This tutorial aims to help you understand the basics of multirotor motors, which will help you choose the optimal and effective motor for your quadcopter build.
Where to Start? Decide on Motor Size First
To begin with, you need to answer these 2 questions:
- What’s your quadcopter’s planned total weight ?
- What’s the size of your frame?
Don’t worry if you can’t get the total weight of the quadcopter right the first time, it can be your best guess as you haven’t built it yet. It should include everything: frame, FC, PDB, wires, motors, ESC’s, battery, payload (such as HD camera and gimbals), and so on. Remember this is just an estimation so you can always go back and update the number when you have a better idea which parts you are going to get.
By knowing the size of the frame, you should know what the maximum propeller size can be used.
Once you’ve worked out these 2 things, you can now calculate roughly how much thrust the motors need to deliver to lift the aircraft in the air, by using propellers of certain sizes.
Read this article about how to choose propellers for mini quad.
2:1 Thrust to Weight Ratio
A general rule is that you should AT LEAST be able to provide twice as much thrust than the weight of the quad. Remember this is the bare minimum to ensure you have a stable copter that is easy to control during hovering. If the thrust provided by the motors are too little, the copter would not respond well to your control, it might even have difficulties to take off.
For example if we had a quadcopter that weights 1kg, the total thrust generated by the motors at 100% throttle should at least be 2kg, or 500g per motor (for a quadcopter). And, of course it would be nice to have more thrust…
For faster flying like drone racing, you should expect the ratio to go much higher than this. It’s not uncommon to see someone build a mini quad that can achieve 8:1, even 10:1 thrust to weight ratio. The quad’s performance would be much more agile and dynamic, it would accelerate much faster and corner better. But when you have excessively high thrust to weight ratio, the quad would become very hard to control, because a little increase in throttle is enough to “shoot the quad in to orbit like a rocket”. :D
My recommendation is, even if you only just plan to fly a slow aerial photography platform, you should aim at somewhere between 3:1 and 4:1. Not only it gives you better control-ability, but also the room for adding extra payload in the future (like heavier cameras, or maybe extra batteries to extend flight time). But if you are into racing, then there is no limit :D Go as high as you feel comfortable with!
The Size of brushless motors in RC is normally indicated by a 4-digit number – AABB. “AA” is the stator width, while “BB” is the stator height, both in mm (millimeter). Basically, taller stator gives you more power at higher RPM while wider stator gives you more torque at lower RPM.
What is brushless motor stator? – A stator is the middle, stationary part of the motor, which is wrapped around by copper wires (windings). It’s made of many layers of metal plates, and in between these layers there is lamination for insulation.
“KV” is another important property of brushless motors. It’s the theoretical increase of motor RPM (rotation per minute) when voltage goes up by 1 volt without load. For example, when powering a 2300KV motors with a 3S LiPo battery (12.6V), the motor would spin at about 28980 RPM without props on (2300*12.6). Typically this just an estimation, or round-up number specified by manufacturer.
Once you mounted a propeller on the motor, the RPM decreases due to air resistance. Higher KV motors would attempt to spin the propeller faster, but lower KV motors normally generate higher torque. That’s why we tend to see larger props paired with low KV motors, and smaller props with high KV motors.
The KV value of a motor is determined by the number of copper wire winding in the motor stator, and the magnetic strength of the magnets. But generally the higher number of winds increases the KV of the motor, while lower number of winds decreases the KV of the motor.
By pairing high KV motors with excessively large propellers, the motor will attempt to spin it fast like they would do with smaller props, and therefore drawing too much current and generating too much heat. Eventually it could burn out the motor due to overheat and shorts in motor coils.
N and P
You might have seen something like “12N14P” printed on the box of a motor. The number before the letter N means the number of electromagnets in the stator, and the number before P means the number of permanent magnets in the motor.
Most motors in RC have the same 12N14P configuration. Some lower KV motors might have more electromagnets and permanent magnets to help increase torque more efficiently (and thus more expensive). While it’s good to know what this is, it’s not an essential piece of information when picking motors especially for mini quad.
Frame Size => Prop Size => Motor Size
Most of the times by knowing frame size, we can estimate what motor size we should use. This is because frame size limits props size, and prop size limits motor size and KV. But anyway, always check motor thrust data to verify the current draw does not exceed safety rating, when pairing the propellers you want to run.
This table below is only a simplified example to give you some ideas. It assumes you are running 4S LiPo batteries. You might also see people using slightly higher or lower KV motors than this table suggests. Frame size is referring to wheelbase (aka diagonal motor to motor distance).
|Frame Size||Prop Size||Motor Size||KV|
|150mm or smaller||3″ or smaller||1306 or smaller||3000KV or higher|
|450mm||8″, 9″, 10″ or larger||2212 or larger||1000KV or lower|
Voltage and Current Draw
It’s important to understand that voltage has a large impact on your motor and propeller choice too. Your motor will try to spin much harder when a higher voltage is applied, and thus drawing a higher current. Always check thrust data first.
Reading Motor Spec
When selecting motors, there is usually specification that comes with the motor provided by the seller or manufacturer. You should be able to find information about the power, thrust, rpm etc. This is an example of the 18-11 2000kv Micro Brushless Outrunner (10g).
