HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Aug 02, 2006, 07:00 PM
Got more toys than my kid
macr0t0r's Avatar
USA, CA, Fremont
Joined Sep 2004
3,676 Posts
Discussion
Juggling efficiency and power at partial throttle (cruise).

Alright, we have had some LOVELY threads discussing max efficiency, how important it is (or isn't) and if max power occurs at that point or not. Lots of charts, lots of beating of chests, yada yada yada.

Theoretically (as best I understand from long-time member, vintage1), max efficiency AND power can be achieved by first discovering the current where max efficiency occurs, then stack up the volts until you get your desired power rating. However, most of us are on LiPos, so you'll have to make 3.7V jumps (no fine-tuning). Many ESCs are limited to 3 or 4 cells if you wish to use the BEC. On top of that, a low-amp, high voltage solution is often only viable for a large-prop setup if you use a gear-drive. Not a desired option for outrunners.

So.....real-world limitations muddy up the perfect solution. No surprise there. Let me throw another wrench in the works:

Most of us NEVER fly at full throttle!

Seriously, I do NOT care how efficient a motor is at full throttle, because I only use that in short bursts. I want to know how it runs at...say....2/3 throttle.

So, I need to find a setup where I get close to max efficiency at partial throttle, while getting max POWER at full throttle (with the same prop and battery, of course).

My solution has been anything from perfect. Since most motors seem to have their peak efficiency at low current, I always plan to run a 3-cell Lipo. I pick a prop and find out what kV will spin it within the current limitations of my battery and ESC. At that point, I find a motor whose PEAK performance (i.e. just below melting) is sufficient to run that setup for a short period. I then assume that partial throttle will scale back closer to the higher efficiency region of the motor. This way, I don't get a bigger motor than I need.

But, I don't know if that's the best way.

There is a website that does a great job of estimating efficiency and power of a motor if you plug in a few measurements: http://www.peakeff.com

Go ahead and pick a motor and look at the charts. My favorite setup is currently a CustomCDR Billet Bullet Single on 3 cells with a GWS 8x4HD prop:
http://www.peakeff.com/MotorDetails....ID=3228&v=10.5

Now, please look at the second plot. The red line is current, while the X-axis is voltage. This is a plot that ties the voltage/current ratio to max efficiency. It appears we'd get max power at 28V and 4.5amps. Hmmmm....7-cell LiPo pack and a 3" propeller! Not a workable solution. So, this is what I did. Please observe my modified chart, since I prefer using the GWS HD props:
http://www.peakeff.com/GraphKIR.aspx...ngle+Cool+Wind

Ignore the statement that the peak thermal is 18.4W. It's actually closer to 30W (for 3 seconds). You can confirm this by looking at the tests done by Dr. Kiwi. Anyhow, I picked a 3-cell pack since obviously high voltage is preferred for efficiency, but I wish to use the built-in BEC of my ESC. Looking at this chart, it appears the GWS 8x4HD is the biggest I can go without motor meltdown and staying within the 10amp limit of my battery and ESC. That gives me over 9000RPM, so I'm not complaining. Now, 2/3 throttle brings me around 5.5 amps at around 11V or so.

Here is the question: does that mean my motor is running close to peak efficiency at 2/3 throttle?

Partial throttle is kind of difficult to rate. The ESC is essentially beating the motor with multiple "pulses" of full throttle. I don't now what the effective voltage-vs-current is being applied in this case. I know what the battery is sending to the ESC, but I don't know what voltage and current is being applied to the motor. If you really want your head to spin, changing the PWM freq on some ESC can improve the efficiency of partial throttle on a motor, so now motor data alone is not good enough.

I'd like to know how I can calculate what the partial-throttle efficiency of a motor-prop combo is so I can setup for an optimum cruise. No parkflyer props for max efficiency at full-throttle, for we usually do max POWER so we have a little extra to pull away from a tree or fence. However, if slightly changing the prop means my cruise is more efficient, that would be good to know.

Alright...so how do you guys handle this? Do you do your prop-matching at full throttle or partial throttle? What values to you plug into motocalc to estimate cruise values? Does partial throttle effect the motor constants? Would plugging in partial-throttle values into the peak efficiency calculator give an accurate chart?

