Motor KV 100% Explained: Why go from 4S ➡️6S➡️8S?! 

I can't help but feel this video was a direct response to the commenter who claimed Chris Rosser doesn't understand motor kv. Well done.

Science: Here's how this works The internet: you are wrong because I don't have time to understand what you are talking about and I think feelings are the same as facts.

When I raced rc on-road cars (for 2 decades) with dc brush 05 motors, many tricks to getting more rpm and power out of the motor. With a torque off-road motor we would turn the arm down (smaller diam) for more rpm which was better for on-road racing. Cut a slot in the arm poles in the right spot (offset from the top of the "T") would effectively advance the timing for more rpm. Slot and short stack arms. All of this changed the effective kv rating. Quite a few tricks with the brushes and brush springs. Special drilled brushes with packing added and special comm drops (this would make the drops last longer, like it had a reservoir). Speaking of brushes, during "handout motor races", the motor (Mabuchi or Johnson) was sealed and had very hard brushes (so they last), but would take the brushes forever to properly seat to the comm (giving more power), we would secretly "water dip" them, run them in a glass of water for 20~30 seconds. The water caused a huge load on the arm, so it had low rpm and high current, the high current setted the brushes quickly and kept the motor cool. Lots of racers always said I get the "good motors" haha. The 6S sub-C nicads we would zap (years before anyone else knew about in (back in the early 80's. We would put a 4000A pulse into each cell, this would weld the internal connections for lower IR, made a huge difference. This was before ESC, we had a servo with wiper arm across a resistor for the speed control. When mosfet esc finally came out, the sport esc was $60, and the racer esc was $250, only difference was the fets (6 forward, 2 rev). I had a friend that worked at IR and would get the best fets (lowest RDSon) and swap them, they were so good we could cut the to-220 tabs off to save weight. I came up with the first balance discharger with led display in late 80's, have some great memories racing, the older you get, the faster you where, haha. Now I just fly LOS as I am almost 60. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-uHoFfpLSb2g.html&t

5 years ago: Is 6S the future? 10 years from now: Is 60S the future?

Science bringing clarity in the confusion of social media. Thanks Chris !

Great presentation! Just a couple of notes on the video: 1. Why not mention Delta and Wye terminations that help us optimize motor KV? I assume to have simpler explanations (2:1 with 4S Vs 8S). That isn’t the whole story… but for the target audience, I would say you did a great job 👍🏻 2. When I design a motor for FPV, I try to have the wires be as neat as possible, because the biggest hit to motor durability is when you get wires crossing and putting force on a single point. Especially when closer to the iron, and more susceptible to high heating, in addition to the shock and vibration we put them through. (Exactly as you described 👍🏻) That’s why you tend to see smaller diameter wires potted in place, which is a net negative (and I really dislike), but is needed if the motor is expected to survive for a reasonable amount of time. Personal notes: 0.3mm is my preferred minimum conductor size, and 0.5-0.55mm is the largest wire I like working with on average. Personally I spec wire turns and termination type, then just select the diameter that will get me the maximum amount of copper, assuming it will handle the required current. I have built quite a few 8S builds the past year+ and am happy to recommend people experiment. The best thing for me is being able to use lighter, less stressed batteries. I don’t use my 8S charger and charging boards, and stopped making 8S batteries. I just use a series pigtail with 2 connectors and just use two 4S batteries, I have found it much easier logistically. 300-450mAh is good for light 3-4” 500-650mAh is good for light 5-6” builds The biggest battery I have used is a 1200mAh that I built with Tattu V5 cells. This was on a 6” chase build, and it is the point where you start having too much battery weight. I generally keep my batteries sub 850mAh. Thank you for your work Mr. Rosser.

Another quality Rosser joint. Even tho' the subject is above my power class, I still learned way more than I thought I would about brushless FPV motors. As is the case with every subject Mister Wizard here covers. Love it.

Awesome explanation on this Chris. Thanks for diving in to this.

Fantastic video, Chris!

This was fascinating, thanks for making this

All fine and dandy but still clueless in regards to motors kv. Thank you for taking your time to extra explain this stuff

Great work Chris!

Great vid thanks, Chris!

Chris, thank you for this video. Very well explained and easy to follow.

Chris Fantastic Video!!! Things are starting to makes sense now....

