Downsides of High kV Motors?

Can somebody do the math and elaborate on this build?

Plug 450kV, 6S, 32:12 gearing and 80mm wheels into the equation and you get a theoretical top speed of roughly 55km/h, that’s without weighting. However, the maximum ERPM on that setup is 68040, and the his VESC is set to limit it to 60k. So his top speed should be a little lower.

So I’ve finally done enough research to be able to fully understand (at least I hope I do) the blog post written by Vedder about this topic. Basically, at the same rpm, two motors, one with 150kV and one with 300kV will have the exact same efficiency/losses, but the 300kV motor will be pulling double to current. So in theory, the motor kV only matters when choosing voltage. BUT, since our ESC’s will have the same resistance regardless of the kV of the motor, losses are much higher on the ESC with higher current/higher kV motors. This is why using the lowest kV possible is an ideal setup.

He also points out that other losses start to build up exponentially around 60k ERPM. So, we should be picking a voltage/kV setup that will give us a maximum ERPM of 60k.

Using this, I found a 5055 motor from may tech with 190kV. Running at 12S, this would get a max ERPM of 57456 which is nearly perfect, and the low kV would mean lower losses on the ESC. The motors are rated for 1170 watts. So I think these motors and 12S would be the ideal dual motor setup. Of course we still need some data on watt usage while hill climbing and the like, but I think 2340 should be enough right?

Thanks to everyone who helped explain all of this :slight_smile:

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good to see that people do their research :+1: @chaka is a big fan of double 50mm systems

I have a 300kv dual motor setup running 4.12 dual vescs, split servo, no can bus on 10s battery.

This is on a belt driven mountainboard that was built with the 300kv motors and dual VESC’s on 10s from the builder.

I also have a bamboo GT, that has smaller motors, but the torque from the bamboo GT is much greater than on my 300kv motor board. Granted they have different size wheels and I don’t have the all terrain conversion to put on bamboo GT, but I think there would still be a big difference in torque if they had similar wheels.

I haven’t taken the mountain board to the limit, it just keeps accelerating past my accustomed speed limit of 22mph of the Bamboo GT. Gets scary going that fast.

I have an idea… why not run dual motors with different KVs?

Use a low KV motor to start off and get up hills, and when you’re on the flat and want speed, switch electronically to the high KV motor to take over!

Vedder could even incorporate that in to the VESC where it has two motor outputs, one for high KV one for low KV and depending on amp draw, send power to the relevant motor?

it’s been done!

I forget who, but someone actually tried this. If I remember correctly they said it had a similar effect to something like a sequential turbo setup in a car.

Edit: Jinra beat me to it :stuck_out_tongue:

That would be smart, put one of them on a one-way bearing as well so you don’t have the drag of both belt setups (But still have some braking power)

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