Electric Motors, ESCs, and picking a power system

We hear a lot about electric rc aircraft these days, and there’s good reason for it. Electric powered aircraft have come a long way in the last several years and flying an electric airplane is an easy way to enjoy the hobby since it’s so much simpler and cleaner than flying glow or gas. I’m not saying glow and gas powered aircraft are outdated or that you should get rid of all of your glow or gas airplanes and switch to electric. I own many glow and gas planes, but the beauty of electric aircraft is that it makes for an easy way to get in the air faster and cleaner than the other options.

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Definitions

Before we describe electric motors and discuss how they work, we need to define some words we’re going to use so that we’re all clear on what we’re talking about.

• Volts – a volt is the SI unit of electric force, the difference of potential that would drive one ampere of current against one ohm resistance. In simpler terms, a volt is what pushes the electrons through an object.
• Current – an electrical current is the rate of flow of the electric charge that passes through a certain point.
• Amps – an amp is short for ampere, which is the measurement of the amount of charge flowing through an electrical circuit over a period of time. Current is measured in amps.
• Watts – a watt is equivalent to one joule per second. It’s a measurement of power, or energy per unit time. It measures how much actual electricity is being consumed by a device. A watt is equal to one amp under the pressure of one volt.
• Resistance – simply put, electrical resistance is the measurement of an objects opposition to the flow of electric current. Resistance is measured in Ohms.
• P = V*I – power(watts) = volts multiplied by current (amps)
• E = P*T – Energy = Power(watts) x Time
• Stator – the stator is the stationary part of an electric motor.
• Rotor – the rotor is the rotating part of the motor.
• Inrunner motor – On an inrunner motor, the rotor, or rotating part of the motor is on the inside, so you will normally only see the inner shaft spin when you spin an inrunner motor. Inrunners can normally spin much faster than outrunners and are mostly seen in smaller aircraft. These can be mounted from the front of the motor since the only part of the motor that spins is the shaft.
• Outrunner motor – Outrunner motors are more common in bigger rc aircraft. With an outrunner motor, the center of the motor is stationary and the outside can rotates around the stator. These are generally lower revving motors, but they produce more torque which makes them great for spinning a propeller. 

The Garden Hose Analogy

These terms are going to come up a lot in this article, so let’s try to explain these concepts. Having a basic understanding of this makes it easier to understand how our rc airplane electric drive system works. It’s a lot easier for me to grasp this stuff when I relate it to a garden hose, so let’s make some comparisons.

Volts are supplied by our battery. Volts are what pushes electricity. Think of volts as the pressure you have from your spigot before the valve is turned on. It’s the stored energy you have access to.

An amp is a measurement of current, so that will be the water flow through the garden hose. The higher your amps, the bigger your hose.

Resistance is going to be anything that slows down the water flow, so your spigot valve, the hose nozzle, and even the size of your garden hose, both diameter and length. This is measured in ohms.

You’ll also hear about voltage drop across wire. I saw a prime example of “water hose voltage drop” the other day when my wife was watering her garden. She just bought 3 – 50 ft sections of that expandable garden hose, the kind made out of the surgical tubing. She turned the hose on and all 3 of the hoses expanded to their full length and then she walked out to the garden with the nozzle.

As soon as she pulled the trigger on the hose nozzle, the hose started to shrink and get shorter and the pressure coming out of the end of the hose was lower. So the voltage, or the pressure of the water dropped because of the resistance of the garden hose, and because of that there wasn’t enough pressure to keep the hose fully inflated, or to keep the flow coming from the nozzle at the same rate. The longer the hose, or the longer the electric wire, the higher the voltage drop will be.

If she was watering with a 25 ft hose, the water flow at the end of the nozzle would be much greater than if she used her 150 ft hose, and if she connected more together to make it longer, the flow at the nozzle would be much less.

Electrical power is the product of the voltage and current, and is measured in watts. Think of watts as the amount of water that comes out of the end of our garden hose to water plants, but the water is measured in gallons instead of watts.

Brushed Motors

Brushed motors only have a few parts, so let’s talk a bit about each one. They consist of stator magnets which are stationary and permanent magnets, and they are mounted inside the shell of the motor.

