Motor Kv

[Rappy Freak]

Hi guys

As some of you know I am looking at getting a TBS airframe soonish.  I have been looking round at motors and have a question  which Kv rating is better.  Is a 740Kv motor better than a 940Kv?  Really I do not understand the Kv rating.  I plan to fly on 4400mah 3s batteries for now with it.  Just looking for help really in understand the difference in motor type 

[Gaza07]

The Kv rating tells you the speed of the prop at a given voltage if I read it right that is, Im sure its much more complex but its 1000rpm per volt so a motor with 1200kv at 11.1v would spin at 1332rpm
worked out 1200 x 11.1 = 13320 a 750kv motor at 11.1v would give 8325rpm which is 750 x 11.1 = 8325,
So you can see the lower kv motors are much more suited to a larger prop up to 12" on the 750kv,
where the 1000kv motor is more suited to a smaller prop and runs much faster, I prefer my motors around the 900 to 1000kv and run with 10" props I think that combo suits a wide range of multi rotors before having to change thing to much 

[Gav]

I agree, 900 to 1000kv motors are best.  ^-^

[Rappy Freak]

 
Nice one thanks. Its all a bit clearer now

[homersainty]

i think of KV as  revolution per volt its not your prop rpm its unloaded rpm

[Gav]

basic quad with minimal onboard gear then as a guideline use:

750 - 900kv 12 to 10 " props 
1000 - 1400kv 9 to 8" props

run a quad with extra weight like gimbal and gopro camera or other camera 250 - 400g then :

1000 - 1400kv 10" props


this is based on my quad experiments over a couple of years in a non controlled environment without any safety gear or white coat.  I do not take responsibility if you buy a combo from my findings and crash.

[Rappy Freak]

Nice one Gav thanks 

[Gav]

I`d propably not go higher than 1200kv motors on a quad, but I think Gazza is going to try 1800kv motors!!  Nutter lol

[rob]

Hi I';m totally new to brushless motors and kv ratings, and was unsure why a smaller prop was recomended if you were using a 4s lipo instead of a 3s lipo, i found this info, it relates to gliders but the theory is all the same, I think I understand now , thought it may be useful to others.

http://www.flyelectric.com/ans.kv.html

[XH558]

I just ';searched'; my way into this thread as I was still really unsure about this KV thing myself ... this sort of does it though and I think I have it now ... but as an extension to the question what on earth are the 4 numbers all motor manufacturers use ... 2212 ... 2248 ....3530 ....2230 .... 3639 

You knew where you were with IC engines - an OS 40 was bigger than an OS30 and the 30 and 40 referred to something measurable i.e. capacity.

But brushless motors all have their own system and it does my head in   ..... why - and what are they about??

And something else confusing .... I am looking at 4 nice new motors for my next quad ... now obviously I think I';d like the 920Kv DJI Flame Wheel Motor ..... but they are £20 a pop ..... so what is the difference between those and say this little baby at about £5 a go ....
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8474

They are the same KV rating - so they';ll do the same job won';t they??
Knowing my luck it';ll be to do with these amp thingies 
So if that';s the case and I want to swing say 9 or 10 inch props and seeing the KV thing I think 940 is about right ... and say I have 25 amp escs what';s the rules regarding the amp things on motors .... is cheap ok like the one above or do you need to spend the bigger bucks ... I just can';t help feeling ripped off like paying for designer name badges on clothes   

Do enlighten me .... words of one syllable - no jargon .... my brain hurts just thinking about it ....

[firey1]

not sure what the number mean i always thought it refered to the can size of the motor

anyway that motor you posted is wrong that is designed to be mounted inside a motor board frame
always check to see where the wires come out in relation to shaft

I just got these for my quad running 10.4 props

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=14737

[guest325]

One of the things that is worth looking at is that higher kv motors rotate faster so any vibration is higher frequency and lower amplitude,  I';ve seen a Tricopter running 1600 kv motors and fpv is much smoother than some I';ve seen - camera was direct mounted on the airframe.

[fruitsalad]

them numbers are the diameter of the can and the length..
some bright spark thought lets confuse it more by using stator size and winds instead..

[peter_a]

So a 2212 is a 22mm diameter by 12mm !!! , there is allays a exception to the rule .

[SimonS]

I think it';s the other way round. It started with, for example, a 2216 being the stator size (22x16mm). Then some marketing git decided to try and sell his motors by can size. 2835 sounds far more impressive than 2216. Generally, the makes towards the top end of the market (and DJI) number by stator while the HK';s etc number by can. For example:-



A 2212 and a 2826 but both virtually the same size.

Where you see an additional number at the end such as 2814/12 then usually this is the number of turns of wire per pole on the stator. The lower the number of turns, the higher the kv rating. Low kv motors tend to be more efficient and use less energy.

On the subject of energy, it is a good idea to get into the habit of think of power consumption from your battery in Watts (W) instead of Amps (A) or Volts(V). A multi requires a certain amount of power to hover. In the case of my Hexa it is around 30A using a 4S LiPo. The formula is W=IxV so W = 30x14.8 which is 444W. If I was to use a 6S LiPo then to get the same 444W I would only need 20A (444W/22.2V). Conversely, with a 2S LiPo 444W/7.4V = 60A. From this you can see that with a higher voltage you can use thinner wires and smaller ESC';s to do the same job.  Another advantage is that a 6S 5800mAh LiPo contains approximately 50% more energy than a 4S 5800mAh but less than 50% more cost and weight.

Here ends the physics lesson :)

[guest325]

I think another thing to bear in mind is that generally speaking motors will take higher voltages better than higher currents - so a higher cell count not only is more efficient but is less harsh on your motors (provided they are rated at the higher cell count).
At the moment I use 3S but am currently looking at 4S or higher to give better flight times and be able to use lower Amp esc';s.

[XH558]

not sure what the number mean i always thought it refered to the can size of the motor

anyway that motor you posted is wrong that is designed to be mounted inside a motor board frame
always check to see where the wires come out in relation to shaft

I just got these for my quad running 10.4 props

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=14737

Ok fair point - but I';m sure most shafts could be reversed - anyway it was the principle of cheap versus expensive I was trying to allude to ... is a DJI Flame Wheel 940KV motor any better than a £5 940KV motor ?? If so why??

I';ve always been brought up on the principle of need v want .. I want a Rolls Royce - I need a car .... so why pay through the nose for some fancy dan name is what I was trying to get to .... are we being hood winked like the emporer';s new clothes that DJI is somehow ';better'; ??

I see you use £9 motors - and thanks for the link 

BTW the physics lesson by SimonS in an earlier post was much appreciated and I think I even understood some of it 

[SimonS]

The problem is that the DJI is a £9 motor, not the £24 that DJI charge for it.  You can get it for much less money with someone elses name on it.  I just can';t remember where I saw it.

[peter_a]

They are like the Emax 2213s , Half the price and a set of props for free !!!

http://www.multi-rotor.co.uk/index.php?topic=198.0

[SimonS]

Those are the ones. 

[XH558]

The problem is that the DJI is a £9 motor, not the £24 that DJI charge for it.  You can get it for much less money with someone elses name on it.  I just can';t remember where I saw it.

Exactly the point I was making .... so DJI just aren';t worth the extra 

Emax here I come 

[teslahed]

If you were building a multirotor for professional video work you';d probably spend quite a lot more on high quality motors and you';d expect them to run much more smoothly straight out of the box than your average £10 motor. The reduction in vibrations are likely worth the money if you are doing it professionally.

For hobbyists like ourselves there is a lot to be said for shopping around until you find a nice £10-£15 motor and then spending a bit of time balancing it yourself if you really want good performance without spending silly money. I think the KEDA motors offer good value for money myself and if you ask around you';ll find people that have been flying a while quickly develop favorite brands that they know and trust. Anything with a bolt on prop adapter will probably run better than the ';collet'; style of pinching prop adapter, in my experience.

Cheap and nasty motors have a tendancy not to run smoothly and will produce excessive vibrations. The bearings may be lower quality, they may break more easily and the general machine work may be of lower quality. Often they use enamelled magnet wire on the output wires instead of decent flex cable - so they will work for a bit but if you bend the motor wires repeatedly they will eventually snap off. These things do make a difference - but I';ve not yet found it necessary to spend more than £15 on a motor myself...

[XH558]

If you were building a multirotor for professional video work you';d probably spend quite a lot more on high quality motors and you';d expect them to run much more smoothly straight out of the box than your average £10 motor. The reduction in vibrations are likely worth the money if you are doing it professionally.

For hobbyists like ourselves there is a lot to be said for shopping around until you find a nice £10-£15 motor and then spending a bit of time balancing it yourself if you really want good performance without spending silly money. I think the KEDA motors offer good value for money myself and if you ask around you';ll find people that have been flying a while quickly develop favorite brands that they know and trust. Anything with a bolt on prop adapter will probably run better than the ';collet'; style of pinching prop adapter, in my experience.

Cheap and nasty motors have a tendancy not to run smoothly and will produce excessive vibrations. The bearings may be lower quality, they may break more easily and the general machine work may be of lower quality. Often they use enamelled magnet wire on the output wires instead of decent flex cable - so they will work for a bit but if you bend the motor wires repeatedly they will eventually snap off. These things do make a difference - but I';ve not yet found it necessary to spend more than £15 on a motor myself...

You make some good points ... the sort a salesman or marketing wallah might make I suspect ... .....then you finish with the best line of all ...... love it