Brushless Motor Basics


Written by Greg Gimlick
Selecting the Correct Motor
As seen in the Winter 2019 issue of
Park Pilot


This motor primer is designed to help a normal, everyday park pilot feel comfortable choosing a motor for his or her new project or replacing a motor in an existing airplane. If you can buy an exact replacement part, easy peasy, but what if you can’t? What if you’ve built or acquired an airplane with no motor? You need some basic understanding, and that’s why I’m here!

Keep it simple: Longtime electric fliers sometimes muddy the waters by wanting to lay out all sorts of information regarding torque constants, winding types, armature turns, magnet types, and everything else engineers consider when designing motors. Fortunately, they don’t really need to know all of that stuff.

They do need to know how much power is needed. For that, I like the chart from Common Sense RC (commonsenserc.com). It’s simple to understand and provides a good base from which to start.

The sample airplane: For the purpose of this tutorial, let’s assume I have a simple, ready-to-fly sport model that weighs 3 pounds. I want it to be sporty and capable of solid aerobatics so I will need roughly 100 watts per pound, according to the chart.

I know I want to have approximately 300 watts of power (3 pounds x 100 watts per pound = 300). This is my starting point—it’s all downhill from here. I also want to fly for roughly 6 minutes, and expect to use a common 3S LiPo battery pack found in many airplanes of this size.

Step by step: The main part is figured out and I merely need to work the math a bit to get the details nailed down. It’s easy!

Power (watts) is amps times volts. I know that the 3S pack will provide 11.1 volts under load and I want 300 watts, so I only need to know current (amps). Three hundred watts divided by 11.1 volts = 27 amps, which would be a full-throttle setting, and I know that the average current over the course of a flight is approximately 66% of the full-throttle setting. Two-thirds of the 27 amps would average 18 amps for the flight.



This chart shows watts-per-pound recommendations, courtesy of Common Sense RC.


When figuring what pack is needed to achieve the current demands and flight time expectation, I need to do a bit more math. I don’t want to completely discharge the battery, so I don’t use 100% of the battery capacity. I want to leave roughly 20% in the pack at the end of the flight, so I’ll use 80% as a guideline.

Packs are rated in milliamp hours, but instead of using the full 60 minutes, I only want 80% of that and will figure the requirement based on 48 minutes—that’s 60 milliamp minutes times 80% = 48 milliamp minutes.

Using 60 and dividing that by 6 minutes to get the C-rate of discharge would drain the pack to zero, but by using 48 (80% of 60) and dividing it by 6, I get an 8C discharge rate, leaving 20% in the pack for safety.

To choose a LiPo pack, I take the average current draw for the airplane (18 amps), divide that by the discharge rate (8C), and find that a 2,250 mAh LiPo battery pack is required to achieve a 6-minute flight time. This pack size is common and economical.

Now I need to select a 300-watt motor and propeller combination that delivers what I need using the 3S 2,250 mAh LiPo pack.

The Cobra motor line on the Innov8tive Designs website (innov8tivedesigns.com) offers a 2814/12-1390 motor that is capable of 450 watts of continuous power on a 3S pack. This is a slight overkill because it can handle 40 amps continuous current when the maximum requirement is only 27 amps, but that’s okay. It means I won’t be burning this motor up and I’ll have the ability to push it harder should I decide to increase the aerobatic capabilities of the airplane later or increase to a 4S battery pack.

The propeller selection guide for this motor shows that an APC 9 x 4.5E propeller on 3S power will pull 28.9 amps, providing 321 watts. The goal was 27 amps and 300 watts, so this looks to be nearly perfect!



This chart shows watts-per-pound recommendations, courtesy of Common Sense RC.


Results: A motor giving everything required and then some will power the 3-pound airplane. The math was simple and the process was easy to follow. Nothing is being pushed beyond its limit and there is plenty of room for adjusting later.

Other options: The Cobra 2814/12-1390 motor was selected, but what if I wanted to compare other options? The process is the same, but I can also reference the chart Lucien Miller has provided on the Innov8tive Designs website. In the Turnigy SK3 line is a 3536-1400 motor that is similar and would work too.