The armature which is also known as windings, a commutator, and as the name suggests, a set of brushes.

Brushed motors have 2 external wires that you connect to your power source. As these wires are electrified with dc current, the electricity runs through these brushes to the commutator.

I always had a problem remembering what the commutator did, but one day it hit me that the commutator communicates the electricity from the brushes to the windings. That helped me a lot! But, the commutator is a little electrical pad inside the motor that is connected to the windings, and the windings create an electromagnetic field.

This field is what interacts with the stator magnets in the motor shell, and that’s what causes the rotation of the motor. As the motor spins, the brushes make contact with different parts of the commutator, which reverses the polarity on the windings and pulls the motor farther through its rotation. This continues on and on and that’s what makes these little motors rotate. The problem with brushed motors is that they’re not as efficient as brushless, and they don’t last nearly as long because of the physical interaction of the brushes riding on the commutator; therefore brushes are a wear item, and the commutator can also go bad. That’s about as deep as I want to go into how brushed motors work because for the most part, many of the electric airplanes we deal with today are brushless.

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Brushless motors

Brushless motors are different from brushed motors, both in the way they are constructed and the way they work. Brushless outrunners will be the most common type of motor we see in the hobby so let’s talk about them. So the reason brushless motors run without the use of a brush is because the armature is no longer the spinning part of the motor, and because of that there’s no need to electrify a piece that is moving.

We’ll talk about ESCs later, but the reason these motors are able to operate is because the electronics advanced to where they became in a loose sense a computerized brush. Let me explain and let’s see if that makes any sense. In a brushed motor, the armature had to be the part that was electrified because there was no other way to reverse the polarity of the electromagnetic field to keep the motor spinning. If you were unable to switch the polarity then the motor would just move to the position that the magnets were attracted to and not continue on with it’s rotation. Since the armature and the commutator spun, the electricity delivered to it by the brushes was able to reverse polarity and continue to spin the motor.

Brushless motors don’t have that issue because the electromagnetic field is controlled by the ESC, and since the electromagnets are on the stator and the permanent magnets are on the rotor, there’s no need to transfer any electricity to a moving part.

Therefore, the biggest limitation to brushed motors was eliminated. So no more brushes, no more commutator, no more friction between the brushes and commutator to slow the motor down. No more sparks to foul up the connection between the brushes and commutator. So, a brushless motor is really a simplified brushed motor, almost an inside out brushed motor. Or at least flipped, I guess not quite inside out. Brushless motors last longer since really the only wear item is the bearings on the front and rear of the motor. Brushless motors come in many different sizes, and they each have some specs that are good to know about. So, what do all the numbers mean in the specs?

This is where it can get confusing, so let’s go through it step at a time. We’ll read the numbers from the motor that’s pictured above.

The numbers on the above motor are A2212 1400kv 

• A = the letters don’t really denote anything specific about the motor setup. They usually are a manufacturer’s brand number, or series number of the motor. Sometimes they’ll be labeled with an S for a short can or an L for a long can, but on the whole, nothing really to be concerned with.
• 2212 = This gets a little tricky since there’s not a standard for these motors. This 4 digit number is really a set of numbers that get broken down to 2 separate sets of 2 numbers. The first two numbers, the 22, tells either the exterior motor diameter or the rotor diameter, and the second two numbers, the 12, tells either the motor height or the rotor height. Since there’s no standard in these motors, the only real way to know what the numbers refer to are by looking at the spec sheet or the description of the motor. The spec sheet for this motor tells me that it is a 28mm can, or the outside diameter is 28mm, so that means the numbers written on the outside of the motor are talking about the rotor diameter since the first two numbers are 22, not the motor measurements itself. If it was talking about the motor, it would start with 28 instead of 22. 
• 1400kv = The kv of a motor tells us, under no load, how many times the motor will spin in one minute with 1 volt applied, so it’s the rpm rating of a motor. Our motor here will rotate 1400 times in one minute with one volt applied. The specs for this motor say it’s designed to be ran with a 2s or 3s lipo, so doing the math, since a 2s lipo is 7.4 volts, this motor will spin at 10,360 rpm on a 2s battery, and at 15,540 rpm on a 3s 11.1 volt battery. Remember, those numbers are all no load numbers, so you won’t get that rpm, but it should be close enough for our purposes. 

