How to choose the right size motors & ESCs for your Drone, quadcopter, or Multirotor build

QuadQuestions how to choose drone motor and esc infographic

Many people have asked me how to properly size your motors to your quad.  Especially today, when we have a ton of available motor options.  The following items shold be considered when selecting the appropriate motor for your racing drone or multi rotor:

QuadQuestions Motor and esc selection guide for drones quadcopters and multi rotors step 1

Step 1.  Choose Frame type

Your frame type will determine many factors about your drone, such as what size props you can run, and what size motor you can mount.

The motor size that you need to choose is largely dictated by the size of your drone or quadcopter.  Therefore, the first thing to consider when choosing the right setup is to determine what size drone you want to you want to build, what size props your chosen frame can run, and what batteries you want to use.  Are you building a racing drone? Freestyle acro flyer? Or are you building something to haul a camera and get the best flight time possible?

There are several types of motors available and several Stator/KV combinations available.  The goal is to build a balanced, well engineered system which compliments your flying style, the purpose of the drone, and the drone size.

By properly sizing your motor you will get longer flight times, longer equipment times, less wear and tear, and lower temperatures.

QuadQuestions Motor and esc selection guide for drones quadcopters and multi rotors step_2

Step 2. Determine the prop size that your drone will run.

Your prop size will be determined by your frame.  So if you choose a QQ190 for instance, you could run 5″, or 4″ props, but for this example, we will choose to run 5″ props not only because this is what the QQ190 was designed to use, but also because this is the largest size that we will be running on our build.

QuadQuestions Motor and esc selection guide for drones quadcopters and multi rotors step_3

Step 3.  Choose your stator size

A general rule of thumb is to classify your motors first by stator size and then lastly by KV after other factors have been considered.

We here at QQ have found the following to be a good starting guide for sizing stators to propeller size:

Approx diameter QQ Frame Size Prop Size Recommended stator size Lowest kv recommended Highest kv recommended
150mm & smaller QQ130 3”

1306

3000

4000

150-250mm QQ166 4”

1806

2600

2800

190-220mm QQ190, Sparrow R220 5” 2204-2206

2300

2600

220-270mm QQ220 6” 2204-2208 1960 2300
350mm TBS Vendetta 7” 2206-2210

1450

1600

450mm TBS Discovery 8”

2212

1000

1200

450mm & larger TBS Discovery 9” 2214-2216

900

1000

 

QuadQuestions Motor and esc selection guide for drones quadcopters and multi rotors step_4

Step 4. Determine the important factors for your build.

Selecting the right motor comes down to weight, power, efficiency and torque and the importance of each of these are ultimately determined by your flying style and what you are trying to accomplish with your multi-rotor build.  Ideally, you want to try to engineer your quad to efficiently fly, but also generate enough thrust without completely destroying your battery.  You also want to build your quad for your flying style.  An all-out drag racer is going to have a different ideal motor than a freestyle quad.  A motor with a larger stator will also be heavier, and will have a slower response time due to the moment of inertia.  So ideally, you should be able to choose a motor that is efficient, doesn’t draw too many amps, isn’t too heavy, and has the power to get the job done.

Wept- Weight, Efficiency, Power, Torque

A good pnunomic to remember is Wept- the guys that didn’t choose the correct motor for their quad build wept because they cooked their batteries.

The general idea that you want to adhere to is ordering the following weight efficiency power, and torque in terms of priority for your individual build and flying style.  Once this priority has been made, finding your perfect motor should be easy.

Weight:

A lighter motor is going to have a faster spin-up & slow down, and will yield a faster change in speed.

A heavier motor will take longer to spin up and slow down, and will yield a slower change in speed.

A lighter motor will most likely feel more precise in the air, and pids might be easier to tune.

lighter motors are more prone to damage upon impact, so beginners might be better off with heavier motors.

If you are doing more straight line racing, a heavier motor might be better, however, if you are doing a lot of acrobatics & aerial stunts, go for a lighter motor.

The highest performing quads have the best power to weight ratios.  Motors can also make up a big part of that, so keep that in mind as well.

Efficiency

Efficiency is measured in grams/Watt so think of it in terms of thrust/power required.

Motor efficiency can effect flight times, voltage sag, and battery life.  Ideally, you want to try to choose the motor that is most efficient throughout the range of operation, not just at the highest RPM.

Remember that your battery will have to be able to support whatever amp draw you have at WOT, and that the smaller the battery, the less capability it has to flow high currents.

A higher KV motor can tend to be more efficient in the high rpm ranges, but at the expense of torque.

