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:
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:
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....