Great stuff. Well done

Battery internal resistance doesn't actually heat up less with 8S or 6S over 4S. If you want to have the same capacity in your battery (same Wh), then 1500mAh 4S means 1000mAh 6S or 750mAh 8S. Half the mAh means double resistance per cell, and with double the cells it means 4x total internal resistance. So consequently, if you draw the same amount of power from the same battery size, the battery will heat up the same (cause half current and 4x resistance cancel each other out). The only reason why higher voltage batteries tend to be seen as colder, is that people usually use higher Wh packs if they go to a higher voltage. For example, we had 4S 1500mAh, then 1200-1300mAh 6S, and Bardwell now did 1300mAh 8S iirc. And higher capacity (Wh, not Ah) means less heat. Thanks nonetheless for the great explanation of motors, now I finally understood why 4S tends to have more windings. Because actually, it doesn't.

very informative and well explained, thank you

Outstanding info! Appreciate your knowledge and sharing.

Excellent vid, super informative, thanks so much Chris!

Best explanation I've ever watched. Well done! 👏

Chris, I love your videos, it's just like being back in college. You went over a lot of fundamental electrical knowledge that was a good refresher that I had learned back in my college days in transmission media class! Motor KV has been a mystery to me since I'm only a few months into the hobby. But now I can make the relationship thanks to this video!

This is the side of FPV that I still haven't understood completely and has been a little mystical to me...thanks a lot for all your info and helping us noobs understand!

The higher the voltage the better. I have a buddy that was electrocuted when he was 17, and he said it simply isn't worth it if it's under 400V. Shove that current into those motors lol.

Fantastic, Chris! 😃 Thanks a bunch for the lesson! Stay safe there with your family! 🖖😊

Nicely explained.

Hey Chris, im glad i just found your channel it has been auper informative so far. I come from the esk8 community and hearing detailed explanations on these topics is super helpful. Im sure there are a lot of similarities. I am more used to seeing 10s to 22s voltage systems running 6370 motors all the way up to insane 80100s. Generally in the 100-205 kv range. I wonder if there are any fpv motor manufacturers that have larger ones as well that are worth checking out?

Thanks Chris for your work and your videos! Doing really clear explanations every time! Flying 8s on my own x4 8" cinelifter frame with 2812 1100 kv motors and it flies great!!

Excellent discussion!

Thank you for the in-depth analysis. I am working on my new drone and your video came just on time. I was planing to build it as 4s2p, but I will probably do 8s instead.

Thanks a lot man !

Thanks Chris

Great info. (But I’m still sticking to 4S because I want to feel the sag when the battery gets low, and to not have a whole plethora of battery/quad combinations)

thanks chris i learned so much from this video as i do all of your videos ...Sede ya in the air pilot!!!!!

Brilliant!

Nice job

9 months later, RCinpower has some excellent GTSv4 2207 1500kv. Tattu now has 8s 1050mah and 1300mah 5.0 batteries. I've still not blown a FC or ESC on my 8S 20x20 Foxeer stack. If you go with the same Tattu 31Wh battery from 6s to 8s is only 10g heavier. Some people forget to realize the 8S equivalent of a 1400mah 6S is 1050mah.

Hi, In your "what are the benefits of higher voltage" you mention the battery internal resistance. Just like for the motor where double the length half the cross section didn't make a difference, the same holds for a battery. If your 8S1P battery is providing 800W each cell is doing 100W. And a 4S2P battery of the same cells also has each cell doing 100W. So for each cell the losses in the internal resistance are the same, so the same holds for the set-of-8-of them. For the resistance in the ESC, it is not obvious it will all cancel out as neatly as for the winding-wire in the motor or the battery cells. For a 4S system you might use 20V MOSFETs and for the 8S system you'd use 40V ones. You can get the 20V ones with lower resistance than the 40V ones. But it is not evident that the ratio of the losses there will exactly cancel out. Could go either way. Of course, (back to your example numbers) 50A requires a thicker wire. from the battery all the way to the motor. So the 8S system clearly has the advantage there.

I built my 1st 6s quad. It's identical to my 4s quad except for the motor kv and the 6s quad had an o3 air unit instead of a Vista. What I notice after a flight is that the motor wires coming off the ESC are pretty warm. The motors aren't getting hot, but I feel a heat in the ESC that I've never noticed in my 4s quads. I wonder what's going on there.

Chris, love your video. What if drop kv motor using betaflight 4s motor to use 6s. Can you explain what is the effect by using this method?

Next time someone asks me about motors, i'm going to direct them to this! Very handy to have a explanation like this for people wanting knowledge. thx Chris! (P.s was there a Dime bar quote hidden in here @5.55?) 🤭

Love the work you do for the hobby, would be great to see a motor strength analysis as in impact resistance. We're all getting tired of these soft alloy's and motor shafts that bend or snap.

