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HomeAircraftHelicopterMain Discussion › Whats More Efficient?
06-03-2009 02:42 PM  9 years agoPost 21
T-Rex-Flyer

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Panama City, Fl

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I was under the impression (maybe mistakenly) that the motor/ESC combination work most efficiently at 80-90 percent. Either way you'll never get the flight times you really really want, before it's time to land again.

If the wings are traveling faster than the fuselage, it's probably a helicopter.

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06-03-2009 02:56 PM  9 years agoPost 22
jbdww

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Round Rock, Texas

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My example earlier is more for brushed motor, and Nguyen has the right answer. Let me see if I can put it in clear terms.

When you start any motor from 0 they draw very high amount of current to start spinning because they have to move mass. So what Nguyen is saying is with a brushless motor you have to send it pulses which will determine the motor speed. So basically if there is a longer period in there where the voltage is a 0 then the motor mass slows down and because of this when the voltage pulse comes back which is from 0 to the full bat voltage the motor draws more amp to speed the mass back up.

When you have 100% throttle curve then the pulses are mostly on with very small amount of 0 voltage time but there is still pulses as the ESC switches the pulses around to keep changing the magnetic poles in the motor. If the Throttle is at 50% then you have half the time at 0 voltage and the other half at full bat voltage. This means that the turn on pulse requires higher amps to keep the motor mass moving as friction has slowed the motor down during the 0 voltage time.

So in short. There is a sweat spot somewhere between 85% and 95%.

Joe

Skids Up
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06-03-2009 02:58 PM  9 years agoPost 23
Richardmid1

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Leeds, England

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You want to spin the motor as fast as you can and have the headspeed you want, 2500rpm is high on a 600, are you struggling to get a smaller pinion? Running a smaller pinion and upping your throttle to 100% will be far more efficient than running a bigger pinion at 80% throttle.

What size pinion are you currently using?

60% of the time, it works every time!

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06-03-2009 03:30 PM  9 years agoPost 24
Sean Williams

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Santa Clarita CA

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I believe it is a 10 tooth pinion on a 83 tooth main gear, but I don't think they make any other pinion options. This is on a Knight 600e.

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06-03-2009 03:35 PM  9 years agoPost 25
Big Fil

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Santa Rosa, CA

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Let me try to put this in layman's terms since it's all I know how to do anyway. The lower you run your throttle the larger amount of energy will be lost through heat rather than tranlated directly into power at the motor. As you run higher on the throttle curve you will lose less energy through heat generated at the ESC and motor.

When people talk about efficiency at higher throttle curves it is in relation to heat loss. That said if you run the throttle higher your not going to gain flight time since you are still consuming more power.

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06-03-2009 04:05 PM  9 years agoPost 26
Hamo

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Ireland

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Nguyen has the right answer
I don't think so. Nguyen is talking about standard DC motors with pulse width modulation speed control, the type used in golf trolleys. He is talking about variable width pulses of current driving the motor which is wrong. Brushless DC motors used for RC are in fact 3 phase AC motors, miniature versions of the big industrial motors, except the 3 phase AC is generated electronically from the DC. Speed variation is achieved by changing the frequency of the 3 phase AC applied to the motor and not the duty cycle of the applied DC as Nguyen suggests.
The ESC receives pulses of 1 - 2 millisecond wide, every 20 milliseconds. 1 ms means stopped, 1.5 mS means 50% speed and 2 mS means 100% speed. The ESC then uses these to alter the frequency of the generated AC, which alters the speed of the motor. 3 phase industrial motors operate at a frequency of 60 Hz, brushless DC motors operate at a much higher frequency.
Hamo

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06-03-2009 04:45 PM  9 years agoPost 27
Cope

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South Lake Tahoe CA

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Wow this is good stuff here. ^^^^

Thanks guys I always wonder about this type of thing.

Fear is the little death,The mind killer. I will allow my fear to pass through me.Only I will remain

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06-03-2009 04:56 PM  9 years agoPost 28
Terrabit

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Seattle, WA - USA

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I think you two should arm-wrestle.

