How to choose drone motors? KV and torque
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- Опубликовано: 15 авг 2019
- Support my work / pawelspychalski
One of the mistakes pilots makes when they build their first drone is they use too small motors. People think that if a motor works fine with 5-inch props, it will work with 7-inch props as well. It's not true. In this longish video, I will tell you how to choose motors for your drone: 5-inch, 6-inch, and 7-inch. We will talk about torque, power, a moment of inertia, what happens when you put a too big propeller on a motor. And finally, some numbers that will help you pick correct motor for propeller size. Both in terms of motor size/stator volume and motor KV.
Errata: I forgot to multiply by Pi i stator volume calculations! All values are around 3.14 times to small, but at least I forgot about it everywhere! Pc in video for 6 and 7 inches are swapped, they are fine in the description.
Stator volume of different motor sizes (all values are divided by Pi, it slipped me and now it's too late to edit. Just do not multiply by Pi :) )
2204 - 484mm^3
2205 - 605mm^3
2207 - 847mm^3
2208 - 968mm^3
2306 - 793mm^3
2406 - 864mm^3
2506 - 937mm^3
2507 - 1093mm^3
2508 - 1250mm^3
Desired stator volume for propeller sizes:
5-inch props - 600mm^3
6-inch props - 700mm^3
7-inch props - 900mm^3
--------------------------------------------------
How to choose KV:
KV = Pc / S
KV - motor KV
Pc - propeller coefficient
S - number of LiPo cells
Pc for different prop sizes:
5-inch props - Pc = 9600
6-inch props - Pc = 7600
7-inch props - Pc = 6400
More about a moment of inertia and KV is here
KV is a lie • Motor KV explained - K...
Moment of inertia • Video 
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Drone PID tuning made simple ruclips.net/video/yA8p2FxIwqU/видео.html
Wonderful! I love your pacing. You hit topics that no one else does--and with real understanding of the math/engineering. Thanks you for sharing your expertise and love of the subject.
I guess you forgot to multiply by Pi in your calculations. Other than that this is a good video.
Oh my, you are correct, Pi is missing in a excel sheet I made for the calculations
I least I missed Pi everywhere and at the end it kind of still makes sense
He also said the squared instead of cubed. But nonetheless the man’s a big brain and makes amazing videos
Hey Pawel, great video. It was just what I needed for my current build.
I've watched multiple videos on this subject now at its very obvious to me that its a case of build and see what happens?! I love this!
Paweł, this is an awesome video on motor selection. Thank you
you are welcome
Fantastic video. Class dismissed. This is going to be a great series.
yeah just spent 5 minutes trying to figure out why I was getting 1,900mm3 as the volume of the 22x5 motor, lol, until I started reading the comments!
i was getting 2492mm3 for the 2306
I'm at this point now. Damn it now I have to go find those comments
I love my 2508's (Racerstar/T-Motor) on Chameleon Ti 7" LR, that I recently finished and flew. Mine are the 1200kv & am running 4's 1500mah 95C's through T-Rex 60a esc's..... It flies nicely on the 4's but intend to run large 6's for longer flights and range, once I've flashed iNav and connected GPS..... Great info as always... Thank you
OMG! I've needed this video for soo loooong! Thank you sir!!!!!!! Now I can make informed decisions next time I'm looking for motors
Damn Good Vid Mr. - I have just finished my first build myself,I am in 'pre first flight mode'. Thanks for all your knowledge, it really helps a newbee like me. Greets from Denmark
awesome explanation
Some very valid info in this video. Thanks.
Can't thank you enough! Thanks for makin the vid :)
Good I needed a Hug ! was skeptical if I screwed up on my new 7 cruiser build order. Got RaceStar 2508 1200 KVB and 7x4.5 two blade props coming. I'm a total noob so thanks for the Hug!
This motor is ok. You can easily put any 7" prop on it no problem
Ein sehr gutes video!!!!!! Bardzo informatywny film, bardzo feinie wytlumaczony wontek!!!
Whew, that's something! Thanks man )
Yes great video thank u for showing us how to get the right motors very important
Very good informative video, love your approach with that. I was trying to run a 5 inch prop on a 18 mm stator motor on my plane. Smoked that thing trying to get some altitude
yup, too small, heat accumulates and everythjing overheats
Thanks for this video, help me a lot.
