Batteries for your quadcopter have a few more areas of consideration than simply how long it will keep your craft in the air. This guide is to help inform you of what those considerations are, and to help you decide which is the best lipo battery for your drone and individual style of flying. 

Voltage and capacity are the most important things to consider, but weight and discharge rate (also called ‘C’ rating) are almost as important.

LIPO BATTERY BASICS

As we know from Christmas mornings as kids, not all batteries are the same. After unwrapping the biggest gift under the tree, and fitting the “no-name” batteries that came free with your toy, the buzzing, and beeping, which drives parents to distraction, gets quieter and more distorted, until the batteries no longer work at all. We learn pretty early on that it is worth spending a few extra dollars on good-quality batteries so our playtime doesn’t get cut short.

Lipo Battery Size Chart

Number of Cells	Battery Voltage	Applications
1S	3.7V	Indoor whoops
2S	7.4V	30-75mm micro brushless
3S	11.1V	100-220mm brushless
4S	14.8V	220mm brushless
5S	18.5V	220mm+ brushless
6S	22.2V	220mm+ brushless

Battery Voltage (Cell count)

The nominal voltage of a LiPo cell is 3.7v, cells are put together in series to increase the voltage, and the number of cells used in a LiPo pack is shown by a number followed by the letter ‘S’. So a 2S battery has 2 cells wired together in series to create a 7.4v battery, and a 3S has 3 cells to increase the voltage to 11.1v. The most common voltage for quads at the moment is a 4S 14.8v battery.

Capacity

The capacity of a LiPo has the greatest effect on flight times, the higher the capacity, the longer the flight time you will get from your craft, but the higher the capacity, the heavier the battery will be. As the LiPo is the single heaviest component on your quad, you will reach a stage where you get diminishing returns, and the battery is too heavy for your craft to carry efficiently. The most common capacity for racing drones with 5-inch propellers is 1300mAh, which seems to find the best balance between performance, flight time, and weight, but there are, of course, exceptions to this.

Discharge Rate (C Rating)

The discharge rate is shown by a number followed by the letter ‘C’, the higher the discharge rate, the better. The discharge rating shows how quickly you can safely discharge your battery. A higher C rating means that you will use less throttle input to get your craft to hover, and it will provide more amperage to the motors at full throttle, making your craft faster and more punchy.

There is a phenomenon known as ‘Voltage Sag’ – The higher your throttle input, the faster you deplete your battery, but this depletion is not linear. At very high throttle the voltage drops even faster, but as you decrease throttle, the voltage will recover, the lower the C rating of your battery the more pronounced the voltage sag will be, and the longer it will take for the voltage to increase again.

With a high C rating, the voltage drop at a very high throttle will be reduced. I recommend 45C as the very minimum to fly slowly, a 75C pack will be better for freestyle, but to get the best out of your quad, and particularly for racing you should be looking at C ratings of 80-100C and higher.

Note – Batteries with a higher C rating will usually be slightly heavier, than others with the same voltage and capacity.

Buyers tip – Some manufacturers inflate the numbers of the C rating of their batteries, which is why it is recommended to purchase your batteries from a reputable source.

Important Buyers Note – Unscrupulous manufacturers often inflate the advertised C ratings of their batteries, as such, it is highly recommended to purchase batteries of a well-known brand from a reputable retailer.

Battery Chemistry

Batteries store electrical energy by using a reaction between different chemicals, such as Lead and Acid – which is what is used for car batteries. Li-Po or Lithium – Polymers have a good power density, they can be made in various shapes, and also inherently have good discharge rates, which make them ideal for our hobby/sport.

There are 2 other common chemistry types used for drones, these are LiHv (Lithium High Voltage) and Graphene. LiHV cells have a higher nominal voltage of 3.8v per cell, which provides a little more punch at full charge. Graphene batteries are said to have a slightly longer lifespan as they build internal resistance slower than a standard LiPo or LiHv.

Batteries don’t last forever

Like propellers, LiPo batteries are consumable in the hobby, however, they should last longer than your props, as long as you treat them well! I mentioned internal resistance earlier, this is what kills your batteries over time. The more you use a LiPo, the more the internal resistance increases. Internal resistance can be thought of as a component within your battery that uses electrical energy, leaving less power for your motors.

Overcharging, and over-discharging your batteries will cause internal resistance to increase more quickly, also leaving your batteries fully charged or discharged (past 3.2v per cell) for extended periods will also cause internal resistance to build faster. Unless you are going to fly tomorrow, I would recommend that you re-charge/discharge your batteries to storage voltage, which for standard LiPo, LiHv, and Graphene batteries is between 3.7 and 3.95v per cell, most say 3.8v for LiPo and Graphene and 3.85v for LiHv.

4S BATTERY OVERVIEW (14.8V)

A 4 cell/4S (14.8v) pack is the most common voltage for flying almost any size drone (apart from micro >120mm frames) at the moment. This voltage is very versatile and provides great performance for racing and freestyle on almost any quad using 2.5” props and over.

There are now components hitting the market that will support 5S and even 6S voltages, but at the current time, these are quite specialized. A few years ago 3S (11.1v) was the most common and as such, some are slightly behind the times and suggest that this voltage is better for the beginner pilot.

However, the throttle curve can be adjusted on your transmitter and/or flight controller (FC) so that at full stick input, your model is not running at its full thrust capacity.

If you do feel that your 4S model is a bit too fast for your current skill level, rather than buying 3S packs, you can adjust the settings to slow it down to a more manageable pace, and then revert or re-adjust these settings over time, to increase the thrust of your model as you feel.

3S BATTERY OVERVIEW (11.1V)

As I mentioned above, up until a few years ago 3S was the most common voltage used in the sport, but as the skill of pilots improved and technology advanced, we saw the introduction of 4S compatible parts, until subsequently, the higher voltage became commonplace.

As you may have learned, much of the hobby is a balancing act, the most important factors to balance are the thrust generated by your motors and props, and the All Up Weight/AUW, (basically, the weight of your model, including the battery).

These days generally 3S packs are used for smaller, lighter models and motors. Models designed for 3S voltages usually have a specific reason for using a 3S pack over a 4S, namely the additional thrust from the higher voltage battery will not show as much improvement as the decrease in weight of the battery.

Of course, there are exceptions to every rule, the exceptions here are that some larger crafts with 8 inches or larger props, will use a high-capacity 3S battery.

Remember that as capacity increases, so does the weight of the individual cells that make up your pack. Some long-range craft, designed to optimize efficiency, will sacrifice the additional thrust from a higher voltage, for an increase in mAh capacity.

Spending extra on long-range components for a craft that only has a flight time of 5-6 mins is a needless expense because you won’t be able to fly to the limits of your range in 2.5-3 minutes, remember you always have to save enough battery power for your return journey!