I am not into model aircraft much ATM. I have built a few off grid solar systems and the analogy to 12 v VS 24 v (or 48 v etc) is quite illuminating. I always thought of motors as transformers where the secondary winding has a variable Resistive load. The more turns on the secondary the higher the resistance on the Real Load to deliver the same amount of Power Watts. With no load the Primary is generating a BACK EMF that cancels the applied AC exactly (assume spherical cow) and hence no current flows .... Let's no mention phase angle shhhhh ... Anyways a beautiful clear exposition on a non trivial subject. Well done Sir... Subbed!!

I know you are right because I did this exact test on motor cooling comparison a year ago. But the thing that never sat right with me was the slot fill ratio argument. Seems like slot fill ratio would be better for multi strand because of the fact that smaller circles pack better than fewer big circles. Thoughts. One thing that I didn't think of was the fact that the thermal transmissibility is worse with multi strand due to the increased number of paths to exit the heat. Solid conclusion. Solid Video.

Would it help for cooling multi-strand motors if you would squeeze thermal paste into the lower layers?

Try silver wire with fiberglass sheathing. If you can find it. My best bet is the coil heated and melted the insulating layer.

We noticed that 6s batteries had a much higher chance at imbalanced cells after 500 cycles (about half the life we get out of 4s), so 8s has to be even worse. I have returned to 4s and with a 6yo build that has 9:1 thrust to weight I am fine, especially since I cannot break it (less 37 cent kk5050-3 props). Yes the light weight guys are getting 15:1, but they break easily and are hell on their packs I am fine where I am at (4s-1300 on rcx 2207-2400), and after 1000+ flights on a pack, I then use the pack for another 1000 flights on my 3d foam planes.

Thank you for explaining the math so a dummy like me can grasp the concepts 😊

Great video! i like 4s setup but you got me wanting to build a 6s setup just to see if my lipos will last longer! i destroy 4s lipos on the daily!!!!

Ive always went with the rule of 1s for every inch of prop if you can, overweight builds seem to do a bit better on higher voltages too

Very interesting video. So, there's a battery cell count "sweet spot" that balances electrical efficiency against motor cooling. I'm wondering how you would expect that sweet spot to scale with drone (and hence motor) size? For example, if the optimum for a 5 inch freestyle drone is 8S, would the optimum for a 3.5 inch freestyle drone (with similar disc loading & power-to-weight ratio) be expected to be 8S? 6S? 4S?

Thank you for the video and clear and scientific explanation of the topic!) Still there is one thing that i can't figure out (due to the lack of knowledge) how KV correlates with the fact that speed of brushless motor spinning controlled by the frequency of alternating current applied to the coils. Or will it be correct to take KV as a motor revolutions per volt with the assumption that ESC changes frequency proportionally?

Finally I understand kv

I think more cells mean more cell packaging, more solder tags that results in more weight and less efficiency. And the whole battery pack is only that strong as its weakest cell. Is that right ? Nearby, i love your channel . Thank you.

Regarding Back EMF and Motor KV, am wondering what effect the number of magnetic poles and coils have on power and efficiency of a motor? Thinking number of coils would allow optimizations for wire thickness and/or lengths.

I went with rcinpower smoox 2206/1350kv with GF4937's on my 8S 5" build. Came in at 610g AUW.

Great video. The 8s price is a nonstarter for me. Heck, I am still dragging out as much life as I can from my ancient 4s quad. Since I am cheap and don't want to replace my batteries at extra cost I am probably going to stay at 4S. I don't plan to ever do competitive racing, just not that good.

It makes sense in terms of motor and output. However, you need to think a little about the FC's power supply. If the voltage is 8S, a voltage of up to 33V is supplied, which requires a dc/dc downconverter and an accompanying inductor and noise-reducing voltage configuration. In order for Vbat to be directly supplied to the FC to generate a 3.3V 5V etc voltage, an intermediate dcdc down converter is required. I think it is not easy to implement in a limited size FC.

Science!!

I’m quite keen on the high kV, high voltage approach. This is more from a e-mobility perspective, not so much relevant to drones, but the motors are essentially the same. High kV has the benefits of nice windings and more speed, while high voltage results in being able to use less current, so you get more heat dissipation along with less heat. It also strains the batteries less; reducing voltage sag etc. Better to just put strain on the bearings than on the whole electrical system. It’s also nice that you can overvolt a system without major consequences most of the time but driving too much current through it will kill it.

with all the knowledge we have now, i´m looking forward to have the flight times we had 10 years ago with 3S 2200 30C. ;-)

I'd love to build a UL7 8S long range rig, but the main problem is that there are no widely available large 8S batteries for it. I hope 8S catches on, it makes so much sense in many respects and especially for LR builds, efficiency is everything.

