We hear a lot about electric RC airplanes these days, and there's good reason for that. Electric planes have come a long way in recent years, and flying an electric plane is an easy way to enjoy the hobby as it is much easier and cleaner than flying on glowing fuel or gasoline. I'm not saying that glow plug and gas powered planes are obsolete or that you should ditch all your glow plug or gas powered planes and go electric. I own a lot of shiny and gas planes, but the beauty of electric planes is that they offer an easy way to fly faster and cleaner than the other options.
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Before describing electric motors and discussing how they work, we need to define a few words that we are going to use so that we are all clear on what we are talking about.
- Volt– A volt is the SI unit of electrical force, the potential difference that would carry a current of one ampere against a resistance of one ohm. In simplest terms, one volt pushes electrons through an object.
- Current– An electric current is the flow rate of electric charge flowing through a given point.
- amplifier– An ampere is short for ampere, which is a measure of the amount of charge flowing through an electrical circuit over a period of time. Current is measured in amps.
- Watt– One watt equals one joule per second. It is a measure of power or energy per unit of time. It measures how much electricity is actually being used by a device. One watt is equal to one ampere under the pressure of one volt.
- Resistance– Simply put, electrical resistance is the measure of an object's resistance to the flow of electrical current. Resistance is measured in ohms.
- P = V*I– Power (Watts) = Volts times 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 engine.
- Inrunner-Motor– With an inrunner engine, the rotor or rotating part of the engine is on the inside, so you usually only see the inner shaft turning when you turn an inrunner engine. Inrunners can generally turn much faster than outrunners and are mostly seen in smaller aircraft. These can be mounted from the front of the engine as the only part of the engine that turns is the shaft.
- external rotor motor– Outrunner engines are more common in larger RC planes. With an outrunner motor, the center of the motor is stationary and the outer part rotates around the stator. These are usually lower RPM motors but produce more torque making them great for turning a propeller.
The garden hose analogy
These terms will appear frequently throughout this article, so let's try to explain these concepts. A basic understanding of this will make it easier to understand how our RC plane electric propulsion system works. It's much easier for me to understand these things when I associate them with a garden hose, so let's do some comparisons.
Volts are provided by our battery. Volt conducts electricity. Think of volts as the pressure you have at the faucet before the valve turns on. It is the stored energy that you have access to.
An ampere is a measure of current, i.e. the flow of water through the garden hose. The bigger your amp, the bigger your hose.
Resistance will be anything that slows down the flow of water, so the faucet valve, hose spout, and even garden hose size, both diameter and length. This is measured in ohms.
You will also hear about voltage drop in cables. I saw an excellent example of "water hose sinking" the other day when my wife was watering the garden. She just purchased 3- to 50-foot sections of this elastic garden hose made from surgical tubing. She turned on the hose and all 3 hoses expanded to their maximum and then she went into the garden with the spout.
As soon as she pulled the trigger on the splinter of the hose, the hose began to shrink and become shorter and the pressure coming out of the end of the hose was less. So the voltage or pressure of the water dropped due to the resistance of the garden hose and therefore there wasn't enough pressure to keep the hose fully inflated or to keep the flow out of the nozzle at the same rate. The longer the hose or power cord, the greater the voltage drop.
If she was watering with a 25-foot hose, the water flow at the end of the nozzle would be much greater than when using the 150-foot hose, and if she connected more joints together to make it longer, the flow would be at the nozzle. bigger, much smaller.
Electrical power is the product of 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 the plants, but water is measured in gallons instead of watts.
Brush motors consist of just a few parts, so let's talk a little about each one. They consist of stationary stationary magnets and permanent magnets and are mounted inside the motor housing.
The armature, also known as the windings, a commutator and, as the name suggests, a set of brushes.
Brush motors have 2 external wires that you connect to your power supply. As these wires are electrified with direct current, current flows through these brushes to the commutator.
I always had trouble remembering what the commutator did, but one day I noticed that the commutator conducts current from the brushes to the windings. It helped me a lot! But the switch is a little electrical pad on the motor that connects to the windings, and the windings create an electromagnetic field.
This field interacts with the stator magnets in the motor frame and causes the motor to rotate. As the motor rotates, the brushes touch different parts of the commutator, reversing the polarity of the windings and pulling the motor further into its rotation. It just keeps going and that's what makes those little engines turn. The problem with brushed motors is that they are not as efficient as brushless motors and do not last as long due to the physical interaction of the brushes in the commutator; So the brushes are a consumable item and the commutator can also break. This is the deepest I want to get into how brushed motors work, as many of the electric planes we're seeing today are largely brushless.
