This tutorial aims to help you understand the basics of quadcopter motors, which will help you choose the optimal and effective motor for your next mini quad or multirotor build.
Index of Content
- The basics – Quadcopter Weight VS Motor Thrust
- Motor Size
- Frame Size => Prop Size => Motor Size
- How to read mini quad motor spec
- Basic Performance Factors
- Advanced Performance Factors
- Features of Mini Quad Motors
- CW and CCW Motors
- Motor Balancing
- Mini Quad Motor Recommendation
Looking For Quadcopter Motors, Where to Start?
If you are new to mini quad and FPV, you should understand the relationship between motor thrust, and weight of your copter. This is useful to building any sizes multirotors.
However if you are just building a mini quad, feel free to move on to the “Motor Size” section.
Total Weight of the Craft
First, of all, estimate the total weight of your quadcopter. Don’t worry if you can’t get the total weight precisely the first time, it can just be your best guess. It should include everything you plan to take on-board: frame, FC, PDB, motors, propellers, ESC’s, LiPo battery, payload (such as HD camera), and so on.
Second thing you need to know is the size of the frame, from which you can then understand what the propeller sizes can/should be used in your quadcopter.
Once you’ve worked out quadcopter weight and frame size, you can now calculate roughly how much thrust the motors need to deliver in order to lift the aircraft, by using propellers of certain sizes.
Read this article about how to choose propellers for mini quad.
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 or stator diameter, both in mm (millimeter). Basically, the taller stator the more power at higher RPM, while the wider stator the more torque at lower RPM.
What is brushless motor stator? – A stator is the stationary part of the motor in the middle, which is wrapped around by copper wires (windings). It’s made of many layers of thin metal plates that is laminated together with very thin insulation layer in between.
The size of propellers the motor is designed for determines the prop shaft size. Motors for 4″, 5″ and 6″ propellers have M5 motor shaft. Modern motors has the prop shaft integrated into the motor bell. Older generation motors might have separate prop adapters.
Currently, common stator sizes for 5″ propellers are 2204, 2205, 2206, 2207, 2305, 2306, 2307, 2407.
“KV” means velocity constant. It’s an important parameter of brushless motors, which indicates the theoretical increase of motor RPM (rotation per minute) when voltage goes up by 1 volt without load (i.e. propeller). 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 decreases the KV of the motor, while lower number of winds increases 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 Numbers
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
By knowing the frame size, we can estimate what motor size we should use. This is because frame size limits props size, and each prop size requires a different motor RPM to generate thrust efficiently. This is where the KV of your motor comes into play. Then you’ve got to make sure that you’ve got enough torque to spin the propeller, this is where your stator size comes into play. Generally bigger stator size and higher KV means more current draw.
This table below is a general guideline. It assumes you are running 4S LiPo batteries. It’s not hard-set rule, 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||1105 -1306 or smaller||3000KV or higher|
|180mm||4″||1806||2600KV – 3000KV|
|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.
Now you have got a good understanding of what motors you want to run and estimated current draw, you can select ESC of the required current rating.
How to Read 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).
Basic Mini Quad Motor Performance Factors
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 faster acceleration, 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).
Advance Quadcopter Motor Performance Factors
Many motor properties are not mentioned by the manufacturers and can only be found through user testing.
- Response Time
- Vibration and balance
Torque determines how fast a motor can change/increase RPM. It affects how precise and responsive your quad feels in the air. A motor with high torque gives more snappy response, because of the faster change of RPM. You might even see less prop wash. High torque also means you are allowed to run heavier props in the cost of sucking more current. However if you put that heavy prop on a low torque motor, it simply won’t have the torque to spin it (can’t achieve the desired RPM) and results in lower thrust and lower current draw. However one drawback of high torque motor is oscillation that is hard to tune out. Because the high torque motors react so fast and able to change RPM so rapidly, it might amplify the error and cause oscillation especially in yaw axis.
