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Old Sep 06, 2012, 07:50 PM
Shore Huckin'
KE Spin's Avatar
Ohio
Joined Aug 2010
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The motor doesn't really demand power, it accepts it from the ESC and the ESC is the limiting factor. Conversely, if one were to use an ESC that is too large, the motor could fry pretty quick (depending on how many amps are being supplied to the ESC by the battery) illustrating that the motor just takes what is supplied.

Anyhow, really enjoyed flying my Extra 330 tonight. Winds were light and I think the wing loading and power setup on my 45" (clip wing) Extra 330 is just right using my 6 oz 2200mah lipos

What??? another vid...coming soon.
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Old Sep 06, 2012, 08:03 PM
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Burke, VA
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Quote:
Originally Posted by KE Spin View Post
The motor doesn't really demand power, it accepts it from the ESC and the ESC is the limiting factor. Conversely, if one were to use an ESC that is too large, the motor could fry pretty quick (depending on how many amps are being supplied to the ESC by the battery) illustrating that the motor just takes what is supplied.

Anyhow, really enjoyed flying my Extra 330 tonight. Winds were light and I think the wing loading and power setup on my 45" (clip wing) Extra 330 is just right using my 6 oz 2200mah lipos

What??? another vid...coming soon.
You missed the point I was trying to make, if you take a power system capable of handling more juice than either of your test batts are capable of producing and then ran this system with both batts at wot you would see that the larger batt would produce more power than the smaller one. I believe this is what the discussion was about, not the semantics of whether a motor demands power or not.
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Old Sep 06, 2012, 08:29 PM
Shore Huckin'
KE Spin's Avatar
Ohio
Joined Aug 2010
2,692 Posts
I believe that I agreed to that in post #268.

I'm happy with my current setup. Gonna try and get some flight time in tomorrow as we have demo flights for the C.A.P. kids on Saturday and our annual fall Funfly on Sunday. Looking forward to a good time.
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Old Sep 06, 2012, 10:54 PM
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United Kingdom, Stoke-on-Trent
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Originally Posted by winndaddy View Post
I guess I learn something new all the time. Thanks for clearing that up.
How embarrassing
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Old Sep 07, 2012, 01:29 AM
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United Kingdom, Aviemore
Joined Feb 2005
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Quote:
Originally Posted by KE Spin View Post
The motor doesn't really demand power, it accepts it from the ESC and the ESC is the limiting factor. Conversely, if one were to use an ESC that is too large, the motor could fry pretty quick (depending on how many amps are being supplied to the ESC by the battery) illustrating that the motor just takes what is supplied.
Really? I thought that it was the motor prop combination that decided how many amps it was going to draw at wot. Say you have a setup that runs at around 35A so you put a 40A ESC on it are you saying that if you changed that ESC to an 80A one you would get more power but risk burning your motor out? Gona have to test that principle I'd have though you would still be running at 35A but your ESC wouldn't be working as hard.
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Old Sep 07, 2012, 03:19 AM
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Originally Posted by Aviefly View Post
are you saying that if you changed that ESC to an 80A one you would get more power but risk burning your motor out?
EDIT: misread your quote, I agree with you that this is not the case at all!


-------------------------------------

At max throttle, the output FETs in the esc are almost completely on.

However, to understand what is going on you can really simplify the whole operation to batteries and bulbs and ohms law.

The motor is like a resistive load. The battery is your current source, and it has an internal resistance, which is like a current limiter. Although the current across the circuit is the same all the way round, there is a voltage drop across the motor, and a voltage drop across the (imaginary) internal resistance resistor.

At low loads, the "virtual" resistance of the motor is much higher than the internal resistance of the battery, and consequently the motor sees almost the entire unloaded pack voltage.

As the load on the motor increases, the "virtual" resistance of the motor decreases, and approaches the internal resistance of the battery, What happens now is quite different - the voltage drop across the internal resistance of the battery is much greater, and accross the motor is much lower. This is why you wattmeter/voltmeter (which, on load, is measuring the voltage across the battery) shows a voltage drop under load.

The rest of the voltage has not disappeared however, it's now dropped across the battery's "virtual" internal resistance. And, since ohms law tells us that power = volts x amps, the now non-negligable voltage dropped inside the battery mulitpled by the current in the circuit results in quite a bit of power (i.e. heat ) being generated inside the pack. Your pack gets hot.

If the pack has a lower internal resistance (which a pack of larger capacity does) then it follows through in the example above that the voltage drop in the pack will be lower, the pack will not get as hot, and the voltage at the motor under load is greater. which equates to more power at the motor.
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Old Sep 07, 2012, 03:25 AM
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Originally Posted by KE Spin View Post
The motor doesn't really demand power, it accepts it from the ESC and the ESC is the limiting factor. Conversely, if one were to use an ESC that is too large, the motor could fry pretty quick (depending on how many amps are being supplied to the ESC by the battery) illustrating that the motor just takes what is supplied.
That's not correct.
The ESC can be thought of as a switch that is turning on and off very rapidly. The amount of power the motor sees is proportional to how long the switches are turned on vs how long they're turned off. This is called "duty cycle".

At full throttle, the switches are on almost all the time. If you ignore the brushless magic that the ESC also does, basically at full throttle, the ESC just connects the battery directly to the motor.

The power consumed by the motor is a function of it's impedance and the voltage applied. Nothing more, nothing less. (the impedance of the motor varies with the load on the motor (i.e prop)).
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Old Sep 07, 2012, 04:47 AM
Shore Huckin'
KE Spin's Avatar
Ohio
Joined Aug 2010
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So, we're in agreement then (the motor accepts what the ESC will send it).

Appreciate the other information.

Getting back to the airframe and setup:

With the extra weight of the 2200mah lipos I saw an improvement in waterfall to inverted elevator/harrier. Previously the 330 did this really well with a little help from a head wind but tonight it was much more stable in near zero wind.

Looking forward to a little more stick time in the 10-15mph winds forecast for later today.
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Old Sep 07, 2012, 05:01 AM
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Originally Posted by KE Spin View Post
So, we're in agreement then (the motor accepts what the ESC will send it).
You could use a 200A ESC on this model, and nothing would blow up. It'd just be a bit on the heavy side ;-)
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Old Sep 07, 2012, 12:24 PM
Shore Huckin'
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Ohio
Joined Aug 2010
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Well, the Extra performed very well (using the 2200mah lipos) with the winds as forecast at 10-15mph.

Very stable and maneuverable in the warm and bumpy air.
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Old Sep 07, 2012, 05:47 PM
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Carlsbad, New Mexico
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Well, I have mine finished. I deviated from the build. I installed the rudder servo just above the horizontal stab. I seen this as a much better location as now I am actuating the rudder from the middle instead of the bottom. I am hoping I don't get too much twist. May not have been an issue anyways. With Glacier 2200 packs (6.7oz) the AUW is 27.5 oz. I am running a 12x6E apc prop. I have not flown it as of yet, but will tomorrow. Will report back my thoughts.
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Old Sep 07, 2012, 08:39 PM
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United States, CO, Denver
Joined Jun 2011
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Originally Posted by noahb View Post
Well, I have mine finished. I deviated from the build. I installed the rudder servo just above the horizontal stab. I seen this as a much better location as now I am actuating the rudder from the middle instead of the bottom. I am hoping I don't get too much twist. May not have been an issue anyways. With Glacier 2200 packs (6.7oz) the AUW is 27.5 oz. I am running a 12x6E apc prop. I have not flown it as of yet, but will tomorrow. Will report back my thoughts.
Noahb.... grats on your newest addition, which motor did you end up installing? Looking forward to seeing your pictures...
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cdee
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Old Sep 07, 2012, 09:07 PM
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Carlsbad, New Mexico
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I installed the 104g torque motor. I think 2818/900. If I remember right, I am pulling around 31-32 amps at full throttle, around 360 or so watts. Don't remember exactly.
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Old Sep 07, 2012, 09:41 PM
Shore Huckin'
KE Spin's Avatar
Ohio
Joined Aug 2010
2,692 Posts
Using the Telink 79g motor, I'm getting 350 watts with the battery at 12.6v so that's 28 amps.

Where does the battery wind up mounting (to establish CG) when using the 100+ gram motors?
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Old Sep 07, 2012, 10:55 PM
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Originally Posted by KE Spin View Post
Using the Telink 79g motor, I'm getting 350 watts with the battery at 12.6v so that's 28 amps.

Where does the battery wind up mounting (to establish CG) when using the 100+ gram motors?
When switching to the 100g motor I moved my 1600mAh batteries back about 1 inch.

Been flying with the 100g motor now for about 20 or 30 flights, going back to the 78g motor for tomorrow morning.
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cdee
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