Jan 03, 2014, 04:49 AM
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Australia, VIC, Melbourne
Joined Feb 2010
1,448 Posts
Quote:
 Originally Posted by tjarko It seems the scale effect is already very pronounced at these dimensions. From 150 grams to 750 grams motors the efficiency goes from 90 to 95%! Do you know where the technical reason is for this scale effect? Tjarko
You are looking at it backwards, a increase in efficiency from 90 to 95% represents a halving in the power losses (and thus, for the same thermal limit a doubling of maximum power handling), it is not a small thing.

From that perspective, the efficiency of the motors has actually doubled.
 Jan 03, 2014, 08:00 AM homo ludens modellisticus near Nijmegen, Netherlands Joined Feb 2001 10,179 Posts Efficiency governs power/weight ratio Higher efficiency does not only mean that the motor makes better use of the batteries' power, it also means the motor is able to handle a higher power input before hitting its maximum temperature mark i.e. a the power/weight ratio will be higher. An example Say the motor has an efficiency of 70% and it can handle 50Watt input. That means it can get rid off 30%*50=15Watt excess heat. Now, by cramming in thicker wire (and/or using better stator-iron, segmented magnets), efficiency increases to say 75% (I'm a bit optimistic here). The motor's ability to loose those 15Watts has not changed (by radiation, convection and conduction). This means the motor now can handle 60Watt before it hits the 15Watt (25%*60Watt) losses mark. An efficiency increase of 5% gives an increase in the power to weight ratio of 20% (from 50Watt to 60Watt). That's why efficiency plays such an important role, in any motor design: efficiency governs maximum power. The motors weight may have increased a bit due to more copper. A rather extreme example, just for calculation's sake/fun: going from 80% to 90% efficiency would increase the input power the motor can handle by a factor two (a.k.a. 2). Vriendelijke groet Ron
Jan 03, 2014, 10:51 AM
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Eindhoven
Joined Jul 2007
144 Posts
Quote:
 Originally Posted by desertstalker You are looking at it backwards, a increase in efficiency from 90 to 95% represents a halving in the power losses (and thus, for the same thermal limit a doubling of maximum power handling), it is not a small thing. From that perspective, the efficiency of the motors has actually doubled.
Hi desertstalker, I do not see why the F5D perspective would be Backwards?
Perhaps you are not familiar with the FIA rules?. We have a 60KJ energy limit, or in other words 1 minute 1000 watts to fly 10 laps of 400m.
5% efficiency means half a lap of free energy!
If you have once been in the situation where the limiter kicked in at pylon 1' lap 10 you know exactly how much 5% is for F5D
My question still stands...
Tjarko
 Jan 03, 2014, 11:10 AM Registered User London UK Joined Jun 2006 100 Posts I think tjarko's question stands, too. I've never seen a comprehensive explanation. The only reference I can recall is in the Austin Hughes book: http://books.google.co.uk/books?id=j...ffects&f=false Last edited by MilesH; Jan 03, 2014 at 11:18 AM.
Jan 04, 2014, 05:32 PM
Registered User
Australia, VIC, Melbourne
Joined Feb 2010
1,448 Posts
Quote:
 Originally Posted by tjarko Hi desertstalker, I do not see why the F5D perspective would be Backwards? Perhaps you are not familiar with the FIA rules?. We have a 60KJ energy limit, or in other words 1 minute 1000 watts to fly 10 laps of 400m. 5% efficiency means half a lap of free energy! If you have once been in the situation where the limiter kicked in at pylon 1' lap 10 you know exactly how much 5% is for F5D My question still stands... Tjarko
You can use a motor of half the weight to handle than power (a 95% efficient motor can handle twice the power of a 90% efficient motor with the same heat dissipation). This would surely be of some benefit given the power limits.
 Jan 05, 2014, 05:50 AM 186mph Funjet Ultra nieuw vennep, the netherlands Joined Feb 2010 460 Posts That would only work if he had no minimum weight restrictions, and he can build a smaller airframe. René
 Jan 06, 2014, 01:01 PM Registered User Eindhoven Joined Jul 2007 144 Posts Exactly Rene, We are going the opposite way in F5D, installing more efficient drives at the cost of more weight, and reduce the total weight to 1 Kg with other means, like using 5S 1580 mAh batteries at > 1000 Watts... Not very nice, but the way to win. If we would not have the F5D race regulations you would be correct desertstalker. Tjarko
Jan 19, 2014, 02:33 PM
Ace
Joined Jan 2007
339 Posts
Quote:
 Originally Posted by tjarko Hi Christian, It seems the scale effect is already very pronounced at these dimensions. From 150 grams to 750 grams motors the efficiency goes from 90 to 95%! Do you know where the technical reason is for this scale effect? Tjarko
larger size does make gaining efficiency easier, mostly because of scale and precision. the easiest example is airgap; if you make a larger motor your air gap is likely to stay the same but now the air gap relative to the area of the interface is smaller, and efficiency is gained. Similarly your flux density has gone up with more copper and you edge effects go down ect.

