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Old Oct 09, 2012, 12:52 PM
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Honest thrust measurements for Hextronic 5g 2000kv

Hey guys, did anyone measure the thrust of the Hextronic 5g 2000kv motor ?

I measured it by suspending it downwards from a scale and got these results:
-GWS EP-5030: 54.2g
-GWS EP-4540: 47.2g
-GWS EP-4025: 36.6g
using a 2s Nanotech 180mAh battery and a HobbyKing 6A ESC.

The measurements reported on the HobbyKing page are much better:
-GWS 5030: 76g
-GWS 4540: 65g
-GWS 4025: 53gr

I'm a bit suspicious of those results though.

Can anyone post their measurements here ?
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Last edited by flylowfast; Oct 09, 2012 at 02:05 PM.
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Old Oct 09, 2012, 08:31 PM
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I looked at that motor on HK. Since you've had it apart, how many poles, magnets, wire dia and turns, no load current and no load rpm at a measured voltage. I think you said its wound "y" and has 8 ohms res.
I would like to run the numbers and see how efficient it is.
George
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Old Oct 10, 2012, 07:03 AM
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I measured the current and power consumption with the same configuration as before:
-motor Hextronic 5g 2000kv
-ESC: HobbyKing 6A ESC: unknown settings
-battery: Nanotech 180mAh, 2s, 25-40C
-wattmeter: Turnigy 130A from HobbyKing

The measurements were peak power only, lasting only a few seconds.
The battery was freshly charged at the beginning but not recharged between measurements.

Results:
-GWS EP-5030: 1.72Ap, 13.3Wp, 7.72Vm
-GWS EP-4540: 1.82Ap, 14.0Wp, 7.67Vm
-GWS EP-4025: 1.35Ap, 10.5Wp, 7.79Vm

No load measurements were done with the motor mount on but without the plastic ring that holds the prop. in place:
Same config as above:
-0.34Ap, 2.7Wp, 8.12Vm, 8.20V at start, 8.18V at the end


P.S.:
The notation used are the same as reported by the wattmeter:
Ap = peak current in amps.
Wp = peak power in watts
Vm = the min. voltage in volts, not necessarily taken at the same time as Ap and Wp
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Old Oct 10, 2012, 07:04 AM
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Then I tried to measure the no load current using:
-3s 2200 old battery from an ArtTech SU-27 with 12.55V at the start
Results:
-ESC: HobbyKing 6A ESC unknown settings: 0.33Ap, 4.1Wp, 12.53Vm
-ESC: HobbyKing BlueSeries 30A ESC unknown settings: 0.40Ap, 5.0Wp, 12.52Vm

Problem: for both ESCs, at maximum throttle, the rpm was not constant but varying at about 1Hz, giving a "wailing" sound.
I'm not sure why that happened, maybe the rpm was too high for the ESCs ? Or maybe I need to change their settings ?

I'll discount these (the no load with 3s battery) results for now since they obviously are not correct.
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Old Oct 10, 2012, 07:10 AM
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Quote:
Originally Posted by Plantoflap View Post
I looked at that motor on HK. Since you've had it apart, how many poles, magnets, wire dia and turns, no load current and no load rpm at a measured voltage. I think you said its wound "y" and has 8 ohms res.
I would like to run the numbers and see how efficient it is.
George
George, see the posts above for some of that data.
I have ordered an rpm meter but it didn't arrive yet, so no rpm data for now.
I'll measure the rpm when it arrives.

For the other data you need see this thread:

http://www.rcgroups.com/forums/showthread.php?t=1735549

Here is some relevant data from the first post:

The original configuration of this motor was a 12 pole stator with 16 magnets wound with 2 strands of 37 gauge with 17 turns (ABC wye)

What software are you using to compute the efficiency ?
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Old Oct 10, 2012, 02:57 PM
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Did some calculations with the online calculator at

http://www.ecalc.ch

Configuration:
-custom motor with Kv=2000, res=1.6Ohm, no-load: 0.34A at 7.94V
-LiPo: 150mA, 20/30C, 2s, res=0.15Ohm, voltage=3.7V
-controller: max 10A with res=0.015 Ohm
-propeller: custom, 0 deg., diam=5", pitch=3", blades=2, prop. const.=1.18, gear=1.0

Results:
-thrust=79.5g, current=1.53A, voltage=6.94, power=11.32, eff.=50.6%
For the same config. but pitch=1.5":
-thrust=54.1g, current=1.14A, voltage=7.06, power=8.44W, eff.=54.1%

The calculator seems to indicate a much higher static thrust than what I'm getting.
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Last edited by flylowfast; Oct 10, 2012 at 03:17 PM.
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Old Oct 10, 2012, 03:17 PM
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I did some calculation using similar data with this online calculator:

http://brantuas.com/ezcalc/dma1.asp

Configuration:
-custom motor with Kv=2000, res=1.6Ohm, current: 0.34A
-LiPo: 180mA, 2s, res=0.15Ohm, voltage=3.7V
-controller: Castle Phoenix 10A: res=0.013 Ohm
-propeller: custom, diam=5", pitch=3", blades=2, prop. const.=1.18, gear=1.0

Results:
-thrust=76.41g, current=1.637A, voltage=6.89V, power=11.28, eff.=45.2%

So the thrust is a little less than that of the previous calculator, but very close to it and very far from real life measurements.

Any ideas on why the big difference between the calculators and real life ?
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Old Oct 10, 2012, 03:42 PM
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flylowfast,
The rpm is not needed to find the efficiency, nor is the true kv because you know the Io amp draw and Rint.

