Choosing the Right Motor

With so many motors on the market, choosing the right motor these days is becoming tougher and tougher. My aim here is to make the choice a little easier for both beginners and experienced pilots. Although I will be going through the motor selection process for a 5-6″ speed quad, this process can be applied to any size quad. It can also be applied when looking for run time rather than speed (see analysis). It sounds like a straightforward process, but a balance has to be met. We can narrow down the motor choice to a few candidates, however there is still some fuzzy decision making (a lot of buts). We’ll start from the most general characteristics of motors and move on to more specific aspects.

Contents:

    1. The Process
    2. We Want It All
    3. Aren’t We Forgetting Thrust?
    4. Motor Numbers
    5. Does Size Matter?
    6. Early Summary
    7. Crunching the Numbers
    8. Analysis
    9. Conclusion

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The Process

The only way to make a good comparison between motors is to have test data that comes from the same source and the same set up. Since there is no standardized test for generating motor performance, the numbers given by the motor manufacturers are useless. They can give us a general idea, but the comparison is not ideal.

Two great sources of motor performance numbers are:

  • Miniquad Test Bench – Tons of data, charts, etc.
  • EngineerX – YouTube videos of bench tests with links to spreadsheet data
    • NOTE: RPM data is very important! The spreadsheet links of EngineerX does not have the RPM data, but you can get it off the video.

When comparing motors, be sure to use numbers from one source. Also, the most important thing to remember is to compare data that uses the same propeller between all the motors.

For this post, I used the numbers from MQTB. I didn’t download all the data, just most of the more recent and popular motors (27 motors). Also, I only used the data from 100% throttle and 75% throttle. From there I combined the data in a spreadsheet, made a few calculations, and then sorted the data. From there, I could make a more educated decision. Before I get into the results, there is a little info to understand.

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We Want It All

We want our motors to be quick (acceleration) and fast (top speed). What is limiting us from getting this? To be quick and fast, we need torque. It may come as a surprise to some, but brushless motors have gobs of torque regardless of the size or kv value. However, we only have gobs of torque if we can feed enough amps to the motors. This brings us to our true limiting factor: the battery. Not the voltage, but the amount of amps our batteries can provide. Too much amp draw kills batteries and kills performance:

  • Amps kill batteries by irreversible chemical reactions – the higher the load, the shorter the life of the battery.
  • Amps kill performance by reducing voltage (voltage drop) – have you ever noticed the voltage drop under hard acceleration? The higher the amp draw, the larger the drop therefore you lose precious RPMs

The battery problem

Having witnessed independent battery testing on batteries of various C ratings (C rating is explained here), it is easy to see that the C ratings from the manufacturer do not reflect reality. I have seen 30C batteries out perform 70C batteries (of the same mah). The best approach to choosing a good battery is to see what is popular or by word of mouth.

The bench test problem

Although the bench tests show voltage drops, they do not reflect reality and fall short of what we actually see when flying a quad. If a motor shows a voltage drop of 1.0 volts, multiply that number by 1.5 to get a more realistic voltage drop.

On the same note, bench test amp draw is much more than we see when flying. A good rule of thumb is to cut the amp draw on a bench test in half. But remember, once you cut it in half, multiply it by 4 to get the total amp draw of all 4 motors.

When it comes to what we want, it’s all about the amps… but we have to strike a balance – we need to be able to turn some high torque RPMs without dropping too much voltage and/or killing the battery.

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Aren’t We Forgetting Thrust?

Although thrust isn’t what gives us speed, it still gives us a good comparison between motors using the same prop since the only way to produce more thrust with the same prop is by spinning it at a higher RPM. However, as we stated above, what price do we pay for this extra RPM? If it’s significant (more amps), then the extra amp draw will cause a larger voltage drop. This can actually cause your quad to go slower since the volts aren’t there.

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Motor Numbers

First the basics. Most multirotor motors are defined by 2 sets of numbers such as 2205 2300kv. Here is a quick look at what the numbers mean.

Motor Size

The motor size is defined by the first set of numbers which is usually a 4 digit number in the form XXYY where XX is the diameter of the motor stator and YY is the height of the stator. For example, a 2207 motor has a stator of diameter 22mm and stator height of 7mm.

Stator
The green part with the spokes (the wire is wrapped around the spokes) is the stator.
Stator Side
The side view of the stator. The vertical height of the spokes (greenish gray rectangles) is the stator height.

The size of the motor can give us a general idea of the torque a motor can produce, but this comes at a price: amp draw and weight which we will get to later.

