Capi's Brushless Motor and ESC 101

[Bill Williams]

Thanks for the discussions regarding my question. I understand the point of working around by storing and extracting inertial energy from the blades. I also see the point of having an electrical system capable of delivering the power the motor requires. I guess what I am still struggling with is, what makes one motor more powerful than another? If two motors are powered with the same amount of voltage and each of them is loaded down with the same amount of resistance (blade pitch, weight etc.) it seems to me, the one capable of drawing the most amps (current) would consume the most watts of power and put out the most power. Something like this has to be happening or all motors running on the same amount of voltage would produce the same amount of power. I would think the motor that could pull the most current, without burning up, would have larger wiring and less resistance. As you can see, I am no electrical engineer, but I would like to know what to look for when I want a more powerful motor. My practical application is comparisons of motors for my Hurricane 550. I will probably select my motor,power pack,ESC,and gearing based on the experiments of others posting on the 550 site. Still it would be nice to tidy up my thinking on this motor power output thing.

[Chizel]

I'm an electrician by trade so technically this is all old hat to me. But I work with high voltage three-phase motors (208-480v) and VFDs. A Variable Frequency Drive is essentially a giant size ESC.

I have a pretty good grip on the behind the scenes stuff, but what want to know is how does it relate to RC helis specifically.

In looking at the Scorpion line, they have the -6, -8, -10, -12, etc. I'll be building my first heli soon, a Furion 450, so which motor to choose? I won't be doing any 3D for quite some time while I learn the basics, so I'd assume I don't need all the power of a -6. I was thinking more along the lines of a -8 or -10, but in practical use I have no experience with them.

Will one of the less powerful motors leave me wanting (or needing) more power? I would certainly appreciate a longer runtime and compromise the output, but what is a happy (and acceptable) medium?

[trickybit]

With respect, this is probably not the best thread for asking this question. If' you're asking about a specific heli (Furion 450) you should ask in the appropriate forum for best results.

Better still, search said forum for long extant threads on the subject. I think you might find more traffic in the T-Rex 450 forum, and the answers are probably compatible, so you might want to post (and search!) there.

I think the answer you get will be that the -6 is a beast but chows batteries and stresses ESCs, or at least requires an upgrade over (T-Rex kit) stock gear, the -8 is adequately beastly, plenty of headroom for your (and my) flying style.

But take it to the better place and you'll get better info.

Happy flying.
Jim

[Pinecone]

Several things.

1) Efficiency. This depends on the wire used, the metal used for the core, the quality of the magnets, and the tolerance of the coils to magnets. Can also be the bearings. SO more watts in, doesn't mean more power out. Also the efficiency may change with RPM, there is an RPM range where the motor likes to run.

2) Gearing. If you take two motors with the same voltage and same heli, you may end up with different headspeeds, unless you change the gearing. And changing the gearings, changes the torque multiplication. And this also changes efficiency.

[Donald Dicharry]

Please explain why two motors of the same or similar Kv ratings can be so vastly different in terms of amp draw and physical size. The 4800 Kv brushless outrunner on my Mini Zoom Pro 2 (100 size heli) is dimunitive compared to other 3800 to 4800 Kv brushless outrunners. Is torque related to physical size, diameter and length, and number of magnets and windings?
I'm sure there must be a simple explaination, but even Einstein died searching for the Grand Theory Of Unification. Strings?
Thanks,
Don

[99grandtouring]

Guy I'm trying to understand brushless motors better but here is my big hang up...
lets say on a 3s lipo system you take a 1400kv motor and put a 12tooth pinion on it and get a head speed of 3krpms...
(just for comparison these numbers aren't exact)
then you can take the same 3s with a 1000kv motor and put a different pinion on it and still end up with a head speed of 3krpms....

So what would be the differences in how the heli flys? battery life / flight times? performance?
I would think one would have more torque / power usage but don't understand which one would and why?

Thanks!

[motorhomev10]

Would a 35 amp esc work with a 3500 kv ele. motor?

[iceman09]

great job it helps to know things like that if onewants to fix there own stuff thanks again great job i really appreciate your taking the time to school me and the others thank you very much

[michael.mccomas]

FROM FLIGHTLINE MIKE; FINLESS BOB IS A GOD. MEN LIKE DOMINIC D ARE HIS DECIPLES. YOU GUY'S HAVE HELPED GUYS LIKE ME SO MUCH. I AM SO GRATEFUL FOR YOU. YOU HAVE EARNED THE RESPECT AND ADMIRATION. THANK YOU, THANK YOU, THANK YOU.

[zen]

I read this thread a couple of years ago and barely understood it. Now it makes a lot more sense, especially that back EMF is sensed from an unused phase loop of the motor! Also why rpm's and torque don't come in the same package.

Thanks again Capi!

[O-tec]

Hi Dominic,
I am writing you from Sweden and I am also in this hobby (as a biginner and 51 year) I am an electrical engineer and worked a bit with 3 fas asynchronous motors. First of all I tank you for a nice article. I have a question regarding the moving picture. For me this is a bruched motor and not a bruchless motor as you stated under the picture. I am right?
Best regards
Olle

[DominicD]

The line "A brushless motor uses an electronic commutator. " wasn't a caption for the figure. I edited the line to make it more clear, thanks.

[KouDy]

Nice write up really.

I am still kind of puzzled. How can i co-relate RPM/V (ie 1000 RPM/V means that when supplying 5V rotor will spin 5000 RPM/V) and Kv (ie 1210 Kv)? Also all this should be related to the ampers (ESC) right?

EDIT :
Also to calculate headspeed from motor and gear attributes should i proceed like this :

headspeed = [(RPM/V)*(V)]/(teeth count on main gear)*(teeth count on pinion)

So for instance on my MT :
3559.4 = [(3700)*(11.1)]/(150)*(13)
This would be value for maximum (100% throttle).
For modifying for ie 80% throttle, i would just multiply by 0.8 in the end getting :
3559.4 * 0.8 = 2847.52

Is this correct thinking and calculation?

[KouDy]

So yes it's good old physics for 2 gears relation is as i posted above.

n1/n2 = t2/t1
n1 - RPM of main gear
n2 - RPM of motor
t1 - number of teeth on main gear
t2 - number of teeth on pinion

n1 is value you want to get hence expressing it is

n1 = (t2/t1) x n2

Adding throttle variable (ie your throttle is 80% then you multiply by 0.8 in the end).

Real RPM of main blades will be lower due to the load (resistance of the air considering the angle of blades etc.

So the last thing is relation between Kv and RPM/V...

[Darell]

Great artical.
I am an electrical engineer but have spent little time studying motors. Thanks

[DominicD]

If i can ever find some time from my new job I want to read this (588 page) book on syncro motors and drives, eventually I would like to sum it up and do a 201 of this thread.

[skyl3r_2k]

My mind is just honestly blown May be a little too in depth for my self!

[NJFlyer1]

Great article. I know a lot more now regarding brushless motors and inrunner / outrunner versions. Thanks a lot, Dominic!

[DominicD]

This article is rather old and some of the info is obsolete. I would like to find the time to write something better but in all honesty I am out of my depth alittle. My day job is designing circuits that run off 0.85V or less.

I have actually downloaded and put a good brushless/esc book on my Kobo, but I have to find the time/energy to read it. learning anything other than what i need for work just seems like work since i finished school.

[domenic dales]

Great job.