Good stuff man
Very interesting, and may have answered why my 7” quad has this wobble on throttle punch. Originally, this quad came with 30A esc’s, and it flew on a rail, but eventually desynched twice out of the air, SO, you have to change something. I changed the entire stack, and motors, to a 4in1, and it’s never flown the same. I replaced the motors with the same size 2306 EMax Eco. Still unstable. So, watching this, I think I may replace with 2508, and see what sticks! What’s ironic is I have a EMax hawk 5, mod with the 6” arms, and that thing is an absolute rocket, that flys on a dime on 4s, it is rated for no higher lipo, but on 4s with 6” arms, running the Azure prop, this thing kicks ass!
This is so useful
Very educational 👍👍
thank you
nicely said.
Great content, Pawel! Thanks a lot! 😃
BTW, there's a new Avenger motor from BrotherHobby, bigger than the V2 2507s. (2608 I guess, I don't remember.) Anyway, Kabab FPV tried them in a 7 incher and loved them! 😃
I would like to build 7" quad. Looking forward to more of your videos.
This helps thank you!
you are welcome
@@FPVUniversity I was wondering if I could pick your brain real quick. Based on your algorithm, 7 inch 2807 motors 1700kv for 4s should be sufficient. Now would it be efficient and powerful enough for some high speed cruising? Or would something like 6s 1300mah be better suited?
Thanks man!
Happy to help!
cześć Pawel, thanks for the video, very interesting since I'm building a 7-inch Source One atm.
if you're saying that the limiting factor with smaller motors is the ability to accelerate the props rather than in providing power for steady-state thrust, then surely the inertial mass of the prop is going to be a major factor? & that if you were to substantially lighten the prop by, say, running a two-blader rather than a three-blader (or by drilling lightening holes in the blades ;) ), then smaller motors - eg 2306 - might be suitable?
I love your videos, and all of the information. I am currently building a 7" Long Range Super G+. I haven't picked Motors and Props yet. I'm torn between BH Avenger V2 2507-1500kv and the 1850kv. I mainly want to fly with 6S Li-Ion homebrew packs. What would you recommend? And if it's not too much of a hassle, would love to know if there's a prop that would pair nicely with this build. Cheers!
I'm using 2207 on my 5" which give me more control over the 2306 I was using because of the increase in stator volume. I'm planning a 7" build and was thinking 2807 emax motors. Is there a frame you like or a frame I should stay away from? 7" probably has a lot of vibration because of the longer arms and heavier motor and prop.
You seem to be the man when it comes to 7 inch. I have the Emax 2807 1500kv motors and the Geprc mk4 7 inch frame. 5s will keep my prop tips below the speed of sound. 5mm arms though. I'm going to glue more carbon to the arms to thicken them up and I'm hoping the glue, which is softer than the epoxy resin will help to dampen vibrations.
Just found your channel, great video. Maybe you covered this some place else, but in terms of long flight times given the same size 4 or 6s battery and all else being equal which motor size yields longer flights - does a motor with bigger volume producing more torque or does a smaller lighter motor fly more efficiently and yield a longer flight time?
Wish this video had been around a few months ago. Tried 2306 1600kv on 6s on my 7", then 2407 1700kv, and now I'm on the sweet spot of 2508 1200kv 6s with 7x4x3 props. 7min hard freestyle, 16min cruising.
Well, me too... I would not spend money on motors that were just not good for the job
@@FPVUniversity I did spend the money. d'oh! :D You'll save people a lot of pain and money with this video, thanks!
@@vaughnd your comment has helped me figure out what to buy now! I am just having a hard time finding that stater size with the bolt layout to fit a chameleon Ti LR 7 inch... :(
RossHasAdrone the racerstar 2508 1250kv fit my chameleon 7” 👍
@@vaughnd sick! just ordered 6 of them. nice that they're so cheap!
Hi Pawel, I really enjoy your videos and would like to ask you a question about my Tyro129. The original motors 2507 - 1800KV came with an aluminum shaft and are now bent without crashing at all. So I purchased XING 2207, 1800KV. My Tyro129 was flashed with Arducopter, therefore no flips and 3d stuff for me.
QUESTION: Will these motors work fine for me at 4S having in mind my calm flying habits? Thank you!
I fly 2208 1800kv Xing E motors on 7056! At first i was worried about the 7 inch prop on the 2208 but... i calculated the 2208 to have 968 mm of volume which is morr than enough. Been flying hard with no issues even on 6s but need to keep an eye on throttle
For 4",3" and 2" what Is the PC propeller coeficient?