But it is important to note that the same square/cube laws which grant larger motors higher efficiency also reduce the surface area to volume ratio and therefore reduce the amount of heat dissipation relative to volume. Which is why electric motors have more or less the same power to weight regardless of size, even though/because the efficiency (generally) increases with size.
Jan 19, 2014, 03:03 PM
Registered User
London UK
Joined Jun 2006
100 Posts
Quote:
 Originally Posted by atmk .......... the easiest example is airgap; if you make a larger motor your air gap is likely to stay the same but now the air gap relative to the area of the interface is smaller, and efficiency is gained..
I'm not sure I understand this point. I see that there's scope for reducing the airgap size, relatively, but I don't see how that impacts efficiency. No matter the scale, you still have the choice to load the magnetic circuit vs the electrical circuit in whatever ratio you want.
Jan 19, 2014, 08:59 PM
Got shenpa?
Los Angeles
Joined May 2004
10,940 Posts
Quote:
 Originally Posted by Dr Kiwi I agree with this.. I've tested hundreds of hobby-grade motors and only rarely does DriveCalc compute more than 80% efficiency.. the vast majority peak in the 70%s and even then only at very low amp draw. By the time they get to a reasonable maximum power they are down in the 60%s.
My observations and measurements show the same thing. Most of the "cheap and cheerful" brushless DC motors are in the 60% - 70% range at usuable current levels, really no better than the old ferrite-magnet brushed Speed 400 and Speed 550 motors we were all using circa 1999 or so! The only differences are better cooling, lower Rm (no brushes), and lower overall weight compared to those old brushed can motors.

But this thread is about something else, not the cheap and cheerful motors, but the very best that are on the market. It's very impressive that a select few manufacturers are actually pushing their motors past 90%!

Just imagine if the internal combustion engines in our cars and trucks could manage to be so efficient - it would change the world! (Too bad that fundamental laws of thermodynamics forbid this from ever happening at practical combustion temperatures. Still, it's fun to dream, though.)

-Flieslikeabeagle
 Jan 20, 2014, 03:56 AM Life begins at transition Australia, WA, Perth Joined May 2007 3,521 Posts I think the aussies are still out in front on this one. I don't think anyone's come up with anything since?
 Jan 20, 2014, 10:12 AM Registered User Eindhoven Joined Jul 2007 144 Posts ATMK, thanks for your constructive reply Airgap is indeed an easy to understand parameter. Here the Tango could be improved as 0.5 mm Airgap is not really very challenging. Replacing air with copper would bring the largest gain probably. An aspect I was thinking of is the frequency. I have the feeling the small motors switch polarity much vaster, and therefore the iron losses, both Eddy current and hysteresis losses would be much higher. end effects I do not understand why they scale, could you explain in more detail? Tjarko
Jan 20, 2014, 03:21 PM
Ace
Joined Jan 2007
339 Posts
Quote:
 Originally Posted by tjarko ATMK, thanks for your constructive reply Airgap is indeed an easy to understand parameter. Here the Tango could be improved as 0.5 mm Airgap is not really very challenging. Replacing air with copper would bring the largest gain probably. An aspect I was thinking of is the frequency. I have the feeling the small motors switch polarity much vaster, and therefore the iron losses, both Eddy current and hysteresis losses would be much higher. end effects I do not understand why they scale, could you explain in more detail? Tjarko
you have a point about frequency's effect on efficiency, but that is dependent on pole count as well as rpm and a larger motor is likely to have proportionally more poles.

As for edge effects...The point of all the copper and iron in a motor is to create magnetic flux to bush and pull on the magnets. The part of the coils that wrap around the end of the stator don't create flux as efficiently as the copper in the stator slots(even if there isn't slots), but still incurs losses. By this logic a longer motor would be more efficient(same reason a long thin wing is more efficient). If I'm not mistaken the primary efficiency benefit for a larger diameter is better proximity of any copper loop to the magnet it's working on relative to the loop size ; since loop area follows a square rule while coil depth is linear and air gap is likely constant.
Jan 21, 2014, 03:27 PM
Got shenpa?
Los Angeles
Joined May 2004
10,940 Posts
Quote:
 Originally Posted by atmk you have a point about frequency's effect on efficiency, but that is dependent on pole count as well as rpm and a larger motor is likely to have proportionally more poles.
I measured Io (no load current) vs motor rpm (unloaded) for several cheap brushless outrunners, and in every case Io was more or less proportional to the rpm over most of the motors usable rpm range. It appeared to be a pretty linear relationship. ( http://www.rcgroups.com/forums/showthread.php?t=674875 )

A side-effect of thes substantial iron losses I observed in inexpensive brushless motors is that the classic "3 constant motor model" with the three constants Io, Rm, Kv is quite a poor fit to these sorts of motors. The motor constant Io is supposed to represent the constant no-load current, but as I found, it is not a constant at all, and is in fact not even remotely close to being a constant, instead varying quite strongly with rpm.

I Added one more motor constant which produced a model with a much better fit to the experimental data, and that model is discussed in the thread I linked to above.

-Flieslikeabeagle
 Jan 21, 2014, 09:44 PM Registered User Joined Dec 2013 196 Posts It was hard to answer this question. For different use, you can found the highest brushless DC motor. Even though, just for a RC helicopter, can be found several highest BLDC motor can meet a certain needs, like last, accelerate, etc.