
Aug 02, 2006, 07:00 PM  

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 longtime 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 finetuning). Many ESCs are limited to 3 or 4 cells if you wish to use the BEC. On top of that, a lowamp, high voltage solution is often only viable for a largeprop setup if you use a geardrive. Not a desired option for outrunners. So.....realworld 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 3cell 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 Xaxis 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....7cell 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 3cell pack since obviously high voltage is preferred for efficiency, but I wish to use the builtin 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 voltagevscurrent 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 partialthrottle efficiency of a motorprop combo is so I can setup for an optimum cruise. No parkflyer props for max efficiency at fullthrottle, 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 propmatching 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 partialthrottle 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 

Aug 02, 2006, 07:47 PM  

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

Aug 02, 2006, 10:03 PM  
Joined May 2003
764 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. 
Aug 03, 2006, 12:10 AM  

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.

Aug 03, 2006, 12:14 AM  
Joined May 2003
764 Posts

Quote:


Aug 03, 2006, 12:33 AM  

Quote:
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 recalculate your current for best efficiency with the new voltage value. Larry 

Aug 03, 2006, 01:41 AM  

This is one reason why Lehner motors are so highly regarded~good partthrottle efficiency. I guess its due to the stacked magnets on the rotor, which i hear Lehner bought the tooling to make inhouse. 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 polepiece 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 
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