hi,

I have been trying to understand the KV rating of Brushless motor

If i have a 3500KV (11.1V, 17A) outrunner motor and i want to take the function of 1000KV does that mean i can run it on 1/3 throttle and enjoy the speed and power equivalent to 1000KV

Maybe try to use a 9 X 4 prop instead of 6 X 3 that i am currently using on 3500KV and control throttle

kv rating means number of rotations/volt
for 1v your motor rotates 3500rpm
for 11.1 your motor rotates  11.1*3500=38850
by varying the throttle you are varying the current not the voltage
so at 1/3 throttle u will still be at same rpm. 
 




there is some safe loaded current for 3500KV
if you use a 9*4 prop it will exceed the maximum current
and burn the motor.

i think 3500kv with 6*3 prop will be running at its safe maximum
 current as specified by the manufacturer.



high kv motors are used to spin small props at higher rpm,
low kv motor can turn larger props at lower rpm.

Try this thread

http://www.rcindia.org/electric-power/understanding-electric-power-systems/

Regards

Let us make this direct and interesting

1.  We know that KV rating is rpm/volt.  So when one varies throttle, does the current vary with the voltage ?

2.  Doesn't the KV rating of a motor hold the same through various throttle ranges ? In the example above, is it even possible to reduce the "perceived KV" of a motor, just be reducing throttle ?  If a motor rated at 3500KV at 12.6v (3s) spins at 3500 * 12.6 = 44100rpm, when one reduces throttle to 1/3rd, which means the voltage drops to 4.2V (or does it?)... wouldn't the motor now spin at 3500 * 4.2 = 14700 rpm  there by maintaining its KV rating ? 

3.  When one reduces prop size, it only changes the Amps draw from the motor, but does nothing to the KV itself, right ?

Let us see what everyones response to these are

1)AFAIK,  since BL out runners are AC motors. the pulse width sent to the motor increases at the same voltage at more throttle. This makes the motor turn faster.

2)I dont think voltage reduces, the pulse width reduces when u reduce throttle. Correct me if iam wrong.

Also you can run a bigger prop on a high KV motor at low throttle. The down side would be BAD EFFICIENCY and motor heats up faster than expected due to huge current draw. Ask me how I burnt my finger at static tests

3)The amp draw increases with the prop - doubling the prop dia means there is 16 times more current draw - not 2 times. SO one should make sure 9X8 is not same as 8X9, the KV slightly reduces on load due to the resistance in the coil. So lower the coil resistance, better the efficiency and better it performs.

Vinay you have express even what opinion I have. BL motors are 3-phase motors and the actual RPM (not Kv- that capability of the motor, not a dynamic measure) depends on what Pulse rate the ESC is supplying.
The Cell rating suggest what Voltage range the Motor can Tolerate. Kv suggest what would the be Max RPM if the max volt is supplied.

That means that one should not use a 2S motor with a 3S Lipo even on Low throttle.

I posted couple of question above in a way that is more applicable to brushed/dc motors (as the simpler case).

4.  For a brushless motor, does the concept of "max" voltage even apply ?  So if the voltage is constant and only pulse width is changed, does the rpm of the motor change in such a way that the KV rating is maintained a constant ?

5.  The question still remains (for both brushed and brushless motors) : "Is KV rating a physical characteristic, that does not vary with applied throttle" ?

4)A high current for low duration or a high voltage for a low duration wont burn a motor. The KV rating is always maintained. The pulse does not have enough energy to accelerate the motor to that KV.

5)The KV rating AFAIK is a physical characteristic. To say in simple words, If a motor had no Physical resistance in bearings, 0 coil resistance and if the motor had 0 weight, then even the slightest throttle would keep the KV constant. But in real world since all the three vary, its more ideal to visualize it the "pulse width" way. Just my 2 cents paise.

BTW, I started with BL motors in RC. So Never bothered to read about Brushed motors. 

May be electronics gurus can explain better/correct me.

Well according to me
4) sometimes yes ,because voltage even depends upon thickness of wire used for winding and people you forgeting here pole,even pole is a main issue ,even number of poles also decides voltage and speed(rpm).in ESC the pulse which is controlling the motor is also controlling the power device like MOSFET ,which in turn controls voltage and current switching time to motor!