Brushless motors differ from brushed motors in both their construction 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 that. The reason brushless motors work without the use of a brush is because the armature is no longer the rotating part of the motor and therefore there is no need to electrify a moving part.
We'll talk about ESCs later, but the reason these motors can work is because the electronics have progressed to where they've become vaguely computer-controlled brushes. Let me explain and see if this makes sense. In a brushed motor, the armature had to be the electrified part, as there was no other way to reverse the polarity of the electromagnetic field to keep the motor running. If you couldn't change the polarity, the motor would simply move to the position the magnets were attracted to and not continue its rotation. As the armature and commutator rotated, the electricity supplied by the brushes was able to reverse polarity and keep the motor turning.
Brushless motors don't have this problem because the electromagnetic field is controlled by the ESC, and since the electromagnets are in the stator and the permanent magnets in the rotor, there's no need to transfer current to a moving part.
Therefore, the main limitation for brushed motors has been removed. So, no more brushes, no more commutator, no more friction between the brushes and the commutator to slow the motor down. No more sparks that could disturb the connection between the brushes and the commutator. So a brushless motor is really a simplified brushed motor, almost a brushed motor inside out. Or at least the opposite, I don't think exactly the other way around. Brushless motors last longer because the only wear part is the motor's front and rear bearings. Brushless motors come in many different sizes, and each has some specifications worth knowing. So what do all the numbers in the specs mean?
This is where it can get confusing, so let's go through it step by step. We read the engine numbers in the photo above.
The numbers on the motor above are A2212 1400kv
- A= the letters don't really denote anything specific about the engine configuration. They are usually the manufacturer's brand number or the engine serial number. They are sometimes marked with an S for a short can or an L for a long can, but this is generally not a cause for concern.
- 2212= This gets a little tricky as there is no standard for these engines. This 4 digit number is actually a series of numbers divided into 2 separate sets of 2 numbers. The first two numbers, the 22, indicate the outside diameter of the motor or the rotor diameter, and the second two numbers, the 12, indicate the height of the motor or the rotor. As there is no standard for these engines, you can only know what the numbers refer to by looking at the data sheet or the description of the engine. The datasheet for this motor tells me that it is a 28mm can or that the outside diameter is 28mm, which means that the numbers on the outside of the motor speak to the rotor diameter as the first two numbers are 22, not the engine measurements themselves. about the engine, it would start with 28 instead of 22.
- 1400 kV= The kv of a motor tells us how many times the motor turns in one minute without load when 1 volt is applied, so it is the RPM of a motor. Our motor here turns 1400 times in one minute with one volt applied. The specs on this motor state that it is designed to run on a 2 or 3 second lipo. Since a 2s lipo is 7.4 volts, this motor will spin at 10,360 rpm on a 2s battery and 15,540 rpm on a 3s 11.1 volts battery. Keep in mind that these numbers are all idle numbers, so you won't get that RPM, but it should be close enough for our purposes.
The datasheet for this one also gives us some other important information.
- Maximum efficiency= 80%. The higher the efficiency, the more efficiently the motor turns a propeller, which means less wasted energy. The less efficient a motor is, the more heat it generates, as heat is a by-product of an inefficient electrical system.
- Maximum efficiency current– 4-10A (>75% at full range)
- current capacity-12A/60S to withstand 12 A bursts for one minute.
- No charging current at 10V- 0.5A So if it is idle it will draw half an amp at 10V.
- cell number– 2-3 Lipo.
- engine dimensions– 28x30mm
- shaft diameter– 3.17 millimeters
- Weight- 47g.
Let's look at the numbers for another engine. It's called CF-2822/14 1200kv. There is a small difference between it and the last engine we talked about. There's an extra number there. So let's go step by step.
- CF- doesn't really tell us much
- 2822– tells us that the diameter of the motor or rotor is 28mm, and the 22 tells us the height of the motor or rotor. As we don't have the technical sheet of the engine in front of us, we can measure the can to find out what the numbers refer to. It measures 28.54mm, so these numbers refer to the can size or actual engine diameter measurement.
- /14– The next two numbers were not on the first engine we talked about. The /14 refers to the number of revolutions of the engine. The greater the number of turns, the lower the KV of a motor. Turns refer to the number of times the copper wire has been physically wound around the motor stator.
