what kind of motor can I choose
08-26-2019, 12:58 AM
#1
Junior Member
Thread Starter
Join Date: Aug 2019
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
what kind of motor can I choose
As many of you know, a motor cannot have high torque at the same time that KV is high. Because KV is high and torque is high requires a larger diameter and a longer copper wire and a stronger magnet, rotation requires a larger current and voltage, and therefore the power is greater, the motor will be heavier, and weight is a very important factor for things flying in the sky. So as a novice, what kind of motor can I choose?
08-30-2019, 10:18 AM
#4
Join Date: Sep 2002
Location: Nijmegen / Nederland
Posts: 628
Likes: 0
Received 22 Likes
on
17 Posts
Note that Kv says absolutely nothing about max.power or max. current a motor can handle, or about max. torque. A 1:1 train motor and the motor in your toothbrush or in a bedroom appliance can have the same Kv of 2200rpm/volt.
It's all about what the motor wants to do versus what the motor can do.
Kv matches rpm and voltage, there's nothing more to it.
- Kv and voltage determine how fast motor wants/tries to run
(rpm_noload = voltage × Kv). - RPM and prop determine torque, which in turn determines current
(current = torque × Kv, in SI units!). - Max.current and power determine whether that battery/motor/rpm/prop combo can run without going up in smoke.
An e-motor tries to keep a constant rpm: as load goes up, current goes up.
Get/copy a proven plane/battery/controller/motor/prop combination.
But if you want to choose yourself, a steep learning curve, below an excellent quote about motor selection from brushless motors Kv - RCG?.
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.
Minor lay-out changes by RvS
... Kv is actually the item one should choose last.
- Decide your peak power requirement based on the weight of the model and how you want to fly it.
- Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
- Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
- Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
- Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as eCalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.
Minor lay-out changes by RvS
Last edited by ron_van_sommeren; 08-30-2019 at 10:31 AM.
08-30-2019, 10:25 AM
#5
Join Date: Sep 2002
Location: Nijmegen / Nederland
Posts: 628
Likes: 0
Received 22 Likes
on
17 Posts
Trainer and training suggestions:
Advice for getting into flying RC airplanes, and how not to - The Ampeer
Some well-structured reading for rainy/windy days, including two e-books, and some handy e-tools as well. Will save you, and us
a lot of questions. Notably the 'what went wrong?' kind of questions 
Will also prevent you from burning up several controllers and/or motors and/or battery:
E-flight primer and tools
And pleasepleaseplease, do your RC equipment, wallet, ego, battery, controller, motor, house/garage/car a big favour ... get a watt-meter. It will more than pay for itself, will save you at least one fried motor and one fried controller. Will also help you finding the best setup.
Keep battery-/-watt-/multi-meter wires short!
too long wires batteryside will kill ESC over time: precautions, solutions & workarounds
Prettig weekend
Ron
• Without a watt-meter you are in the dark ... until something starts to glow •
• e-flight calculators • watt-meters • diy motor tips&tricks • Cumulus MFC •
Advice for getting into flying RC airplanes, and how not to - The Ampeer
Some well-structured reading for rainy/windy days, including two e-books, and some handy e-tools as well. Will save you, and us
a lot of questions. Notably the 'what went wrong?' kind of questions Will also prevent you from burning up several controllers and/or motors and/or battery:E-flight primer and tools
And pleasepleaseplease, do your RC equipment, wallet, ego, battery, controller, motor, house/garage/car a big favour ... get a watt-meter. It will more than pay for itself, will save you at least one fried motor and one fried controller. Will also help you finding the best setup.
Keep battery-/-watt-/multi-meter wires short!
too long wires batteryside will kill ESC over time: precautions, solutions & workarounds
Prettig weekend
Ron• Without a watt-meter you are in the dark ... until something starts to glow •
• e-flight calculators • watt-meters • diy motor tips&tricks • Cumulus MFC •
