Written by Greg Gimlick Selecting the Correct Motor As seen in the Winter issue of Park Pilot
This motor primer is designed to help a normal, everyday park pilot feel comfortable choosing a motor for his or her new project or replacing a motor in an existing airplane. If you can buy an exact replacement part, easy peasy, but what if you can&#;t? What if you&#;ve built or acquired an airplane with no motor? You need some basic understanding, and that&#;s why I&#;m here! Keep it simple: Longtime electric fliers sometimes muddy the waters by wanting to lay out all sorts of information regarding torque constants, winding types, armature turns, magnet types, and everything else engineers consider when designing motors. Fortunately, they don&#;t really need to know all of that stuff. They do need to know how much power is needed. For that, I like the chart from Common Sense RC (commonsenserc.com). It&#;s simple to understand and provides a good base from which to start. The sample airplane: For the purpose of this tutorial, let&#;s assume I have a simple, ready-to-fly sport model that weighs 3 pounds. I want it to be sporty and capable of solid aerobatics so I will need roughly 100 watts per pound, according to the chart. I know I want to have approximately 300 watts of power (3 pounds x 100 watts per pound = 300). This is my starting point&#;it&#;s all downhill from here. I also want to fly for roughly 6 minutes, and expect to use a common 3S LiPo battery pack found in many airplanes of this size. Step by step: The main part is figured out and I merely need to work the math a bit to get the details nailed down. It&#;s easy! Power (watts) is amps times volts. I know that the 3S pack will provide 11.1 volts under load and I want 300 watts, so I only need to know current (amps). Three hundred watts divided by 11.1 volts = 27 amps, which would be a full-throttle setting, and I know that the average current over the course of a flight is approximately 66% of the full-throttle setting. Two-thirds of the 27 amps would average 18 amps for the flight.
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This chart shows watts-per-pound recommendations, courtesy of Common Sense RC.
When figuring what pack is needed to achieve the current demands and flight time expectation, I need to do a bit more math. I don&#;t want to completely discharge the battery, so I don&#;t use 100% of the battery capacity. I want to leave roughly 20% in the pack at the end of the flight, so I&#;ll use 80% as a guideline. Packs are rated in milliamp hours, but instead of using the full 60 minutes, I only want 80% of that and will figure the requirement based on 48 minutes&#;that&#;s 60 milliamp minutes times 80% = 48 milliamp minutes. Using 60 and dividing that by 6 minutes to get the C-rate of discharge would drain the pack to zero, but by using 48 (80% of 60) and dividing it by 6, I get an 8C discharge rate, leaving 20% in the pack for safety. To choose a LiPo pack, I take the average current draw for the airplane (18 amps), divide that by the discharge rate (8C), and find that a 2,250 mAh LiPo battery pack is required to achieve a 6-minute flight time. This pack size is common and economical. Now I need to select a 300-watt motor and propeller combination that delivers what I need using the 3S 2,250 mAh LiPo pack. The Cobra motor line on the Innov8tive Designs website (innov8tivedesigns.com) offers a /12- motor that is capable of 450 watts of continuous power on a 3S pack. This is a slight overkill because it can handle 40 amps continuous current when the maximum requirement is only 27 amps, but that&#;s okay. It means I won&#;t be burning this motor up and I&#;ll have the ability to push it harder should I decide to increase the aerobatic capabilities of the airplane later or increase to a 4S battery pack. The propeller selection guide for this motor shows that an APC 9 x 4.5E propeller on 3S power will pull 28.9 amps, providing 321 watts. The goal was 27 amps and 300 watts, so this looks to be nearly perfect!
This chart shows watts-per-pound recommendations, courtesy of Common Sense RC.
Results: A motor giving everything required and then some will power the 3-pound airplane. The math was simple and the process was easy to follow. Nothing is being pushed beyond its limit and there is plenty of room for adjusting later. Other options: The Cobra /12- motor was selected, but what if I wanted to compare other options? The process is the same, but I can also reference the chart Lucien Miller has provided on the Innov8tive Designs website. In the Turnigy SK3 line is a - motor that is similar and would work too.
This Cobra motor has its name and information inscribed on the case. Cobra motors suggests a matching ESC, making it easy to put a system together.
What about Kv? Unlike what many think, Kv doesn&#;t stand for kilovolt. It refers to a velocity constant that tells how many rpm a motor turns per volt with no load. Apply 11.1 volts from a standard 3S pack and you&#;d expect the /12- motor to turn 15,429 rpm with no load. Of course, efficiency, load, and other factors play into it, but for planning purposes, that&#;s close enough. One problem new fliers run into is when an &#;expert&#; at the field tells them Kv is all they need to know. You might as well tell someone gravity is a state of mind. Kv means nothing by itself. It&#;s a useful tool as part of a total package, but not alone.
These two motors have the same Kv, but are obviously different in size, and consequently, capability. Don&#;t believe people who say you only need the Kv to select a motor.
What do all of those numbers in the name mean? Some manufacturers use the size of the stator and others use the outside dimensions of the motor case to come up with the numbers. We need a way to compare them and that&#;s where Kv and weight come in to even the playing field. With a motor such as a Cobra /12-, I know that the dimensions of the stator are 28 mm diameter and 14 mm length with a 12-turn design and 1,390 Kv. To find another motor similar to that, we might check out the Turnigy SK3 -. This is essentially the same motor, but the measurements reflect the case size in diameter and length, then the Kv. If Cobra listed its motor using this naming convention, it would be roughly -, so they are roughly the same size. Comparing the weights, the Cobra is 107 grams and the SK3 is 110 grams. With similar Kv, weight, and size, the motors are nearly the same.