The spec sheet for this one also tells us some other important information.

• Max efficiency = 80%. The higher the efficiency, the more efficient the motor is at spinning a propeller, which means less wasted energy. The lower efficiency a motor is the more heat it produces since heat is the by-product of an inefficient electrical system. 
• Max efficiency current – 4-10A (>75% throughout that range)
• Current capacity -12A/60S, so it can handle 12 amp bursts for one minute.
• No Load current at 10V – .5A So when it’s just sitting idle, it uses half an amp at 10V.
• Number of cells – 2-3 Lipo.
• Motor dimensions – 28mm x 30mm 
• Shaft diameter – 3.17mm
• Weight – 47g. 

Let’s look at the numbers from a different motor. It’s labeled CF-2822/14 1200kv. There’s a little difference between it and the last motor we talked about. There’s an extra number in there. So, let’s take it piece by piece.

• CF – doesn’t really tell us much of anything
• 2822 – tells us the motor or rotor diameter is 28mm, and the 22 tells us either the height of the motor or the rotor. Since we don’t have the motor spec sheet in front of us, we can measure the can to find out what the numbers are referring to. It measures 28.54mm, so these numbers are talking about the can size, or the actual measurement of the diameter of the motor.
• /14 – The next two numbers were not on the first motor we talked about. The /14 refers to the number of turns in the motor. The higher the turn number, the lower the KV of a motor. The turns refer to the number of times the copper wire has physically been wrapped around the stator of the motor.
• 1200kv – this is the rpm/volt of the motor, so for every 1 volt applied, the motor will spin 1200rpm under no load.

So, like I said, higher turn motors have lower KVs, but they have more torque. Because of that they are able to spin larger props. Also, as the physical motor size increases, most of the time the KV of a motor decreases because of the way it’s constructed. But that’s okay, because the bigger, lower KV motors run on higher voltage batteries, so even though the KV is lower, the overall performance increases.

Let’s look at one last motor.

Sometimes a motor will not give you very many specs, but instead it will be labeled with the size nitro motor it is designed to replace, like the below Super Tigre .10, which is designed to replace a .10 sized nitro motor.

How can you tell a brushed motor from a brushless motor?

If you have a motor sitting around, or you see one and you’re not sure if it’s brushed or brushless, the easiest way to tell is by how many wires are coming out of it. Brushed rc motors have 2 wire leads, and brushless rc motors have 3 wire leads.

What if I hook up a brushless motor and it spins backwards?

One last thing on a brushless motor. To change the rotation of a brushless motor, simply swap any two of the three wires going to the motor from the speed controller. So, if you hook your motor up and it spins backwards the first time you power it up, don’t fret. Just switch two of the wires then you’re all set!

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Electronic Speed Controllers, or ESCs

So, now that we know all about our brushless motors, we need something to make them spin. The brushless electronic speed controllers, or ESCs that we all know and love take the direct current, or DC, from the battery, and convert it to the 3 phase alternating current, or AC, that the motor needs in order to spin. The terms ESC, speed controller, and electronic speed controller are all interchangeable. ESCs interpret the PWM signals, or the pulse width modulation signals from the receiver. That means the width of the signal pulse determines how fast the ESC spins the motor. The ESC sends the electricity to the motors at different frequencies based on what speed the motor needs to spin based on the signal from the receiver. The motors are in sync with the frequency being produced by the speed controller, which is why the motors are called synchronous motors.

Speed controllers are pretty complicated little electronics, but we really don’t need to concern ourselves with how they work, we just need to know what to look for when deciding on which one we need. Most speed controllers have a few settings that can be programmed based on your application, and the programming instructions will either come in the package with the ESC when you buy it, or will be available on the website where it came from. Each ESC’s programming sequence is going to be different. Some are programmable from the transmitter, some have a little card that plugs into it to program it, some connect to a computer for programming, and there are some that aren’t programmable at all. Sometimes there’s nothing really that you need to change so programming isn’t necessary and it’ll work just fine for what you’re doing straight out of the package. You’ll have to refer to your manual, or look at the manual online for specific directions for a particular ESC. 