These 3S packs have been chosen to reflect the current popularity of micro models, you will notice that the capacities of the packs in this list are lower than the 4S list above due to the limited thrust of smaller props.

2S BATTERY OVERVIEW (7.4V)

2S packs are commonly used for quads with a wheelbase of between 100-120mm, but nowadays there are quite a few craft in this size category that will happily run on 3S and even 4S voltages.

Finding information on the craft that works best with 2S packs is not that easy as smaller capacity 3S and 4S packs become more common. Some ultra micro craft such as brushless tiny whoop’s, which often run single cell (1S) packs, are now using components to support upgrades to 2S voltages.

These packs will generally have a very low capacity between 200-400mAh, which is not enough to handle the current draw of 2” or larger props. Another thing to consider with 2S batteries is the connecter that is used, I think 2S voltage has the widest range of different connecter types as you can find Walkera/LOSI, JST PH, JST RCY, XT 30 and XT 60 are all available, so make sure you order the correct type for your model.

1S BATTERY OVERVIEW (3.7V)

1S packs are where many of us start, powering a toy-grade model such as the Eachine H8 mini, blade indicatrix, or one of its many clones. There are many different non-branded types of 1S batteries which is what taught me (the hard way) that not all Li-POs are equal.

Some of the no-name brand 1S packs I bought worked fine, others, from their very first use, failed to provide enough power to sustain a hover! Due to the popularity of Tiny Whoops, there are now many well-known companies providing high-quality 1S packs and relatively cheap too.

Most of the packs listed here are available in standard Li-Po and Li-Hv chemistries, so ensure that your charger and model are compatible with your selection. 

This guide will explain how can you make an informed choice while shopping for your next drone build. Motors are one of the most complex to choose from due to their seemingly overwhelming performance factors.

We’ll delve into the working of brushless motors in quadcopters and how they impact flight performance. We will examine different types of motors, design variations, weight, total power, and other factors that affect quadcopters’ motor performance.

Where to Start?

Before choosing motors, you should at least have a rough idea of the size and weight of the drone you want to build. I will explain the process of determining motor size based on drone size.

Some of the most important considerations are:

• Motor weight
• Power (thrust)
• Efficiency (g/watt)
• Torque and response (RPM changes)

Brushless Vs Brushed Motors

There are two types of motors used in RC, Brushless and Brushed motors. Generally, we use brushless motors on larger models (such as racing drones, and any bigger models), and Brushed on micro-drones and toy drones. We will focus on brushless motors in this guide.

To begin, there are two types of motors used in drones: brushed and brushless. Brushless motors are more powerful and have a longer lifespan compared to brushed motors. For larger quads, brushless is the preferred choice.

However, for micro and nano quads, cheap brushed motors are also an option. Both brushed and brushless motors operate on the principle of electromagnetism. When the motor windings are energized, a temporary magnetic field is created which repels or attracts the permanent magnets inside the motor. This magnetic force generates a repulsive force in the coil, which in turn rotates the shaft.

Efficiency-wise, brushless motors are typically 85-90% efficient whereas a brushed DC motor is 75-80%. Windings are present on the rotor for a brushed motor as compared to in a stator for a brushless motor

This difference in efficiency means that more of the total power used by the motor is being turned into rotational force and less power is being lost as heat.

A brushless motor lasts longer because there are no brushes to wear out, while the brushed motor wears out quickly. That’s one of the reasons brushed motors came out cheaper than brushless motors. 

This guide is mostly oriented on brushless motors used in racing/freestyle FPV drone configurations. 

THE BASICS OF BRUSHLESS MOTOR

There are 2 major parts to a brushless motor called stator and a rotor. A picture is shown below for reference.

The stator is the stationary part of the motor(windings) and the rotor is the rotatory part of the motor(bell with magnets). Also, there are a lot of other minor things such as bearings, coils, magnets, shafts, etc.

Motor size is based on the stator size (diameter and height). For example, if a motor is sized as 2207, it means that the stator is 22mm in width and has 07mm in stator height. 

Also, there is something called a KV of a motor. What it means is the speed at which the motor rotates for every volt applied to the motor, theoretically.

Thrust to Weight Ratio

Brushless motors come in all shapes and sizes. The general rule of thumb is to aim for a 2:1 thrust-to-weight ratio. You aren’t going to be able to do hard-core racing with it. The higher the better. The thrust to weight to ratio depends mostly on the size of the quads themselves.

There are pre-built quads such as from Diatone Crusader GT which has a thrust-to-weight ratio of 8:1. There are people who have achieved 13:1. But there are certain limitations for the motors because they can only spin so fast, and spinning them any faster makes them inefficient.

Even for a photography rig, you should aim at least for 3 or 4:1 in case you decide to upgrade your setup in the future or add an HD camera or a bigger battery for longer flight times or something, then you’ll have some reserve power left to compensate.

For a drone racing beginner 4 or 5:1, thrust would be the sweet spot. If you’re on a tight budget then build a beast of a quad and limit the throttle limit on Betaflight or the transmitter.

Motor Size Explained

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 millimeters.

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.

When increasing the stator height, increases the permanent magnet size more than the coil size, while increasing the stator width, increases the electromagnetic coil size more than the permanent magnet.

Increasing either the width or height of the motor stator will increase the stator volume, as well as the size of the permanent magnets and electromagnetic stator coils. This will result in higher overall torque of the motor – it’s able to spin a heavier prop faster, and produce more thrust (but draws more current). The downside of a bigger stator is it’s heavier, and also less responsive.

Taller Stator vs. Wider Stator

A wider motor has larger inertia when spinning. The mass of the motor is further away from the rotational axis, therefore it takes more energy to change RPM. For this reason, wider and shorter motors are generally less responsive than narrower and taller motors even if they have the same stator volume and generate the same amount of torque. A wider and shorter motor also means the magnets on the motor bell are smaller, which hinders the power of the motor.

However, the advantage of a wider motor is better cooling thanks to the larger surface area on the top and bottom. Temperature is crucial to a motor’s performance. As the motor heats up, its ability to generate magnetic flux decreases which impacts efficiency and the ability to produce torque.

It might be oversimplified, but the width and height of a motor stator are a balance between responsiveness and cooling, the decision comes down to the type of flying you do. For example, for a relatively slow cinewhoop carrying a heavy camera, because the motors are constantly working hard, you might want to consider motors with a wider stator for better cooling. For fast and snappy racing/freestyle drones where responsiveness is crucial, taller stators are preferred.

A wider stator also allows for a larger bearing to be fitted. Larger bearings are better for efficiency, smoothness, and longevity.

