This tutorial aims to help you understand the basics of mini quad motors, which will help you choose the optimal and effective motor for your next quadcopter or racing drone 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 multirotor.
However if you are just building a mini quad, feel free to move on to the “Motor Size” section.
First of all, you need to know the size of the frame. This will determine what propellers and motors you will be able to use in your quadcopter.
Total Weight of the Craft
You can estimate the total weight of your quadcopter once you have selected all the components. It should include everything you plan to take on-board: frame, FC, PDB, RX, VTX, antenna, motors, propellers, ESC’s, LiPo battery, additional payload such as the GoPro, and so on. Adding 10-20 grams to compensate for wires, buzzer, zip ties etc is also a good practice. You probably won’t get it exact to the gram, and it’s better to overestimate the weight and have extra power than underestimate, and not be able to fly.
Once you’ve worked out your quadcopter weight and frame size, you can now calculate roughly how much thrust the motors and propellers combination needs to deliver in order to lift the aircraft.
Further Reading: how to choose propellers for mini quad.
Thrust to Weight Ratio
A general rule is that you should be able to provide at least twice as much thrust as the total 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 is too little, the copter will not respond well to your controls, it might even have difficulty taking off.
For example if we have a quadcopter that weighs 1Kg, the total thrust generated by the motors at 100% throttle should be at least 2Kg, or 500g per motor (for a quadcopter). Of course it’s always nice to have more thrust available than needed…
For faster flying such as drone racing, you should expect the power to weight ratio to go much higher than this. It’s not uncommon to see someone build a mini quad that can achieve 8:1, or even 10:1 thrust to weight ratio. Generally speaking, for acro flying, I would recommend to have at least 5:1.
With a higher thrust to weight ratio, a quad’s performance will be more agile and dynamic, accelerating and changing direction much faster. When you have an excessively high thrust to weight ratio, however, the quad can become very hard to control. Just a little touch of throttle can be enough to “shoot the quad into 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. This not only gives you better control, but also provides room for extra payload in the future (such as heavier cameras, or larger batteries for extended flight time). But if you are into racing, then there is no limit :D Go as high as you feel comfortable with!
Motor Size Explained In a Diagram
The Size of brushless motors in RC is normally indicated by a 4-digit number – AABB. “AA” is the stator width (or stator diameter) while “BB” is the stator height, both are measured in mm (millimeter).
What is brushless motor stator? – A stator is the stationary part of the motor, this has ‘poles’, which are wrapped around by copper wires (windings). The ‘poles’ are made of many layers of thin metal plate that is laminated together with a very thin insulation layer in between.
- Taller stator = more power at higher RPM
- Wider stator = more torque at lower RPM
Increasing the size of either will increase both the size of the permanent magnet and the electromagnetic stator coils. However increasing the stator height increases the permanent magnet size more than the coil size, and increasing the stator width increases the electromagnetic coil size more than the permanent magnet
The size of propellers a motor is designed for determines the prop shaft size. Motors for 3″, 4″, 5″ and 6″ propellers all have M5 (5mm diameter) motor shaft. Modern motors have the prop shaft integrated into the motor bell, while 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” is the velocity constant, and it commonly translates to “thousand RPM per volt”.
It’s an important parameter of brushless motors, which indicates the theoretical increase of motor RPM (rotation per minute) when voltage goes up (without load, i.e. propeller). For example, when powering a 2300KV motor with a 3S LiPo battery (12.6V), the motor will spin at around 28980 RPM without props on (2300 x 12.6). Typically this is just a round up estimation specified by the manufacturer.
Once you mount a propeller on the motor, the RPM decreases drastically 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,more torque is required to turn a heavy prop, while smaller and lighter props are better suited to high KV motors.
The KV value of a motor can be determined by the number of copper wire windings on the motor stator. Generally the higher number of winds decreases the KV of the motor, while lower number of winds increases the KV of the motor.
The magnetic strength of the magnets can also affect the KV value of a motor, stronger magnets will increase KV.
By pairing high KV motors with excessively large propellers, the motor will attempt to spin fast like it would with a smaller prop, but this will require more torque. As the motor tries to produce the required torque it will draw more current and subsequently generate too much heat. This will eventually lead to overheating and burn out your motor. This is because when the motor overheats, the coating on the coil will start melting and causing electrical shorts in the motor.
N and P Numbers
You might have seen specification such as “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 (these will generally be more expensive to manufacture). While it’s good to know what this is, it’s not an essential piece of information when picking motors especially for mini quad.
How to Decide on Motor Size?
You can find out the component sizes to use in this order: Frame Size => Prop Size => Motor Size
By knowing the frame size, we can estimate what motor size we should use. Frame size limits props size, and each prop size requires a different motor RPM to generate thrust efficiently, this is where motor KV comes in.
You also have to make sure that the motors produce enough torque to spin your choice of 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’s not a hard-set rule, you might also see people using slightly higher or lower KV motors than this table suggests.
It assumes you are powering the motors with 4S LiPo batteries, and frame size is referring to the 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. Your motor will try to spin faster when a higher voltage is applied, and thus draw more current. Ensure you are aware of how much thrust your motors produce and how much current they will draw.