I have been thinking of building a 8s quad but just running 2 4s packs in series.

Hi Chris. I'm into security. Sorry about this off topic question. I've noticed your lips aren't sync'd with your voice in all or most of your videos. I was thinking you might be evading some scrapers, hehe. I would like to know the reason why you're doing it. Anyways, does it require a new esc for 8s? Just finished the whole video. Very cool! And yes, I totally agree with you on price. If it's expensive, I'll stick with 6s

Would be cool to see how d term affects motor temperature

So did i choose wrong when i bought 1800 kv for me 6s 5" quad? Im running 1700 now and wanted more power that was more like the 1750 motors that were on my quad but i figured it would just be more responsive with a little more draw from the battery . I also have a quad with 1950 kv or maybe 1900 but i noticed it was fast but i only get like 2 min of flight time. Do i need to cancke my order? Help!!!

I am contemplating changing my 10in from 6s to 8s. I am running BH Avenger 2812 1115kv motors, would it be necessary to change or would it be worth running those motors? If I can get away with the same motors, all I really need to change is the ESCs.

Does the higher voltage systems make more sense for larger aircraft? It seems like 12S is pretty common for larger VTOL aircraft like the MFE Fighter. Thanks for the really interesting video.

@Chris Rosser. I’ve noticed that different motors seems to use different motor insulation because some have either a dark or lighter color in the windings than others. Is there a certain motor winding insulation that’s better than other motors to prevent smoking? Thank You.

Well, if you start higher the voltage and lower the KV then you'd be pulling less amps you won't need a Lipo, battery and cells wouldn't need to be so big, just more of them to make the voltage lithium ion would be the way start keeping disposable vape pens lithium ion in side them and small 👍🏻

I WAS content with my 4S setup. So, I should see better motor efficiency and cooler temps with 6S, lower KV motors. Correct?

5s 2450 kv is my go to. 49000 rpm at 4v vs 48000 rpm at 4v for 8s 1500. Sure 8s less sag so probably little better but can new components handle that higher voltage?

I would say there might be another very important thing that stop me from 8s, safety... I can poke my fingers to both ends of my 4s bat for no reason and safe.. but think that will be not safe for 6s,the zap sound of 6s bat ask me for quite a while to get use to it , not sure if people is ready for 8s' consiquences.

I think long range quads will move to 8s, but freestyle and racers will stay in the 4-6s range; weight is still a major factor in performance, and really more importantly the feel.

Well damn....I learned something to.....hey!!! wait a second!! :D

On my 6s quads capacitors blowing on the esc more offens than other from other drones. In full throttle pictuture is more stable in 4s(f*ng induction). Less vibrations. For 2004 motors in 5 inch quads, 4s batteries more availaible in shops, and more lightweight, centerweight drone< and accurately airflow than on the 6s analogs...

Would it work to soak the coils in thermal paste, the stuff they use to help transfer heat from GPUs and CPUs into the heatsink in computers, to make a motor run cooler?

If motors typically have 12coils & 14magnets, when setting 'Motor Timing' of the ESC, should we use a multiple of 12, 14, is there a formula that takes both these numbers into account, or is the number of coils & magnets not a consideration, when setting the ESC Motor Timing?

What about inductive efects? Higher count of windings means higher overal inductance of the motor what means two things - you reduce how fast you can change the current and if you try reduce /switchoff the current you produce much more voltage spikes - at 6S compared to 4S it's quite noticeable.

Even when I didnt understand shit in motors I understood one simple thing: if automotive manufacturers and other seriouse businesses like power companies try to run their systems at as much of a voltage as they can, then higher voltage must have some benefits

hairpin motors and new technology.. 80v whoops baby!

I don't understand why these simple motors don't implement Halbach arrays on the outer magnet ring -there must be a ton of this radial flux 'going into space'... Is it a cost/complexity thing? Isn't this one of the reasons why axial flux designs are so much more powerful?

What happens if I use 7 magnets and 6 windings, I would have more space for thicker wire, better cooling and etc. I guess the downside would be more desyncs right?

Ok all nice and good, but what I really want to know: how does this effect the motor torque? I mean in the inside of the motor it is only the Current that produces torque. So how does this work with the higher voltage?

Thank you, Engineering videos are the top!