Response Time – this is a product of torque, high torque usually means fast response time. One easy way to measure response time is to see how long it takes for a motor to go from 0 RPM to full RPM. And obviously, response time is going to be different from prop to prop
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
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 in Mini Quad Motors
There are so many variables that affect the performance of a motor, it can get complicated very quickly. For example for motors that are the same stator size and KV, you might get very different thrust and current draw running the same props, because of the differences in design and hardware. (different magnets or air gaps and so on)
The following features contributes to better performance in some aspects of the motors, and might also change motor characteristics.
it allows stronger/heavier metal for more robust shafts yet the weight remains the same (or even results in a lighter motor).
Type of Magnets
Magnets used quadcopter brushless motors, such as N52, N54 etc, are a grade of the magnet according to their magnetic field strength; the higher number, the stronger they are; stronger magnets provide a stronger magnetic field and thus help generate higher torque, power 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.
The thickness of the wires wrap around the stator has a big effect on the motor.
Thicker winding has a lower impedance, thus a larger current can pass through. While thinner wires result in lower currents as there is a larger resistance.
However, thicker wires take up more space which means you cannot fit as many wraps in,and as a result the back EMF produced is lower. This affects the “KV” constant. The more winding you have the greater the back EMF voltage produced at a given rotational velocity, thus your KV reduces.
“Air gap” in a motor generally refers to the distance between the permanent magnets and the stator. Magnetic force degrades non-linearly with distance, so shortening the gap between the two significantly boosts the power of the motor. Smaller air gap Not only makes the motor more energy efficient and powerful, also it improves torque and response.
Arc magnets (aka curved magnets), is a technique to bring the magnets even closer to the stator; allows more consistent and smaller air gap.
Wire gauge/wraps in winding
The motor winding determines how much power and current a motor can take.
Fewer winding = less resistance = more current can get through = higher RPM. That’s why higher KV motors generally have fewer winding int he stator.
But the downside with that is the reduced magnetic field and thus lower torque. Higher current passes through also means more heat is generated that could lead to lower efficiency.
To tackle these downsides when increasing the power of mini quad motors, manufacturers choose to increase the number of windings and use thicker copper wires. This will effectively reduce the resistance in the winding, improve the power of the motor without sacrificing efficiency and torque. Larger wire gauge allows larger current without burning up.
However, thicker wire and more winding means heavier motor. Also the winding takes up more physical space and that’s why we are seeing bigger and bigger motors (stators), and that’s also why bigger motors are more powerful.
C-Clip / E-Clip / Screw Shaft
To hold the motor bell to the base, motor manufacturers use one of these methods on the bottom of the motor to lock the shaft in place: C-clip, E-clip or a screw. Each of these ways has their pros and cons, and it’s hard to say which one is the best.
Generally speaking, screws are better for user maintenance as it’s easier to remove a screw than a C-clip or E-clip. But screws suffer from risk of over-tightening and locking the shaft (making the motor harder to spin).
You might hear stories about C-clips popping off during flight, which causes the motor bell to fly off and crash. However, screws are not immune to this problem.
Naked bottom or closed bottom?
In motor base design, there is the more traditional “closed bottom” approach, and the more recent “naked bottom” style. There are pros and cons to both of these designs.
The “closed bottom” design means a stronger motor base due to the excess metal, however the “naked bottom” motors tend to be lighter as less material is used.
With naked bottom motors, it’s easier to clean dirt from the bottom of your motor, but it’s the complete opposite with closed base motors as dirt could get trapped between the stator and the motor bell.
With naked bottom motors, you can see clearly how far the motor screws are going in, and you have less chance of shorting the motor winding if the screws are too long. (it happens often with close bottom motors to beginners)
However, the closed bottom motors provides better strain relief to the wires in case of crashing and stretching.
- Soldering tabs on motor
- ESC integration
- Cooling design
All these features make modern motors more powerful and robust, yet draws a lot more current too! 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, because they simply draw so much amps! You might also notice a high current draw with heavier props, putting a lot of stress on LiPo batteries and ESC’s.
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.
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)
Mini Quad Motor Recommendation
There are so many options out there, it will give you an headache :D in this mini quad parts list I searched for all the popular motor options for mini quad.
Here is a list of top 5 motors from the community.
- Oct 201 – Article created
- Nov 2016 – updated with new facts
- May 2017 – updated with “Motor Features”