A very important thing to know is the current, both the continuous and the peak ratings that an esc is rated for. You will also need to know what battery it is designed to take. You will need to know if it has a BEC (battery elimination circuit), and if it does, its rating for voltage and current. The BEC is designed to eliminate the flight battery that we use in our nitro airplanes to power the radio equipment and the servos. When you look at the spec sheet or the label if it has one, it will tell you the amps and voltage the BEC provides. So, in a typical wiring setup for a small electric airplane, your battery will plug directly into the ESC, and the motor also connects to it. Some ESCs have a short lead with a power switch on it to turn it on or off. That switch would be mounted in an easily accessible area on the airplane. The last set of wires that come out of an ESC are the leads that go to your receiver, which get plugged into the throttle port on the receiver. Electricity from the battery is ran through this lead to power the receiver and all of the servos in the plane. If you remember from our receivers episode, we talked about how the power bus bar on the receiver could take input voltage from any of it’s servo ports, so you don’t necessarily need to have anything plugged into the battery port if you’re running an electric setup. However, you don’t have to use the BEC to power your radio, and if you have a bigger airplane, you can’t use the BEC to power your radio. The BECs aren’t designed to handle a lot of amps, a lot of them are in the 2-3 amp range, but sometimes more. If you have a bigger airplane, you’ll want to power your radio equipment with a separate flight battery, just like you were running a nitro engine. If you do it that way, it will be set up just like a nitro airplane, with the engine and fuel tank separate from the radio gear. Fuel tank in this case would be the big battery powering the electric motor. Tom’s Christen Eagle is set up that way. Even though it’s a fully electric airplane, he has to charge and take care of the flight battery the same way he would any of his nitro planes. If you don’t want to have a separate flight battery, you could use what’s called a UBEC (universal battery elimination circuit). All battery elimination circuits are just voltage regulators, so essentially with a UBEC, you’re just adding a separate voltage regulator from your battery to the receiver. These separate UBECs can handle more current than the ones built into the ESC, and can also handle more voltage, so they can usually be used with bigger lipo batteries than you could with a normal BEC on an ESC. Most of the BECs built into the ESCs are only good up to a 3s lipo. If your ESC has a BEC, and you plan on powering your receiver with a flight battery, or a UBEC, you must cut the positive red wire that goes from your ESC to the receiver. Leave the other two wires alone. You don’t want 2 positive sources of electricity going into the electronics.

Some ESCs have brakes on them, and the brake is normally able to be switched on and off in the programming. Sometimes the brake is used on airplanes with foldable props, like for glider pilots that turn off their motors to glide. 

ESCs for RC airplanes usually have a built in safety feature that will turn off the electric motor once the battery gets too low, but it will continue to power the radio and servos through the BEC. This is called the LVC (low voltage cutoff). Some ESCs are programmable and can be set to different voltages, and some are set from the factory and can’t be programmed. It’s a better idea to fly with a timer and use this safety feature as a last resort, but it’s a nice feature to have to save your li-po batteries from accidental over discharge without turning the whole airplane off. 

Let’s take a look at a speed controller and see what it says. It is an inexpensive hobbyking ESC. It’s labeled with HK-30A ESC, has a big 25 printed on it, with a small 30 in subscript below it. It says Cells 2-3s(auto detect), Max Current 30A, BEC 3A. It does not say if it’s for a brushed or brushless motor on the label, but we can figure that out by looking at it. Since it has 3 wires on the motor side, we know it’s designed to run a brushless motor. If it only had 2 wires on the output side, it would be for a brushed motor. Now, that’s a lot of information on the label, but it’s important information to understand.

