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[alvinlwh]

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[Wooders28]

As far as I'm aware , it's to do with Watts (unit of power) and also RPM capability plays a part (but that's motor limit not esc)

Watts = Volts X Amps 

So, as the volts increase, the amps need to decrease to keep the Watts the same.

 

With regards to the RPM, motors are max speed rated, the cheap brushless motors are only rated to 50,000rpm, whilst the Castle motors are 100,000rpm, so you need to keep within those ratings , or risk the magnets coming detached off the rotor, and/or bearing failure.

[alvinlwh]

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[cyclonecap]

Higher voltage with lower current is more efficient but the trade off is weight and packaging.

[Wooders28]

Can only presume, more volts and less amps ,means less heat and also, if using less amps , the battery would last longer? 🤔

 

 

[ad456]

@alvinlwh torque would be a big player in this. A 2s 4500kv might struggle with a heavier car while a 3s 3000kv with a smaller pinion might run cooler and faster. This example is extremely arbitrary but I think the theory is right although it’s all about balance for the particular application. 

[alvinlwh]

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[ad456]

I’d think that you may not be wrong there. @Mad Ax has one and maybe able to advise better that me. 

[El Gecko]

Yes, it's all about balance, but I think it's even simpler than watts.

Ohm's law: voltage = current X resistance

When you increase the voltage of the battery, but keep the resistance of the motor the same, the current must increase. ESCs can only handle so much current or they overheat.

A 550 motor at 12t has more material in its copper windings than a 12t 540, so the resistive load is higher.

So the 550 will not move as much current through the ESC as the 540 would, for the same voltage of battery.

That is why you can run a lower-turn 550 with a higher-spec battery on the same ESC as a 540 with a lower-spec battery. The current moving through the ESC is roughly the same, or at least within acceptable values for both.

[alvinlwh]

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[alvinlwh]

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[El Gecko]

On 11/5/2022 at 10:20 AM, alvinlwh said:

However I am not keeping the resistance the same in my example (or am I? 🤷). I am comparing 2 different motors, 3000kv vs 4500kv with 2 different batteries, 2S vs 3S resulting in the same end RPM. 

Voltage is the measurement that's proportional to RPM.

4500Kv/10.5t on 2S/7.4V = 3000Kv/15.5t on 3S/11.1V = 33,300RPM in both cases

The higher voltage turns the "slower" motor faster, reaching the same RPM as the "faster" motor with a less powerful battery.

The advantage of the bigger battery and 3000Kv motor is in heat management. The 4500Kv motor will have less copper in the windings than the 3000, 10.5 turns vs. 15.5. So the resistance (ohms) of the 4500Kv motor is less than the 3000Kv motor. The ESC needs a certain resistance on the motor circuit, and the less resistance, the more heat is generated because the more current is allowed through. So the ESC will run cooler with the 3000Kv motor, and the motor itself will run cooler with the larger battery, even though the top RPM is exactly the same for both on paper.

Taking it a step further, the reason they give you RPM/motor turn limits on the ESC is for heat and current. If you tried to run the 4500Kv motor on 3S on that ESC, the resistance would likely be too low and there would be too much current flowing through the ESC, resulting in fairly immediate thermal shutdown.

The 3000Kv will generally have more torque than the 4500 on either battery, because of the higher resistance. More resistance in the motor generally means more torque.

[alvinlwh]

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[El Gecko]

On 11/8/2022 at 6:36 AM, alvinlwh said:

However, I noticed (I could be wrong) if the 3 and 2S are the same size, the 3S suffers from lower capacity and therefore usually lower C/A due to the smaller capacity right? 

Yes, indeed. Same reason that 5000mAh NiMH cells are just a wee bit longer than 3000 or less: energy density. You can only pack so much energy into a given physical space for any type of battery, so in this case if you're trying to fit three 1S cells in the space of two, those three cells must necessarily be smaller than the cells used in the 2S setup. Which as you suspected means less mAh available and possibly lower C and discharge rates depending on the cells. So for example a 3S 2000mAh may end up being the same physical size as a 2S 5000mAh. The 3S in that case would still have more power from the higher voltage, but since the mAh is less than the 2S, it wouldn't last as long (runtime).

[LongRat]

Just compare the total stored energy in Watt-hours.

2S 5000 mAh -> 7.4V x 5Ah = 37Wh.

3S 2000mAh -> 11.1 x 2Ah = 22.2Wh.

Do with that energy what you will, by specifying the motor/ESC/gearing to draw the power of choice.  Either run longer or run faster.  Differences in discharge rates probably make little difference.  If the higher voltage battery has a lower C rating, that's not really a problem because it doesn't need to discharge such high current to develop the same power.  So this balances out.