motor and look how the rated amps change with KV. I don’t understand why Flipsky/Maytech and others don’t properly rate their motors when the KV changes. They all say 80A regardless of KV which doesn’t match up with more detailed motor manufacturer specs.
Selecting KV should really be based off max motor RPM (a 63mm motor can do ~18k), battery voltage and pulley size limitations. esk8 recommendations went off track when the early VESCs had a low RPM limit coupled with gearing limitations on small urethane wheels. If you put a 140 KV motor on a board that had a 170kv motor you may notice higher torque right away, but if you change the gearing to match the same top speed, you end up with less.
If voltage could be anything we wanted, lower KV motors can make perfect sense. Notice how the lower the KV, the higher the volts and lower the amps… but they still recommend like 22s on 195kv to get maximum power because these motors can spin to 18k.
The thing is our drivetrain is completely the same. Not adjusted to the KV. We both use a 1:4 etoxx gear drive. He may pull lower motor amps because of the higher resistance motor but shouldn’t he at least have the same torque?
The whole reason he went for lower KV motors is to get better low end torque because we don’t really need that much speed in the woods.
The 140kv motor may be running at a lower efficiency at the same km/h speed, turning power in to heat instead of wheel torque. These motors can run as high as 85%, and lower than 60% if they are too heavily loaded at low RPMs. Pedro did some great work on efficiency charts but we don’t yet have a good way to put that into practical use like the ebike motor simulator based on the loads we actually see.
There are other factors that could also cause him to be slower too:
Larger tires
Heavier rider
Lower battery amp setting will limit total watts to the motor
Different throttle curves
Lower voltage
Most likely, a higher KV motor will improve performance though
Maybe because the higher Kv motors have typically lower winding resistance which allows the same voltage to push more current
Look at your motor detection results and there is a resistance number. I suspect yours is significantly lower.
Make sure both sides of the vesc are programmed, you have to do all the settings twice (once per motor)
Its like the 10th build I helped setup. Im sure everything is correct. I even adjusted the PWM calibration so it hits 100% even if the remote is not pressed all the way to rule out remote problems.
We have exactly the same battery, wheel size vesc settings and gear drive.
The new VESC-Tool will have a mythbuster buit into it. It is called motor comparison…
You can basically plot your motor efficiency in a graph and play with voltages and gearing etc.
In order to measure efficiency we need to measure mechanical power output correct? I see a “torque out” and “torque shaft” field in there so can it accept a load cell input for example (HX711/Arduino)?
Im still on vesc tool 3.00 and im wondering how the “KV” command works. It only outputs 0 KV but it would be great to get KV from vesc tool instead of calculating it myself.
You need to spin up the motor, then run the KV command.
Smh I always knew this in the back of my head but never really believed it as my 140kv build didn’t have a particularly impressive amount of torque. Glad we have numbers for this.
This is still kinda confusing me. If I’m running a 140kv and a 190kv motor at the same settings with the same ratio and everything, they’ll have the same amount of torque?
KV, and KT (voltage constant and torque constant, respectively) are inversely proportional - as one goes up, the other must go down. In an ideal world where winding resistance didn’t exist, and motors run at 100% efficiency, a lower KV motor would produce more torque at the same current due to its higher KT.
Just like KV is related to voltage (RPMs/volt), KT is related to current (newton-meters per amp).
The problem is, that for a given motor design, to decrease the KV, you have to wind more turns of wire on each stator tooth, and there’s only a finite amount of room in there. To fit more turns of wire, the wire has to be thinner. Thinner wire has more resistance, so lower KV motors generate more heat at a given current level.
This means that a lower KV motor should produce more torque due to its higher KT, but in practice, this gets offset by the increased winding resistance and heating, which means that lower KV motors can’t be pushed quite as hard.
I suppose this only applies if the wattage is the same… and since lower kv motors seem to produce less watts you probably don’t get a noticeable increase
Looks like for high voltage builds were gonna need 190kv motors that are made for high RPM, and gear drives with a really high ratio.6+
Or use other methods to lower the KV - stronger magnets, thinner rotor-stator gap, that kind of thing. Winding number is just one of the variables that can be adjusted.
Motors are too complicated 