A bigger stator is not always better. For example, 2207 motors are more than capable of handling typical 5″ propellers, by using the much heavier 2506 motors of the same KV is most likely not going to provide any benefits since they’d have the same thrust or even worse responsiveness. If you want to get more performance it’s perhaps better to go higher KV since it doesn’t add any weight. The 2506 motor would probably work better for 6″ props than 2207 though as the bigger prop requires higher torque.

Quad Motor Sizes – Taller Vs Wider stators

A motor is indicated by a set of 4 numbers like 2207 or 2306 or whatever it may be. It denotes the diameter and the height of the rotor in millimeters (mm). The bigger the motor gets the higher the thrust it generates.

• Taller stator = higher top speed and terrible low-speed handling
• Wider stator = lower top speed and better handling at lower speeds

The main cause is the increased magnetic field from the stators. The taller stators have larger magnets as compared to smaller and wider stators.

2207 vs 2306 motors

A good comparison would be the typical 2207 vs 2306 motors. It is a hugely debated topic as to which is better and one can’t be recommended over the other as both have their advantages and disadvantages which will be covered in another article.

Brushless Motor Size Chart

The motor selection is determined by how large you want to build your quad. Hence the name Frame size = Motor size.

By determining the frame size we can define how large of a motor we should use. 

Frame size also limits the prop size and each prop requires a different motor to spin it and generate the thrust efficiently.

Also, the KV of a motor plays an important role in the selection of the motor. As mentioned earlier higher KV draws more current.

The table below shows the nominal frames and quadcopter motor numbers:

Frame Size	Prop Size	Motor Size	KV
150mm and smaller	3″ and smaller	1105-1306	3000 and higher
180mm	4″	1806	2600-3000
210mm	5″	2204-2208/2306	2600-3000
250mm	6″	2205-2208/2306	2000-2300
300mm	7″	2208/2212	1600

How to Decide on Drone 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 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’s not a hard-set rule, you might also see people using slightly higher or lower KV motors than this table suggests. Anyway, it’s a good starting point.

It assumes you are powering the quad 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 and higher
180mm	4″	1806, 2204	2600KV – 3000KV
210mm	5″	2205-2208, 2305-2306	2300KV-2600KV
250mm	6″	2206-2208, 2306	2000KV-2300KV
350mm	7″	2506-2508	1200KV-1600KV
450mm	8″, 9″, 10″ or larger	26XX and larger	1200KV and lower

Performance Factors

Brushless motors have many factors that define their performance. New drone enthusiasts, may feel overwhelmed. That’s why we will try to explain in easier terms.

1) KV – Velocity Constant

Theoretically, RC brushless motor KV rating represents the speed at which the motor rotates for every volt applied to the motor. For example, if a motor is 2300kv with a 3S quadcopter battery motor applied to its ends then the motor will spin at 2600×12.60 =32,760 rpm (Revolutions per minute – number of turns in one minute), without propellers. The rpm decreases gradually because of air resistance.

Motor KV represents the speed at which the motor rotates for every volt applied to the motor.

Higher KV means lower resistance and higher current draw and lower efficiencies. Lower KV means higher resistance and lower current draw and considerably better efficiencies.

If you’re starting somewhere around 2300-2400 KV would be desirable.

For example, if a motor is rated at 2300kv with a 3s battery voltage applied to it, then the motor will spin at 2300×12.60 =28,980 rpm without the propellers, and it is the maximum rpm it can attain at no load.

The rpm sharply decreases when a propeller is mounted because of air resistance offered to the motor while spinning the propeller.

2) Torque

Moving on the next factor that comes into the picture is the torque produced by the motor. Torque is the spinning force or the rotatory force that spins the propeller. It doesn’t matter much if you’re starting.

Motor torque is affected by factors such as:

1. Stator size- the bigger the stator higher the torque

2. Materials such as quality of magnets and copper windings in the rotor

3. Motor construction factors such as air gaps between stator and rotor.

The torque produced by the motor significantly affects the performance of the quad. It also determines how the quad is going to feel for the inputs. Higher the torque produced by the motor, the more responsive the quad will perform.

Torque also governs how fast a quad changes its direction mid-flight which will greatly help to do tighter turns in a race. Relatively if a motor produces less torque and is fitted with heavier props, the motor can’t spin the propeller and resulting in reduced efficiencies and also thrust. The current draw in an over-propped motor will be significantly high.

The one major disadvantage of high torque motors is even though they feel more snappy and responsive to the controls they have bad oscillation. Since motors with high torque can change their rpm more rapidly they amplify the error (oscillation).

Oscillation is hard to get rid of in Betaflight even with PID tuning, especially on the yaw axis of the quad.

3) Efficiency

The efficiency of a motor is calculated by dividing the thrust produced by the motor at 100% throttle by the power produced by the motor.

This is measured by grams per watt (g/w). The higher this ratio, the more efficient the motor is essential. We are not going to be flying 100% throttle the whole time; therefore it is important to consider the efficiency of the motor through the whole throttle range from 0% up to 100% throttle.

Some motors may be efficient at the lower end of the throttle curve and some motors may be more efficient at the higher end of the throttle curve. Therefore it is important to choose the right motor depending on the style of your flying.

4) Current Draw

The current drawn by the motor is important because it helps us to determine the quadcopter esc size required for that particular motor.

For example, an 1104 motor draws 10A max at 100% throttle whereas some 2306 motors draw 40A max at 100% throttle. An esc must be selected accordingly for 20% more rating than the current drawn at 100% by the motor.

Ie; if a motor draws 30A max at 100% throttle, an ESC rated for 36A constant current would be ideal.

There is also known as the burst rating of an ESC. It’s the maximum amps of current the ESC can handle for a short period without damaging itself.

5) Temperature

Temperature or heat in general is a killer of motors over time. If the motors are exposed to prolonged heating, the magnets in the rotor lose their magnetic field strength over time. They demagnetize over time when exposed to constant heat and consequently resulting in a reduced lifespan of the motor.

The main reasons for a motor to heat are over-propping and using higher throttles for long periods. If you’re a pro-level drone racer, you will be running at higher throttles, but if you are just starting and the motor heats then the motor is over-propped.

But motor manufacturers compensate for this issue by designing cooling fins to help the motor to suck in air into the motor and cool itself equating to longer life spans, provided you don’t crash and destroy the motor beforehand.

Additional Factors to consider

In addition, there are some more factors that control the performance, smoothness or compatibility of a BLDC motor.

1) N and P numbers

A typical 22xx or 23xx motor will have 12 poles and 14 magnets. This number will be denoted something like 12P14N. As depicted in the diagram, the poles are present on the stator and the permanent magnets are found on the rotor of a motor.

2) Single & Multi stranded wires

Single-stranded wires as the name suggests are made of a single wire of copper whereas multi-stranded wires are made of 3 smaller wires to replace the single thicker wire. Single-stranded wires are thicker and handle the heat produced much better when used on high-voltage builds.

