Motor Kv selection seems to be a common question, and after lots of lurking I think I might have it generally figured out. Hopefully, I’m not wrong on the subject and maybe this thread will be worth adding to the stickied guide list.
When you design your board, you generally have a top speed in mind, and will pick your motor, gear ratio, and wheel size to hit that target. So basically the question “what Kv should I pick” really boils down to
- should I pick a high Kv motor and gear it up/use small wheels or
- should I pick a low Kv motor and gear it down/use larger wheels
The confusion comes from the fact that Kv is essentially constant/Kt, i.e. a high Kv motor will have a low Kt torque constant. Now some people think that this means high Kv motors have less torque than their low Kv siblings, but this is not the case, because the effect is exactly offset by the changes in gear ratio that you will make in order to achieve the same top speed.
Let’s try a real world example, let’s say 50 kmph top speed target, with a 10s LiPo: Configuration 1: a 200 Kv motor at 36 V turns a 7200 rpm. Assuming 83mm wheels, that’s 36V *200rpm/V *3.14 *83mm *60min/hr 1km/1000m 1m/1000mm = 112 kmph with no gearing. To get this down to spec, we have to gear down to 2.24:1. Now Kv 200 rpm/V 23.14radians/rotation 1min/60s = 20.93 A/Nm, or 0.04778Nm/A. Assuming the battery maxes out at 50A, the max torque at the wheel is therefore 0.048Nm/A *50A 2.241= 5.38Nm
Configuration 2: a 300 Kv motor at 36 V turns a 10800 rpm. Assuming 83mm wheels, that’s 36V *300rpm/V *3.14 *83mm *60min/hr 1km/1000m 1m/1000mm = 169 kmph with no gearing. To get this down to spec, we have to gear down to 3.38:1. Now Kv 300 rpm/V 23.14radians/rotation 1min/60s = 31.4 A/Nm, or 0.03185Nm/A. Assuming the battery maxes out at 50A, the max torque at the wheel is therefore 0.03185Nm/A *50A 3.38= 5.38Nm
So theoretical torque is the same regardless of what Kv we choose, because we will just pick a different gear ratio to compensate. How do we pick a motor then? Well, have you ever noticed how two sk3 6374’s with lower Kv’s have lower wattage ratings listed by Hobby King? I think that is because the way you actually achieve a lower Kv for a given motor housing/stator is by winding the stators with thinner and therefore longer copper wire. Long, thin wire has higher electrical resistance than short, thick wire, wasting more energy in the form of heat losses, which not only decrease energy efficiency, but also cause increased wear on the motor.
So in short, the old adage that a low Kv/high Kt motor is good for torquey builds and bad for top speed is kind of off the mark, since your pulley gearing changes all that anyway. You should always pick the highest Kv available as long as you can find motor/wheel pulleys to support the intended top speed.
Well… Except for the fac that the VESC (pre v6) seems to have an erpm limit of about 60k, so if your Kv is too high relative to your battery voltage, you risk damaging your speed controller. Also there’s some thought that maybe needs to go into the possibly higher mechanical losses associated with a more dramatic gear ratio, maybe some weird inertial effects due to increased rotational mass of a larger wheel pulley too, I dunno, haven’t hammered it out. But I bet these are minor effects, so in the end they can probably be ignored.
I will find them though because this is a good resource!