Flying style also plays a role when choosing efficiency. For an all out, high budget drag racer that is at WOT all of the time,  efficiency might have less of an impact on your decision, whereas if you are trying to build a freestyle flyer that gets great flight time, can use cheaper batteries and lower cost escs, efficiency will have more of an impact for you.

Power:

The thrust output of your motors has a lot to do with which ones to select.  Looking at all-out thrust is a good thing to do, but it doesn’t mean everything.  A lighter quad with motors that have less thrust can feel just as good in the air as a heavy quad that has motors that output max thrust.  In the end, you will want to make sure that you have enough thrust to fly well. A good rule of thumb for a quadcopter is to aim for a 4:1 power to weight ratio.  It is common to see that number double to 8:1 or more in racing quads.

To figure your power to weight ratio, you will want to calculate the max static output of the motors divided by the weight of the quad.  For example, the power to weight ratio of the QQ190RTF can be calculated by taking the max thrust of the QQ 2205 2450 kv motors with HQ5x4x3 props ( 4241.56g ) and divide by the total weight of the quad with Tattu 4S 1300 mAh 75C battery. (471.2g).  The result is a whopping 9:1 power to weight ratio.  This is more than double the power needed for acrobatic flight and can make for a truly awesome flying experience.

We could have chosen motors for the qq190RTF with a higher thrust output to go on this quad, but this wouldn’t have been a good choice because the motors on the QQ190 only draw 24A at full thrust with HQ 5x4x3 props.  This is important not only to keep our special 2-in-1 ESCs happy, but also to protect the battery.  By using this combination, a 1300 75C battery which can handle 97.5A is perfect for this quad.  Had we used a motor that draws more than 30A, we would have had to redesign our ESCs, battery recommendations, and possibly the overall design.

Torque

An often overlooked and very important thing to consider when choosing motors is torque.  Torque can effect the time it takes the propeller to reach a desired speed. So a motor with higher torque will be easier to tune, and will make the quad feel more precise in the air.  Stator size effects the torque output, and larger stators tend to produce more torque, whereas smaller stators produce less.

Advances in mulitoror motor technology such as N52 magnets, curved magnets, tighter clearances, and .15mm stator laminations are allowing smaller motors to generate more and more torque, and is one reason that we have seen a shift in racing drone performance in the last few years.

KV is not what you think it is- but it is still important.

This is where things get a little bit complicated.  Kv is typically thought of as RPM per volt.  While this is a very good way to quickly calculate unloaded motor speed when voltage is applied, it isn’t technically the right way of thinking about it.  Motor KV doesn’t tell you how powerful the motor is, or how much current it can handle, or how efficient the motor is!   You have to find thrust tests to figure that out.

KV explained:

to think about KV, let’s think about the basics…

When a magnet is pulled over a copper wire, electricity is created in the form of voltage.  This means that when a motor spins, it creates back-emf that can be measured as voltage.

The correct way to think about Kv is to think of it as a motor constant.  If you spin a motor, it will generate 1V at a certain RPM.  The resulting RPM is the Kv of that motor.

So, think of it like this:

If a motor generates 1V of back-emf at 2300 RPM, then we can determine that it is a 2300 kv motor.  If the motor were to generate 1V at 1900 RPM, then we would be able to determine that motor to be a 1900 kv motor.

! Remember, Kv doesn’t tell you how powerful the motor is or how much current it can handle or how efficient the motor is.

A small motor and a big motor can have the same Kv constant.  The large motor will most likely turn out to be more powerful than the small one.  So there is no way to use Kv to tell you how powerful your motor is.  What Kv does do is  help us to understand the current requirements of a motor to produce a certain amount of torque… because Kv is directly related to the torque constant.

Torque & it’s relationship to KV.

KV can help us understand the current requirements of a motor to produce a certain amount of torque…

Without going into too much detail, all that you really need to know is that the torque constant Kt is the inverse of Kv. So as Kv goes up, Kt goes down.

Kt=-kv

Torque can be calculated using current and kt, the inverse of kv.

Torque=Kt*I

Current, torque and Kv are all related, Lower Kv motors require less current to spin heavy props and therefore have more torque, but loose efficiency at high rpm-Vice-versa, High Kv motors require higher currents to spin heavy props, but can run at high rpm more efficiently.

In other words:

High kv motors require more current than lower kv motors to produce a certain amount of torque.

QuadQuestions Motor and esc selection guide for drones quadcopters and multi rotors step_5

Step 5: Looking up the motor information:

Now that we know what we should look for, we can start scouring the internet for DATA!  Google motor thrust test INSERT MOTOR NAME HERE.

Prop considerations-

Props can greatly effect performance of your drone, and some props are much better suited to specific builds than others.  Although, beyond the scope of this post, I will only point out that the popularity of the 5x4x3 has to do with the fact that it is a very well balanced prop in terms of thrust, rpm and speed.  This props is perfect for freestyle flying, but isn’t quite the fastest prop out there, and a steep pitched, 5×4.5 bullnose prop allowed us to reach higher top speeds than the 5x4x3.  For the purposes of this post, we will try to use 5x4x3 because we are theoretically building a freestyle flyer.

Pick the motor

Let’s say that we think we want to use the ZMX 2205-2300

You need to find a chart of a given motor’s Volts/rpm/Watts to make proper decisions about whether or not a motor is a good choice for your mulitrotor build.   Without the chart, you will really be shooting in the dark.

Use the chart in step 3 to pick a good stator size starting point, then find a motor that you think will work.  Maybe you like the look of the ZMX Fusion Purple top 2205-2300kv motors. So let’s use them as an example.

Find the data

The ZMX Fusion 2205-2300kv  has been tested in numerous places, and it’s data can be seen here: check out the miniquadtestbench.com for more data.

Summary Data  full throttle  ZMX Fusion 2205-2300kv
100% Thrust (g) Current (A) Voltage High RPM Pass Efficiency
Prop Tested: Max(g) Avg Max(g)* Max (A) Avg Max(A)* Avg Min(V)* Max(V) Max Min Avg Max* Watts G/W
HQProp 4x4BN 723 712 57.80 15.58 15.37 16.68 34562 33351 33504 240 3.0
HQProp 4×4.5BN 827 815 74.30 20.55 14.91 16.65 32467 30612 31027 306 2.7
HQProp 4x4x3BN 836 830 68.30 18.99 15.05 16.61 33351 31512 31953 286 2.9
Diatone Ghost 5×3 953 938 78.10 16.90 15.22 16.58 34149 32590 33014 257 3.6
HQProp 5x4GF 1032 1017 59.00 21.32 14.80 16.56 31282 30075 30534 315 3.2
GemFan 5×4.5BN 1115 1102 67.20 23.36 14.71 16.66 30181 28957 29390 344 3.2
GemFan 5×4.6BN 1157 1129 77.90 28.54 14.18 16.60 26702 25210 25961 405 2.8
HQProp 5x4x3GF 1177 1151 90.30 25.48 14.49 16.63 28860 27649 28176 369 3.1
KingKong 6×4 1388 1345 92.10 30.27 13.98 16.57 25510 24144 24768 423 3.2

Interpret the data

Looking at the data, we can see that this motor with the HQ5x4x3 averages 1151g, pulls 25.48A, and has an efficiency factor of 3.1

If we look at the same graph, but at 50% throttle, we see that the efficiency goes way up.

50% ZMX Fusion 2205-2300kv Thrust RPMs Amps Volts Watts G/W
HQProp 4x4BN 261 19815 5.15 16.27 84 3.1
HQProp 4×4.5BN 309 18964 6.12 16.21 99 3.1
HQProp 4x4x3BN 315 18963 6.10 16.19 99 3.2
Diatone Ghost 5×3 336 19826 5.25 16.23 85 4.0
HQProp 5x4GF 385 18741 6.26 16.13 101 3.8
GemFan 5×4.5BN 438 18286 6.94 16.15 112 3.9
HQProp 5x4x3GF 468 17790 7.54 16.08 121 3.9
GemFan 5×4.6BN 489 17094 8.50 15.97 136 3.6
KingKong 6×4 565 16817 8.75 15.91 139 4.1

Compare the options

lets look at the zmx fusion 2206-2300kv blue top motor for comparison.

100% throttle ZMX Fusion 2206-2300kv Thrust (g) Current (A) Voltage High RPM Pass Efficiency
Prop Tested: Max(g) Avg Max(g)* Max (A) Avg Max(A)* Avg Min(V)* Max(V) Max Min Avg Max* Watts G/W
HQProp 5x4GF 1146 1135 76.10 24.36 14.82 16.67 34013 32345 32739 361 3.1
GemFan 5×4.5×3 1237 1226 108.90 30.50 14.24 16.54 30832 29254 29754 434 2.8
GemFan 5×4.5BN 1265 1247 104.40 26.10 14.65 16.64 32715 31397 31851 382 3.3
HQProp 5x4x3GF 1332 1308 118.00 30.44 14.26 16.58 30721 29455 29919 434 3.0
GemFan 5×4.6BN 1362 1340 118.00 33.54 14.02 16.60 29055 27919 28233 470 2.8
KingKong 5x5x3HBN 1401 1370 116.30 36.24 13.85 16.60 27560 26619 27061 502 2.7
DAL 5×4.5x3HBN 1403 1390 89.50 33.03 14.14 16.63 29556 28382 28880 467 3.0
DAL 5x4x4 1435 1399 98.60 33.27 14.16 16.67 30075 28382 28847 471 3.0
HQ 6×4.5 1546 1502 102.80 39.26 13.54 16.52 25895 24701 25343 531 2.8
KingKong 6×4 1643 1603 119.30 36.78 13.79 16.56 27919 26455 26938 507 3.2

Ok, same prop, generates 1308g of thrust, but it draws 30.44A.  :(. The efficiency of this motor is also a little less than the purple top 2205 2300 KV.

Lets’s check it at 50% throttle.

50% throttle ZMX Fusion 2206-2300kv Thrust RPMs Amps Volts Watts G/W
HQProp 5x4GF 360 18677 5.86 16.25 95 3.8
GemFan 5×4.5BN 413 18288 6.51 16.20 105 3.9
GemFan 5×4.5×3 421 17766 7.22 16.07 116 3.6
HQProp 5x4x3GF 448 17588 7.43 16.08 119 3.8
GemFan 5×4.6BN 478 17103 8.04 16.06 129 3.7
DAL 5×4.5x3HBN 485 17230 8.08 16.10 130 3.7
DAL 5x4x4 495 17172 8.22 16.11 132 3.8
KingKong 5x5x3HBN 514 16384 9.05 15.99 145 3.5
KingKong 6×4 555 16795 8.66 15.98 138 4.0
HQ 6×4.5 595 16194 9.51 15.90 151 3.9

So what can we learn from this?  Using the same prop, the 2206 motor takes a lot more current at WOT because of the larger stator.  It is also a power hungry motor at WOT, however, because the stator is larger, it will have more torque.  At 50% throttle the motors both look good in terms of efficiency and power consumption.

Weight wise, the 2206 weighs 30.5g vs the 2205 at 27g.

How to choose?

Both motors will work, the 2206 are an amazing motor, and so are the 2205s,  partially due to the strong magnets.  The 2206 would be a better choice for pushing heavy props, but battery performance would be put to the test.

What it really comes down to is personal preference.  I would run a gemfan bullnose with the 2206 because the efficiency approaches 3.3 and the current drops to around 26A at full throttle.  This would give me the best option whilst still providing me with the greatest torque. However, If using hqprop 5x4x3, then the purple top 2205 is a much better choice.

QuadQuestions Motor and esc selection guide for drones quadcopters and multi rotors step_6

Step 6- Size your ESCs

last, but not least, use the data that you find to make sure that you choose an ESC that can handle the current demand that your motor/prop combination will be requiring at WOT.  Give yourself some room here- so if you are drawing 25A, choose a 30A esc, 18A, choose a 24A esc. Your ESCs will thank you!

CW vs. CCW motors:

CW vs CCW nuts

Q: Do I need motors with opposite threads on my counter-clockwise spinning motors?

A: No- Clockwise with locknuts are fine.

Some motors are labeled as CW, and CCW.  What this means is that the motor nut and shaft is either handed CW (right hand thread)or CCW (left hand thread)  In the early days of using prop nuts, this was more important.

In our testing and experience, however, we have found that it is not necessary to use motors with different thread directions on racing drones and quadcopters.  In theory, it makes sense to use a CCW motor on all of your right hand motors, but in practice, a locknut does a fine job of keeping your prop tight regardless of rotation.

We have even used locknuts on 3D setups without issue.

Counterclockwise vs clockwise propnuts for multi rotors make no difference.  So why not use a motor that is cheaper, easier to produce, and easier to find replacement parts (CW motor) on all of you quads, instead of using CCW motors which complicate things?

Conclusion

There is a lot of hype that is influencing people to build quads using strange component combinations, and the best quad is one that uses components that are well engineered to work together.  This is up to you, the builder to figure out.

Find the motor testing data, and let that guide you through the engineering process.  Decide your frame type, prop size, flying style, and then use the data, your prop choice, and your personal preferences to determine the best motor for your build.  If you still have questions, please post them at ask.quadquestions.com so that we can help you choose the best motor for your next multi-rotor build.

Remember, A good pnunomic to remember is Wept- the guys that followed the hype train and didn’t properly engineer their quads wept because they cooked their batteries.

Weight,

Efficiency,

Power,

Thrust,

Happy flying!

4 replies on “How to choose the right size motors & ESCs for your Drone, quadcopter, or Multirotor build

Leave a Reply

Your email address will not be published. Required fields are marked *