Motor kv

Motor kv value is defined by the 2nd set of numbers (easy to see since “kv” follows these numbers). The kv value of a motor is a guideline to tell us how many RPMs a motor will give us per volt. This number isn’t set in stone; typically these numbers vary +/-100kv.

Now we have an understanding of the numbers. Since I am concentrating on 5-6″ type quads, the motor sizes typically range from 22-24mm stator diameter with a stator height of 5-7.5mm.

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Does Size Matter?

As we will see in the analysis section, motor size doesn’t follow a perfect trend; the difference between sizes is a little fuzzy. In fact, I was having a hard time finding any true trend in the spreadsheet numbers. However (with the motors I analyzed), there is a very loose trend: the larger the stator diameter, the more efficient the motor. That didn’t quite satisfy me though. If they are more efficient, why don’t we see all motors with larger stator diameters?

Stator Diameter

What was the drawback to a larger diameter? In an effort to find a trend, I went to the Data Explorer page on MQTB and scrolled down to the “Motor Comparison” graph at the bottom of the page. I chose 3 motors with a similar kv, but with a different stator diameter. Since I knew the performance of the motors differed once they reached max RPM, I was looking any difference in the time it took each motor to reach max RPM and there was very little difference. However, I did notice that the amp draw at startup differed greatly: the larger the diameter, the larger the amp spike at startup.

Chart-4

Stator Height

For the most part, this startup amp spike also held true for the stator height: the taller the stator, the larger the spike.

Also of note is that when comparing motors with the same kv value and same stator diameter, the taller stators have more torque which means that the motors RPMs are less affected by the air drag on the props: it won’t slow down as much and is able to handle more aggressive and larger props.

Chart-5

The thing to take note here on motor size is that the larger the motor, the larger the spike in the startup amp spike.  This isn’t as evident in the spreadsheet numbers as is the efficiency and RPM gain.This spike in amps probably holds true for acceleration too – just an assumption.

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Early Summary

Not so much a summary, but more of a reflection on what we learned so far: not much in terms of choosing a motor. Although we now know that amps are the key, how do we know what is too much or what is worth sacrificing? The answer is still subjective. We can organize and crunch a few numbers (which we will do shortly), but even after that, it’s not clear cut. But it will definitely narrow down the field.

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Crunching the Numbers

To start, we can go to MQTB and get some numbers from the motor test results page, or get the numbers from another source that has the thrust, amps, and RPMs. I chose a common motor prop and put the data into a spreadsheet. NOTE: I have a lot more numbers in there than needed; all that is required is thrust, amps, and RPM.

Spreadsheet-1

Next, I added 3 columns. The first column calculates the RPM per amp (simply RPM÷amps). Since we are comparing the same prop, it works for thrust per amp also.

The next one calculates the velocity of the prop (see a more detailed explanation here) at the given RPM. This is calculated by:

Prop pitch X RPM X 60 ÷ 12 ÷ 5280

The last column is just a copy of the amps just so I can easily see them next to the other 2 columns.

Spreadsheet-2.jpg

Next, you want to sort the data according to the RPM/amp. Using MS Office, this is done by highlighting all the data as shown:

Spreadsheet-3.jpg

On the data tab, click on the sort button:

Untitled-3

In the sort dialog box, select sort by (whatever column your RPM/amp data is in) and order values from largest to smallest and hit OK:

Untitled-4

Now the data looks like this. NOTE: I highlighted a few things:

Spreadsheet-6

A few things I highlighted:

  • In the last 3 columns, I highlighted the the best in each with green text and the worst with red text.
  • In the theoretical velocity column, I highlighted a few of the motors that stood out (to me at least! See the Analysis below): Light green are runners up and dark green is the winner.

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Analysis

Although it is the most important attribute, there is more to motor selection than just the RPMs per amp. The next significant aspect is the theoretical velocity. You could have a large amount of RPMs per amp, but what if the motor is only drawing 3 amps? It might be efficient, but also very slow. So what I like to do is go down the velocity column and highlight the ones that have a nice bump in speed. Generally, it’s only worth going halfway down this column since, by the time you are halfway down, the amp draw is getting to be a little too much.

The Emax RS2306 2400kv, Cobra Champion 2207 2450kv, and EFAW 2407R 2500kv are all very, very close. The Emax falls short 4.5mph of the other 2 motors, but they require 10.2% more amps to gain 3.6% more speed. However, both the Cobra and EFAW have over 100g more thrust than the Emax. But still, they need 10.2% more amps to gain 9.2% (for the Cobra, 8.5% for the EFAW) in thrust. Very close.

