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motoxaust6
04-03-2007, 03:40 AM
Hey,
i have a blade cp pro and i want to buy a brushless motor setup. There is two motors on the e flite website.
one is 4100 Kv (rpms/volt)
the other is 5400 Kv
Which one would be better for the BCCP?
What is the performance difference btwn higher and lower Kv?

I hope someone knows about this stuff b/c i don't know a whole lot about it.

thanks a whole lot

Balil
04-03-2007, 03:45 AM
the higher kv will give you more power which means a higher head speed... if you want extreme 3d performance id go with the 5400kv motor. but if you want the higher head speed i do recommend the aluminum head.....

motoxaust6
04-03-2007, 04:48 AM
if im just doing some simple stuff like rolls and loops will that screw up the original head?

this info helps me a lot!
I think i will go with the 5400 motor

thanks a ton

Balil
04-03-2007, 05:32 AM
the plastic head will hold up for mild aerobatics. but its always a good idea to have the aluminum head incase of the "hard upside down landing"......

ron_van_sommeren
04-03-2007, 07:16 PM
the higher kv will give you more power...Not quite. For two identically built motors, but with a different number of winds, both motors can handle the same power (depends mainly on size and surface). However, the motor with the higher Kv (less winds) will deliver it's maximum power at higher rpm than the motor with lower Kv (more winds).

Vriendelijke groeten ;) Ron
• diy motor building tips & tricks (http://www.rcgroups.com/forums/showthread.php?t=240993)
• diy brushless motor discussion group (http://www.yahoogroups.com/group/lrk-torquemax)
• Drive Calculator (http://www.yahoogroups.com/group/D-calc/) download & discussion group
• int. E fly-in & diy outrunner meet (http://home.hetnet.nl/%7Eronvans/), Aug.26 2007, Nijmegen, the Netherlands

PerlAddict
04-03-2007, 08:55 PM
So you get a wider range of RPMs with a higher Kv motor, but the both deliver the same amount of power?

Something's not clicking right in my head, because it seems like with what you just said, you get more power sooner out of the lower Kv.

I'm sure I'm getting lost in translation somewhere (getting confused somewhere between "the power they can handle" and "deliver its maximum power." Not sure if you're talking about output in both places, or input in the first and differing outputs in the second).

ron_van_sommeren
04-03-2007, 09:02 PM
No, not a wider range. Higher Kv means it will rev more for a given voltage. Therefore it will output (and consume) more power (keeping the prop the same) for the same voltage. But ... the maximum power both motors can handle is the same.

Vriendelijke groeten ;) Ron

r_kopka
04-04-2007, 11:19 AM
Not quite. For two identically built motors, but with a different number of winds, both motors can handle the same power (depends mainly on size and surface). However, the motor with the higher Kv (less winds) will deliver it's maximum power at higher rpm than the motor with lower Kv (more winds).
But the motor with less winds can use thicker wire and therefore max Amp is higher, internal resistance is lower -> more power or better efficiency. At least if you have some kind of gear which can be adjusted to the right values.

RK

Team MPP
04-04-2007, 12:31 PM
But the motor with less winds can use thicker wire and therefore max Amp is higher, internal resistance is lower -> more power or better efficiency. At least if you have some kind of gear which can be adjusted to the right values.

RK

Yes, but what Ron stated is still true. I believe the motors top power is dictated mainly by the magnets and flux ring. Either can only attain a fixed upper limit and once reached the motor cant produce anymore power. If a motor with 6 winds reached that point on say 11.1v it might take 22v for the motor with say 10 winds to attain the same level. Either way they both topped out at the same power level.

ron_van_sommeren
04-05-2007, 09:40 AM
Maximum power depends on a motor's ability to loose heat (size & surface) on one side and it's efficiency on the other side.

B.L.E.
04-06-2007, 07:11 PM
A motor with a kV of 4100 rpm/volt being run with 5.4 volts will turn the exact same rpm as a kv of 5400 rpm/volt motor would run on 4.1 volts.
They both will perform the same but will need different cell counts and will have different maximum amp ratings.
Going to a higher kV motor and changing nothing else will make your plane more powerful because the higher kV motor will be overloaded more. You can do the same thing by increasing the number of cells in the battery or going to a higher gear ratio (lower numerically), or a bigger prop.

mysticplayer
05-02-2007, 11:59 PM
You have a fixed prop and gear set, battery pack, ESC but two motors rated at different kV (assume same effeciency - which is unlikely).

The higher KV motor will try and turn faster (higher RPM) then the lower kV motor for the same input voltage.

Since the load is the same (prop and gear), the amp draw must be higher as the motor is trying to do more work (spin faster). Assuming nothing is being stressed, you will end up with higher rotor speed and shorter flight duration.

If you change the prop (diameter or pitch or both) and/or gearing, you change the load the motor sees which affects the amp draw, run time, etc.

By changing different parameters, you can have both motors doing pretty much the same thing as far as load/current draw/flight time goes. What will be different is the flying characteristics of the heli.

It has been a learning curve for me too trying to figure out a good compromise for my planes.

Jerry

skiman762
05-03-2007, 02:26 AM
Hey,
i have a blade cp pro and i want to buy a brushless motor setup. There is two motors on the e flite website.
one is 4100 Kv (rpms/volt)
the other is 5400 Kv
Which one would be better for the BCCP?
What is the performance difference btwn higher and lower Kv?