Multi-stranded wires break or melt easily at higher operating temperatures. Typically multi-stranded wires are more efficient because they pack the wires much tighter and closer together, therefore giving stronger magnetic fields and resulting in more powerful motors.

3) Magnets

The magnets in a motor play an important role in deciding how powerful a motor is going to be.

Cheap motors will have weaker magnets and produce less thrust as compared to an expensive motor which will have more powerful magnets.

Some higher-end motors even have curved magnets that are contoured to the shape of the rotor.

Magnets used in brushless motors are graded based on magnetic strength such as N52, and N54 etc…, the higher the stronger the magnetic field produced by the magnets.

4) Motor windings

Another factor to consider is the quality of the windings of the motor. If the motor has high-quality copper windings they’re going to offer less resistance for the flow of currents and thus offer better efficiencies and longer flight times.

5) Motor Weight

The weight of a motor is governed by the size and materials of the motor. The bigger the motor the heavier it is. Keep in mind the motors used on a 5” quad usually weigh with wires around 30-40 grams. There are some very light motors such as the Emax RSII which weigh around 25 grams for a 2306 motor with a couple of grams for the wires.

There are some very heavy motors such as the cobra 2204 motors which weigh around 34 grams. As they say, every gram counts, especially with the motors. Why? Because the moment the arm increases the heavier the motor gets. Simply put it takes a significantly large amount of force to turn a quad with an increase in every gram.

But that doesn’t mean lighter is better. Lighter motors are not as durable as heavier ones because they would be made of lighter materials to save weight. Hence it all boils down to what are you going to use the motor for.

6) Motor mounting patterns

The mounting patterns of a motor also matter because they should be compatible with all the frames you choose to put the motor in. Most 5” quads these days use motor sizes from 2205 to 2407.

All the motors either have (16×16) mm or (16×19) mm mounting patterns. All the modern frames support all those mounting patterns and this shouldn’t be much of a concern.

The above image shows a brushless motor in a quad. The screw holes will be present on the stator of the motor. The above-shown motor or the stator has a 16×19 mm mounting pattern and uses 4 m3 screws for securing the motor to the frame.

Measurements of Motor Performance

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 can consider the following factors:

• Thrust
• Efficiency and Current Draw
• Weight

The decision here really depends on your application, flying style, and how you want your aircraft to perform.

Thrust

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 to ensure acceptable flight time.

While thrust is an important aspect in selecting a motor, it’s not the only thing to consider.

Motor Weight

The weight of a motor often gets overlooked, which can be 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, the motors must work harder and requires more torque (not just thrust) 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 slow down. That’s why it feels less responsive. If your flying involves more fast changes in direction (e.g. freestyle and racing), motor weight matters more than just cruising in a straight line (cinematic cruisers).

Efficiency and Current Draw

Motor efficiency is typically calculated by dividing thrust by power at 100% throttle, measured in grams per watt (g/w). The higher this number is, the more efficient the motor is.

It’s important to look at efficiency through the whole throttle range, not just the top end. Some motors might be efficient at the lower throttle but could lose efficiency by drawing increasingly higher currents as they approach their limits.

Another good way to look at efficiency is to use “grams per 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.

Every motor responds differently to different propellers, carefully choosing your propeller is the key to balancing thrust and efficiency.

Efficiency and current draw affect your choice of battery. An efficient motor with a large current draw might abuse your battery and cause voltage sags.

Advanced Motors Performance Factors

Many quadcopter motor properties are not mentioned by the manufacturers and can only be found through more technical testing.

• Torque
• Response Time
• Temperature
• Vibration and balance

Motor Torque

Torque is the force that turns the propeller, it determines how fast a motor can increase and decrease RPM. In other words, 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 a 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 can change RPM so rapidly that it can amplify error (in PID loop), causing oscillation that can be hard to eliminate even with PID tuning, especially on the yaw axis.

Torque is directly affected by stator size, bigger stator size = more torque. Other factors that can increase torque are:

• Stronger magnets
• Minimizing the air gap between permanent magnets and stator, such as using arc magnets
• Thinner stator laminations

Another benefit of high torque is higher tolerance to propeller weight (thus runs better with a wider range of props). But you should benefit from using lighter props as RPM changes quicker.

Response Time

Motor Response Time is also dependent on torque, high torque motors often have a faster response time. One easy way to measure response time is to see how long it takes for a motor to reach maximum RPM from 0.

Response time will be affected largely by the weight and pitch of your propeller choice. Remember that atmospheric conditions can have an effect too. At low altitudes, for example, the air is thicker, which means that there is a greater number of air molecules that the propeller must physically move, to produce thrust. At high altitudes, your props will spin faster and react quickly to changes in throttle, but the overall thrust will be reduced because there are fewer air molecules for the prop to interact with.

Temperature

Temperature affects brushless motors because the magnets used in our motors have a weaker magnetic field when operating at high temperatures, 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 that aid cooling often equates to a longer lifespan. That is, of course, provided you don’t destroy it in a crash beforehand!

Vibration

Vibration caused by the motors can have several unpleasant side effects on 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 ESCs.

Many have successfully soft-mounted motors, and the flight controller to reduce vibration, with some positive results.

Remember that damaged, bent and unbalanced propellers can also cause problematic vibrations.

FEATURES TO LOOK FOR IN MOTORS

Motor Shaft

Newer motors nowadays use hollow shafts as opposed to solid shafts in an attempt to reduce the weight of the motor. This has its positives and negatives.

Hollow shafts reduce the weight of the motor but they’re less durable during crashes.

You can’t replace shafts without replacing the whole rotor of the motor. For budget builders, hollow shafts are a bad thing but for those looking to save every gram hollow shaft are the way to go.

Air gap

Another thing for a better-performing motor is the air gap between the stator and the rotor. The closer the rotor is to the stator (magnets to the windings) the more efficient it is in converting the current. The smaller the air gap the higher the thrust that the motor produces as the stator slices through magnetic fields better.

Motor wires

The next thing that affects the performance of a motor is the wire gauge of the motor. The motors either use 20 AWG or 18Awg (American wire gauge). The Emax RS series claim that changing from a 22 gauge to a 20 gauge wire increased the power output by 5%. But this is no big deal when buying a motor and getting started in the hobby.

Retainer clips

The next thing we are going to talk about is the retainer clips or retaining methods of the stator and rotor. There are mainly 3 types we use in our hobby.

• E clip
• C clip
• Screw retainers

Each has its advantages and disadvantages.

For example E clips are difficult to remove without breaking the clip itself. Screw retainers are easily removable and hence giving easy access to the stator and rotor.

