Low KV Experiment

[Hector_Fisher]

I 100% agree when it comes to the math and science.... talking about real quality testing where the scenarios and loads are a real fair comparison.

In the real world observing people having issues with heat. Lower KV fixes the issue time and time again while maintaining the speed/power they want. I believe they will ultimately have a less responsive car but maybe they don't notice.

Fair enough, having swapped to a lower kv motor in my MT410, i have to concede that I no longer run into heat issues. However, power is a fickle thing to measure. The moment you change gearing or the motor you change the potential power draw. I don't believe the typical driver can "feel" power, but rather changes in torque. Whose to say that in these instances with lower kv motors that power isn't being "better" applied to an RPM range where it's more efficiently used? In my anecdotal experience with my MT410, my truck is now slightly slower (still more than sufficient for my needs), but runs cooler and has more torque. I'd say that's a brilliant example of misapplied power.

I think my point is that the generality goes: at the same power level, the lower/higher kv motor will not perform the same thermally.

 

[KnowAir]

Fair enough, having swapped to a lower kv motor in my MT410, i have to concede that I no longer run into heat issues. However, power is a fickle thing to measure. The moment you change gearing or the motor you change the potential power draw. I don't believe the typical driver can "feel" power, but rather changes in torque. Whose to say that in these instances with lower kv motors that power isn't being "better" applied to an RPM range where it's more efficiently used? In my anecdotal experience with my MT410, my truck is now slightly slower (still more than sufficient for my needs), but runs cooler and has more torque. I'd say that's a brilliant example of misapplied power.

I think my point is that the generality goes: at the same power level, the lower/higher kv motor will not perform the same thermally.

Sounds like my experience with the 1450kv. It's definitely slower overall than it was with 2000kv but still satifies my needs.. I should have clarified that. I'm a "seat of the pants" guy so, all the technical talk "kills my rc buzz", LOL. I just throw $hit at the wall till something sticks✌

 

[Dan B.]

Lower KV fixes the (heat) issue time and time again while maintaining the speed/power they want. I believe they will ultimately have a less responsive car but maybe they don't notice.

100%. What do you do when your rig runs too hot? You reduce KV and gear accordingly. Leaving fans, ambient temps, etc. arguments aside for the moment. In practical IRL experience that approach just quite simply works.
And a “less responsive” car is for me a total non-issue. At the very least, it certainly is subjective. When you have a small, 20# toy car with 8-12hp capable of 200+mph speeds, than what exactly is “less responsive?”

 

[Diem Turner]

Do you ever sleep? What time is it in Wienerslider, anyway? And don’t come back with, “it’s Wienerslider time!!!” Just sayin’…

I sleep at the end of the month...whether I need it or not.

"At the tone it will be 2:02AM..................................beeeeeeeeeeeep."

I agree that there isnt much objective evidence for this, but thats why i really want to test it! I just really don't want to spend $130 on a new motor for it to fail, and run hotter than what I have now LOL. But then again, if it worked, it would be well worth the money.

Fair play. FWIW, you have my blessing and support in your quest (I know that doesn't help to pay for the motor but, it's the best I got atm).

 

[Lennart]

My beliefs:
Running a motor with high rpm will decrease efficiency because of Eddy currents and not primarily friction. The faster the motor spins the faster the stators magnetic field has to be changed and a fast-changing magnetic field induces stuff. Those currents will generate heat in the steel stator and inside the rotor With MAGIC. Just like an induction cooker.
On the new castle 1515 v2 motor you can turn the rotor quite easily by hand, which could mean more efficient top end rpm.

Im no speedrunner but I think when running max voltage on a motor (often 8s lipo) they don't get their top speed in the upper 10-20% of the motors rpm range.

But what I know is that every motor has different characteristics and you can only figure things out by testing them and measuring while driving. Or ask castle for a Power/ Torque vs speed motor dyno graph.
And when gearing up by 1.2 you need 1.2 times more amperage resulting in 1.2*1.2 (1.44) times the heat created by the amps in the motor wires. P = (I^2) R

 

[ARRMA King]

I’ve already experimented with this in my infraction up to 14s:

- pick a kv range for your esc application, definitely keep rpm’s under 70k for run of the mill as rotor magnets will not hold and jam stator (still have to test TP’s 100k claim some day)

- gear down as low as you can and let rpm’s do the work, let the motor sing! I’ve ran as low as 9t. Again amps stay low.

Upon this, heat stays low in the summer, 120’s, ran up to 80 in a parking lot and run times of 30+ min continuous runs easily (sometimes you’ll get bored already on the 2nd set of batteries) but the high voltage efficiency is undeniable.

Using a low kv motor to gear it back up is theoretically the same amount of work performed, no real difference there.

Using higher voltage on the same kv yet geared for the same capped top speed, the motor is still doing motor work with a wider power band, expect more heat. Think about how the motor is spinning higher rpm’s unloaded, that’s more mechanical work there.

All decent brushes less motor should have a constricted efficiency rating in the 90+. A quality built motor helps the most here.

The auto industry has already moved onto and trying to standardize 800v. There’s already your sign. HV is the way!

 

[Diem Turner]

I’ve already experimented with this in my infraction up to 14s:

- pick a kv range for your esc application, definitely keep rpm’s under 70k for run of the mill as rotor magnets will not hold and jam stator (still have to test TP’s 100k claim some day)

- gear down as low as you can and let rpm’s do the work, let the motor sing! I’ve ran as low as 9t. Again amps stay low.

Upon this, heat stays low in the summer, 120’s, ran up to 80 in a parking lot and run times of 30+ min continuous runs easily (sometimes you’ll get bored already on the 2nd set of batteries) but the high voltage efficiency is undeniable.

Using a low kv motor to gear it back up is theoretically the same amount of work performed, no real difference there.

Using higher voltage on the same kv yet geared for the same capped top speed, the motor is still doing motor work with a wider power band, expect more heat. Think about how the motor is spinning higher rpm’s unloaded, that’s more mechanical work there.

All decent brushes less motor should have a constricted efficiency rating in the 90+. A quality built motor helps the most here.

The auto industry has already moved onto and trying to standardize 800v. There’s already your sign. HV is the way!

This mirrors my own thoughts on the subject.

On more modest gearing, 90+ efficiency is definitely no problem. But as you push the over gearing for 3S speed runs, you're getting into 0.8:1 and lower ratios where efficiency falls to 80, sometimes even a little less.