This Cobra motor has its name and information inscribed on the case. Cobra motors suggests a matching ESC, making it easy to put a system together.


What about Kv? Unlike what many think, Kv doesn’t stand for kilovolt. It refers to a velocity constant that tells how many rpm a motor turns per volt with no load. Apply 11.1 volts from a standard 3S pack and you’d expect the 2814/12-1390 motor to turn 15,429 rpm with no load. Of course, efficiency, load, and other factors play into it, but for planning purposes, that’s close enough.

One problem new fliers run into is when an “expert” at the field tells them Kv is all they need to know. You might as well tell someone gravity is a state of mind. Kv means nothing by itself. It’s a useful tool as part of a total package, but not alone.



These two motors have the same Kv, but are obviously different in size, and consequently, capability. Don’t believe people who say you only need the Kv to select a motor.


What do all of those numbers in the name mean? Some manufacturers use the size of the stator and others use the outside dimensions of the motor case to come up with the numbers. We need a way to compare them and that’s where Kv and weight come in to even the playing field.

With a motor such as a Cobra 2814/12-1390, I know that the dimensions of the stator are 28 mm diameter and 14 mm length with a 12-turn design and 1,390 Kv. To find another motor similar to that, we might check out the Turnigy SK3 3536-1400. This is essentially the same motor, but the measurements reflect the case size in diameter and length, then the Kv.

If Cobra listed its motor using this naming convention, it would be roughly 3534-1390, so they are roughly the same size. Comparing the weights, the Cobra is 107 grams and the SK3 is 110 grams. With similar Kv, weight, and size, the motors are nearly the same.



These motors are disassembled to show the difference in how numbers reflect sizes. The stator (the part with wire wound around it) is much smaller than the case itself. Find out whether your motor manufacturer uses stator or case dimensions in its nomenclature.


It’s important to know which method a manufacturer uses to identify its motors, otherwise you end up comparing apples to oranges. If you’re not sure, compare the Kv and weights. If a kit calls for a certain size, find out whether the manufacturer means stator or motor case dimensions.

Bottom line: Don’t get caught up in the “Kv only” argument. Define your requirements for the airplane and do some simple math to figure out the right motor to use. It only takes a few minutes to search manufacturers’ websites for a match. Sites such as Innov8tive Designs provide the type of information that’s needed to make the right choice. If a site doesn’t provide the necessary information, you might want to inquire.

This is a basic course to get you going successfully. Down the road we can look more closely at details.






Article: 

40 comments

I went to commonsenserc.com and could not find the sizing chart.

ncbob75@gmail.com

Thank you for your comments on motor Kv. It is a secondary information in motor selection process. The first thing as you pointed out is required power capabilities. Unfortunately many (if not most) manufacturers specify motor dimensions and Kv. Nothing about power capability. Sadly even most articles about electric motors and reviews of electric powered models mention Kv like a prime consideration. Hopefuly more people will read your article.

For years I have been flying with Graupner direct drive speed 600 and 700 brushed motor on 6 and 7 NiCad cells. Weighing 38 oz to 55 oz Playboy and Lanzo Bombers as examples.

I like the silence of these brushed motors but like the prop noise. Also, don't like the heavy ni-cads and poor reliability and life expectancy of the brushed motors. I need to change all to brushless.

What brushless motors, "in or out runners" would you select/suggest?

Thanks for any guidance, Richard rschaum1@verizon.net

Great article! Clears up all the long standing questions I had. You are right, long time flyers will get into the weeds faster than a plane into a solitary tree. Thanks!!

It would be great if you would show the actual math for battery and motor selection.

Motor Basics

Great for using to increase power in a plane and power for vertical flight.

It might have been useful to mention the definition for stator and rotor (magnets). The part with the wire wrapped around is the field. The field is used to control the motor by adjusting the energy applied. (Inrunner / outrunner) The wires are connected to the field (stator) and the propeller is connected to the rotor ( magnets).