But screw retainers are prone to unscrewing and loosening over time under the constant vibration of the motor and also run the risk of over-tightening the shaft and making it harder for the motor to spin. One clip can’t be recommended over the other. It all depends on what the motor is going to be used for and its applications.

CW CCW motors

There are 2 types of motors- clockwise (CW) and counterclockwise (CCW) rotation motors. They vary only by the direction by which they rotate with the rest of the design parameters of the motor being the same. The below diagram shows a motor orientation for a quad, hex, and octa drone. We can conclude that opposite-side motors spin in the same direction. It is the same in hex and octal drones.

Naked bottom OR Closed bottom motors

The latest trend in quad motors is naked bottom motors. They save a lot of weight (2g in general), it may not seem much but that’s a lot in terms of drone racing. It may be the difference between winning a race or losing one.

Those are the pros of the naked bottom motors, there are also a few cons for these types of motors.

Firstly during a crash, small stones and debris may get inside the bell and damage the magnets and coils. Even pro racers crash often so it’s a good choice for beginners to buy closed-bottom motors when starting in the hobby because they’re prone to crash more. The below photographs depict naked and closed bottom motors.

Knowing the Total Weight and Frame Size

The total weight of the quadcopter should include all the components: frame, FC, ESC, motors, propellers, RX, VTX, antenna, ESCs, LiPo battery, GoPro, and so on.

It doesn’t have to be 100% accurate, an estimation would be fine. It’s better to overestimate the weight and have extra power than underestimate and struggle to take off. Adding 10-20 grams to compensate for the electrical wires, buzzer, zip ties, etc is also a good idea.

By knowing the frame size, you can determine the maximum propeller size allowed. Further Reading: How to choose propellers for mini quad.

Calculating Thrust Required

With the estimated total weight of the craft, you can work out the minimum amount of thrust required for the motor and propeller combination to produce.

A general rule is that the maximum thrust produced by all motors is at least double of the total weight of the quadcopter. If the thrust is too little, the copter will not respond well to your control, it might even struggle to lift 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 thrust-to-weight ratio (or power-to-weight ratio) to be much higher than the example above. It’s not uncommon to see 10:1, or even 13:1 thrust-to-weight ratios. Generally speaking, for acro flying, I recommend having at least 5:1.

With a higher thrust-to-weight ratio, a quadcopter will have greater agility and acceleration, but it might become harder to control as well. Just a little touch of throttle will be enough to “shoot the quad into orbit like a rocket” 😀 Of course, this depends on piloting skills too.

Even if you only just planned to fly a slow and stable aerial photography rig, you should aim at somewhere between 3:1 and 4:1 ratios. This not only gives you better control but also provides room for extra payload in the future.

Connecting Brushless Motor

You need an ESC (electronic speed controller) to drive a brushless motor. Unlike brushed motors which only have two wires, there are three wires in a brushless motor, and you can connect these wires to the ESC in any order. Simply swapping two of the three wires will reverse the rotation direction. It’s also possible to reverse motor direction in software.

Motor Torque

A high torque motor allows for a more rapid change of RPM and faster response time, you will get less prop wash oscillation and it will give you that instant and snappy response.

The torque of a motor is determined by many factors, for example:

• Stator size (volume)
• Materials: the type of magnets, quality of the copper windings
• Motor construction: such as air gap, number of poles, and so on

But since motors in the FPV industry are manufactured with pretty similar specifications and designs in recent years, stator size is the easiest way to quantify the torque of a motor.

Stator size can be calculated the same as the volume of a cylinder:

volume = pi * radius^2 * height

For example, for a 2207 motor, the stator volume would be

pi x (22/2)^2 x 7 = 2660.93

The bigger the stator volume, the more torque a motor can generate.

Now let’s take a look at a 2306 motor, the volume would be 2492.85, which means a 2207 motor has more torque than 2306.

So when choosing a motor, what you want to do is to compare their motor stator volume, as well as the motor weight. A lighter motor with the same volume would be preferred, assuming other factors are equal.

So why don’t we just pick the biggest motor available? The answer is weight. Motors with bigger stator volumes are also heavier, so it depends on the application.

For example, for a lightweight drone, doesn’t need that much throttle to stay in the air, this gives them more left-over torque. Paired with lighter-pitch propellers, the motors can spin them with less torque. So the motor torque requirement is low in this case, and you can probably get away with lighter/smaller motors which also keep all up weight down.

The only time you don’t want a motor that is too powerful (too much torque) is when you prioritize “smoothness” over responsiveness. That’s because high torque motors are capable of changing RPM so fast, they can feel jerky and less smooth sometimes. It also creates more voltage spikes and electrical noise to the power system, which could affect gyro performance and overall flight performance if noise filtering isn’t optimal in your drone, contributing to oscillations.

KV

“KV” is how many revolutions per minute (rpm) a motor turns when 1V (one volt) is applied with no load (e.g. propeller) attached to that motor. Here is a more academic explanation of KV. For example, when powering a 2300KV motor with a 3S LiPo battery (12.6V), it will spin at around 28980 RPM without propellers mounted (2300 x 12.6). Typically KV is just a rough estimation specified by the motor manufacturer.

Once a propeller is mounted on the motor, the RPM will decrease drastically due to air resistance. Higher KV motors would attempt to spin the propeller faster, and produce more thrust and power (while drawing more current). That’s why we tend to see larger props paired with low KV motors, while smaller and lighter props are better suited for high KV motors.

The KV of a motor can be determined by the number of copper wire windings in the stator. Generally, the higher number of turns of winding decreases the KV of the motor, while lower number of turns increases the KV. The magnetic strength of the magnets can also affect the KV value, stronger magnets will increase KV.

By pairing a high KV motor with an excessively large propeller, the motor will attempt to spin fast as it would with a smaller prop, but this will require more torque. As it tries to produce the required torque it will draw more current and subsequently generate too much heat. This will eventually lead to overheating and it can burn out the motor – when the motor overheats, the coating on the coil will start melting and causing electrical shorts inside the motor.

That’s why a higher KV motor is likely to run hotter than a lower KV one of the same motor size.

KV affects the current and voltage limits of the motor too. As explained, higher KV motors have shorter windings and thus lower resistance. It lowers the maximum voltage rating and increases the current draw for the motor and propeller combo. Anyway, when you buy a motor, it should specify what voltage you are allowed to use, and what the maximum current it can take on the product page.

There is a “Motor Output” limit in Betaflight that allows you to reduce the motor signal and use higher voltage batteries, e.g. you could fly 4S motors on a 6S battery.  This might be a workaround but it’s more likely to blow your ESC with high KV motors. The way the MOSFET in ESC works is by switching on and off, so the output voltage is always either the battery voltage or 0V. By limiting the motor output you are just setting a limit to how long you keep the MOSFET switched on, but you are still exposing the same higher voltage to the motor, and it’s more likely to cause issues than a lower KV motor that is rated properly to that higher voltage. It’s recommended to get the right KV motors for the battery voltage you plan to use.

