Downsides of High kV Motors?

So after looking at @VladPomogaev’s data on watt usage, it seemed like the potential wattage of the motors we are using is much higher than it has to be. So I went looking and found a 5045 450kV Turnigy SK3, and it was rated at 1260 watts. From what I’ve seen 1260 should be enough, especially if you are running dual motors.

Using the esk8 calculator, I found that if you use a 6s li-ion pack, and a 40:12 gearing ratio, you would be able to hit a weighted top speed of 40km/h.

Since you’d be running 6s, you would have more space to run cells in parallel, helping solve issues with low discharge rates of li-ions. Also lower discharge li-ions have higher capacities.

Also, since BLDC motors have higher efficiency when they are at higher rpms, the large reduction ratio helps the system be more efficient.

As far as I can tell the only thing stopping something like this from working is the ESC. However I’ve heard the VESC v6 will be able to accommodate much higher kV motors as well as higher current.

So what gives? Why does high voltage/low kV seem to be the standard for high performance boards? It seems like we could be lowering the cost and weight of our boards with this kind of setup. Or am I missing something?

Higher kv motors tend to have less copper in the stator, which in turns means less power/torque. Higher voltage leads to less amp draw and better efficiency due to less energy lost through heat. That’s why high voltage + lower kv is a good combo for this hobby.

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does high kv motors always tend to have less copper in the stator? or you mean smaller motors tend to have less copper in them? I say that because low kv motors have more turns so the wires are thinner so there is higher resistance so higher heat. for example, look at the resistance, max amps and watt values of these two motors: http://www.hobbyking.com/hobbyking/store/__18184__Turnigy_Aerodrive_SK3_6374_149kv_Brushless_Outrunner_Motor.html
http://www.hobbyking.com/hobbyking/store/__18129__Turnigy_Aerodrive_SK3_6374_192kv_Brushless_Outrunner_Motor.html

@NickTheDude I think 40:12 gearing ratio might be problematic because you have small contact area between the belt and the motor pulley. may be fixed with a tensioner.
I think that generaly low kv motors are more effecient, because they have lower resistance, for example, look at big and heavy are bicycle hub motors that are less powerfull than rc motors.

Jinra nailed it. Also, keep in mind that my power consumption tests were done on perfectly flat land.

In order to go up a hill, your power consumption increases according to W = mass x acceleration due to gravity x speed of climb + regular power consumption at a certain speed. Meaning that in order to climb a 10m hill in ~20 seconds, the average person would be pulling an additional 343 watts, on top of the power they use to move along that hill. Since motors and ESCs are not 100% efficient, the actual power consumption is even greater.

Once again, that’s why cheap boards use low powered motors, it’s because they can’t go up hills. Also, @Hillso is right, there’s a limit to how much you can bend a belt (or chain) while keeping the grip and not destroying the belt in the process.

Why are the motors we use between 150kv and 300kv but then there is a whole area of motors which are like 2000kv +?

Why such the big jump in KV and what could those 2500kv motors potentially provide over lower kv like 200kv. (My local RC shop only sells 1000kv+ motors)

higher kv motors are used for planes/heli’s. They need a lot of rpm per volt since they use less batteries due to weight limitations. There’s not nearly as much resistance to spin a propeller vs propel a human.

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So if you had two boards, one with the 149kV motor, and the other with the 192kV motor, and they were both geared for the same top speed, would they not have identical performance?

The reason I think that a high kV motor would be more efficient is because of the graph below. Is the heat lost from the higher amp draw is enough to cancel out the gain in efficiency that comes with higher RPM?

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Higher RPM may be more efficient if you were going a constant speed. However, with esk8, you’re constantly slowing down and speeding up. The longer you spend overcoming resistanc, the more amps you pull and the worse your efficiency. Not to mention, high speeds run into a lot of resistance which will increase current draw exponentially.

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Does that not apply to both high and low kV motors though? Or does the relationship between amp draw and efficiency get worse as you increase kV?

Well, like I said earlier, higher kv motors tend to have less copper due to the thicker windings. You can’t fit as much copper in there with big windings, much easier to fit more copper with thinner windings, which result in lower kv. More copper = more torque = gets you up to speed faster.

You can try accelerating or going up hills with a high kv motor, but I doubt you’ll get up to speed in a reasonable time, assuming your motor or esc doesn’t burn out first.

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Isn’t the lower torque compensated for by the gearing though? I get that the higher kV motors tend to have less copper, but in the examples @Hillso had, the higher kV motor is listed as having a higher wattage than the lower one.

There are a bunch of things that affect torque to varying degrees. While gearing is important, motor size (ie copper content) is more so.

U can get the same copper in regardless of kv. If anything you should be able to get more in with higher kv as it needs less turns so less insulation but it’s splitting hairs. Most manufacturers use multistrand copper and many windings in parallel so don’t have to bend a big wire. If u did bend a big wire instead of the multistrand you could get a motor with lower resistance to inductance and more copper and a better motor.

A motor can put out the same power regardless of the kv. Running a high kv and fewer volts and more amps is just as good as long as the esc can do it and your battery wires are thicker. You get the same torque to heat ratio in the motor regardless, which defines the motor’s limits

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I have a question. current limit of a motor determined by heat? what determines voltage limit? Thanks.

The wire enamel. And the bearings more so. The bearings max speed

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I’m using 2 of these motors on 6s with 2 vescs :smile: great power and speed and don’t get too hot. Would recommend :joy:

Really? What kind of gearing around you running? Top speed?

Currently I’m using 12t-32t, 60k limitied erpm on the VESC with 70mm wheels, but I’m upgrading to kegels 36t soon. I’m 75kg and it takes me up pretty steep hills, and the top speed is insane - like 50+km/h

What motor u use?

The little turnigy sk3 5045s