- 1200 kV– This is the RPM/volt of the motor, so for each volt applied, the motor will rotate 1200 rpm without load.
So, as stated, higher RPM motors have lower KVs but have more torque. This allows them to rotate larger props. As the physical size of the motor increases, the KV of a motor tends to decrease due to its design. But that's ok because bigger and lower KV motors run on higher voltage batteries so the overall power goes up even though the KV is lower.
Let's look at a final engine.
Sometimes an engine will not give many specs but will instead be marked with the size of the nitro engine it is designed to replace, such as the .10 Super Tiger below which is designed to replace a .10 size nitro engine.
How to distinguish a brushed motor from a brushless motor?
If you have a motor lying around or you see one and you're not sure if it's brushed or brushless, the easiest way to tell is 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 connect a brushless motor and it spins backwards?
One last thing about a brushless motor. To change the speed of a brushless motor, just change two of the three wires that go from the speed controller to the motor. So if you plug in the motor and it spins backwards when it first starts up, don't worry. Just swap two of the wires and that's it!
Electronic speed controls or ESCs
Now that we know everything 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 direct current or DC power from the battery and convert it into the three-phase AC power that the motor needs to run. The terms ESC, cruise control and electronic cruise control are interchangeable. ESCs interpret PWM signals or pulse width modulation signals from the receiver. That is, the pulse width of the signal determines how fast the ESC turns the motor. The ESC sends current to the motors at different frequencies depending on how fast the motor needs to rotate based on the signal from the receiver. Motors are synchronous with the frequency generated by the speed controller, hence the motors are called synchronous motors.
Speed controls are pretty complicated little electronics, but we really don't need to worry about how they work, we just need to know what to look for when deciding which one we need. Most ESCs have some settings that can be programmed based on their application and programming instructions are provided with the ESC when purchased or are available from the website they came from. The programming sequence of each ESC will be different. Some are programmable from the transmitter, some have a small card that plugs in to program, some connect to a computer to program, and still others are not programmable. Sometimes there's nothing you really need to change, so no coding is needed and it works fine for what you're doing out of the box. You will need to refer to the manual or search online for specific instructions for a specific ESC.
One very important thing to know is the current, continuous and peak, that an ESC is rated for. You also need to know which battery it is designed for. You need to know if it has a BEC (Battery Elimination Circuit) and if so, its voltage and current ratings. The BEC was designed to eliminate the flight battery we use in our nitro planes to power the radio equipment and servos. If you look at the datasheet or the label, if any, you will be told what amplifiers and voltage the BEC is supplying. So in a typical wiring setup for a small electric plane, your battery will connect directly to the ESC and the motor will also connect to it. Some ESCs have a short cable with an on or off switch. This switch would be mounted in an easily accessible area on the aircraft. The last set of wires to come out of an ESC are the wires that go to your receiver, which connects to the receiver's throttle port. Battery power is routed through this wire to power the receiver and all of the aircraft's servos. If you remember from the receiver episode, we talked about how the receiver's power rail can take input voltage from one of its servo terminals, so you don't necessarily need to connect anything to the battery terminal when doing an electrical setup. However, you don't have to use the BEC to power your radio, and if you have a larger aircraft, you won't be able to use the BEC to power your radio. BECs are not rated for many amps, many of them are in the 2-3 amp range, but sometimes more. If you have a larger aircraft, you should power your radio equipment from a separate flight battery, just as you would a nitro engine. Doing it this way constructs it like a nitro plane, with the engine and fuel tank separate from the radio equipment. The fuel tank, in this case, would be the large battery that powers the electric motor. Tom's Christen Eagle is structured like this. Despite being an all-electric aircraft, it needs to charge and maintain the flight battery like any of your nitro planes. If you don't want to have a separate flight battery, you can use the so-called UBEC (Universal Battery Elimination Circuit). All battery disposal circuits are voltage regulators only; so with a UBEC you basically add a voltage regulator separate from your battery to the receiver. These separate UBECs can handle more current than those built into the ESC and can also handle more voltage, so they can typically be used with larger lipo batteries than a regular BEC in an ESC. Most BECs built into ESCs are only good up to a 3s lipo. If your ESC has a BEC and you plan on powering your receiver from a flight battery or a UBEC, you will need to cut the positive red wire that runs from the ESC to the receiver. Leave the other two wires alone. You don't want 2 positive power sources going into the electronics.