Hi Pawel and everyone, I do have a question on my next buil. In have a 6 and 7 inch freestyle frame and I do have two kinds of motors with different KV’s but same brand. First ones are the EMAXX RS-2306 KV-2750 and the other ones are EMAXX ECO 2306 2400KV. Which motors suits the 6 inch and 7 inch kwad? I would like to do a little bit of freestyle while doing long range. Any answer would be much appreciated
nice rule of thumb to estimate the motor requirement...hoping next you can make video how to optimizing the flight time , regarding to battery weight , capacitance and total weight of quad to reach the optimum flight time...
And I'm sure it's just a coincidence that the abbreviated PC could also be the Pavel Coefficient. 😉 good video though I'm more of a 6" flier and I thought that 2000kv on 4s felt about what I wanted, so a little bit higher than calculated, but it's good to have roughly translatable baselines.
An informative video!
Then I almost guessed right.
I have chosen 2207 motors with 2050KV for my 6 inch / 4S quad. I currently use Gemfan Cinelifter 6030 propellers.
Should I remain with ~30 pitch because of the slightly higher KV number, or should 42 pitch also be ok?
Are the T-Motor F40 PRO III to weak for a 7 inch LR on 4S? Like the fact that I can put them in a 6S racer later if I’m bored;-)
You can also see the propeller as the load in the system. A propeller with more mass, aerodynamic friction, momentum and diameter will require more torque/power. More propeller blades mean more aerodynamic friction, so this also requires more power. If you can effectively transfer this power to the air, you have respons.
sorry for the silly question the thing that confuses me most is the voltage on these motors . lets say im running 4s how come the " kv " is the same . you go to any wesbitr and it tells you from 4-6s 1700kv or 4-6s 2400kv so doesnt the kv differe or shouldnt there be for each voltage a kv i. e 4s 1700kv , 6s 1700kv , 4s 2400kv and 6s 2400kv . different motor for each voltage ??
Hi, thanks for the video!
You believe that I will have good performance with these motors: T-Motor Velox VELOCE SERIES V2208-V2 1750KV and 4S battery, 7 inches props?
Hi, Pawel, do I have to alter these values if i have a 5" tri-bladed prop?
Thanks, Steve
Good question, since I'm thinking of using bi blade 7" on a 4s | 2207 1950kv
I am a noob, can I use same size but slightly different kv in a same build, one of my drone lost one motor and I can not find the same size same kv motors, thanks
I'm using some NIN 2806.5 1350kv motors, and they work great on my 7", but at $30 a pop they are pretty pricey.
I need a substitute for my next build that's a lot cheaper but just as good. Maybe the Brotherhobby Avengers.
Thank u
my pleasure
So would it be good to have 2806.5 1800k with a 4s running it ?
I understand your idea and it makes sense a larger motor has more torque. But if you compare say a 2306 to a 2507 (motor dependent) the 2306 can produce more watts of power and at times more thrust. Motor design, quality, number of cells plays a huge part .
More watts in heat not torque ,,,
hi pawel, fantastic video very informative. i watched another quad expert and he said that if 2 motors had the same/similar stator volume but 1 motor was higher stator, with smaller diameter stator vs shorter height and larger diameter, motor 1 will spin up faster hence producing more torque, do you agree with this? any chance of elaborating on this? thanks again for the great videos!!!!!!
That is just wrong :) If both motors have same KV, the higher diameter motor will probably have more torque since the arm of the force will be longer. Also, relation between torque and RPM is inverted. The faster motor is rotating, the less torque it has available. The power might grow, but not the torque
Hey bro can u tell me if the dji fov prop adp is compatible with the bebop2 motor I'm having a lot of issues with the bebop prop it burst in mid air and cause crash I really need to change these prop to gemfan
I have absolutely no idea about that. Sorry
what motor could be advised for a 9" set up ? the new 2806.5 or are these to small and I should go up to 2814 ??
2806 is probably too small. 2814 should be a better option
How do you calculate these numbers? V=pi*r^2*h. V for 2206 for example is 11781 mm^3.
I also would like to add that the propellers seems to be very big for the volume of these tiny engines, even if the calculation had been right. I never seen such propellers on such small engines.
I just came here looking for someone to tell me what should I use for n my 6s 6inch freestyle cinematic quad and end up with headache and more confusion
So PC for 3" comes in at around 12,500????
I use 2806.5 xing on my 7" true X and although it flies just fine, I've always suspected they are too big. What is your opinion? 6s and 1300kv.
For 7 inch those motors are fine
Which motor brand is reliable ?
hi i fly 5 inch quad... my setup is 2205 with gemfan 5045 and have bobbing...which is correct setup?