Gonna have to spend some quality time with the Tach and Wattmeter. Any help, questions, or explanations would be greatly appreciated. The quest for the optimum power package continues....

- Jim
macr0t0r is offline Find More Posts by macr0t0r
RCG Plus Member
Reply With Quote
Sign up now
to remove ads between posts
Old Aug 02, 2006, 07:40 PM
Registered User
vintage1's Avatar
East Anglia, UK
Joined Sep 2002
29,703 Posts
I have been brain-bending about this as well. For scale I certainly look to place the best efficiency at cruise, and sacrifice it at WOT.

The pulsed current makes the maths more complex...If I get a chance I'll try and figure out what is happening in a part throttle motor/ESC.
vintage1 is offline Find More Posts by vintage1
Reply With Quote
Old Aug 02, 2006, 07:47 PM
Dieselized User
gkamysz's Avatar
Chicagoland
Joined Feb 2000
7,387 Posts
Quote:
Now, 2/3 throttle brings me around 5.5 amps at around 11V or so.

Here is the question: does that mean my motor is running close to peak efficiency at 2/3 throttle?
We don't really know. We can calculate the torque needed for the prop RPM at 2/3 throttle. Then we know how much current is needed, and we know what the motor voltage is. but it's not that simple as you can easily lose 10% efficiency, that we can't really define easily.

http://www.rcgroups.com/forums/showthread.php?t=360287

Greg
gkamysz is offline Find More Posts by gkamysz
Reply With Quote
Old Aug 02, 2006, 10:03 PM
Registered User
Joined May 2003
768 Posts
We can get a pretty good feel for how things work out with a quick experiement.

I just ran a BP21 motor on my video easystar with GWS 0843 at full throttle and measured:

11.3V 9.3A 9990 RPM. DriveCalc sez that's 73.4% efficient.

Running again around half throttle:

12.1V 2.3A 5520 RPM. Drivecalc sez that's 47.1% efficient.

Hmmm. So does that mean I went too far to the left of the efficiency peak? Looks like I did since efficiency falls off rapidly below peak efficiency.

Running again and targeting 6A (which is roughly the predicted peakeff for BP21 based on an earlier run I did) I measured these:

11.3V 6.5A 8880 73.9%

11.5V 4.1A 7380 66.2%

11.1V 7.2A 9180 75%

Interesting: Higher efficiency at part throttle.

IOW, it looks like reducing the current via the ESC PWM action will slide you up and down the efficiency curve just as a smaller/bigger prop would. If a 10x4.3 is hitting 60% eff at full throttle and 2*Ipeakeff, then pulling the throttle back to achieve Ipeakeff will increase the efficiency.

And that says there is indeed value in knowing where peakeff sits and propping to hit that at, say, 3/4 throttle. What is a bit surprising is that on my FutabaT6XH that half throttle is about 1/4 WOT current (not sure, though if this is a function of the ESC, which is a TBird 18 or the radio...). So if prop half throttle to be peak efficiency, then full throttle means the motor will smoke pretty quick On my BP21, that'd roughly be half throttle at 6A, full throttle would be 24A.

Someone should do this experiment on their own equipment and report back.
matttay is offline Find More Posts by matttay
Reply With Quote
Old Aug 02, 2006, 10:39 PM
hot air rises...
jfinch's Avatar
Pleasant Grove, UT
Joined Jul 2005
7,713 Posts
How are you measuring the voltage to the motor? It should be varying more then your showing. You can't just use the voltage from battery if you're not at full throttle (the esc is dropping voltage).
jfinch is offline Find More Posts by jfinch
Reply With Quote
Old Aug 02, 2006, 11:27 PM
Got more toys than my kid
macr0t0r's Avatar
USA, CA, Fremont
Joined Sep 2004
3,676 Posts
Sort of...that's the problem. The ESC is pulsing full voltage, so there should be an RMS voltage of sorts. With a Wattmeter, measuring the battery is all we can do. However, he's showing that his efficiency values closely match the curve as if only the current is being altered. However, I've heard there will still be an additional loss in efficiency due to heat from the hard-pulsing ESC and inductance losses due to transmitting a pulsating current. Explaining that is over my head, though.

