KV and torque

PART 2 is here:

Online there's a great debate about whether a low KV motor has more torque than a high KV motor. In this video I'll prove that the torque does not strictly depend on the KV number. Yet there will be something else to say in the next video that will hopefully change everything once again :)
Bear in mind we are talking about the generic "torque", be it starting torque or half-speed torque does not matter.
I am an electrical engineering student and my goal is to make engineering stuff easy and enjoyable for anyone in this hobby.

[h]

Liked the mathematical explanation, the style of presenting information and the shortness of the video !
I hope you continue making videos :D

[paulmartin]

Its dead simple. If you have a propeller say a 32 " diameter one for example, and you have a 130Kv motor and your run it at 70V to get maximum power then the blade will spin at 9100 rpm. That means that the tips are totally supersonic and it's really inefficient. So to bring it to just below Mach 1 you have to run it at 60V which means that the motor is running at about 60/70 or 86% of its peak power. In fact its still quite inefficient and if you said that you wanted the tip speed to not exceed M0.7 to avoid compressibility losses you would have to run it at about 5600 rpm so you are now down to 61% of max power i.e 43 V. If instead you decided to use an 80kV motor then at 70V its running at 5600 rpm so its the sweet point. As Power = Torque x Omega where Omega is the rotational speed. So in the low Kv case you are running max power and in the 130kv you are running 61% of max power. For the same rpm the lower kv gives you a much higher torque. It's not about electrical theory and winding etc it's about matching your motor speed as close as possible to the rotational speed you need to operate whatever you are driving for it to be at its design point. If you geared the higher kv motor so it drove the propeller at the speed you wanted then there would be no difference.

[LSUtiger]

I'm missing something here. You said Torque is related to current and KV, but where does Voltage (Power) come in to play? Certainly 5A at 10V produces more torque than 5A at 5V (for a given KV) right? Or maybe I'm confused.

[FPVPortugalTV]

amaazing video, only 2min but so informative. subscrived.

[Nick-_-]

These are awesome. I love the sketching. More of a sped up sequence would be nice vs a time lapse. Less Jumpy

[paulmartin]

1/4 current? if say the kV is reduced by half so the number of turns (and wire length) doubles and the cross sectional area is reduced to a quarter to package it all and the resistance is proportional to length and inversley proportional to cross sectional area and the limit is really a thermal one i.e I2R (current squared x resistance) then isnt the maximum current reduced not to 1/4 but to 1/8?

[sciencebug]

you forgot that wire length is 4 times what is was before. so resistence goes up 16 times in total, and torque at same voltage is actually only a forth of what it was in the higher KV motor. but power is a sixteenth, so a lower KV is more efficient, in your simplified example

[janeas]

I have a motor which is 36v 280kv 3 Nm so do you mean if I will winding the motor for 90kv 36v it will have the same 3 Nm like before?

[HoangLinhBuiSilverDrone]

Lower KV motors has more torque at the same amount of current. Which means 1000KV x 6S has more torque than 1500KV x 4S at its peak draw

[Zarr..a]

Why are you speaking so fast? If you do not want to learn anything it is not necessary boring yourself. Ok.