E = Efficiency = Po/Pin Io=no load amps Rint= internal resistance, should include esc fet resistance since they are in series with the motor leads.

Pin=Vin* Ain Po=( Vin-(Ain*Rint)) * (Ain-Io)
E = ( ( 1- (sq root of (Io * Rint / Vin )) )^2

Amps at max power = ( Vin + (Rint*Io)) / (2*Rint)

Io increases with rpm, its a curve. when you get your new meter graph kv and Io at various rpm. Graph the calculated amps, power and efficiency too.

George
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Old Oct 10, 2012, 04:55 PM
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Originally Posted by Plantoflap View Post
flylowfast,
The rpm is not needed to find the efficiency, nor is the true kv because you know the Io amp draw and Rint.

E = Efficiency = Po/Pin Io=no load amps Rint= internal resistance, should include esc fet resistance since they are in series with the motor leads.

Pin=Vin* Ain Po=( Vin-(Ain*Rint)) * (Ain-Io)
E = ( ( 1- (sq root of (Io * Rint / Vin )) )^2

Amps at max power = ( Vin + (Rint*Io)) / (2*Rint)

Io increases with rpm, its a curve. when you get your new meter graph kv and Io at various rpm. Graph the calculated amps, power and efficiency too.

George
Thanks, but what I really need to know is how to get in real life the thrust predicted by the calculators.
Or, failing that, some ideas on where the difference comes from.
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Old Oct 10, 2012, 06:17 PM
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If there is nothing wrong with your motor, prop or scale then use your numbers, but the more accurate your measurements are the closer you can get to the truth.

Trying to understand or explain someone's calc program without knowing everything about it is a waste of time.

The equations I posted have been used long before brushless motors were available.

George
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Old Oct 11, 2012, 08:44 AM
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Not every motor is created equally. I've seen lots of variation in hextronic motors. In twin motor airplanes, I have to sometimes go through 6-7 motors to get two of them that run close to the same rpm. It does seem like the lower the KV and the larger the motor, the smaller the differences. The actual voltage supplied under load could be different too.
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Old Oct 11, 2012, 11:27 AM
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Originally Posted by flylowfast View Post
Any ideas on why the big difference between the calculators and real life ?
Those calcs use a too simple motor model, they forget a big part of the losses that can be seen as a resistance in series, increasing with rpm... Rm (internal resistance of the motor) is not enough !, so it results in increased rpm predictions, so over-estimated thrust, higher amps draw and better efficiency than real life. Also the propeller "drag" might be incorrectly estimated by the calcs, and Io also increases with rpm... the batteries Ri can be wrong too...
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Old Oct 11, 2012, 12:51 PM
Jack
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Thrust generated by a prop in flight is really hard to measure realistically. And we are often using props that, regardless of what the markings on them may say, often have measured pitches that are quite different from the markings.

Not sure if it is helpful to you or not but I like to go to the prop database at flybrushless.com and look up a prop by brand and model name or markings. Then I can see a graph of the static thrust obtained in test by various prop testers who voluntarily feed the motor and prop databases there.

That also leads me to a list of the motors that thrust data was obtained with and that is useful information. And if I'm contemplating a prop choice for a plane it gives me a potential thrust to weight ratio to consider and also a theoretical pitch speed.

I don't consider any of it to be empirical but it is certainly enough to get me in the ballpark as far as what might be good to try for props and motors.

Another thing I like about the data there is that the RPM readings for a given prop are all safe or practical to use numbers. They don't report readings that are in excess of safe RPM limits or that caused the motor temperatures to become excessive.

Jack
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Old Oct 12, 2012, 04:49 AM
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Quote:
Originally Posted by flylowfast View Post
I measured it by suspending it downwards from a scale and got these results:
-GWS EP-5030: 54.2g... 1.72Ap, 13.3Wp, 7.72Vm
Compare this to ssatoru's test data in the thread New 5 gram outrunner/ unitedhobbies HXM1400-2000,
Quote:
Motor #1
GWS 5x3 CL400 2S 7.63V 1.91A 14.00W 9,390rpm
Extrapolating from my own tests of the EP5030, Motocalc predicts 73g static thrust at 9400rpm. 54.2g corresponds to about 8100rpm.

Either your motor is running significantly slower then it should, or your thrust measurement is inaccurate (perhaps a little of both?).

Code:
Motocalc In-flight Analysis
Sea Level, 101.3kPa, 20C

Motor: HXM1400-2000 (ssatoru); 2020rpm/V; 0.35A no-load; 1.55 Ohms.
Battery: Test Lipo (100C); 2 cells; 10000mAh @ 3.82V; 0 Ohms/cell.
Speed Control: Castle Creations Phoenix 10 (cc); 0.013 Ohms; High rate.
Drive System: GWS EP5030 (ba); 5x3 (Pconst=1.23; Tconst=1) direct drive.
Airframe:  None.

AirSpd  Batt Motor Motor  Input   Loss MGbOut MotGb   Prop Thrust
 (m/s)  Amps  Amps Volts    (W)    (W)    (W) Ef(%)    RPM    (g)

   0.0  1.91  1.91  7.61   14.5   7.27   7.24  49.9   9390   72.8
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Old Oct 18, 2012, 02:01 PM
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Plantoflap, Joe 1320, yomgui, jackerbes and Bruce,

Thanks a lot for the very useful information you provided.
Since the discrepancy between what others reported and what I measure is big, I need to take some additional steps to validate my results.
Once the rpm meter I ordered arrives, I will be able to get some additional data, i.e. the rpm at various voltages and thus reduce the number or possible causes.

As Bruce also suggested, I might need to validate the way I measure the thrust.
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