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Pumpkin drop event

episode request about motors and batteries

atsugi

Junior Member
#1
I have a request, as someone getting into the hobby. I am often overwhelmed by the numbers being thrown around. I know that you guys give battery and motor recommendations for the planes you test. but what about an episode on how you chose the right battery and ESC for the motor. Or how to chose the right motor for what you have lying around. As much as Mr. Scott hates math can you find a simple way to do some calculations for those of us who want to experiment with different power plant setups.
 

atsugi

Junior Member
#3
yep watched it again but it still have a question. So here is a example. I have a 9x6 prop driving a NTM Prop Drive Series 28-26 1100kv / 252w motor Model: NTM Prop Drive Series 2826 1100kv
Kv: 1100rpm/v
Poles: 12
Motor Wind: 17T
Max current: 20A
Max Power: 189W @ 11.1V (3S) / 290W @ 15V (4S)
Weight: 57g
ESC: 20~30A
Cell count: 3s~4s Lipolywith a 30A ESC

does that mean I can use any 3-4 cell battery that has a discharge less than my esc max Ampres and more than my motor ampres draw??
 

Epitaph

Ebil Filleh Pega-Bat ^.^
Mentor
#4
Less than your ESC no... More!!

And when you look at discharge, it's not the C rating as such, but the amperage. In other words, if you have a 30A ESC, it doesn't mean you use a 30C battery or more, as the C rate is for you to calculate the discharge, not the discharge itself.
 
#5
you can break it down like this:
the motor should alway be the limiting factor, every part that follows in the line should be able to deliver more Ampere.

Example:

If your motor draws max. 10A you should get an ESC that delivers AT LEAST the same amount of Ampere. usually you add 10-25% to be sure, since you can't trust the specs of the manufactureres.
So you go with a 12A ESC (or even a 15A ESC these small ones are cheap and lightweight so why not add 50% to be sure)
So your battery should be able to provide at least 12A CONTINOUSLY.
so you get a 1000mAh Battery with at least 12C (here again to be sure, add some Amperes to be sure. so go with 20C or even 25C - you might want to use it in another plane with a stronger motor Combo. also when your ESC burns out, your plane crashes and you might need some glue. if your LiPo goes up in Flame you just can watch your plane burn)
the C rating tells you how much of it's cacpacity a battery can deliver. 1000mAh = 1Ah * 20C = 20A so a 1000mAh 20C battery is more than enough for a 12A ESC


short example:
Motor needs 10A, ESC could deliver 12A, Battery could deliver 20A = Your Plane and you are happy ;)

Background:
(very roughly broken down)

The ESC regulates the volts that go into the Motor to tell him how fast to spin.

The motor draws the amount of A that he needs for that (depends on differnent factors, such like resistance coming from prop etc.)

Motor doesn't care what the ESC has to say about that.

ESC has to be able to deliver that much A or more (just delivers as much as motor asks for) or it goes bye-bye from overheating.

ESC draws that much Amps from Battery

ESC doesn't care what the Battery has to say about that.

Battery has to be able to deliver that much A or more (just delivers as much as ESC asks for) or it brings down your plane in a ball of fire.



hope that helps ;)
 

Montiey

Master Tinkerer
#6
Tell me if I'm wrong, but the KV of a motor is not the power, but efficiency? So you could have a 1300KV motor the size of a small car, but still turn 1300 times pm/v? of course, this would be freakishly efficient due to the jumbo resistance in the coils, and most likely confiscated by the FBI to be investigated as possible alien technology...

KV=efficiency?
Phisical Size=allowable power output?
Smaller Motor=Less posible output?(same efficiency?)
 

Craftydan

Hostage Taker of Quads
Staff member
Moderator
Mentor
#7
No. KV is a direct measure of the motor's top speed (RPM = KV * V) , but implies the size of the prop for a given voltage, and can imply the model's top speed from the prop's pitch and the battery voltage.

For a fixed KV and voltage, you know the RPM, then knowing the recommended watts out of the motor you can begin to guess how long of a prop it can throw.

