understanding KV to RPM

[dasboata]

I am having a hard time figuring what KV motor I need and at what RPM it will run at. I know alot has to do with the # of cells/Volts but as I see the motors on OSE it doesn't realy tell you the voltage that it runs on and sometimes the rpm's I just see the KV and I know you times the KV by the voltage and this will give you the rpm it this true ??? I also see recomendations for the 4074 Lepard which is suggested to run 20-30M when a neu with run 60,000 I would think that is the motor I want please enlighten me thanks chris

[Chuck E Cheese]

in boating 30-35,000 rpm is what you want. kv = rpv per volt. so if you have a 1000 kv motor with a 2 cell battery (7.4v) that is 7400 rpm. a basic set-up is 1500-2200kv motors on 4s that is everything from the 1500kv proboat motor, 1800pb, 1800aquacraft, 2030aq, 2000 leopard, 2200 neu. all good choices for 4s.

60k rpm is in a jet, that is just what the bearings in the motor can handle. boats like around 30k. this topic goes verry deep if you search it.

what boat and battery set are you trying to find a motor for?
the best way to find out the set-up is find someone that has done what you are trying to do (same boat, batteries, speed). we have all been experimenting for a while and there is a good chance someone has allready made the mistakes for you (at their expence). if it didnt work for someone else it probablly wont work for you. you can save alot of money with this theory

[dasboata]

I am just trying to get all of this in my head so I can understand the differences this is what boat I have on the build table I also have 2 new CC 2028 but this boat is too smal also what do you think this is my mystic http://forums.offshoreelectrics.com/...ad.php?t=21956 thanks Chris

[Darin Jordan]

I am having a hard time figuring what KV motor I need and at what RPM it will run at. I know alot has to do with the # of cells/Volts but as I see the motors on OSE it doesn't realy tell you the voltage that it runs on and sometimes the rpm RMP's I just see the KV and I know you times the KV by the voltage and this will give you the rpm it this true ??? I also see recomendations for the 4074 Lepard which is suggested to run 20-30M when a neu with run 60,000 I would think that is the motor I want please enlighten me thanks chris

It's pretty simple, really... First, what is your target RPM? I usually figure the following:

25-30,000 => Mono Racing, Sport Running
30-35,000 => Sport Hydro (Racing)
35,000+ => Hydro, SAW, 2-Lap...

Once you know your target RPM, then you need to figure out what class you are going to run in. I'm thinking you are considering 1/8th scale?? Either way, once you know what voltage range your class is running, then you simply divide the desired RPMs by the voltage.

For example, if you are running 1/8th scale on 10S, then you will be running 10 * 3.7V/cell (nominal... it's actually more like 4.2V/cell fully charged) = 37V

If you target 30,000 RPM, then 30,000/37V =~ 810RPM/Volt = 810KV

Things like torque (number of motor poles), etc., can also come into play, but in general, this is how it's done.

Hope this helps.

[properchopper]

Chris,

This may help in choosing the right KV range motor for different cell counts.

[dasboata]

Big help guys thank you all

[TotalPackage]

KV is also used to calculate the torque of a motor the lower the kv usallly means higher torque. Not considering quality of materials etc... if im not mistaken 1/kv = kt u take the kt And mutiply it by the max usable ampdraw of the motor to get torque of that motor. Somewhere out there theres an octura prop chart that shows you what rpm range the different props like and their applications. A rule of thumb with fe is to start with a small prop and work your way up. I remeber in car stereo there is a " Golden Rule" for building boxes . You could appThis rule and get a sweet spot for you particular driver or motor if you will.Its obvious this was derived through alot of trigonometry and calculus, I dont know if anyones derived one for the sine-motors we run, but out there somewhere, in math it exist.

[Fluid]

I have to respectfully disagree with some in this thread. First, two and four pole motors can behave differently under load. For two-pole motors like Feigao and Hacker the following is probably a good guide:

Sport running - 20,000 to 25,000 rpm (Kv x nominal 3.7 volts)
Oval racing - 25,000 - 30,000
SAW racing 35,000-40,000

It is possible to run outside these parameters but most run in this range with success. Most two-pole motors run close to the calculated rpm under reasonable loads.

For four/six pole motors - Neus in particular - the motors will actually run slower than the stated rpm when under load. The exact rpm reduction will depend on the motor and use, but for many multi-pole motors:

Sport running - 25,000 to 30,000 rpm (Kv x nominal 3.7 volts)
Oval racing - 30,000 - 35,000
SAW racing 35,000-45,000

You also have to have the correct size motor for the hull. You canot just pick a 380-sized motor and expect it to power a 36" mono, even on 10S. The motor has no torque to swing a large enough propeller and will overheat quickly. Likewise, trying to run a 2215 Neu in a 29" mono is not a good idea. It's too heavy and it needs a large prop to work efficiently, large enough to roll a narrow hull over.

Excessive prop rpm, particularly with larger props, is not necessarily a good thing as it requires a very small prop to keep the rpm up. In rough or race water the small prop can unhook easily, stressing the ESC and loosing speed. Wear on the drive train is higher too, and flex cable friction increases.



.

[Darin Jordan]

For two-pole motors like Feigao and Hacker the following is probably a good guide:

Sport running - 20,000 to 25,000 rpm (Kv x nominal 3.7 volts)
Oval racing - 25,000 - 30,000
SAW racing 35,000-40,000

I find it rather hard to believe anyone actually runs 2-pole motors any longer... Even WAY back when brushed motors were the thing... guys like Ed Hughey were using 3X and 4X winds on their motors. Torque is where it's at, and most 2-Pole motors don't have that....

I'm down with what Jay said... He's spot on. I'm not sure who he's disagreeing with... If it's me, that's odd, because we said nearly the same thing... He just elaborated further. I also never even consider 2-Pole motors, so my targeted RPMs are for 4 or 6-Pole. However, the only 6-Pole motors that I really know of are the P-Spec/Limited series of RTR motors... Most others I've seen are 4-Pole... (Neus, etc.)

[TotalPackage]

It would also seem that the higher rpm makes balancing all the more critical too. Darin werent those brushed motors still two pole? They only had two magnets in them.I thought the double and quad winds only increased the current and the magnetic field between the stator and the magnets,but with the multiple wind motors you lost rpm partly because of the extra weight of the multiple winds.