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06-03-2009 05:00 PM  9 years agoPost 29
jbdww

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Round Rock, Texas

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Skids Up
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http://twitter.com/jbdww/

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06-03-2009 05:11 PM  9 years agoPost 30
AirWolfRC

rrProfessor

42½ N, 83½ W

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The lower you run your throttle the larger amount of energy will be lost through heat rather than tranlated directly into power at the motor.
This is only true of the very old linear type DC controllers
. . . that nobody uses any more.

The current technology is burshless and pulse modulated control. The controller does not act as a resistance in series with the motor. The power is switched on and off as needed for the desired throttle setting.

Nothing runs at 100% efficiency.

The controller typically runs at about 80% to 90% effeciency.
The motor about the same.

Those efficiencys vary depending on the operating point and have the best efficiency at some specific load and speed.

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06-03-2009 06:04 PM  9 years agoPost 31
DKNguyen

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N/A

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That's all fine and dandy Mr Nguyen. I believe what you are saying is that it depends on the motor and the ESC. Now, can you clearly express that in laymen's terms and shorten it down to four sentences or less? I have a very short attention span! Like a nat.

When will you engineer types learn how to speak to us administrative types?
He did ask for a technical explanation. Ask and I definately won't even try and hold myself back lol.

I did! Then again it's really buried in there. I went back and bolded my ONE summarizing sentence.
I don't think so. Nguyen is talking about standard DC motors with pulse width modulation speed control, the type used in golf trolleys. He is talking about variable width pulses of current driving the motor which is wrong. Brushless DC motors used for RC are in fact 3 phase AC motors, miniature versions of the big industrial motors, except the 3 phase AC is generated electronically from the DC.
Hamo
THese are not like most big industrial motors- those are usually induction motors. A brushless motor and synchronous AC motor are the same thing. And a brushed motor is the same as a brushless motor except that the brushes have been removed and the phases have been run out of the motor with wires.

All three are the same electrically and work the same way:
-the induced BEMF opposing the battery voltage is the same
-the current ramping in the winding inductance is the same
-All three also run off AC internally (brushed motors automatically convert AC to DC internally with their brushes while brushless and synchronous motors need this spoonfed to them by the ESC.)

THe *only* difference between brushless motors (which are AC synchronous motors with permanent magnets instead of field windings and brushed motors), is that brushed motors have brushes to automatically commutate. They all work the same way and can all windings of all can be driven by a AC sine-wave or a square-wave approximation of a sine-wave (though it would be mechanically tricky to bypass the brushes on a brushed motor to do so).
Speed variation is achieved by changing the frequency of the 3 phase AC applied to the motor and not the duty cycle of the applied DC as Nguyen suggests.
The ESC receives pulses of 1 - 2 millisecond wide, every 20 milliseconds. 1 ms means stopped, 1.5 mS means 50% speed and 2 mS means 100% speed. The ESC then uses these to alter the frequency of the generated AC, which alters the speed of the motor. 3 phase industrial motors operate at a frequency of 60 Hz, brushless DC motors operate at a much higher frequency.
Only a very few German ESCs actually vary the PWM duty cycle so that the average voltage seen by the winding during a commutation cycle is a sinusoid. The vast majority, just keep the PWM duty cycle the same during a commutation cycle resulting in a constant average voltage during any one commutation cycle. The PWM duty cycle/average voltage is updated every commutation cycle.

Increasing the voltage of the sine wave, or the PWM duty cycle of the square wave DOES increase the speed of the motor. It does this in the same way as a brushed motor. Higher duty cycle means higher average voltage increases current through the winding which strengthens the magnetic field. This accelerates the rotor increasing RPM. But since this is a synchronous motor, this must be accompanied by the proper increase in commutation frequency (or sine wave frequency) in order to keep the motor synched with the rotating magnetic field. Otherwise, you start getting false commutation and jerkiness.

Brushed motors take care of all of this automatically since the faster the motor spins, the faster the brushes spin, the higher the commutation rate.

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06-03-2009 07:33 PM  9 years agoPost 32
Micro-Maniac

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Pasco,Washington Formerly: Captain Chaos

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It's my understanding that rather than throttle to a particular motor rpm we should be loading to a particular motor rpm to get the most power and efficiency and coolest run - Run full throttle and gear to load the motor as close to 90% of it's freerun rpm as possible - So that seems to leave no choice in a helicopter but to run full throttle and gear for the head speed and performance desired since nothing less than 100% throttle is ever going to put the motor nearer to 90% of it's freerun rpm under load

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06-03-2009 07:45 PM  9 years agoPost 33
DKNguyen

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N/A

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Yup. Best to do things right the first time and match the battery voltage with motor kV and gearing so that the motor runs at max efficiency at the right headspeed, than to go back and fix it with extra low throttle settings.