For the top speed estimate, you can find out the RPM, then the pitch of the prop is how far forward the prop needs to move per revolution (in inches) so theoretical top speed = pitch * RPM (in in/min -- you'll have to do the math to convert it to mph)

Keep in mind, KV is an unloaded rating. The RPM *will* slow down as you load the motor, and over-proping the motor will cause it to seriously bog down.

One other thing to toss into this mix is Props -- The motor will only draw what the load demands of it, and that's the prop. It's hard to estimate what a prop will demand, but easy to compare between props if you know data for one. Power required to swing a L"xP" prop is proportional to P*L*L. this number (P*L*L) won't tell you how much power you need, but it can give you an idea of whether a 8x6 prop requires more power than a 9x4.7 (suprisingly it does by narrow margin, but the 8x6 will have nearly 30% higher top speed)
 
Last edited:
#8
No. KV is a direct measure of the motor's top speed (RPM = KV * V) , but implies the size of the prop for a given voltage, and can imply the model's top speed from the prop's pitch and the battery voltage.

For a fixed KV and voltage, you know the RPM, then knowing the recommended watts out of the motor you can begin to guess how long of a prop it can throw.

For the top speed estimate, you can find out the RPM, then the pitch of the prop is how far forward the prop needs to move per revolution (in inches) so theoretical top speed = pitch * RPM (in in/min -- you'll have to do the math to convert it to mph)

Keep in mind, KV is an unloaded rating. The RPM *will* slow down as you load the motor, and over-proping the motor will cause it to seriously bog down.

One other thing to toss into this mix is Props -- The motor will only draw what the load demands of it, and that's the prop. It's hard to estimate what a prop will demand, but easy to compare between props if you know data for one. Power required to swing a L"xP" prop is proportional to P*L*L. this number (P*L*L) won't tell you how much power you need, but it can give you an idea of whether a 8x6 prop requires more power than a 9x4.7 (suprisingly it does by narrow margin, but the 8x6 will have nearly 30% higher top speed)
you can break it down like this:
the motor should alway be the limiting factor, every part that follows in the line should be able to deliver more Ampere.

Example:

If your motor draws max. 10A you should get an ESC that delivers AT LEAST the same amount of Ampere. usually you add 10-25% to be sure, since you can't trust the specs of the manufactureres.
So you go with a 12A ESC (or even a 15A ESC these small ones are cheap and lightweight so why not add 50% to be sure)
So your battery should be able to provide at least 12A CONTINOUSLY.
so you get a 1000mAh Battery with at least 12C (here again to be sure, add some Amperes to be sure. so go with 20C or even 25C - you might want to use it in another plane with a stronger motor Combo. also when your ESC burns out, your plane crashes and you might need some glue. if your LiPo goes up in Flame you just can watch your plane burn)
the C rating tells you how much of it's cacpacity a battery can deliver. 1000mAh = 1Ah * 20C = 20A so a 1000mAh 20C battery is more than enough for a 12A ESC


short example:
Motor needs 10A, ESC could deliver 12A, Battery could deliver 20A = Your Plane and you are happy ;)

Background:
(very roughly broken down)

The ESC regulates the volts that go into the Motor to tell him how fast to spin.

The motor draws the amount of A that he needs for that (depends on differnent factors, such like resistance coming from prop etc.)

Motor doesn't care what the ESC has to say about that.

ESC has to be able to deliver that much A or more (just delivers as much as motor asks for) or it goes bye-bye from overheating.

ESC draws that much Amps from Battery

ESC doesn't care what the Battery has to say about that.

Battery has to be able to deliver that much A or more (just delivers as much as ESC asks for) or it brings down your plane in a ball of fire.



hope that helps ;)
This is my first message in this forum, I'm so new to this hobbie that my first Tx/Rx is still being shipped home :)

I just cant do anything but thank you for such good explanations, that really helps new joiners. The Joshes and David also do a great job for this community!!
 

Craftydan

Hostage Taker of Quads
Staff member
Moderator
Mentor
#9
MyMaps,

Welcome to the Forum!

We've all been there, and a helping hand or suggestion has been all the difference between sitting frustrated at the workbench and a beautiful day of flying! FT is a good show, but it's really excelled at creating a place where the best in the community could grow.

When you get that radio gear in and start building/flying, feel free to show off what you're up to :)