When you know the current draw of the the motor and prop combination, you are now ready to choose ESC for your drone.
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 probably still have many options available 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, and 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 be aware of current draw and efficiency. Don’t abuse your batteries with an amperage-hungry motor/prop combo.
If your quad draws a lot of current at high throttle, the maximum discharge rate of your battery has to be able to keep up. The battery must also have a large enough capacity so it doesn’t discharge too quickly.
While motor power and thrust are important when selecting a motor, these stats are not the only things to consider.
One thing often gets overlooked is the weight of a motor, which is a very important factor for acrobatics and racing drones.
Since the motors are mounted at the four corners of the frame, they have a strong influence on the responsiveness of your quad. Heavier motors increase the angular moment of inertia of your quad, this makes the motors work harder in order to change attitude.
In practice, when your quad is doing flips and rolls, it takes time to pick up angular acceleration, move to the desired position and stop. Heavier motors will take longer to pick up that angular speed, and also longer to slow down therefore it might feel less responsive.
The motor itself also has moment of inertia (The shaft and the bell) since it works by spinning. The heavier the motor is, the more torque it requires to spin, therefore taking longer to change RPM. This affects the responsiveness of the motor and subsequently, the characteristics of the quad as a whole. The delay in changing RPM makes the quad feel less precise and makes PID controller work harder.
Motor efficiency is typically calculated by dividing thrust by power at 100% throttle, measured in gram/watt. The higher this number is, the more efficient the motor.
It’s important to look at efficiency through the whole throttle range though, not just the top end. Some motors might be efficient at lower throttle, but could lose efficiency by drawing increasingly higher current as they approach their limits.
With less efficient motors not only are you wasting energy and flight time, your batteries might also suffer from voltage sag, this effect will be compounded if you are using poor quality LiPo batteries.
Remember that Watt is calculated by multiplying voltage and current, as we know the voltage of the battery we will be using. Another good way to look at efficiency is to use gram/amp (thrust/current).
Generally, the more thrust generated, the larger the current drawn to produce that thrust, so motors with high thrust and low current are preferred. Inefficient motors either generate too little thrust or draw too much current.
Advance Quadcopter Motor Performance Factors
Many quadcopter motor properties are not mentioned by the manufacturers and can only be found through more technical testing.
- Response Time
- Vibration and balance
Torque is the force that turns the propeller, it determines how fast a motor can increase and decrease RPM, i.e. how easy it is for the motor to move the mass of the rotor, prop, and most importantly, the air.
Torque greatly affects the performance of your quad, specifically, how precise and responsive it feels in flight. A motor with high torque gives more snappy response, because of the faster change of RPM. You might even experience less prop wash with more torque.
High torque also means you can run heavier props (at the cost of drawing more current). If a low torque motor is driving a propeller that is too heavy for it (aka over-prop), the motor will be unable to produce enough force to spin it at the desired RPM, resulting in poor efficiency and overheating.
One drawback of high torque motors though, is oscillation. Motors with high torque are able to change RPM so rapidly that it can actually amplify error (in PID loop), causing oscillation that can be hard to eliminate even with PID tuning, especially on the yaw axis.
Motor Response Time is also dependent on torque, high torque motors often have faster response time. One easy way to measure response time of a motor is to see how long it takes for a motor to reach maximum RPM from 0.
Response time will be affected by the weight and aggressiveness of your choice of propeller, and remember that atmospheric conditions can have an effect too. At low altitude, for example, the air is thicker, this means that there is a greater number of air molecules that the propeller must physically move, to produce thrust. At high altitude, your props will spin faster and react quicker to changes in throttle, but the overall thrust will be reduced, because there are fewer air molecules for the prop to interact with.
Temperature affects brushless motors because the magnets used in our motors have a weaker magnetic field when operating at high temperature, they also demagnetize faster at the motor gets too hot which affects lifespan.
Over-propping your motors and using full throttle excessively, will cause your motors to run hot. This will degrade the performance of the motor and the magnets over time, therefore motor designs which aid cooling often equates to a longer lifespan. That is of course, provided you don’t destroy it in a crash beforehand!
Vibration caused by the motors can have a number of unpleasant side effects to the performance of your quad.
If a motor has poor balance or build quality, you might experience vibration that can affect your PID controller. As the frequency of the vibration changes at different throttle levels, this can make your quad very difficult to tune.
A motor suffering from vibration will also produce a greater amount of electrical noise than one which is running smoothly. This electrical noise can affect your Gyro sensor, making flight performance even worse, and it will also degrade your FPV video quality if you are powering your FPV system from the same battery as the motors and ESC’s.
Many have successfully soft mounted motors, and the flight controller to reduce vibration, with some really positive results.
Remember that damaged, bent and unbalanced propellers can also cause problematic vibrations.
Features in Mini Quad Motor
There are so many variables that affect the performance of a motor, it can get very controversial and complicated. For example, motors with the same stator size and KV, you can have very different thrust, current draw and response time even using the same prop. Differences in the design and material both have a great impact on performance.