• HK-30A ESC -This is the model number of the ESC. Not all speed controllers will have a model number on them, so if yours doesn’t, it’s okay.
• 25 – Now, the big 25 printed on it is telling me what it’s constant current rating is, which is 25 amps.
• 30 – The subscript 30 , along with the max current info printed on it, tells that it has a 30 amp burst rate, so for short bursts of time, 15-20 secondsish, it can handle 30 amps without being damaged.
• Battery – 2s or 3s, auto detect -The label also tells us that it is designed to be ran on either a 2 cell lipo battery, or a 3 cell lipo battery, and it can auto detect which one is plugged in so it knows when to enable its low voltage cutoff, since the low voltage cutoff will obviously be at different voltages between 2s and 3s lipo batteries. 

Can you use the same speed controller for a brushless motor as a brushed motor? 

Something we’ve been asked is whether or not you can use the same speed controller for brushed motors as you can for brushless motors. And generally speaking, the answer is no. Brushless motors work in a completely different way than brushed motors, and because of that require a special speed controller to drive them. Brushed motors run on dc, or direct current. Brushless motors, on the other hand, are 3 phase ac synchronous motors. I say generally speaking because there are very few that can handle either type of motor based on firmware.  The speed controllers that can do both are not common, but not unheard of.

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Selecting the right motor

So now that we know and understand the basics of brushless motors and speed controllers, let’s put it to use and figure out how to figure out what electric setup you need for your airplane. 

Remember the garden hose from earlier? Let’s use that analogy again to quickly walk us through what we’re going to do to figure out what electric motor, battery, and speed controller we need for our airplane. 

First thing we need to figure is the motor size. We’ll talk about that in a minute, but once we know the size motor we need we have a starting point for the rest of the setup. For the water hose analogy, let’s say we need to fill a 1 gallon bucket with water every minute to do whatever it is that we need to do. Now that we know what our goal is, we can work backwards to figure out the rest of the equation. The next thing we would figure is how much pressure we need from the spigot to do that, and remember the spigot pressure is our voltage from our battery. And the last thing to do is figure out what kind of hose we need to get to be able to continually supply that water at that pressure, or how many amps our ESC needs to be. So, without knowing what our end goal is, we can’t pick any of the components of our system. You wouldn’t use the same water hose or pressure from the spigot if you needed to fill a 55 gallon drum in 30 seconds. 

Let’s talk watts for a minute. What are watts, and how do those numbers relate to their capability? Watts = voltage x amps. The higher the number, the more powerful the motor will be. Think of it like a car motor. To get more power out of one of those, basically, you need to be able to get more fuel through the engine to burn (more cylinders, bigger displacement). The watts are a bit like that. The higher the wattage, the more power the motor makes, but it will also drain the battery, aka your fuel tank, faster, so you’ll need a higher capacity battery in order to get a longer flight.

Okay, so, we start with picking the right motor for our airplane. To do that, we need to know a couple of things. Let’s use my Kavalier as an example. 

First thing we need to know is the weight of the airplane. According to the manufacturers website, it’s weight will be between 5.5 and 6 lbs, so we’ll use 6 pounds for our calculations. The next thing you want to do is figure what kind of flying you’re going to be doing. 

The old fashioned general rule is to stick with the following guidelines.

Trainer	80 watts per pound
Sport	100 watts per pound
Sailplane	80-150 watts per pound
3D	200 watts per pound
EDF	200-300 watts per pound

I want to fly it in a sport manner, so I’m going to aim for 100 watts per pound. Taking 100 watts per pound and multiplying it by the 6 pounds my airplane should weigh, I will need a motor capable of producing around 600 watts. Another thing to consider is prop RPM. Nitro engines give or take spin around 10000 rpm in the air. 