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KV Vs Torque Constant

While motor KV does not have a direct effect on torque, it does torque constant. The torque constant of a motor defines how much current it costs to create torque. KV doesn’t affect how much torque can be created. There are factors like magnet strength, air gap, and coil resistance that have a much more significant impact on torque production.

Higher KV motors have higher torque constant, which means they require more current to generate the same amount of torque compared to a lower KV motor. To generate the same amount of torque, the higher KV motor would require more current, resulting in extra losses in the ESC, battery, and wires. Even worse, there would be more heat building up in the motor due to the higher current, making it harder to generate magnetic flux. Overall the higher KV motor is less efficient if you were to fly at the same speed as the lower KV motor.

Therefore it’s wise not to go crazy on KV, try to keep it moderate. It’s especially important if you are building a long-range drone that prioritizes on efficiency and flight time.

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Mounting Holes

Common mounting patterns (hole distance) for 22xx, 23xx, and 24xx motors are 16x19mm and 16x16mm. Modern 5″ racing drone frames should support both patterns. The mounting holes of these motors use M3 screws.  Use screws with a thread length 2mm longer than the thickness of the arms, for example, for 3mm arms, I use 5mm screws; for 4mm arms, I use 6mm screws.

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Poles and Magnets

You might have seen specifications such as “12N14P” printed on the box of a motor. The number before the letter N means the number of electromagnets in the stator, i.e. poles, and the number before P means the number of permanent magnets in the bell.

Different sizes of motors have different numbers of poles, 22XX and 23XX motors generally have 12 poles and 14 magnets.

The number of poles determines the spacing between the poles if you have fewer poles, you can fill in more iron content in the stator, so you get more power out of the motor. But with a higher number of poles, the magnetic field is spreads out more evenly, and therefore you have a smoother running motor because you have more fine control over the rotation of the bell.

• More poles = Smoother
• Fewer poles = More powerful

The pole configuration has to be a multiple of 3 because it’s a 3-phase motor and there are 3 wires in the motor, therefore the pole numbers have to be 9, 12, 15, 18, etc. That’s why the pole number is not easily changed, and thus it’s not an essential piece of information when picking motors, especially for mini quad.

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Motor Windings

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 in 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 without sacrificing efficiency and torque. The motor would also be able to handle the high current without burning out with a larger wire gauge.

However thicker wires and more windings mean a heavier motor, and the winding takes up more physical space so it requires a larger stator. That’s why we are seeing bigger and heavier motors, and that’s also why bigger motors are generally more powerful.

Multi-Stranded vs. Single-Stranded Windings

Single-stranded windings are thicker, therefore manage heat better and are better suited for those who run higher voltages like 5S or 6S. But you cannot pack as many wires around the stator because the gaps are larger between the thicker wires.

Multi-stranded windings use 3 smaller wires to replace the 1 thicker wire in single-stranded windings. Due to the thinner wires, they don’t carry as much heat and they will break easier physically.

But generally, multi-stranded windings provide better performance than single-stranded windings because you can pack the wires more tightly around the stator thanks to the smaller gaps between wires, and this will give you a stronger magnetic field and a more powerful and efficient motor.

Note that the neatness of winding is also important, not only aesthetically, but also electrically. If the winding is messy and has a lot of wire crossings, the wires don’t cross the stator perpendicularly and the resulting magnetic field will be less efficient.

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Bearing

Motor bearing isn’t talked about a lot because there isn’t much info available, but I thought I should give you a basic introduction anyway.

The size of the bearing is not the outer diameter or the inner diameter, but the difference between the outer and inner diameter. The wider it is the larger the marbles/balls can fit inside it. Larger balls can take more abuse to break, and hold up better to a crash. But smaller balls are more stable and smoother at high speed/RPM.

There are motors marketed as using “Ceramic Bearings” – they use ceramic balls instead of steel balls, and they are indeed smoother, but easier to break.

The diameter of the hole in the bearing (inner diameter) also determines how big a shaft you can use.

9mm x 4mm is a good balance for durability and smoothness.

Popular bearings used in FPV drone motors are Japanese NSK, NMB, EZO, etc. While the EZO bearings are hyped up to be the best, it’s difficult to measure how much better it is for others. Not to mention you don’t know if the manufacturer is using the genuine thing or just cheap clones.

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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 with a higher voltage, thus drawing a higher 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 motor and prop combination, you are now ready to choose ESC for your drone.

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Features in FPV Drone Motors

There are so many variables that affect the performance of a motor, it can get very controversial and complicated. For example, for motors with the same stator size and KV, you can have very different thrust, current draw, and response times 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 contribute to better performance, which can also change the characteristics of the motor.

Motor Shaft

To mount a propeller, you put a hole in the center through the motor shaft. Most brushless motors for 3″, 4″, 5″ and 6″ propellers have M5 shafts (5mm diameter).

The construction of motor shafts is changing over time. It used to be a solid aluminum rod, later manufacturers start using a hollow shaft with titanium. Similar in weight but is much stiffer and harder to bend. However, drilling the hole in the middle of the titanium shaft increases the cost of manufacturing significantly.

And more recently they’ve come up with a new motor shaft design, by inserting a steel rod in the hollow shaft for extra strength.

Magnet Type

Magnets used in brushless motors are graded according to their magnetic field strength, such as N50, N52, N54, etc. The higher the number, the stronger the magnetic field. For example, N52SH will be better than N50SH.

A stronger magnetic field is theoretically capable of generating power more efficiently, providing more torque and a faster motor response time.

When you turn a motor by hand, you can feel the notches, the stronger you can feel them is a bad thing because it tells you how strong the magnetic force is, and how weak it is in between magnets, which tells you the magnetic field is not even. Weaker notches usually mean a smoother motor.

Magnets will lose magnetic strength when they reach a certain temperature, therefore N52H is used to prevent this problem. The letter at the end has to do with operating temperature. It’s said that N52SH can withstand even higher temperatures, but there is no data at the moment to indicate how much better N52SH is compared to N52H and N52.

Magnets may get loose in crashes or due to vibration. You can glue it back in the bell using Loctite 438.

Arc Magnets

Using arc magnets (aka curved magnets) is a technique to bring the magnets closer to the stator; allowing for a smaller and more consistent air gap (We will explain what air gap is later).

The way a permanent magnetic field works mean 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 (nonarc) magnets.