I hope someone knows about this stuff b/c i don't know a whole lot about it.

thanks a whole lot


Here a good site for cp pro upgrades
http://www.e-fliterc.com/Products/Default.aspx?ProdID=EFLH1300

Dr Kiwi
05-03-2007, 07:58 PM
If you look at charts posted by a reputable motor manufacturer (e.g. Medusa Afterburner) you'll notice that maximum power for every motor of the same diameter and length (no matter what the Kv) is exactly the same.

The difference lies in the ability of the higher Kv versions to cope with higher amp draw (but of course, the watts limit still holds, so that higher amp draw has to be balanced by lower voltage).

watt_the?!
09-22-2007, 10:17 PM
But the motor with less winds can use thicker wire and therefore max Amp is higher, internal resistance is lower -> more power or better efficiency. At least if you have some kind of gear which can be adjusted to the right values.

RK

same number of poles?...i.e. dont forget total wire length when comparing.

watt_the?!
09-22-2007, 10:25 PM
You have a fixed prop and gear set, battery pack, ESC but two motors rated at different kV (assume same effeciency - which is unlikely).

......

If you change the prop (diameter or pitch or both) and/or gearing, you change the load the motor sees which affects the amp draw, run time, etc.

.....

be careful when comparing setups like this as although you are chaning some of the variables that make up the power setup- you are not changing all of them.

particularly mising in this example is the relative stress on the motor, battery and ESC. In fact these contribute heavily to the setup and shouldnt be left out if you are comparing.

bottom line is- power is the only thing that matters and governs your flight experience.

efficiency comes mostly from the relative stress you expose your equipment to. i.e. if one of your setups uses 10A and you have a 30A ESC then you'll run a cool ESC. Similarly you might have a 2000mah battery pack so that too will be cool and wont exhibit V drops under high relative load.

One pf the things people get wrong when making these comparisons is to not change the battery mah and ESC in their comparison.

Thus they find a setup that runs much better than the other not because it is "better"..but rather because it now isnt stressed as much.

Basically now there is too much ESC and battery mah so theres more $ and weight that is responsible for the improvement.

Tim

Bill G
09-23-2007, 08:57 AM
I don't know how the motors perform in a heli, but in EDF I've found the the short can 5400kv is not a very efficient motor, and you get the same if not better performance, at the same if not even slightly lower current levels, with a long can 4200kv.

If it works out anywhere close to my fan experiences in a heli, I'd go with the 4100kv short can 20mm motor, since the 5400kv is a pig, with respect to current draw. In an EDF55, I can draw 25A with a Park 370-5400.:eek:
Considering that the heli flys brushed, the 4100 will be a good upgrade.

watt_the?!
09-23-2007, 11:46 AM
Bill- what fan/motor?. like anything else, this could be a matching issue as most- if not all BL E Motors produced are about as efficient as each other.

some people run a setup that has them at the 55% efficient part of their curve and another person might have the same motor on a setup that nails 80% efficient for most of the flight.

Tim

Bill G
09-24-2007, 10:40 AM
This fan and motor:

I don't know how the motors perform in a heli, but in EDF I've found the the short can 5400kv is not a very efficient motor, and you get the same if not better performance, at the same if not even slightly lower current levels, with a long can 4200kv.

If it works out anywhere close to my fan experiences in a heli, I'd go with the 4100kv short can 20mm motor, since the 5400kv is a pig, with respect to current draw. In an EDF55, I can draw 25A with a Park 370-5400.:eek:
Considering that the heli flys brushed, the 4100 will be a good upgrade.

Warren T
03-07-2008, 01:14 AM
I was told that a 2cell 20c discharge battery will give you more torque to use a biger prop and a 3cell battery will give you more rpms and less torque. Is this correct? Who can make sence of all these numbers? WarrenT


No, not a wider range. Higher Kv means it will rev more for a given voltage. Therefore it will output (and consume) more power (keeping the prop the same) for the same voltage. But ... the maximum power both motors can handle is the same.

Vriendelijke groeten ;) Ron

watt_the?!
03-07-2008, 01:37 AM
you were told bulls***. all that the battery gives you is the amount of energy available per unit time (when combined with the discharge rate).

the motor characteristics determine torque. You can run a gearbox to generate torque elsewhere for example.

but a load of voltage and current into a motor and youll get lots of torque.

so it is related to power and kv.

the higher that ratio, the more torquey a motor is.

i.e. a 5000kv, 300W rated motor (10V and 30Amps) delivers

[5250 x (300/745) (this is horsepower conversion) ] / 5000 x10volts

0.042 ft-lbs at 50000rpm.

but maybe another, say 800kv motor rated at 600W (10V x 60A) has torque = 0.53ft-lb.

both of these ignore inertial effects.


Now this does give a guide to prop size although thats a little more complex. The prop load increases as rpm increases and the equivalent load does not occur at the same place on the prop for all rpm. It also changes according to the efficiency of the prop at different rpms and flight conditions.

to simplify it you could assume that the prop load occurs at a point about 2/3 the way down the prop blade.

so then you can match the prop to the torque..

where Torque = Force x Radius (where the equivalent load is)

Determining force comes from the prop coefficient.- up to a point as even this changes versus rpm.

hope this helps.