These motors are disassembled to show the difference in how numbers reflect sizes. The stator (the part with wire wound around it) is much smaller than the case itself. Find out whether your motor manufacturer uses stator or case dimensions in its nomenclature.
It&#;s important to know which method a manufacturer uses to identify its motors, otherwise you end up comparing apples to oranges. If you&#;re not sure, compare the Kv and weights. If a kit calls for a certain size, find out whether the manufacturer means stator or motor case dimensions. Bottom line: Don&#;t get caught up in the &#;Kv only&#; argument. Define your requirements for the airplane and do some simple math to figure out the right motor to use. It only takes a few minutes to search manufacturers&#; websites for a match. Sites such as Innov8tive Designs provide the type of information that&#;s needed to make the right choice. If a site doesn&#;t provide the necessary information, you might want to inquire. This is a basic course to get you going successfully. Down the road we can look more closely at details.
Remember the first day you fell in love with an RC? I bet you would have appreciated the way the car performed on the floor. Later, when time made you a pro, you would have realized that sustainability of the driving motor matters more than its performance.
Now that you are here, we know that you would for sure be planning to upgrade your favourite RC's motor or build an RC customized for your unique needs from scratch. Either way choosing an RC motor can get tricky as there are a lot of parameters to be considered before you invest your money on one.
This article is written with an intention of being a comprehensive guide to RC owners to help them find the best RC motor. We will try keeping it simple and easy so that you can have more time using the motor than reading about it.
Choosing Between a Brushed vs Brushless DC Motors
If you are an RC car enthusiast, you might be knowing that Electric RC cars use DC motors to convert the electrical energy into mechanical energy. There are four different types of DC motors for RC cars available in the market viz; Brushed motor, Brushless motor, Servo motor & Vibrating motor.
Out of these motors, brushed & brushless motors are the commonly found one in RC vehicles. To keep things simple, we will focus on these two types of motors for now.
Brushed & Brushless DC motors
Brushed & Brushless DC motors are the two configurations of DC motors. They are essentially the same when it comes to fundamental working principles, the only difference between them lies in the way they convert electrical energy into mechanical energy.
As the name suggests, Brushed DC motors use a carbon brush to convert electrical power into mechanical energy whereas Brushless motors do not.
The fact that brushless motors do not use a brush to produce mechanical energy, makes them highly efficient than brushed motors. Apart from this, they are also faster than brushed motors counterparts.
Note that brushless motors tend to fall on the pricey side. But if you were to look for an upgrade, we highly recommend you to invest money on a Brushless motor for your RC vehicle.
What Are the Parameters to Consider Before Choosing an RC Motor?
As said earlier, there are several factors to be considered before selecting a motor for your RC. This section will discuss in detail the various parameters that are to be considered.
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The Size, weight & aerodynamics of your RC are crucial parameters you need to consider before buying a motor as they determine the speed, torque and power requirements of the electric motor.
There are two current values one need to consider before buying a motor.
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Operating Current
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Stall Current
Essentially it is the current that powers the motor, and you want just the right amount of current on your motor windings. Operating current is the average amount of current the motor is expected to draw under typical torque.
Stall current is for RC planes & helicopters. It is the maximum amount of current the motor should be able to draw to apply enough torque for the plane/helicopter to run at stall speed or 0RPM.
Finding the optimum operating current & stall current of the motor is important in the design of heat sinks to prevent the motor coils from overheating or melting.
The physical size of the motor has to be a decisive factor in choosing a motor for your RC. Look for the size of the motor on the motor case to ensure that the new engine can fit perfectly. Generally, the motor's size will be indicated by a number that looks something similar to '24-07'. 24 is the width of the motor, and 07 is its height.
It is important to note that bigger motors produce more torque, and people who are relatively new to the world of RC tend to buy bigger motors for more power. Well, there is a disadvantage to using bigger motors that most people are unaware of.
The larger the motor, the heavier your RC vehicle is going to be. With an increase in weight, your RC is going to have a lot more momentum, and you are going to need a more robust chassis to withstand accidents. You would also require better suspensions & breaks.
Which increases the momentum of your RC. This means that you would need better chassis for impact protection, better suspensions for stability & new breaks!
Watt is the unit of the overall power of a motor. Higher the wattage, the powerful the engine will be. If you are only planning to upgrade your RC vehicle's motor, make sure that you check the rating of your vehicle's Electronic Speed Controller (ECS).
Ideally, you should have an ECS with a rating that is at least 20% larger than the wattage of the motor. For instance, if a motor were to pull 20A, you would need a 30A ECS.
Now that you have an idea on the basic parameters, you need to keep in mind before investing in an RC motor, know that it is essential to know the manufacturers recommendation while upgrading your engine. Similarly, suppose you are planning to build an RC vehicle from scratch. In that case, we suggest you refer the manufacturers' recommendation for the model you expect to have once you are done building!
Conclusion
You also need to keep in mind that upgrading your RC car isn't all about the motor you use. It's just that the other parameters are beyond the scope of this article. If you still have doubts regarding choosing the best RC motor, feel free to contact us.
So, if you are still with us with a better understanding and clarity, we recommend you to have a look at the collection of RC motors we have with us here.
Are you interested in learning more about how to measure rpm of a dc motor? Contact us today to secure an expert consultation!
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