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Selecting the "right" brushless motor

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Old 01-12-2006, 02:47 AM
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LarryB22
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Default Selecting the "right" brushless motor

I'm designing an electric powered r/c plane that will weigh around 18 lbs and need to select a motor/gearbox combination. For our application a motor rpm of between 7000 and 11000 would be acceptable. Most motor manufacturers supply no load RPM/V (Kv), idle current (lo), and resistance (R) for their motors. From P=V/R, I can find the power output for a given number of cells, and then I can find current using I=P/V. I can get RPM from the number of cells using RPM/V and from that determine what gear ratio I would require. Aveox specifies a max continuous current, max peak current, and max rpm for each of their motors. Hacker does not supply this information, so I was wondering if this information or similar information is given somewhere for Hacker motors. Also, the manufacturer specified RPM/V is for no load I believe (w/o prop/gearbox), so is their a method for determining the resulting motor rpm with a prop and gearbox and does adding a prop/gearbox affect the resistance or current draw of the motor?
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Old 01-12-2006, 04:40 PM
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Default RE: Selecting the "right" brushless motor

You may try following link:
https://rcplanes.online/calc_motor.htm
It's not 100% accurate, but it may put you in the ballpark and besides the price is right...

Last edited by adam_one; 12-22-2017 at 02:12 PM. Reason: Changed URL
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Old 01-14-2006, 05:30 PM
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Default RE: Selecting the "right" brushless motor

I want to correct a mistake in my first post. P=V^2/R, not P=V/R . Thanks for the link to that site. It is helpful. Also, I did end up finding more information on Hacker's website, though it is confusing to me what exactly the units are for Resistance (Ohms?) and Idle current (Amps?). I need to know this, so if anyone out there knows, please transfer the knowledge along. Thanks.
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Old 01-14-2006, 08:17 PM
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adam_one
 
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Default RE: Selecting the "right" brushless motor


ORIGINAL: LarryB22

I want to correct a mistake in my first post. P=V^2/R, not P=V/R . Thanks for the link to that site. It is helpful. Also, I did end up finding more information on Hacker's website, though it is confusing to me what exactly the units are for Resistance (Ohms?) and Idle current (Amps?). I need to know this, so if anyone out there knows, please transfer the knowledge along. Thanks.
The Resistance (Rm) is just what it means, the coil's resistance in ohms.
The Idle current (Io) refers to the current drawn with no load (no prop).
Such figures may be used to do some useful calculations.
The following equation can be used to calculate the motor's Output Power:

Pout = (Vin - Iin * Rm) * (Iin - Io)
Pin = Vin*Iin

The motor's Efficiency is the ratio of the Output Power to the Input Power:
% Efficiency = 100 * Pout / Pin

Also, for instance, a motor with a Kv of 1000, a coil resistance Rm of .04 ohms and
with a terminal voltage of 8 volts at 12 amps will have the following RPM:

1000 * (8 - 12 * .04) = 7520 RPM, instead of 8000 RPM (if the motor coils had no resistance Rm) that means a loss of 480 RPM from the ideal in this case.

RPM Loss = Kv * (Iin * Rm)
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