Motor formulas - Page 4 - RC Groups
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Apr 20, 2011, 09:41 AM
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motor constants by Joachim Bergmeyer

i think adding his formulas could help to increase the knowledge of our motors. i have been using them and learned a lot about motors.
here is the link:
he also has another formulas for a simplified method: .
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Apr 25, 2011, 01:11 AM
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manuel v's Avatar
I have this ecuations in Excel.

Manuel V,
May 02, 2011, 03:36 PM
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very nice thanks for writing all of the formulas down, very helpful
May 08, 2011, 07:02 AM
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Hi guys,

I hope this is not much OT.

For more than a year we have been using so called tax power formula for evaluating the input power in our local e-sailplanes competitions.

The general formula is

Pd is tax power [W]
kv is motor constant [RPM/V]
U is battery nominal voltage [V]
D is propeller diameter [inch]
d is air gap diameter [mm]
L is air gap length [mm]

For "small outrunners" (with a bell diameter of 28 mm) it is slightly modified as
Pd = 10*L*(kv*U*D/100000)^2

The calculation sheet is here:

Axi 2814/10, 2S lipol, propeller 9.5x5: tax power is 0.4*28*14*(1640*7.4*9.5/1e5)^2 = 208 W, measured 210 W (this is what I fly in the AH Castor)
TG 2826-1650, 2S lipol, 10x6 (this outer 2826 = inner 2209): tax power = 10*9*(1650*7.4*10/1e5)^2 = 134 W, measured 135 W (my "small outrunner" sailplane).

Now, while the measured (static) values correspond to the calculated "tax power" values reasonably well, I am not able to get anything close to this formula theoretically. Would someone of you more educated experts be able to help?

May 08, 2011, 10:14 AM
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Fourdan's Avatar
Hi Jan
Normally the power at prop shaft is INDEPENDENT of the motor
Power is a function of prop design + rpm

Regarding the total power at the battery level
1) Power at prop shaft (could be around 80%)
2) + Ohmic losses (motor) (could be 10%)
3) + Iron losses (motor) (could be 6%)
4) + ESC losses (brushless controler) (could be 4%)

Air gap dimensions are acting on 2) and 3)
May 08, 2011, 12:57 PM
Registered User

Thanks for your reply - it is clear to me. However, my problem is a different one .

Using measured data for various combinations of motors/props/batteries and a "brutal computing force" I arrived in a relatively simple formula for calculating the input power of a drive unit. Well, it is not 100% accurate but it is, say, usable and we have been flying competitions based on it.

The formula is "simple" with only linear and quadratic functions. I would guess it should be possible to derive it theoretically. However, here I have failed repeatedly. Very likely I have been using too simplified models, or even more likely I am not clever enough. So, I would appreciate any help in getting that formula just from the desk or notebook - I would sleep better then .

Jul 17, 2011, 05:44 PM
Registered User

No Receiver

I am building an RC foam plane and trying to decide if I need to connect a receiver inside the plane if I already have a receiver in my Spectrum DX6i Radio. And if I do not connect a receiver inside the plane to connect the ESC to the Servo's can I connect the ESC directly to the Servo's? A simple question I cannot find an answer to.
Jul 18, 2011, 01:39 AM
Registered Aircraft Offender
Truglodite's Avatar
Try asking over in the beginner threads, or better yet go by your local hobby shop and asking a few questions. They'll get you started in the right direction. This particular thread is not related to what you are looking for.

Sep 21, 2011, 12:33 PM
Lost but making good time
To any Electricians,

When you add voltage by going from a 2S pack to a 3S pack, do the amps increase along with the voltage?

Somewhere in this section I seem to remember >Amps=(%>V)squared. Please advise.


Sep 23, 2011, 09:37 AM
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Fourdan's Avatar
Originally Posted by hankg
To any Electricians,

When you add voltage by going from a 2S pack to a 3S pack, do the amps increase along with the voltage?

Somewhere in this section I seem to remember >Amps=(%>V)squared. Please advise.


Hi Hankg
Generally when voltage increases the rpm are also increased
Speed (rpm) higher means higher consumed power + higher motor losses.
If the load is a propeller you could have an exponent around 3.0 upto 3.3
See the picture (Power versus rpm)
For an e-car it could be different
Last edited by Fourdan; Sep 23, 2011 at 10:41 AM.
Sep 23, 2011, 10:32 AM
Lost but making good time

Thanks for the reply.


Oct 16, 2011, 04:44 AM
Registered User

I see all of you appear like RC motor experts because you discuss about motor formulas.

I understand this forum place is for DIYers where advanced members like to help others in theirs fields. I have many technician skills, but RC aircrafts is field where I am not familiar so, and I am here new.

So please, go in my thread "The basics of building RC aircraft motor where rotor is permanent magnet?" and help me something there.

Thanks in advance :-),

Aug 07, 2012, 04:07 AM
Registered User

brushless outrunner motor

hi i could not find out to how to post my question,so im writing here sorry.
how to find the max rpm of the motors. ie Turnigy 1811 brushless Outrunner 2900kv. is it 2900rpm?
whats the max air distance on 2.4ghz rx tx?
Oct 26, 2012, 06:31 PM
Bombs away!!!
quitcherbitchen's Avatar
I have a question about inductance? I have been messing with the timing on my ESCs to better match the pole counts of my motors and having good results. Now I am thinking of changing the PWM Pulse width Mod, but am hesitant because most of my ESCs Eflite pros tell you to check a motors inductance before changing. My problem is I don't know if the Ohm resistance reading is a measure for the Inductance which I understand as a magnetic field induced by electricity. Seems none of the motor manufactures list the inductance so I am assuming it needs to be interpolated.
Old Nov 04, 2012, 09:54 AM
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