However, it looks like you CAN use partial-throttle analysis. So, find a prop that gives you max efficiency current at 2/3 throttle. If that prop is too large for the motor to handle it at max, then reduce the prop size. Otherwise, you've found the perfect prop!

Unfortunately, the online charts can only tell you the best current at a specified voltage. The prop labels are for full-throttle only. You'll still have to experiment.

Here may be a reasonable rule of thumb:
1. Take the current at max efficiency for your battery voltage.
2. Add 50% to it (multiply by 3/2)
3. If that current is within the ratings of the motor, use that prop. Otherwise, slide down to the largest prop it can handle.

If current is roughly proportional to throttle, then that means you are propping for 2/3 throttle cruise.

Let's try it with my motor I have listed in my first post\ (I'm afraid I'm not at home to test this).
1. Max efficiency at 4 amps.
2. Target amps at 4(3/2) = 6 amps.
3. Hmmmmm....this is a 3D plane, so I need wide diameter and shallow pitch. Looks like I need something BETWEEN 7x3.5 or the 8x4 prop. It does appear I'd be running much more efficiently with the 7x3.5, though. Wish there was an 8x3 prop. Ah well.

- Jim
macr0t0r is offline Find More Posts by macr0t0r
RCG Plus Member
Reply With Quote
Old Aug 02, 2006, 11:38 PM
Flying motor mount master
fly_boy99's Avatar
San Jose, California, United States
Joined Oct 2004
8,745 Posts
Want more efficiency from your motors?

Pull back on the throttle!!! Just like letting up on the gas
pedal!!

fly_boy99 is offline Find More Posts by fly_boy99
Reply With Quote
Old Aug 02, 2006, 11:59 PM
Got more toys than my kid
macr0t0r's Avatar
USA, CA, Fremont
Joined Sep 2004
3,676 Posts
We knew that pulling back on the throttle reduces power consumption, we just weren't sure it would increase efficiency (if you were starting off beyond the peak efficiency current draw at full throttle). Originally, I was assuming that the motor would always run at less efficiency than WOT, since partial throttle really is "pulsed" WOT. Apparently, it is more complex than that, and that there IS an advantage to marginal over-propping.....so far. We need a little more raw data.

- Jim
macr0t0r is offline Find More Posts by macr0t0r
RCG Plus Member
Reply With Quote
Old Aug 03, 2006, 12:00 AM
Flying motor mount master
fly_boy99's Avatar
San Jose, California, United States
Joined Oct 2004
8,745 Posts
That was a tic remark...

he would be proud!!

heh,
B
fly_boy99 is offline Find More Posts by fly_boy99
Reply With Quote
Old Aug 03, 2006, 12:10 AM
Registered User
Joined May 2003
768 Posts
Quote:
Originally Posted by macr0t0r
We knew that pulling back on the throttle reduces power consumption, we just weren't sure it would increase efficiency (if you were starting off beyond the peak efficiency current draw at full throttle). Originally, I was assuming that the motor would always run at less efficiency than WOT, since partial throttle really is "pulsed" WOT. Apparently, it is more complex than that, and that there IS an advantage to marginal over-propping.....so far. We need a little more raw data.