Some ESCs have brakes, and the brake can usually be turned on and off in programming. Brake is sometimes used on planes with folding propellers, e.g. B. for glider pilots who turn off their engines to glide.
ESCs for RC planes usually have a built-in safety feature that shuts off the electric motor when the battery runs low, but still powers the radio and servos through the BEC. This is known as LVC (Low Voltage Cutoff). Some ESCs are programmable and can be set to different voltages, others are factory set and cannot 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 protect your Li-Po batteries from accidental deep discharge without shutting down the entire aircraft.
Let's look at a speed controller and see what it says. It's a cheap Hobbyking ESC. It is labeled HK-30A ESC, has a big 25 printed on it with a small 30 in the index below. It says Cells 2-3s (autodetect), Max Current 30A, BEC 3A. The label doesn't say if it's a brushed or brushless motor, but we can tell by looking at it. Since it has 3 wires on the motor side, we know it is designed to run a brushless motor. If it had only 2 wires on the output, it would be for a brushed motor. Well, there's a lot of information on the label, but it's important information to understand.
- HK-30A rules-This is the model number of the ESC. Not all speed controls have a model number, so it's okay if yours doesn't have one.
- 25- Well, the big 25 printed on it tells me what its constant current is, which is 25 amps.
- 30– The index 30 , along with the maximum current information printed on it, indicates that it has a burst rating of 30 A, allowing it to withstand 30 A for short bursts of 15-20 seconds without being damaged.
- Battery– 2s or 3s, automatic detection – The label also tells us to operate with a 2 cell lipo battery or a 3 cell lipo battery and it can automatically detect which one is connected to know when to activate the low voltage cut because obviously the cut low voltage is at different voltages between 2s and 3s lipo batteries.
Is it possible to use the same speed controller for a brushless motor and a brushed motor?
We were asked if you can use the same speed controller for brushed and brushless motors. And, in general, the answer is no. Brushless motors work completely differently than brushed motors and therefore need a special speed controller to drive them. Brushed motors run on direct current or direct current. Brushless motors, on the other hand, are three-phase AC synchronous motors. I say in general terms because there are very few that support both types of firmware-based engines. Speed controllers, which can do both, are not common, but not uncommon either.
Choosing the right engine
Now that we know and understand the basics of brushless motors and ESCs, let's apply them and find out how to determine what electrical configuration you need for your aircraft.
Do you remember the old garden hose? 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 plane.
The first thing we need to figure out is the engine size. We'll get into that in a moment, but once we know the size of motor needed, we have a starting point for the rest of the setup. For the water hose analogy, let's assume we have to fill a 1 gallon bucket with water every minute to do what we need to do. Now that we know our goal, we can work backwards to figure out the rest of the equation. The next thing we would figure out is how much tap pressure do we need to do this, and remember tap pressure is battery voltage. And the last thing we need to figure out is what kind of hose we need to be able to continuously supply that water at that pressure, or how many amps our ESC needs to be. Therefore, without knowing what our ultimate goal is, we cannot choose any of the components of our system. You wouldn't use the same water hose or faucet pressure if you had to fill a 55 gallon keg in 30 seconds.
Let's talk about watts for a moment. What are watts and how are these numbers related to your wattage? Watts = voltage x amps. The higher the number, the more powerful the engine. Think of it like a car engine. Basically, to get more power out of either one, you need to be able to get more fuel through the engine to burn (more cylinders, higher displacement). Watts are a bit like that. The higher the horsepower, the more power the engine makes, but it also drains your battery, aka the fuel tank, faster, so you'll need a larger capacity battery to get a longer flight.
Okay, so let's start by choosing the right engine for our plane. To do this, we need to know a few things. Let's take my knight as an example.
The first thing we need to know is the weight of the plane. According to the manufacturer's website, the weight will be between 5.5 and 6 pounds, so we will use 6 pounds for our calculations. The next thing you want to do is figure out what kind of flies you are going to be making.
The old-fashioned rule of thumb is to stick to the following guidelines.
|Trainer||80 watts per pound|
|Sport||100 watts per pound|
|glider||80-150 watts per pound|
|3D||200 watts per pound|
|FED||200-300 watts per pound|
I want sport flying so I'll aim for 100 watts per pound. If I take 100 watts per pound and multiply that by the 6 pounds my plane must weigh, I need a motor that can produce about 600 watts. Another thing to consider is the Prop RPM. Nitro engines spin at 10,000 rpm in air.
