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Motor Efficiency vs Performance

JDConover

Junior Member
#1
If a 180W 1400kv brushless motor has a Max Efficiency of 75% what kind of performance should I expect?

Is it correct to assume that 180W*75%= 135W of power at the shaft and 45W given off as heat?

Would the same be true for RPM or kv?

1400kv*11.1v= 15,540RPM

15,540RPM*75% = 11,655RPM
 

earthsciteach

Moderator
Moderator
#2
Oh man, you are getting deep quick with this question! My best and not-at-all adequate answer is, it depends on the prop and throttle setting. Honestly, I don't know how much stock you can put on the published efficiency of the motor because they don't tell you the conditions that it was determined under. My bet is that the efficiency is determined with no load on the motor. Since they don't publish curves, there is no way to tell.
 

squishy

Pirate ParkFlyer
#3
I don't think of it as motor efficiency, but power system efficiency because the other components have just as much influence on efficiency as the motor..
 

squishy

Pirate ParkFlyer
#4
Is it correct to assume that 180W*75%= 135W of power at the shaft and 45W given off as heat?

Would the same be true for RPM or kv?

1400kv*11.1v= 15,540RPM

15,540RPM*75% = 11,655RPM
I don't think it works like that, not to mention all these numbers are dynamic while in flight, and which prop are you using? lol

Physics, just isn't as cut and dry as that. If that stuff interests you, you need some advanced electrical and aerodynamic books, I am not even going to pretend to understand the math behind what you are talking about but I bet there's many interacting equations and dynamic answers or even subjective answers, like how fast is the airplane moving when you measure this? What is the ambient temperature and what is the air pressure... do you even know at what RPM, temp, resistance (prop) voltage that 75% number came from? How can you trust it?

My personal view of the subject goes like this. I just know and realize that the power systems are not 100% efficient. Some more than others. That is why it is so important for us to get all the numbers right where we can so we are not hurting the efficiency further. We will never reach 100%, but there is a peak to every system and it is usually adjusted and found by using the proper propeller...The airplane should be designed around the prop, and the power system should be designed to spin that prop at it's max efficiency...call it simple if you want, it's actually quite complex...

Best way to do this is to change props and test, change props and test, change props and test...just make sure you have a pile of props, a notebook to record all the numbers and a watt meter to read them. This is all you need to reach the max efficiency of a power system..

And most of the time, you will find that the manufactures recommendations, are spot on...
 

earthsciteach

Moderator
Moderator
#5
Translation of Squishy's post: Buy a BUNCH of stuff! You want to, anyway. Even if you don't want to, this hobby is going to suck you in and you'll walk into your workspace one day and wonder how the hell you accumulated all this crap. If you accumulate willingly, you can better plan and organize. Before you know it, you'll be the leading expert on building power systems. We need one of those...
 

squishy

Pirate ParkFlyer
#6
Yup, that too...don't ever buy one thing, get two, and get some stuff near it's size as well...it's fun to build up your pile of parts. I now have a plastic bin for everything, one filled with motors, one for ESC's and so on. The big box of props rarely leaves the car though because they are so important to have in the field, I crash a lot and fly combat every week. It pays to live within a short drive of the APC factory, it goes without saying that I buy in bulk...
 

Tritium

Amateur Extra Class K5TWM
#7
Never will approach 100% efficiency, HA! I am shooting for "Over Unity"! I am absolutely certain that some device sold over the Internet will allow an electrical powered flying device come back to earth with more battery charge than it left with! :p

Thurmond
 

squishy

Pirate ParkFlyer
#8
You are correct and it's already been done many times...the FPV endurance and distance record is over 100 miles with solar cells on the wings, he flew for hours and hours and hours using thermals and the sun, landed with a full charge..
 

chaydock

Junior Member
#10
JDConnover,

I don't know if you got the answer you were looking for or not. What helped me quite a bit was reading an article about picking the right motor and prop for a quad. The article was OK, but further down the page is a series of links going over "the science" of motors. It's really basic, but helps put everything into terms that the RC industry uses and builds progressively. It was a bit slow for me because of my electrical background; but, I thought it was good and it helped me with the terms used in RC.

I also spent a lot of time looking at dyno graphs of motors and playing around with eCalc. While not perfect, eCalc helped me quite a bit in understanding how one battery, ESC, motor, prop combination performs with respect to another. And, how the resistance (while quite low) of all the components worked together as a combined system and not just prop+motor. That seemingly insignificant resistance plays a huge part in efficiency, especially at higher power levels. There are so many options to play with on eCalc that it can be overwhelming at times. You can loose yourself for days trying to find the "just perfect" combination. :eek: