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.
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.
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 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.
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.
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.
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.
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.
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.
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.