2205 is just right for 5" props
So this means I would benefit by retrofitting 6 inch props on my Skystars G730L, as the standard motors are only 2306, 1800kv and a good 4s battery - and the 2508's, 1200kv running on 5s, I chose for my Chameleon 7 inch are entirely big enough for 7 inch props?
yes
@@FPVUniversity excellent, I'd ordered some 1300mah Acehe 75C 5S for the 2508's and may try running a couple in parallel, and have a good stock of their 1550 mah 95C 4s for the g730l - good to know, thank you
Are the 2306 1300kv motors OK for 6 inch props though on a 7 inch frame on 6S?
2306 are too small for 7 inch props no matter the voltage.
@@FPVUniversity I know, I am asking about 6 inch props. I tried the 2306 on the 7 inch and it was very unstable! I wonder if I should replace the frame with a new 5 or 6 inch frame...
What's the formula for calculating stator volumes then ? ...
Hey pawel, have you seen kebabs video on motors for 7”?
He recommended the brother hobby
2806.5 motor, which I calculated to be
4002mm volume, any thoughts?
Cheers 👍
for me it's an overkill
In my math I forgot to multiply by Pi, so divide by 3.14 ;)
@@FPVUniversity So will it be inefficient on battery vs something smaller if your flight style is theoretically the same? Because you say "overkill", what if you want that surety of pulling out of dives etc on the 7"?
Thank you very much for the information however the volume is measured in cubic millimetres not millimetres squared
Yes, you are right
For 6" props on 6s I would want ~1300kv according to your KV = Pc / S formula. That seems too low. Most folks are at 1600 to 1900kv I thought.
People have been talking about "bigger motors" for 7" for quite some time now, smaller motors are terrible on 7" they cant even sometimes "catch" the quad, they dont have enought torque to spin the prop up. 3 years back i tried to fly 7" on 2205? and its was not a good combination.
I think current genertion of 2507 and similar is just right for 7 inchers.
Are you omitting shaft size in your calculations? It seems quite important, takes significant volume after all.
bearing hole not the shaft but it doesn't matter for this context. Size does matter,,,
To keep it simple we can just skip that
What is the difference between a motor and a servo ?
The same as between horse and a cow. More less
So stator volume formula is : r2 * 3.14 * h ? So for 2204 should be 1900mm3?
d = 11, h = 4, r = 11
11 * 11 * 3,14 * 4 = 1519
@@FPVUniversity Yeah, sorry, I just counted 2205 instead 2204 (multiplied by 5mm height instead of 4mm)... Thanks
Wrong on the volume calculation and said squared instead of cubed. You totally had my interest right up until then. Good info on the physical aspects though. Thanks.
Why the Pc for 7" is higher than for 6" ?
Indeed, 6 and 7 are swapped in the video. They are correct in the description
The Pi number is missing in V computation. Roughly the result should be multiplied by 3.
yes I know, there is an errata in the video description
will 2306 1700kv with bi-bladed props will be good enough for long range?
on 5-inch? 7-inch?
@@FPVUniversity 7 inch
@@FPVUniversity 7035 prop
2306 are too small for 7-inch
I think you need you re-do this vod.. No mention of 2807 and co.
I was also thining about it. A few years passed and it might need an update
Just ordered brother hobby 2510 1380kv using your guide to run 5s 2200mah for 7 inch props on Armattan Productions DIY DRL racer with HGLRC T-rex 60a esc's and fflightone FalcoX, just for the hell of it. Can't wait for it all to arrive, get it built and maiden armattanproductions.com/pages/kit_detail/2554
How does the propeller blade count affect this torque/kV calculations? What about the propeller pitch?
This just a guideline, not the detailed way to choose motor
Sounds too big ... but try the new Brotherhobby Avenger 2806.5 on 1300kv ... but you need a stiffer prop than the HQ 7x3.5x3 ... the motor is not even heavier than a 2507
for 6s ... the 1300kv
16:18 - you're number for 7" looks really off for me... for one thing, they are turning back upwards, and you're making a bell curve with 6" at the apex, instead of a slope with just 6" as one point along the line. According to your bell curve, 1" props and 12" props would have both like super high kv motors. You're 1" props will be doing fine, you're 12" props might be a strugglin'... *shrug* not a prop expert.
choppergirl.air-war.org
You said 605 (squared) but wrote 3. That's cubed. You might want to fix that :)
Apologies for being a pedant but mm^3 is mm cubed not mm squared. Power of 3 not power of 2
yeah, I know. I should probably record a new version of this video
The PC values seems incorrect?
