How to choose ESC for quadcopter | Electronic Speed Controller
In this article we are try to cover what an ESC is, 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 controling the speed of the motors.
ESC is just like a gear box in a car, the gear box 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 ESC’s are so advanced and filled with features they put the older generation ESC’s to shame. Without wasting our time let’s get look into the world of an ESC
quadcopter esc MOTOR wiring
A Brushless ESC has 3 wires coming out from it which directly plugs or gets soldered onto the 3 wires coming from the motor.
As the diagram illustrates, connecting any 3 wires will make the motor to 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 to spin clockwise and swapping any 2 wires will make the motor to spin in anti-clockwise direction.
BLheli, SimonK and KISS
There are 3 major firmwares that most of the ESC’s run- BLheli, SimonK and KISS. KISS is a closed source ESC which means that the KISS firmware is exclusive to KISS ESC’s whereas BLheli and Simonk are open source. Since Simonk is an outdated firmware and has become so obsolete that it is not really used anymore. But some airplane ESC’s still use this firmware. So the popular choice nowadays is BLheli firmware, as it is feature rich and user friendly interface.
BLHeli_S vs BLHeli_32
BLHeli_S is the second generation of BLHeli firmware developed for ESC’s. They are 8 bit processors, have a simpler interface as compared to previous generations.
BLHeli_S delivers a smoother response curve thanks to the hardware pwm. It also has 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 of firmware written for ESC’s with 32 bit MCU (micro controller unit) and has gone closed source since their release in late 2016.
These 32 bit ESC’s have more processing power and have increased processing power than their older 8 bit counter parts. With this increased processing power, faster input signals and much lower latencies can be achieved.
32 bit ESC’s are more future proof than their 8 bit previous generations as developments for even faster protocols are being tested and under development. These ESC’s now include a current sensor where you can monitor your current consumption of the individual ESC’s during flight.
There is a downside for all these extra features, since it is closed source the ESC manufacturers have pay a license fee for the developers at BLHeli and what this means an increase in cost of the ESC’s themselves.
Protocols are like the OS (Operating systems) in the world of ESC’s. 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 upto 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.
List of ESC protocols from slowest to fastest is shown below with their average or approximate latency
As seen above it is evident that Dshot 1200 has the lowest latency. Even though they are barely noticeable by humans it is noticeable in terms of machines. Some of the advantages if the system is capable of running it of Dshot1200 are more accurate data at higher resolution, data error rejection, higher speed and lower latency.
Dshot600 is still the most popular choice and widely available. If asked 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 looptime 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 ESC’s for quadcopters- single or individual ESC’s and 4 in 1 ESC’s.
4in1 ESC’s are basically 4 individual ESC’s soldered together stacked below a FC to save some complexity in wiring and a little bit of weight too.
With advancement in technology 4in1 ESC’s 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 cost effective in the longer run.
Replacing a 4in1 ESC at $50 is not 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 ESC’s to start with as you will be crashing more and replacement ESC’s cost much cheaper and is economical too.
Weight and Size
The weight and size of an ESC is dependent on the current rating of an ESC. Most ESC’s on the market today have more or less similar dimensions and weight ranging between 4-6g each. Its challenging to make ESC’s 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 ESC’s heat up quickly and cannot be effectively cooled without constant airflow over the ESC. Even though the smaller ESC’s 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 to a 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 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 of your flying, this is why ESC’s have something called as 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 ESC’s.
The voltage rating of an ESC is the maximum amount of voltage the ESC can handle. Some low cost ESC’s are rated upto 4S, but most ESC’s today can handle voltages upto 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 on a tight budget, you can pick up an ESC rated for 4S and save a few dollars.
You may be wondering 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 ESC’s. But the limiting factor here is the size and weight and it is totally unnecessary in my opinion. If a motor draws 18A at full throttle, then there is no way it is going to draw more instead reduces as mentioned earlier. Hence it makes sense to buy the 20A over a 30A ESC.
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.
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 really feel the difference with and without a capacitor. Though ESC manufacturers try to integrate capacitors in the ESC’s, they usually don’t do a great job in reducing these spikes. Hence using a capacitor on a build is a good practice.
Typical examples for these type of capacitors
With or without BEC (opto ESC’s)
BEC stands for battery elimination circuit. The function of a BEC is to provide constant current at a specific voltage. Airplane ESC usually have BEC as they provide power for the planes needs like powering the electronics.
But in the world of quads we really don’t need ESC’s with BEC as the power necessities like powering a VTx or powering a camera is taken care of a dedicated PDB (power distribution board) or PDB integrated into the FC’s. The ESC’s lacking a BEC tend to be much less noisy, lighter and smaller in size.
ESC and Thrust
The thrust produced on a motor is dependent on the ESC itself, as an ESC is responsible for spinning a motor. Two different ESC’s can produce different amounts of thrust with the same setup (ie.., 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 knock off. The thrust can also depend on factors such as build quality, quality of solder joints etc…. Another factor that could affect an ESC is it’s size.
Smaller ESC’s have smaller heat sinks which do not cool as efficiently as their larger counter parts 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 ESC’s from reputed manufacturers perform very similarly with very little difference in thrust produced.
There are literally hundreds of ESC’s 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 over the other. Unless you buy older generation ESC’s you won’t have any trouble.
Hope you enjoyed reading this article and give you some basic knowledge an insight into the basics of an ESC’s. HAPPY FLYING!!!