5)Even brushed ESC controls the motor in pulsed format ! but it will be in fast rate where like connecting LED to a 50hz supply even though it looks like continiously glowing but it will be fluctuating or blinking for 50 times per sec!
and between there is no KV rating for brushed motor because here in this case voltage only considered as normal DC motor ,and voltage depends upon again coil and number of poles bcoz as the pole increases the commutator size also increases!

The poles help in deciding the torque and/or RPM. But it does not matter when the question is about RPM per throttle.

if over all rpm increases  obviously the rpm/volt also increases! correct me if i am wrong!

and any how Vinay are u coming to jakkur this sunday

Planning man, My cousin is here and he wants to see jakkur badly. The worst of all is that all the 8 2200 packs are fully charged. Its more easy to discharge them using a heli rather than a charger for long term storage. But let me see how it goes. hope I get some free time.

As a simple electrical characteristics the brushless motor are working on principle of AC and not DC so KV rating is directly applicable to DC brushed motors only
For Brushless , neither the voltage nor the current is varied , the rotating field is created by varying the pulse width to the FETs using Back emf.
All i understand is does the efficiency of BL motor is same at 1/3 throttle and full throttle.

In online rc stores they mentioned the motors by some numbers like 2908 with kv ratings..Wat does the number mean? How can we know how much power will the motor deliver? For a 700gm auw trainer plane which number's motor i can choose?

1. Varying the throttle changes the pulse width. The peak voltage remains the same but the AVERAGE voltage changes. The current also changes.

2. Effectively, yes.

3.Yes. The current changes because of the change in loading of the motor. Increasing the pitch and/or diameter will increase the draw and vice versa.

i have finalised the following motor for my plane, seems like a appropriate.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=110437384988&ssPageName=STRK:MEWAX:IT

Thanks but I'm looking for the  answer of my doubt too,like how u selected this motor by this data 2822-14 1200KV Outrunner Brushless Motor?? and wat does these numbers mean 2822-14 ??

KV rating is RPM per Volt.  So the more the KV rating, the faster the motor will run for the same voltage. This means that you have to use smaller props for high KV motors, and such a setup is used in faster planes (most pushers fall under this category). The smaller KV motors have less rpm, and can in general turn bigger props. These are used in slow-flyer/3D models. 

2822-14:
28 - stator diameter
22 - stator stack height
14 - no: of turns

The higher the first two numbers are  the higher the power motor can handle.
 
For a motor of given size, Kv is inversely proportional to the number of turns.

Can we calculate the power of tat motor by using those numbers?

There are a lot more variables that determine the power in addition to these numbers. If you want to calculate mathematically, it is too complicated. For our purposes, it is much easier to search for similar motors and guesstimate the power.

Most manufacturers supply a max power rating but they almost never mention how hot the motor gets. It almost always turns out to be unusable at the max power. I tend to look at user reviews to see what is actually usable.

To the best of my knowledge, no. And that is a major grouse I have with electric motors, in general. There is scant - or no - information as the wattage it could or would draw under the typical conditions it would be used in.  

It would help no end if a simple graph showing the motor current for, say 3 types of propellors and 2 or 3 batteries, was available for each motor type. This is where the LHS/online stores could do their bit.

Sushil ,
Some Info :
http://www.rcindia.org/electric-power/full-datasheets-of-all-rcforall-motors/

Sai

If one hunts enough online, you can get datasheets for many motors.  But there are still enough variables, like Rotorzone said.  For example, this motor is available in both 1200 and 1400 KV.

For electrics, a wattmeter/ammeter seems to be the only sure shot way.

If we can't calculate the power using those variables means how can we choose the right motor? The volt meter n ammeter can be used after buying only..So wat can we do if the chosen motor is under powered or over powered for our model after buying the motor?

We go by the published information about the motor.  Start out by the AUW of your plane, find a motor with the appropriate wattage/thrust and go with it.  Overpowered is not a problem , you can always fly at lower throttle.  Overweight is a problem. 

If you feel the setup is underpowered, you can try reducing the AUW (a smaller battery etc).  You can also play around with prop choices.

That has been my grouse. Many motors do not have ANY published information!

The wattage/thrust and recommended prop is available most of the time.  Information like what is the amps draw for various prop choices and lipo cell counts are not always available.

Actually it is a good idea for LHSes to make such information readily available on their websites, as that helps assure the buyers.