Unfortunately, you can’t just google a 600 watt rc airplane motor and find the motor you need. After digging around online, I found a couple motors, so let’s look at them. First thing I found was an E-flite 4250 brushless motor, 540kv. Unfortunately, it didn’t give me any specs on the website, so that one’s out. The next one I found was a Tom Cat G46 5020-680KV outrunner. Let’s look at this a little closer. So, it straight up says on the website that it’s a comparable replacement for a 46 glow engine, and it is rated at 900 watts max. So far, looking good. So, let’s do some math and see how this motor will work for us. We want our propeller to spin at around 10000 rpm. Since the motor we picked is 680kv, we know we will get 680 rpm per volt. To find volts for the battery, we will take the rpm we want, and divide it by the rpms per volt of the motor, so 10000 divided by 680, and that takes us to 14.7 volts. A 4s lipo is 14.8 volts, so that works out perfectly. Now, since we know that P, or power labeled as watts =VI or voltage multiplied by amps, let’s figure out how many amps the motor will draw based on the power we need. So, we have our wattage requirement at 600, and our voltage is 14.8. So, remember P=VI and since we’re looking for amps, we’ll switch that around to P divided by V = I, so watts divided by volts equals amps. . 600 watts divided by 14.8 Volts = 40.5 amps. According to the spec sheet, the motor can handle 42A of continuous current, so we’re good there so far! 

Next is to pick the battery we want to use, and to do that we will need to know what flight time we want to get out of it. Normally, I like to fly for about 7 minutes per flight. So, to find the capacity of the battery needed, you take the time and multiply it by the current, and divide it by 60. So, 7 minutes at let’s say 40.5 amps, divided by 60 is 4.725 amp hours, or 4725 mah. I usually like to only fly batteries to about 80 percent of their capacity, so 4725/.8 = 5906 mah, so a 6000 mah battery will fly this airplane for 7 minutes. When do a quick search for a 6000 mah 4s lipo, I see one that has a 25c continuous discharge, with a 50c peak. So, 25 multiplied by 6 is 150, meaning that pack can supply 150A continuously, and 50 multiplied by 6 is 300, so it can supply 300A in short bursts. Since we’re only pulling 40ish amps out of it, that battery will work perfectly for our application. 

When it comes to picking an ESC, most of the time your motor specs will tell you what amp ESC to get. On the motor spec page, it will give you the max current it’s designed to handle. In our case, the max current is 60A. A good rule of thumb is to have about 15 percent overhead on the ESC for that, so 60 multiplied by 1.15 is 69, so a 70A or 80A speed controller will work just fine. It’s always a good idea to go a little bigger on the electronics than smaller, that way you have some wiggle room if you need it. 

To recap what we just did, we started by figuring out the wattage needed for our airplane by multiplying the weight of the aircraft by the amount of watts per pound recommended for the type of flying we wanted to do. We found a couple motors to consider, and then we made sure the motor would work for what we needed by checking its kv to see if it would spin the propeller at our desired rpm. Then we figured out how many cells we needed for our lipo battery. Then we figured out what capacity battery we needed to fly for the amount of time we wanted, and then we picked the correct speed controller to round out the setup. There’s a lot to it, I know. 

Nitro conversions

If you’re converting a nitro powered plane to electric power, or EP, after you follow the steps we just talked about for picking the right electrical setup for your plane, there’s really not a lot that goes into switching out the motor and electronics. Obviously, you’re going to pull everything out of the frame that was used for the nitro engine. So, the engine comes off, the motor mount comes off, the throttle servo comes off, all of the linkages come off, the fuel tank comes out, and all of the plumbing for the fuel comes out. When it comes to mounting your shiny clean new electric motor to the front of your airplane, you can either buy a pre-manufactured electric motor mount, or make you own, which is what I prefer to do. You’ll need to measure the stand-off distance so you make sure the propeller sits at the same distance from the firewall as it did when it was hooked up to that dingy, dirty, smelly ole nitro motor. After you have the distance measured out from the propeller to the firewall, you can measure how long the motor mount needs to be and design one up. Motor mounts aren’t all that tricky, and since we’re going electric, they won’t be subjected to the high vibrations that a nitro engine produces, so they don’t have to be built as strong. You’ll need to find a place to mount the ESC, and a place that’s easily accessible to mount the battery. Depending on the airplane, you may want to cut a hatch somewhere. It will most likely go in the same place that the fuel tank was removed from. After you have it all laid out and put together, you have to, and I can’t stress this enough… It is imperative that you recheck the center of gravity and make any necessary adjustments. The weights of the nitro gear versus the electric gear aren’t going to weigh the exact same amount, so the cg is going to change and it has to be addressed.