Thanks for the information, this will help.
Ray

My figures show that I want about 25 watts to power my project. Wish you had some data and motor types that fit the range.

It is a simple addition, but I think it would help if there was a bit more talk about ESCs and how they are chosen. You talk about amps, very clearly, but if you are looking for the right ESC, one has to know that Amps are essentially way to choose the proper ESC. And further, what are the consequences of choosing the wrong size ESC - what happens. It may seem "too" rudimentary, but for the novice, I think it would help. Great article!!

I have spent decades in the electronics industry. I get what your tying to say. However, there is so much not delineated in this article that no normal person could follow it.

Very informative.

motor info

great information. I am new to electric planes and this will make it better for me to understand.

Thanks

Great

100w / pound or more helped me understand motor and battery selection. Hobby King has motors listed as "40 size", which is very helpful.
And now I know what kv means.
Thanks

Followed the LINK, but cant find it : (

Great article. Will be very helpful.

good stuff

Brushless Motor Basics

Select a motor for a given model.

This is a great article that does well at explaining what the key terms are and how to select a motor. The bottom line you point out is that we need to know the wattage and Kv that would best fit our desired model weight giving a selected prop and RPM. However where you and nearly all brushless motors suppliers fall short (and have for at least a decade) is that motors aren't listed by wattage, only by Kv. Sure, wattage is sometimes made available, but only by digging through data sheets.
In my opinion, wattage is the primary sorting criteria for motor selection. Searching for 1000 Kv motors is tedious when buyers must sift through individual data sheets in order to find one that is appropriate for their model (by size/weight).
If you could publish a database of brushless motors that includes wattage as a data point, that would truly be useful!

Thank you for very good article about electric motor basics and how to choose one. I suggest a companion article about choosing the proper electronic speed control (ESC). Also some explanation of ESC/motor "timing," very much a not-understood topic.

This is a great article. As im new to the hobby i would like more articles like the explaining how to size servos , esc, for someone who wants to build foam or home built planes.
Thank you
Terry Kloeffler

Being a novice to this world, I found the article fascinatingly interesting.

electric motors

I have been flying since 1976. My first plane was a RCM Sport trainer designed by Joe Bridi. Had a K&B .40 nitro engine on it and flew great until I introduced it to the side of a building. From the very start I just loved the sound and the smell of the sport. Nothing like the smell of burning Caster Oil and alcohol in the morning. I have purchased a couple of electric planes and they are clean simple and QUIET. I can't tell when I need to add power because I can't even tell if the darn this is running. I owned a 1960 Chevy convertible and it had a 283 cu. in. V8 engine. Glass pack mufflers dual exhaust. God did that car sound sweet. Now I ask all of you, would that car have been near as cool if it was powered by an electric motor with batteries? The crowd just got real QUIET. I rest my case. Sorry Tesla. Mea Culpa.

Propeller chart in this article is most useful. Common sense rc website is now on my favorite sites list. With a little time I may even understand electric motors better. Thanks!

Great info. I didn't have any info about motors.

Great!

Great article - covers all the important basics without going overboard. Saving for future refresher. Thanks!

thorough and well written for the new comer to RC. Thanks!!

Always looking for information on how to chose a motor to fit planes I build, this all the info needed, thanks.

Best info to date!

thanks

no ability to print is 19th century tech

Very informative

This is an article in a Park flying publication and you use an example for a 3 lb aircraft????
Park flyers as defined by the AMA as 2lbs or less and under 60 mph speed. I can understand all the model suppliers who sell 3 and 4 pound aircraft in their "park flyer" category as their motive is profit not safety,but why not promote safe flying and use a 2lb example so people don't think it is ok to fly heavy aircraft in in a uncontrolled environment. It is just a matter of time before we end up like Canada who bans flying RC aircraft in most public parks. If we don't regulate our selves we will get regulated! I have experienced unsafe flying in local parks and heavy fast aircraft make it even worse!

Hi Vernon. From time to time, we review aircraft that are over the 2-pound weight limit. Some of our members are regular Adult AMA members who fly aircraft of that size.

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