What to look for in a motor
Once you have decided on motor size, you might already have a handful of motors to choose from. To pick the best motor for your application, you should consider the following factors:
- Max Thrust
- Current Draw
The decision here really depends on your preference, how you want your aircraft to perform.
Thrust and Power
Thrust is probably the first thing people look at when choosing a motor.
Higher thrust gives you top speed, but you also need to look at efficiency, making sure it’s not using a ridiculous amount of power that exceeds what your hardware can support. Don’t abuse your batteries with amperage-hungry motor/prop combo. If your quad could draw a large amount of current at high throttle, your battery’s max discharge rate has to be able to keep up in order to provide the power, as well as they don’t overheat and go buff (check out C rating).
However I don’t believe motor power and thrust are the only things that represent the whole picture of motor performance.
One thing that often get overlooked is motor weight. It’s a very important factor for racing drones.
Since the motors are mounted at the corners of the frame, they have an obvious influence on the responsiveness of your quad. Heavier motors increases the moment of inertia of your quad, making it harder to change angular velocity. In practice, when your quad is doing flips and rolls for example, it would take some time to pick up that angular acceleration and eventually move to the desired position. Heavier motor requires more torque
The motor itself also has moment of inertia since it works by spinning. The heavier the motor is, the more torque it requires to spin, and therefore takes longer to change RPM. And again that affects the responsiveness of the motor and the quad as a whole, the quad would feel less precise and more work for the PID controller.
The weight of a motor matters a lot more for those who do acrobatics and racing.
Motor efficiency are typically measured in gram/watt (thrust/power). It’s important to look at efficiency through the whole throttle range, not just the top end. Some motors might be efficient at lower throttle range, they could lose efficiency by drawing increasingly higher current as they approach their limits.
With less efficient motors not only are you wasting a lot of energy and flight time, your batteries might also suffer from voltage sags. Inefficient motors either generates too little thrust or draws too much current, so another good way to look at it is to use gram/amp (thrust/current).
Other Factors Manufacturers Don’t Tell You
There are so many other motor properties that aren’t printed on the box. They have a big impact on your motor performance.
- Propeller choice
Torque and Response – how fast the motor can change RPM. It affects how precise and responsive your quad feels in the air, and makes PID tuning much easier. A motor with high torque gives faster response, which is related to stator size. One easy way to find out about this is by measuring how long it takes for a motor to go from 0 RPM to full RPM
Temperature – magnets in our motors get demagnetized faster in high temperature, and performance of the motor decreases. Therefore a cooler motor means longer motor life
Prop choice – Different propeller shows different performance on a motor. Response time, max thrust, efficiency… It’s all up to your personal preference. And don’t judge a motor simply because it didn’t fly nicely with 1 type of prop, experiment with some other props to get a better understanding
Vibration – If a motor has poor balance or build quality, you might experience vibration that affects your quad, making PID tuning much harder. Some people tried soft mounting motors, or/and the flight controller to reduce vibration and achieved some really positive results. Bashed and unbalanced propellers could also cause vibrations on the motor.
Features of Mini Quad Motors
The following features contributes to better performance in some aspects of the motors, and might also change motor characteristics.
- Hollow shaft – it allows stronger/heavier metal for more robust shafts yet the weight remains the same (or even lighter)
- Type of Magnets (N52, N54 etc) – the number is a grade of the magnet according to their magnetic field strength; the higher number, the stronger they are; stronger magnets gives higher torque and faster motor response
- Stator lamination thickness – in a nutshell, the thinner, the better as it increaes the power, reduces the heat generated as leads to better efficiency
- Smaller air gap – how far the magnets are from the stator, the closer the more energy efficient and powerful, also it improves torque and response
- Wire gauge in winding – larger wire gauge allows larger current without burning up, but it would also be heavier
- Arc Magnets (curved magnets) – a technique to bring the magnets closer to the stator; allows more consistant and smaller air gap
- Soldering tabs on motor
- ESC integration
- Cooling design
All these features make modern motors more powerful and efficient, yet draws a lot more current too! For example there are some negative impacts from smaller air gap and stronger magnet. Because of the stronger magnetic field, you might not be able to run high voltage such as 5S or 6S without risking burning your gear. You might also notice a high current draw with heavier props, putting a lot of stress on LiPo batteries and ESC’s.
After you received your motors, the first thing you should do is to balance it. Although it’s not always necessary, it’s a good practice. I personally only do this on large motors though like 2212 or bigger. For mini quad motors I normally find balancing not necessary because the build quality are generally very good. (2208 or smaller)
There are so many options out there, it will give you an headache :D For example these are the options for 250 mini quad motors.
CW and CCW Motors
You will sometimes see motors labelled as CW and CCW. They stand for “Clockwise” and “Counter Clockwise”.
However this does not mean they can only spin one direction. They are essentially the same motor that can spin both directions. The only difference of CW and CCW motor is the the direction that the prop shaft is threaded.
The intention is to use 2 CW motors and 2 CCW motors on a quad, so that when the motors spin, all four prop nuts get tightened rather than loosened.
To tell if you have the correct threaded motor on, simply hold the prop nut on the shaft, then start turning the motor with your hand in the direction it should spin. If the nut tightens then you got the correct one :)
However I personally prefer to get all motors of the same threads so I don’t confuse myself with the different prop nuts.
Article was created in Oct 2013, Last updated in Nov 2016.