Aug 02, 2006, 07:40 PM Registered User East Anglia, UK Joined Sep 2002 29,709 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.
Aug 02, 2006, 07:47 PM
Dieselized User
Chicagoland
Joined Feb 2000
7,727 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.

Greg
 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.
 Aug 02, 2006, 10:39 PM hot air rises... Pleasant Grove, UT Joined Jul 2005 8,434 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).
 Aug 02, 2006, 11:27 PM Got more toys than my kid 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
 Aug 02, 2006, 11:38 PM Flying motor mount master 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!!
 Aug 02, 2006, 11:59 PM Got more toys than my kid 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
 Aug 03, 2006, 12:00 AM Flying motor mount master San Jose, California, United States Joined Oct 2004 8,745 Posts That was a tic remark... he would be proud!! heh, B
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.
 Aug 03, 2006, 12:10 AM Registered User 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.
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.
Aug 03, 2006, 12:33 AM
Senile Member
Moab, Utah, USA
Joined Apr 2003
6,669 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
Last edited by Lnagel; Aug 03, 2006 at 12:52 AM.
 Aug 03, 2006, 12:47 AM Registered User Lake Sawyer, Washington Joined Jul 2003 1,100 Posts
 Aug 03, 2006, 01:41 AM ................. Phoenix,Arizona, United States Joined Jun 2002 4,639 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