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Over amping motors by 50%??

Old 10-16-2008, 11:17 AM
  #1  
HugoW
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Default Over amping motors by 50%??

Yeah, yeah, newbie, I know. I'm in the middle of electrifying my Funtana X50, and I just received a tip from a fellow dutchman, he flies the S40 (same thing) with a A4120 motor on 5S. He's pleased with the performance (no-end pull out), which I think is weird 'cause the motor maxes at 55 amps, with 18.5V is 1kW. Which for a 2.600 grams plane means 380W/kg, not too special.

But then he wrote his data logger shows 82(!) amps at 18V, that's almost 1.5kW! Now he doesn't run that full-time, but is it normal to allow motors to run 50% over-amps in short bursts? I was looking at this motor to run at 6S, but the low amps held me back. If I can use that up to, say, 70 amps, I'm a happy camper and save about 150 grams over the drive system I now have.

Cheers,

Hugo
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Old 10-16-2008, 03:15 PM
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Dr Kiwi
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It is perhaps possible to run a motor at way over spec in short bursts, but if you care about your equipment it is not the smart thing to do. Amps = heat... and excessive heat will fry the motor windings and demag the mags far more quickly than if you run at below spec.

I'll bet that, at 1500W, the efficiency of his motor is well below what it would be at 1000W. Basically most of the extra power will be going into waste heat, not driving the prop!

I am sure Andrew won't mind me borrowing this graph - efficiency at modest amps = 80%, efficiency at excessive amps = 55%
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Old 10-21-2008, 03:57 AM
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kyleservicetech
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I ran the Turngy numbers through motocalc. This software program indicated efficiency less than 50% at the 80 Amp level.

Wow, compare that motor to a Hacker A50-12S motor. The Hacker motors run at 85% or higher efficiencies at 50 plus Amps. I've got four of them and all work very well. (I do not work for Hacker!)

These Hackers and similar type motors are more expensive, but when you want performance, it just costs more.
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Old 10-21-2008, 07:30 AM
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HugoW
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Yeah, I've hear good things about Hacker, but the price doesn't validate the difference for me. And in all honesty, the specs on the hacker site don't look all to promissing (http://www.hacker-motor.com/images/C...EB%2016+17.pdf) for the given weight. Burst rate of 55 amps seems low to me.

Here's the one I'm going to fit:
http://www.hobbycity.com/hobbycity/s...idProduct=7871
It really seems in the same league or better for only a few grams more, and costs a third of the hacker.

Can you tell me more about this motocalc?

Thanks

Hugo
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Old 10-21-2008, 05:17 PM
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Dr Kiwi
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http://www.motocalc.com/


I guess you can take the risk with the Aerodrive - it's your call.

I think the specs for Hacker are fairly accurate and well established - what can you read into the Aerodrive "scarcely anything useful specs" - all you get is a Kv, an Io and an Rm which may or may not be accurate... you get a 1475W rating which is probably highly optimistic (you'd need 6s and 80A, or 8s and 60A to get that). No mention of cell # or max amps [other than ESC 45A+].
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Old 10-21-2008, 05:49 PM
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Moxus
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overpowering electric motors is no problem. for short burts, or with sufficent cooling, you can run them as hard as you please.
dip a mabuchi style toy motor into water and you run it at 50 amps continous, provided that you catually could put that many amps through before the voltage of your supply maxed out.
as long as you ceep the coils cool, there is not maximum limit to amp draw. put them in liquid nitrogen, get them superconducting and put the full power of a nuclear power plant through your motor... you will bend the metal in the motor from sheer torque before the coils starts having problems with amps.

og, that was not realistic, but it explains the principle. more cooling, more power. and most engines, at least quality brands, are rated with a good safety margin, so running them significantly over rated power for extended periods of time is all just ok.
if you take the effort to cool them properly, you can squeeze out even more.
it took over 15 seconds for my hacker b40 to burn out its coils at 2300 watts... :S its a motor who shouldnt take over 600 watts.

but all this makes sense, if you think about starter motors for example, they run at 10x rated amp as a daily routine. the only magic being that they run for so short time that the coils isnt getting hot. now if you run the same current continously, and force the coils to stay cool........
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Old 10-21-2008, 06:20 PM
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jooNorway
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At 6S and 55A we talk appr 1150Watt to the system. Max recommended...
Your friend use 5S and 82A equals appr 1435Watt to the system. This is (when talking Watt) appr 25% over spec, not 50%.
If a motor do fry at max specced the spec is wrong.