Indeed, 6 and 7 are swapped in the video. They are correct in the description
@@FPVUniversity In description they are as in video:
"Pc for different prop sizes:
5-inch props - Pc = 9600
6-inch props - Pc = 6400
7-inch props - Pc = 7600"
check again, I see them like this:
Pc for different prop sizes:
5-inch props - Pc = 9600
6-inch props - Pc = 7600
7-inch props - Pc = 6400
The flight become smother with bigger motor
I dislike when you say everyone knows this and that. You suppose that people knows the KV choosing, the inches... you have to be already expert to understand your content
Well that's a very strange, misleading and roundabout way to come to a roughly indicative relationship that trends more or less in the right direction.
More correctly, torque varies with the inverse of kV (you can calculate torque from any published specs, it isn't given separately on data sheets as it would be redundant), with the limiting factors being magnetic saturation and power rating. Power rating is roughly proportional to stator volume for motors of similar construction, sure, and torque at saturation will be proportional to air gap area x moment, which happens to also be proportional to volume... so it amounts to roughly the same thing in practice for motors using the same magnets, gap and stator construction, but it's not very useful if the motors differ much. Different stator materials will change the saturation flux, stronger magnets or better air gaps will increase emf (increasing torque and lowering kv), cooling will affect power limits, better winding efficiency will pack more copper into the same stator volume etc. . The volume may a good rule of thumb if comparing motors of the same series from the same manufacturer, but if you want real results for torque or want to compare different models, it's no substitute for going to the data sheet and doing the maths.
mm³ = cubed
word to the third
Cubic mm..
Very
In mm..r x r x pi x h :1000 ( cubic cm)
The calculation you’re writing down is not correct. This means that the calculations you made prior to this video trying different propellers has to be incorrect as well for the calculation of the propeller size your telling us now to be somewhat “correct”. That is if we also make the incorrect calculation you did. So to everyone watching, you will end up with a volume 3 times higher if you know how to calculate volume, this will get you dead wrong figures.
This guy obviously don’t know that if you write a little 3 after 600 for example it means 600x600x600.
Maybe he mean 600x3, that would make this calculation more accurate.
That was milmiter qube
I use on my 7 inch long range quad 2207 racerstar 1600kv with 5000mah lipo gives me good result.
What flight times are you getting ? Thanks
@@Gw0wvl 17 min
why i do no see the connection between the RPM and the AoA of the propeller? Everything begins here. For highher torgue its enough if you choose lover same size lover KV motor. THere is a connection the highher is the KV, the highher is the Amperge and the lover is the torgue. Connect this with the AoA of the prop + the formula for multibladed prop. Measure the Amperage and thrust to find the optimum prop for the motor. If you know the total weight and the thrust, its easy then. Unfortunatly this video ends with in my opinion and not a fact, like based on the measurement this combination is the best of the sets I have....
KV without values A x B make no sence. This tips might work, might not....
ah, what will be the speed of the miniquad? Thats the first step to chose the right AoA prop before going to the motor, guys.
Do you have prop/motor suggestions for 7 inch long range 60km/h, 4s 1500mAh (maybe 2 batteries parallel)?
@@radekr981 that a low speed...what is the bigest prop will could be fited there? use the rule: (x)*(x/2) like 7*3.5 and if your motor is nominated for max 16A and with this prop it consumes 20A at static measurement on max throtle, selekt smaler prope, try 7*3, or 6x3.5 and measure again to prevent your motor from burning, select high KV motor and later the battery for the flight time accordingly. But lets ask first what will be the flight weight approx? (I am asking because the motor + prop defines the Thrust, what could be approx 1.3-2.0 more compared to the weight, let say 1.5 is enough, and no need for 3 blades, they are less effective)
@@fromika yeah, I just weight all my props and chosen lightest ones with lowest pitch. Have 6.5 inch tmotor props and 6040 two blades. I'll have a look how it flies. 400g quad + 200g battery max 7 inch 2205 2000kv.
@@radekr981 ok, so we have 800g. X1.5=1200/4=300g/motor
2205 could produce it at 50%th with 6045 prop@38W (and approx 550g @100%) I would select 7025 or something like that from elektroprops of APC likes and might smaller motor. Anyway you will have plenty of reserved power for extra battery.
mm3 > Cubed
Bay the way..... you meean cubic milimeters not mm sqare.... pozdro
ruclips.net/video/HAj_WLJkcZg/видео.html Giggity
Twoja wiedza jest duza , ale angielski tragedia nie idzie tego sluchac 🤮
ciekawe, że tylko polacy marudzą na mój akcent... taka narodowa tradycja, co nie?
@@FPVUniversity taka nacja. pozdro.
ung m instead of mm and saying squared instead cubed makes it hard for me to believe the legitimacy of the information in this video