Here I will explain a few different motor design features that contributes to better performance, which can also change the characteristics of the motor.
A hollow shaft saves weight, which allows manufacturers to use superior, albeit sometimes heavier, materials to improve durability.
But drilling the hole in the middle of the shaft can increase the cost of manufacturing.
The Types of Magnet
Magnets used in quadcopter brushless motors are graded according to their magnetic field strength, such as N52, N54 etc. The higher the number, the stronger the magnetic field.
A stronger magnetic field is theoretically capable of generating power more efficiently, providing more torque and a faster motor response time.
Stator lamination thickness
in a nutshell, the thinner stator lamination, the better. Laminations help to reduce a phenomenon known as Eddy Current, which generate heat in a changing magnetic evironment. Thinner laminiations means less power is wasted on generating the eddy currents and making motors more powerful and efficient.
“Air gap” in a motor refers to the distance between the permanent magnets and the stator. Magnetic force degrades non-linearly with distance, so reducing the gap between the two significantly boosts the power of the motor.
A smaller air gap not only makes the motor more powerful, it also improves torque and response. The downside of tighter airgap is the increase in current draw and decrease in efficiency.
Magnet Shape – Arc Magnets
Using arc magnets (aka curved magnets), is a technique to bring the magnets even closer to the stator; allows for a smaller and more consistent air gap.
In fact, the way a permanent magnetic field works means that with a curved magnet, the strongest magnetic point of each pole is no longer on the surface of the magnet, as it is with standard (non arc) magnets.
The ‘epicentre’ of the field of the pole on the outside of the curve, will be below the surface of the magnet, and the epicentre of the pole on the inner curve will actually be above the surface. In this manner, the magnetic fields of the permanent and electromagnets are actually brought even closer together, over and above the physical reduction of the air gap.
Apart from the shape, some manufacturers test mini quad motors with different thicknesses of magnet, often finding that a slightly thinner magnet (therefore a weaker magnetic field) can actually provide better results.
Wraps/turns on stator winding & wire gauge
The number of copper windings or ‘turns’ on a stator pole determines the maximum current a motor will draw, while the thickness of the wire determines how much current the motor can handle before overheating.
Fewer turns = less resistance = higher KV. The downside is a reduced electromagnetic field on the stator and thus lower torque.
The opposite happens when we have more turns in the coil. The increase of copper produces a larger magnetic field on the stator pole and generates more torque. But due to the longer wires and higher resistance, the KV of the motor decreases.
To tackle these issues when increasing the power of mini quad motors, manufacturers choose to increase the number of windings while using thicker copper wires.
This will effectively reduce the resistance in the winding, and improve the power of the motor without sacrificing efficiency and torque. The motor would also be able to handle high current without burning out with larger wire gauge.
However thicker wires and more windings means a heavier motor, and the winding takes up more physical space so it requires a larger stator. That’s why we are seeing more bigger and heavier motors, and that’s also why bigger motors are generally 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).
There are reports about C-clips popping off during flight, which resulting the motor bell flying off and causing a crash. However, be aware that screws are also 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, however the “naked bottom” motors tend to be lighter as less material is used.
Closed base motors are less likely to get dirt trapped inside the bell, against the argument that, naked bottom motors are easier to clean the dirt out.
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. (This happens often to beginners with closed bottom motors.)
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
Motor manufacturers are constantly experimenting with different designs and levels of hardware integration, which has led to advances in cooling and even an integrated motor and ESC in one. Personally I think solder tabs on the motor can come in handy, it allows you to use a lighter gauge wire to save weight on less amp hungry applications. These motors should also be easily repairable if the motor wires get pulled off, which can often spell the end of a motor of typical design.
CW and CCW Motors
You will sometimes see brushless motors labelled as CW and CCW. They stand for “ClockWise” and “Counter-ClockWise”.
While this is important for brushed motors because the brushes wear out quicker when rotating in the wrong direction, brushless motors are not limited in this way.
CW and CCW brushless motors are essentially the same motor that can spin both directions. The only difference is the direction that the prop shaft is threaded.
The motors on a quad spin in different directions, the intention here is when the motors spin, all four prop nuts get tightened.
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 have the correct one :)
Personally I prefer to have the same threads on all my motors, so I don’t confuse myself with the different prop nuts. If you have to replace a prop nut at the hardware store, it can be a real headache trying to find a CCW threaded locknut (or more commonly in the hardware jargon, a ‘left hand thread nut’).
When you receive your motors, the first thing to do is balance them. Although it’s not always necessary, it’s a good practice. I personally only do this on large motors though such as 2212 or bigger.
I find balancing unnecessary for many brand name mini quad motors because the quality is generally good enough. However with cheaper options that are becoming available don’t be surprised to find less attention paid to quality control.
Mini Quad Motor Recommendation
There are so many motor options out there, it will give you a headache :D in this mini quad parts list I have compiled all the most popular motor for mini quad.
And here is a list of top 5 motors options for mini quad voted by the community.
- Oct 201 – Article created
- Nov 2016 – updated with new facts
- May 2017 – added “Motor Features”
- Jan 2018 – Article revised