Could BEFM have anything to do with ESC's blowing?

5s should be discussed!!

What does 4s, 6s, and 8s mean? Is there a basic explanation?

Try to test the same motor on 3s and 6s battery with no load. And look at the current consumption at the same rpm, you will be surprised. Than you can try it with load ))

I wonder why kv and xS scale not linear. For example, usually 6s freestyle build use 1500-2000kv, but if go to 1s and multiple kvs by 6 it should be around 9000-12000kv. But no one flies with such low kv, we usually fly with 19k kv, 25k kv or even 30k kv. Why with increasing voltage 2x we have to decrease kv more than 2x?

Yeah, I will. I’ll fly 8S someday

i have a question. Why when there is noise the motors get hot?

I really can't tell much difference between my 2306 2400kv, 2207 2550kv or 2207 2750kv equipped quads. I am curious about the motor gap, like how Tesla uses carbon to keep the motor gap super tight and not allow it to expand.

I guess higher voltages would be ok if only there was a better battery technology. What I mean with better battery is one that is super-lightweight, maybe smaller and that it can deliver equal or more amps than a lipo because 6S and 8S batteries are practically a weapon by their weight already imo.

To be honest I think we should cut the crap and go straight to 16s job done.

In electric cars, they are switching to the hairpin technology (hexagons fill the room much better than circles -> less air gaps). I wonder why they are not trying that for rc-motors.

Why are motors not wound with square wires. I have seen square magnet wire but have not seen what it’s used for.

$230 for 5200mah a 12s lipo, $74 for two 4s lipos! 3200kv-4400kv are hard to handle at 3s and really hard if it's over 80c discharge. For example right now I have a 2847-4000 KV motor. A 60 esc, 3s lipo 80c and it clocked at 58mph! And has a ton of torque. I may understand what you're getting at I understand the science behind it and stuff but like the 8s kraton! That motor only has an rpm of 32000 with 8s battery so that's lame compared to a 11.1x4000 which is 44,000 Rpms. All I'm getting at is these high-end RC trucks ever since I started building them myself I can buy the parts and put in the motors in the ESC and all that stuff way cheaper than these top end cars and the only difference is it doesn't have a name brand shell. Metal frame every part is metal an alloy or even carbon fiber from aliexpress. I just got one carbon fiber frame everything was metal like the swing arms all that kind of stuff for 130 bucks and it was a 1/10 model absolutely amazing

Dunk.

Great video overall, you nailed most of it. There are a couple of points I disagree with though. The biggest one is related to the assumption that 4S motors mostly use inefficient multi-strand windings. A significant portion of 4S motors still use very thick single strand or dual strand winds, and due to the lower number of turns in the 4S motors, they are often using thicker gauge wire than the 6S or 8S motors. In practical production, the coil resistance of 4S motors does actually end up being pretty significantly lower than the low Kv motors designed for higher voltages, even if it's not quite the 1/3 (6S) or 1/4 (8S) of the resistance. It's still close enough that in real world testing the power dissipation difference is not dramatic. Secondly your conclusion is making the assumption that folks are staying with the exact relative Kv/voltage ratio, which we already know is not true. Most 4S pilots are running 2400-2500kv for example, while 6S pilots are typically running 1900-2100kv. That's the equivalent to 2800-3100kv on 4S. Due to the increase in relative Kv, most of the potential efficiency gains are lost, and in the majority of cases folks are actually losing efficiency when moving up in voltage. Essentially what we see in most cases is that moving up in voltage allows folks to simply draw *more* power overall, and they just keep pushing Kv till the current draw is within eyeshot of what it was at lower voltage. There's also the factor of other losses in the system other than purely resistive losses due to heat dissipation. In practical testing, even with exactly matched Kv/voltage ratios the functional efficiency comparing 6S and 4S was at best break even. We tested this pretty extensively several years ago, and saw no significant efficiency gains in a controlled test environment. The benefit of going to 6S in practical terms is as I said before, access to more power, not necessarily increased efficiency. Now whether that remains true at 8S remains to be seen. 8S wasn't really a thing when we were testing, and it starts getting difficult in terms of controlled test environments. It will be interesting to see of more folks dive in, but going up in voltage comes with other challenges and sacrifices, so we'll see. My gut feeling is there isn't a real benefit for 8S on 5" setups, etc. Really going up in voltage has the best results when you're needing the increased power to lift more weight. 7" and up where you're lifting a pretty decent payload is likely to see the biggest returns, so really Cinelifter territory and that's about it, at least in my opinion. Like I said, we'll see.