Why the Cobra?

There is one more thing to consider. In the Drone Physics series, I talked about the role thrust plays. Since thrust only offsets the force of drag, thrust becomes less important the more aerodynamic your quad is. So, in summary, this is how I would choose:

  • For an aerodynamic quad, the Emax. Is it enough of a difference to ditch my Cobra motors? No.
  • For an everyday freestyle quad, the Cobras. And truthfully, the difference between the Cobra and EFAW is negligible so either one would be good.
2017_0731_00242600
The VXR-190 hit an average of 165.80 mph with the Cobra 2207 2450kv motors (verified by GPS)

What If I Don’t Care About Speed?

Then simply choose a motor that is higher on the RPM/amp column and don’t pay attention (or pay very little attention) to the theoretical velocity column. According to the data here, the F80 is the champion. But… in this case, it would also be helpful to compare motor weight since the Sunnysky is so close to the F80 in terms of efficiency. The Sunnysky weighs in at 29.9g while the F80 is 42g. But (#2) the F80 puts out 53g more thrust.

Weight

Weight should have very little influence on the decision. As seen above, some motors make up the difference, but it is never quite that that straight forward.

Physics: I will eventually expand on this so I’m keeping it light right now… Using Newtons second law F = ma, it can be calculated how much more thrust a motor needs to generate to compensate for added weight. That works out to be approximately 10g of thrust for every 1g increase in motor weight. Usually larger motors more than make up this difference.

What about the increased moment of inertia? This is non existent. Remember that the weight increase is in the motor, not in the frame. Why does this make a difference? Because the center of mass of the motor is in line with the center of thrust, no moment arm is created. Even if we take the center of thrust and center of mass of all 4 motors, they are still in alignment and no moment arm is created.

Update January 31, 2018: I am working on the moment of inertia issue. With linear acceleration, moment of inertia is taken care of with Newton’s 2nd law. However, angular acceleration is affected much more by the moment of inertia, more specifically, the weight distribution.

Response Time: Does weight influence response time of the motor? In most cases, no and in a few rare cases, barely. The benefits of the extra thrust and torque from the heavier motor out weighs any benefit of a light motor.

2018-01-01 15.48.15
Although the Bolt Worx is 7.1g lighter than the Cobra of the same size and kv, it has roughly the same response time but 150g less thrust. Thats a whopping 600g less thrust on a quad. Do you want to be 28.4g lighter or have 600g more thrust? Hmmm….

Further Analysis

Although this has nothing to do with choosing a motor, I wanted to find the trends in motor size vs efficiency. I decided to color code the motors according to stator diameter. It is not an exact trend (due to the different stator heights), but it is relatively easy to see the trend of higher efficiency with larger stator diameters.

Chart-6

The same was done for the kv value and the opposite conclusion can be made: the higher efficiency motors have a lower kv value. This is no surprise as this is generally a well known fact with brushless motors.

Chart-7

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Conclusion

Although the motor choice was easily narrowed down, there is still a good deal of “educated guessing” when it comes to making a final decision. I will never say my choices are always correct since there are still many factors to consider. As it stands, I am actually going to stick with the Cobra’s for now (I don’t get any kick backs from them). There still might be a motor out there that isn’t in the MQTB database that will outperform it so I will just have to keep my eyes open.

Also, I plan on adding more and more motors to the spreadsheet I have made, especially since MQTB lags a little in terms of getting the latest motors tested – I’m not blaming him, it’s not easy doing that stuff, especially with a day job! 2 motors I want to see are the new Emax LS2207s and the new Cobra CPL2207s…

emaxls22072_2

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4 thoughts on “Choosing the Right Motor

    1. downanddirtydrones says:

      Thanks! If batteries didn’t limit us so much, the outcome would be much different. Then it would be the ability of the motor to handle watts AND prop tip speed. I could go on and on about this… I remember seeing at least a couple dual battery builds 🙂 I wonder how well those will do since it will help with delivering amps? Only time will tell!

      Like

  1. downanddirtydrones says:

    No problem. Yes, I am shocked that the Cobra is still holding up. It goes to show how different mixes if stator diameter, height, and kv can yield the same results. Also there are intangibles like the way the motor housing handles the magnetic flux. I think this is how Cobra manages to get great performance out of the CP2207 motors compared to other manufacturers motors of the same size.

    Like

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