The ‘epicenter’ of the field of the pole on the outside of the curve, will be below the surface of the magnet, and the epicenter of the pole on the inner curve will be above the surface. In this manner, the magnetic fields of the permanent and electromagnets are 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 the magnet, often finding that a slightly thinner magnet (therefore a weaker magnetic field) can provide better results.

Air Gap

“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, but it also improves torque and response. The downside of tighter air gap is the increase in current draw and decrease in efficiency. Also, there is concern regarding durability, if the motor bell takes any sort of impact and it gets out of alignment and shifted at, the magnet can run into the stator and end up getting shattered.

Stator Laminations

A lamination is the thickness of the individual sheets of metal stacked up in the motor stator, thinner lamination allows you to stack more layers of stator plates for the same height of the motor stator.

In a nutshell, the thinner the stator lamination, the better. Laminations help to reduce a phenomenon known as Eddy Current, which generates heat in a changing magnetic environment. Thinner laminations mean less power is wasted on generating the eddy currents (leads to an undesired magnetic field) and making motors more powerful and efficient.

C-Clip / Shaft Screw

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 its pros and cons, and it’s hard to say which one is the best.

C-Clip vs. Screw on the bottom of a Motor Shaft

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 the 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.

Aluminium Alloy

The metal used for the motor bell and motor base determines the durability of the motor. There are two common types of aluminum alloy used in FPV motors: 7075 and 6082. The number designates the different series of aluminum alloy grades and chemical compositions.

In a nutshell, 6082 has more ductility and is more formable while 7075 is more rigid and holds up better against crashes. 6082 is used back in the days before 2016/2017, but 7075 is the most common in modern motors and is thought to be stronger against impact.

Unibell

Unibell (unibody top bell) is a preferred moto bell design, which is in one piece of metal instead of separate pieces of metal glued together. The main benefit of unable is better durability, but it costs more to manufacture. Non-unable could split into pieces in a hard crash and it’s irreparable and the damaged motor or bell needs replacing in most cases.

Bottom Design

In the 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 base, however, the “naked bottom” tend to be lighter by removing the excess material, and the weight saving is around 2g.

Closed base motors are less likely to get dirt trapped inside the bell, against the argument that, the naked bottom is easier to clean the dirt out.

With naked bottom, you can see clearly how far the 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.)

Naked bottom motors are easy to get dirt inside the motor, but it’s also easier to clean

However, the closed bottom provides better strain relief to the wires in case of crashing and stretching.

Flux Ring Design

A flux ring is the round steel ring inside the bell that contains the magnets. The bell is usually made of aluminum, while the flux ring is made of steel because it has to respond to magnetic field lines.

The latest flux ring design is a custom shape instead of the usual round shape, which can help direct more magnetic field lines back into the motor and improve the torque.

PoPo Technology

The “Pop on Pop off” system is a motor shaft with a spring-loaded bearing for installing and removing props quickly. For a more detailed overview and product, list checks out this article.

Other Features

• Soldering tabs
• 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 integrating ESC inside the motor. 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. They should also be easily repairable if the wires get pulled off, which can often spell the end of a motor of typical design.

CW and CCW Drone Motors

You will rarely see brushless motors labeled as CW (clockwise) and CCW (counterclockwise).

This does not indicate the direction in the motor spins. Brushless motors can spin either direction. This label differentiates the direction that the motor bolt is threaded. This is done so that as the motor spins, the torque from the propeller pushes the motor nut to tighten rather than loosen. This keeps your props from loosening and coming off while you fly. This means you will need two of each for your 4 motor layout in standard Betaflight rotation.

• Front Left: CW
• Front Right: CCW
• Back Left: CCW
• Back Right: CW

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 nut (or more commonly in the hardware jargon, a ‘left-hand thread nut’). Prop nuts these days are lock nuts (have rubber inside), they stay on relatively well when tightened down and don’t get loose easily.

Balancing FPV Drone Motors

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 e.g. 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.

Conclusion

This article has tried to cover most of the basic aspects of a brushless motor for a quad. It will be modified in the future with some modifications to it as and when it is deemed necessary. There are so many options out there to choose from and we thought it should be covered in another article. We hope this article helped you in some way to help you gain some basic knowledge getting into FPV. Thank you for reading if you stayed to the end.

In this article we are trying to cover what an ESC is, the terminologies relating to it, its functions, and the factors to consider when buying one. ESC stands for Electronic speed controller, connecting the FC and the motor. Basically, they are controlling the speed of the motors. 

ESC is just like a gearbox in a car, the gearbox tells the wheels at which speed it must rotate, in the same way, an ESC controls the speed at which the motor must rotate for the throttle applied. This throttle signal is provided by the flight controller to the ESC spinning the motor.

Most modern ESCs are so advanced and filled with features they put the older generation ESCs to shame. Without wasting our time let’s get a look into the world of an ESC

QUADCOPTER ESC MOTOR WIRING

A Brushless ESC has 3 wires coming out from it which directly plug or get soldered onto the 3 wires coming from the motor.

As the diagram illustrates, connecting any 3 wires will make the motor spin. But the direction in which the motor rotates depends on the order in which the wires are connected. Matching the 3 wires from the top to bottom will make the motor spin clockwise and swapping any 2 wires will make the motor spin in the anti-clockwise direction.

ESC FIRMWARE

BLheli, SimonK and KISS

There is 3 major firmware that most of the ESCs run- BLheli, SimonK, and KISS. KISS is a closed-source ESC which means that the KISS firmware is exclusive to KISS ESC whereas BLheli and Simonk are open-source. Since Simonk is an outdated firmware and has become so obsolete that it is not used anymore. But some airplane ESCs still use this firmware. So the popular choice nowadays is BLheli firmware, as it is a feature-rich and user-friendly interface.

BLHeli_S vs BLHeli_32

BLHeli_S is the second generation of BLHeli firmware developed for ESCs. They are 8-bit processors and have a simpler interface as compared to previous generations.

BLHeli_S delivers a smoother response curve thanks to the hardware PWM. It also has a small step resolution varying between 512 and 2048 steps. It has a signal response delay of 1-2ms.

BLHeli_32 is the third generation and the latest firmware written for ESCs with 32-bit MCU (microcontroller unit) and has gone closed source since its release in late 2016.

These 32-bit ESCs have more processing power and have increased processing power than their older 8-bit counterparts. With this increased processing power, faster input signals and much lower latencies can be achieved.

32-bit ESCs are more future-proof than their 8-bit previous generations as developments for even faster protocols are being tested and under development. These ESCs now include a current sensor where you can monitor your current consumption of the individual ESCs during flight.

There is a downside for all these extra features since it is a closed source the ESC manufacturers have to pay a license fee for the developers at BLHeli and this means an increase in the cost of the ESCs themselves.

ESC PROTOCOLS

Protocols are like the OS (Operating systems) in the world of ESCs. They determine how fast the ESC and FC (flight controller) can communicate with each other which plays a major role in the handling and performance of a quadcopter.