- Jim
WOT is pulsed, too. We definitely do need more data.
matttay is offline Find More Posts by matttay
Reply With Quote
Old Aug 03, 2006, 12:10 AM
Registered User
ryanl2006's Avatar
Ames, Iowa
Joined Oct 2002
2,495 Posts
On all this voltage stuff... why not just find a motor prop combo that spins the same RPM at full throttle as your motor does at half throttle (If you dont have one yourself use drive calc) and figure out the power output. The power output is the same if the RPM is the same no matter if one motor is at full throttle while another does the same RPM on part throttle. They are still outputing the same. So then use that along with the input watts to get your efficiency. In other words use a different motor as sort of a 'middle man' where you can get the true voltage.
ryanl2006 is offline Find More Posts by ryanl2006
Reply With Quote
Old Aug 03, 2006, 12:14 AM
Registered User
Joined May 2003
768 Posts
Quote:
Originally Posted by jfinch
How are you measuring the voltage to the motor? It should be varying more then your showing. You can't just use the voltage from battery if you're not at full throttle (the esc is dropping voltage).
Note that the voltage presented to the motor is either full battery or ground. The ESC is dropping the voltage via PWM, so you can indeed just measure the voltage of the battery. Presumably the meter is true RMS and thus the current you see times the input voltage is you see is the true Pin at any throttle setting.
matttay is offline Find More Posts by matttay
Reply With Quote
Old Aug 03, 2006, 12:33 AM
Senile Member
Lnagel's Avatar
Moab, Utah, USA
Joined Apr 2003
6,141 Posts
Quote:
Originally Posted by matttay
What is a bit surprising is that on my FutabaT6XH that half throttle is about 1/4 WOT current...
Not surprising at all when you consider that the power required by the propeller varies with the cube of the RPM. Current draw will not be linear with throttle position. Equation for determining static propeller power:

P = K * Dia^4 * pitch * RPM^3

Let's say your propeller is turning x RPM and the motor is drawing 10 amps at 10 volts for a total of 100 watts. If you reduce the voltage to 5 volts then the RPM will be roughly x/2 or 0.5 of the original value. The propeller will now need only 0.5^3 * 100 = 12.5 watts. So you should now see 12.5 / 5 = 2.5 amps, or 1/4 of your WOT current.

Yes, it is extremely difficult to measure motor voltage at part throttle. However, one should be able to calculate it by using the RPM formula.

RPM = Kv * (V - Iin * Rm)

rearrange this to:

V = RPM/Kv + Im*Rm

Measuring your RPM at partial throttle and plugging the numbers into the formula should give you a reasonable idea what the motor voltage is at that throttle setting. Then you ca re-calculate your current for best efficiency with the new voltage value.

Larry
Lnagel is offline Find More Posts by Lnagel
Last edited by Lnagel; Aug 03, 2006 at 12:52 AM.
Reply With Quote
Old Aug 03, 2006, 12:47 AM
myk
Registered User
myk's Avatar
Lake Sawyer, Washington
Joined Jul 2003
1,100 Posts
FYI

http://www.rcgroups.com/forums/showp...6&postcount=26

Mike
myk is offline Find More Posts by myk
Reply With Quote
Old Aug 03, 2006, 01:41 AM
.................
Dylwad's Avatar
Phoenix,Arizona, United States
Joined Jun 2002
4,634 Posts
This is one reason why Lehner motors are so highly regarded~good part-throttle efficiency. I guess its due to the stacked magnets on the rotor, which i hear Lehner bought the tooling to make in-house. Im pretty sure all of lehners motors use stacked magnets, except maybe the basic series. Large hackers use stacked magnets, but the smaller ones dont.

I wonder if stacking the mags in an outrunner would do anything at all. I also think bell material plays a part, I get better results using pole-piece material out of high end loudspeakers than i did using iron pipe, my poor little lathe can barely keep up making a 1mm ring out of a solid chunk of steel.

Ive also wondered how the lack of an angular contact or thrust bearing on the back of most(if not all) outrunners affects the power to the prop under load. I know on a cnc router chaning out single row bearings to angular contact on the leadscrews made a huge difference in the smoothness and operation of it. They make em in 3mm id, pricey though. http://www.vxb.com/page/bearings/PROD/Thrust/kit1027

Dylan
Dylwad is offline Find More Posts by Dylwad
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Peak current at partial throttle Lnagel Power Systems 19 Oct 19, 2006 03:18 AM
Discussion less amps with more lipos for a given RPM at partial throttle? Nethole Batteries and Chargers 30 Oct 12, 2006 05:50 AM
Are outrunners more efficient at partial throttle DWS Power Systems 3 Feb 18, 2005 08:57 AM
Is flying at partial throttle with a sensorless brushless motor bad? Mitch G Power Systems 6 Dec 12, 2001 02:15 PM
ZAGI 400-X ; power cutting out to motor after flying at full throttle Joe-z Foamies (Kits) 29 Jul 15, 2001 10:51 PM