But as always, the Amp is the bad wulf which usually lets the magic white smoke out. When in addition the efficiency falls like the banks do nowadays the heat get out of control.

I would recommend to buy a quality motor, in your example an Actro 24-5. I love the Actroes, far better than Hacker
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Old 10-21-2008, 07:31 PM
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kyleservicetech
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Check out www.motocalc.com. They offer a 30 day free trial of their program.

Some of these motors are given ratings by the manufacturers that are rather optomistic. One brand has been accused of providing "Continuous current ratings" of a magnitude that will result in a burned out motor in short order.

The Motocalc program will provide guidence on whether the manufacturers ratings are realistic, or will severely overheat the motor when the ratings are used.

I run my Hacker A50-12S motor on 6S2P A123 cells, pulling 57 Amps full power. Its got 10 1/2 hours of actual flying time so far, the motor only gets warm after a flight. The Hacker is rated at 45 Amps. I do provide full air cooling on this motor!

(I also do NOT do hovering or any other type of flying that makes my Showtime 50 look like a helicopter.)
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Old 10-21-2008, 08:55 PM
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HugoW
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Thanks guys, a lot to learn I see! I feel like being back in school, I thought I knew about RC planes but this is a completely new game.

I looked at the Actro 24-5, nice motor, but at € 249,- (= US$ 313.00 current rate) it is just way above budget. My whole set-up, including batteries, sets me back less. Now you might call me a cheap Dutchman (which I am...) but the difference is too big, and I cannot spend endlessly.

Motocalc looks promissing, I'll dive in and take the trail version. More to learn!

Hugo
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Old 10-21-2008, 10:02 PM
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ekdenton
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Carefull that your ESC will handle the extra current, I just smoked a 40amp ESC running at 40+or- amps, motor was rated at 40amps(right at the limit). Some brands of ESC's and motors will no doubt will take more than others but IMHO (now) it is better to overbuild or stay within the recomended limit than push the limits and regret it later.


If you put a larger prop than the system will handle the motor will be having to work harder and need more current, then if your ESC will only give XX amount of current the motor will start getting hot......so will the ESC. Then it is anyones guess which will get hot enough to go first. Probably the ESC like mine did and then you will be locked out on your controls unless your running a seperate battery for your receiver.
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Old 10-21-2008, 11:35 PM
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ron_van_sommeren
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Originally Posted by HugoW View Post
Yeah, I've hear good things about Hacker, but the price doesn't validate the difference for me. ..,
Chinese manufacturers can be a bit 'optimistic' in their specs And e.g. AXI is known for under-speccing their outrunners

Vriendelijke groeten Ron

Last edited by ron_van_sommeren; 10-21-2008 at 11:54 PM.
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Old 10-21-2008, 11:54 PM
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ron_van_sommeren
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Originally Posted by HugoW View Post
... I looked at the Actro 24-5, nice motor, but at € 249,- (= US$ 313.00 current rate) it is just way above budget. ....
"Want the same power as a actro 32-3 at around half the price?"
Misschien zit er ook een 24-5 in de pijplijn?:
http://www.rcgroups.com/forums/showt...1#post10774561

Originally Posted by HugoW View Post
... Motocalc looks promissing, I'll dive in and take the trail version ...
Calculator compilation, most of them freeware, Drive Calculator and MM_Calc (Scorpion_calc, Dualsky_, Motrolfly_, HiMaxx_, Aeronuts_calc) are popular too
http://www.rcgroups.com/forums/showthread.php?t=606703

The WebOcalc discussion is active:
http://www.rcgroups.com/forums/showthread.php?t=930018

Vriendelijke groeten Ron
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Old 10-22-2008, 07:19 AM
  #13  
HugoW
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Hoi Ron,
dank voor je input. Jij zit ook overal, hè? Wel handig...

ESC-wise I think I'm safe, mine can handle 60A cont., 10 second bursts of 90A. I think if I run 6S 90A for 10 seconds, my plane is out of sight! I've choosen a separate BEC hooked up to the main battery, so if I fry the motor and/or ESC, I should still have control. I also take precoution for when I blow the main battery; I always carry a broom and a dustbin to the field.

I'll put some figures in my Funtana X50 topic when I receive the motor.

Hugo
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