Otherwise...you're just using a battery pack with more volts than you need with none of the high voltage efficiency. It also probably means that you have reduced mAh since all your battery weight is tied up in extra voltage. It's a doubly whammy because you have both higher voltage which means higher peak currents, as well as reduced mAh so your batteries work harder to provide that increased current.

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06-03-2009 09:31 PM  9 years agoPost 34
cbflys

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Nesconset, NY - USA

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You can try a little experiment to setup your ESC for maximum battery efficiency.

The KV rating on the motor and the battery voltage (multiplied by some efficiency constant) determines the maximum rotor RPM (headspeed) with any given gearing. You could measure the headspeed at zero pitch with 100% throttle to get the actual maximum RPM for your setup. Once you know that RPM, you can start decreasing the throttle curve to the point where the RPM just begins to drop off. That will be about the setting the ESC needs to maintain head speed at zero pitch. Any higher and you'll be past the point of maximum efficiency because the PULSE WIDTH MODULTATION duty cycle will be longer than necessary. This keeps the coils energized longer than necessary, waisting energy.

You could do this at all points on your curve to maximize efficiency, that is - finding the throttle setting where the RPMs just begin to decay at a given pitch and then increasing the throttle till max RPM is again realized. Ideally, this is done with an optical tach while flying.

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06-03-2009 10:13 PM  9 years agoPost 35
AirWolfRC

rrProfessor

42½ N, 83½ W

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Efficiency with a brushless motor is not an easy number to come up with. You can experiment all day long and still not get close. Put it on a dyno and measure power out divided by power in and only then can you get a good number. And even then you only have an efficiency measure of the motor/controller combination.

And if both are 85% efficient by themselves, then the combo will be 72% efficient over all (.85 x .85 = .7225).

Then you have the aerodynamic efficiency of the rotor head . . . .
. . . . which is close to best at about 8º blade angle of attack . . . .
. . . . but everybody runs at about 1º or 2º angle of attack . . . .
. . . . which is not the same as hover pitch angle . . . .

About the only thing you can do is make sure that (Kv x V > rpm) the Kv times the battery voltage is higher than the motor rpm you expect to be running at but not excessively so.

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06-03-2009 11:22 PM  9 years agoPost 36
Terrabit

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Seattle, WA - USA

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I think I got it. So it's slightly more than two times the six degrees of Kevin Bacon? The answer is no because motorcycles don't have doors! PROVE ME WRONG!!! I dare ya!

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06-03-2009 11:56 PM  9 years agoPost 37
TaleGunner

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Deer Park WA

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think I got it. So it's slightly more than two times the six degrees of Kevin Bacon? The answer is no because motorcycles don't have doors! PROVE ME WRONG!!! I dare ya!
OK just add door to the motorcycle and a 1/2 side of bacon then subtract gravity from the equation and operate in a vacuum .

You've been proved

Then go get an E-logger and find out were your setup is the most efficient

CRASH! GLUE! REPEAT!
Spectra-G, Ion X-2

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06-04-2009 01:06 AM  9 years agoPost 38
fergus

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Ireland

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Spitfire Is the milk cooked yet

Seriously though I love this stuff

Regards

Fergus

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06-04-2009 02:23 AM  9 years agoPost 39
cbflys

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Nesconset, NY - USA

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My point was not to try to determine the efficiency of the system as a whole, but just to adjust your throttle curve for the most efficient use of the energy. At any given blade pitch, you can increase the throttle to the point where the RPMs will not increase anymore. That is the most efficient duty cycle at that pitch setting.

As AirWolfRC pointed out, as long as the KV, battery, and pinion are capable of spinning the rotor fast enough, then you tweak the ESC (throttle) for maximum battery efficiency.

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06-04-2009 06:10 AM  9 years agoPost 40
Spitfire1

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Perth Australia

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Spitfire Is the milk cooked yet
Oh hell yeah!

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