Thursday, April 26, 2007

Electric motor performance -- Christo vd Merwe

Here's Christo's reply to questions from Neil Allen:

Hi Neil

Correct choice of battery and prop (and maybe a little more diet of the airframe) might just do the trick. Rudolf and Simon may be right in suggesting a stronger motor, but I never give up that easily. I always try to improve what equipment I have - and so far most of my "experiments" seemed to have been very successful.

I'm wondering where you got the "E Flite motor spec of 12a continous" from. . . If it's from the manufacturers, I'd encourage you to "experiment", because "a limit of 12A continuous" is an open-ended statement. . . and experience has taught me to flatly ignore those. We all know that Watts = Amps x Volts. Therefore, to state that you shouldn't exceed 12A is ludicrous. My immediate response would be to ask "at what voltage". Statements like that get my neck hairs up, because it's either a case of the manufacturers not knowing what they're talking about (unlikely) or they're thinking that the users are stupid. And that we are not. So, in effect, it's a plain insult to all of us. The worst is that even major manufacturers, like Kontronik are guilty of doing that. (I could write another 2 pages about what I think of motor manufacturers' useless motor numbering/sizing conventions - like E-Flite's and Hacker's for instance).

Unlike I.C. engines that have limits that can't (or rather shouldn't) be exceeded, e.g. RPM limits, power limits, etc., an electric motor is a wonderful device, in the way that it will ALWAYS try to "give" what's been asked from it. It's almost like a dog that wants to please it's owner, no matter what... Just put a bigger prop on or run an extra cell. So, if you ask 800W from a motor that's been rated at 300W, it will actually do it - maybe just for a few seconds, but it will do it's utmost to deliver 800W. Obviously, with a serious side effect, in the form of heat. There is great truth in what I've just said. The "real limit" to an electric motor's power comes in the form of it's ability to dissipate heat. IOW, "how much heat" a motor can dissipate, is the real-life limit of it's power. In your case I'd say, if the motor doesn't get hot, it's not working hard. Put differently, if your Park 370 doesn't get hot while pulling 14A, then it's fine and I can guarantee that it will last, obviously bearing in mind that the stator is the hottest part of the motor and not the outside casing, which may feel much cooler to the touch. The type of installation also plays a major role. An enclosed motor (inside a cowling) can't get rid of the heat as quickly as a motor mounted in fresh air, like one gets on most shockies.

Sorry about the long post, but I'd like to mention an example that will hopefully prove my arguments above. When I acquired my 85% Shotgun (built by Mike Hirst), I happened to have a brand new, unbuilt kit motor from Torcman in my workshop. In fact, when the unassembled motor (which was untouched for about 6 months), saw me carrying in the Shotgun, it wormed itself out of the packaging and started assembling itself. ;-) It's a 28-15 outrunner (28mm dia x 15mm long stator) and it weights only 108 grams. So it's really a rather small motor for a Shotgun (roughly double the size of the Park 370), even for this smaller electric Shotgun (AUW = ±1040g) that Mike designed for his dad, Andrew Hirst. So, true to my "experimentalist" nature, I decided to give the poor motor a helping hand and wound it to see what kind of power I can squeeze from it. Bearing in mind that the Torcman website "rates" this particular motor at 250W - 300W, I nevertheless persisted and on only my second attempt, got 450W (static). That's a 50% improvement over Torcman's "limit" for this motor. The final test was to fly the motor. The result: in one word, it's PERFECT for the Shotgun. Those who have seen it fly, will bear me out on this. It's a great performer.

Oh, incidentally, Andre Kilian once lent me his flight data recorder, and after the flight that included some real awesome verticals, we downloaded the data and to our amazement the power peaked at 600W. That's a 100% improvement over what the Torcman people rated this great little motor and BTW, after 18 months it's still performing as new.

I'll now rest my case!!! ;-)


Christo - big time experimenter and motor builder

PS Your 14A at 11V (3S) equals 154W, which is spot on target with the Park 370's rating of 150W
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