Unfortunately, you can't just Google a 600 watt rc airplane engine and find the engine you need. After searching online I found quite a few engines so let's have a look. The first thing I found was an E-flite 4250 540kV brushless motor. Unfortunately, it didn't give me any specs on the website, so one is out. The closest I could find was a Tom Cat G46 5020-680KV outrunner. Let's take a closer look. Therefore, the website directly states that it is a comparable replacement for a 46-shine engine and has a maximum power of 900 watts. So far things look good. So let's do some math and see how this mechanism works for us. We want our propeller to spin around 10,000 rpm. Since the motor we selected is 680 kV, we know we are getting 680 rpm per volt. To find volts for the battery, we take the desired RPM and divide it by the RPM per engine volt, then 10000 divided by 680 and that brings us to 14.7 volts. A lipo 4s has 14.8 volts so it works perfectly. Now that we know that P or Power denoted as Watts = VI or Voltage multiplied by Amperes, let's figure out how many Amps the motor will consume based on the power we need. So we have our power requirement at 600 and our voltage is 14.8. So think about P = VI, and since we're looking for amps, let's switch 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 datasheet, the motor can handle 42A of continuous current, so we're fine with that so far!
Then we must choose the battery that we want to use and, for this, we must know what flight time we want to get from it. I generally like to fly about 7 minutes per flight. So, to find the battery capacity you need, take the time, multiply it by the amperage, and divide by 60. So 7 minutes at 40.5 amps divided by 60 is 4.725 amp-hours or 4725 mAh. I normally like to fly batteries with only about 80 percent of their capacity, so 4725/0.8 = 5906 mAh, so a 6000 mAh battery will fly this plane for 7 minutes. If I do a quick search for a 6000mah 4s lipo, I see one that has a continuous 25c discharge with a 50c peak. So 25 multiplied by 6 is 150 which means the pack can deliver 150A continuously and 50 multiplied by 6 is 300 therefore it can deliver 300A in short bursts. As we are only consuming 40 amps from it, this battery will work perfectly for our application.
When it comes to choosing an ESC, most of the time the engine specs will tell you which ESC amp to get. The motor's specification page will tell you the maximum current for which it is rated. In our case, the maximum current is 60A. A good rule of thumb for this is to have about 15% overload on the ESC, so 60 multiplied by 1.15 is 69, so a 70A or 80A speed controller will work fine. It's always a good idea to make the electronics a little bigger than smaller so that you have some wiggle room if needed.
To summarize what we just did, we started by determining the required power for our plane by multiplying the plane's weight by the recommended amount of watts per pound for the type of flight we were going to perform. We found a few motors to consider and then made sure the motor would work for what we needed it to by checking its kv to see if it would turn the propeller at our desired RPM. So we figured out how many cells we need for our lipo battery. We then figured out what battery capacity we needed to fly for the desired time, then selected the right speed controller to complete the setup. There's a lot to it, I know.
If you're converting a nitro-powered aircraft to electrical or EP power after following the steps we just talked about to select the correct electrical configuration for your aircraft, there really isn't much you can do to replace the engine and connected electronics. . Of course you will pull everything out of the frame that was used for the nitro engine. So, the engine comes out, the engine mount comes out, the throttle servo comes out, all the linkages come out, the fuel tank comes out, and all the fuel lines come out. When it comes to mounting your shiny, clean new electric motor to the front of your plane, you can buy a pre-made electric motor mount or make your own, which is my favorite thing to do. You need to measure the distance to make sure the propeller is the same distance from the firewall as it was when it was connected to that dirty, dirty, smelly old nitro engine. After measuring the distance from the propeller to the firewall, you can measure how long the engine mount needs to be and design one. The engine mounts aren't too hard, and as we drive electric they aren't subject to the high vibrations a nitro engine creates, so they don't need to be built as strong. You need to find a place to mount the ESC and an easily accessible place to mount the battery. Depending on the aircraft, you might want to cut a hatch somewhere. It will likely go in the same spot the fuel tank was removed from. Once you have everything organized and set up you must, and I cannot stress this enough... It is imperative that you double-check your center of gravity and make any necessary adjustments. Nitro gear weights versus electric gear will not weigh exactly the same, so the center of gravity will shift and needs to be addressed.