Some of the older PWM protocols had a delay of up to 2ms as compared to the average blink of a human eye of 100ms. While some latest Dshot and Multishot protocols have greatly reduced the latency to just about 5-25µs.

A list of ESC protocols from slowest to fastest is shown below with their average or approximate latency

Protocol	Latency
Standard PWM	1000-2000 µs
Oneshot125	125-250 µs
Dshot150	106.7 µs
Oneshot42	84 µs
Dshot300	53.3 µs
Dshot600	26.7 µs
Multishot	15-25 µs
Dshot1200	13 µs

As seen above it is evident that Dshot 1200 has the lowest latency. Even though they are barely noticeable to humans it is noticeable in terms of machines. Some of the advantages of the system being capable of running Dshot1200 are more accurate data at higher resolution, data error rejection, higher speed, and lower latency.

Dshot600 is still the most popular choice and is widely available. If asked if can one feel a difference between an ESC with Dshot600 and Dshot1200 while flying, the answer is NO. Technically yes, Dshot1200 is better but if you want to run 32K/32K loop time if you have no other choice but to pick Dshot1200.

FACTORS TO CONSIDER

Individual and 4in1 ESC

As the title suggests there are 2 types of ESCs for quadcopters- single or individual ESCs and 4-in-1 ESCs. 

Individual ESC

4 in-1 ESC

4in1 ESCs are 4 individual ESCs soldered together and stacked below an FC to save some complexity in wiring and a little bit of weight too.

With advancements in technology, 4in1 ESCs are getting more and more reliable. But the downside of using a 4in1 ESC is that if you burn out one ESC, the whole board becomes useless.

But that is not the case when using an individual ESC. If you burn one out, you just replace that ESC. Cost wise using individual ESC is much more cost effective in the longer run.

Replacing a 4in1 ESC at $50 is not more cost-effective than replacing a single $15 ESC. So it all boils down to personal preference. If you are reading this guide you are most definitely a beginner then the weight gained from using a lighter ESC cannot be justified.

We would recommend you get individual ESCs to start with as you will be crashing more and replacement ESCs cost much cheaper and is economical too.

Weight and Size

The weight and size of an ESC are dependent on the current rating of the ESC. Most ESCs on the market today have more or less similar dimensions and weights ranging between 4-6g each. It’s challenging to make ESCs any lighter without having to lose performance and effective cooling.

Though lighter is better for racing it is not wise to bargain on one of the important components of a quad. Smaller ESCs heat up quickly and cannot be effectively cooled without constant airflow over the ESC. Even though the smaller ESCs carry heat sinks, they are not enough in most cases.

Voltage and Current Ratings

The current rating of the ESC should be decided after selecting a suitable motor size. The maximum current draw of the motor at 100% throttle helps us select a suitable current rating. If a 2207-sized motor draws 40A at full throttle, these are at ground conditions.

The current draw is approximately 20-25% lower than at ground conditions because of the effective cooling of moving air of both the ESC and the motor. So if a motor draws 40A at the ground, it will draw 32A in the air at full throttle. Also, you will not be at full throttle the whole time you are flying, this is why ESCs have something called a burst rating.

What this burst rating means is that it is the maximum amount of current the ESC can handle for a small amount of time without damaging the ESC itself. For example, if an ESC is rated at 35A, it will have something like a 50A burst current rating. But this burst current rating mostly depends on the quality of the components used on the ESC and is different for different ESCs.

The voltage rating of an ESC is the maximum amount of voltage the ESC can handle. Some low-cost ESCs are rated up to 4S, but most ESCs today can handle voltages up to 6S. But this mostly depends on what battery voltage you are going to run on your quad. If you are going to run a 4S battery and are on a tight budget, you can pick up an ESC rated for 4S and save a few dollars.

You may be wondering if an ESC rated for 20A costs $10 and one rated for 30A costs $13, so why not get the 30A ESC? Of course, it’s only $12 more for all 4 ESCs. But the limiting factor here is the size and weight and it is unnecessary in my opinion. If a motor draws 18A at full throttle, then there is no way it is going to draw more instead of reduces as mentioned earlier. Hence it makes sense to buy the 20A over a 30A ESC.

External capacitors

A typical 5-inch quad draws over 100A at full throttle, this huge current draw strains the battery and causes something known as a voltage spike. Voltage spikes do not affect smaller quads because the batteries are not strained much and can handle the lower current draw.

A voltage spike is a phenomenon where the voltage of the battery increases exponentially due to the huge current draw. This spike can be reduced by using a capacitor in a quad. The capacitor is soldered on the battery leads of the quad and it filters out most of the voltage spikes.

The most commonly used capacitors are known as low ESR capacitors. These capacitors lower the voltage spikes drastically. You can feel the difference with and without a capacitor. Though ESC manufacturers try to integrate capacitors into the ESCs, they usually don’t do a great job of reducing these spikes. Hence using a capacitor on a build is a good practice.

Typical examples of these types of capacitors

With or without BEC (opto ESCs)

BEC stands for battery elimination circuit. The function of a BEC is to provide constant current at a specific voltage. Airplane ESC usually has BEC as they provide power for the plane’s needs like powering the electronics.

But in the world of quads, we don’t need ESCs with BEC as the power necessities like powering a VTx or powering a camera are taken care of by a dedicated PDB (power distribution board) or PDB integrated into the FCs. The ESCs lacking a BEC tend to be much less noisy, lighter, and smaller in size.

ESC AND THRUST

The thrust produced by a motor is dependent on the ESC itself, as an ESC is responsible for spinning a motor. Two different ESCs can produce different amounts of thrust with the same setup (ie.., the same motor and propeller). This is mainly because of the quality of the components used on the ESC.

The difference in the thrust produced can vary as much as 20% between a good ESC and a cheap knockoff. The thrust can also depend on factors such as build quality, quality of solder joints, etc…. Another factor that could affect an ESC is its size.

Smaller ESCs have smaller heat sinks which do not cool as efficiently as their larger counterparts and hence resulting in poorer efficiencies. A hot ESC performs poorly than a well-cooled ESC.

The quality of a solder joint also plays a role in the thrust produced as a bad solder joint may limit the amount of current flowing through it. But the latest ESCs from reputed manufacturers perform very similarly with very little difference in thrust produced.

CONCLUSION

There are hundreds of ESCs to choose from the market from dozens of different manufacturers. You really can’t go wrong with most ESC’c currently. All perform exceptionally with one doing marginally better than the other. Unless you buy an older-generation ESC you won’t have any trouble.

Hope you enjoyed reading this article and giving you some basic knowledge and insight into the basics of ESCs. HAPPY FLYING!!!