How To Buy The Best Quadcopter Motors?
Flying some nice sturdy quadcopter drone is an exciting experience, even for a drone geek. But for the best flight performance and the durability of your drone, you will need some nice matching motors to power your quadrotors.
There is a little complexity in picking the right quadcopter motors; the propellers are powered by four motors so if you make a mistake in choosing the wrong motors for the rotors, you have to multiply the mistake by four. It can be quite costly if you are purchasing costlier motors. Even a cheap multirotor motor costs anywhere from $20 and $25. If you have to replace the four of them, you will be spending at least $80. It could make your hobby needlessly expensive!
Now, you don’t have to be a mechanical engineer or physicist to choose the right quadcopter motor. It is a fairly simple process once you have understood the specs of your drone. You have to match the motor to the weight of the quadrotor otherwise the motor will not deliver the best performance that allows you to have an enjoyable flight experience. If there is a mismatch, it may not even last more than a few hours of flight.
The quadcopter motors cannot be chosen in isolation. You have to get the right match for the recommended specifications of the motors to that of the quadrotors. There is a clearance or range or specifications within which you can hit a “sweet spot” that allows you to get an optimal and smooth flight performance with your quadcopters.
There are things that you need to think of when choosing a quadcopter motor. For example, how much thrust will you need? What is the weight of your quadrotors? What type of motor are you planning to purchase? The brushless ones are generally more efficient but also costlier. Also, you need to consider how aggressively you plan to fly the quadcopter. Will you be making sudden aggressive twists and turns? That might require higher payload capacity. Consider also, the size and pitch of your quadcopter propeller. We will look at how these factors impact your choice of a quadrotor motor in a moment but first, let’s look at the various types of drone motors in the market.
Types of Quadcopter Motors
Quadcopter or multicopter motors can be broadly classified into brushed and brushless motors. Most quadcopters generally use the brushless DC motors, abbreviated as BLDC. The cheaper designs might however use the brushed DC motors
Brushed DC Motors
The brushed DC motors can be found in some of the cheapest multicopters, generally the toy-grade drones. They have a coreless design and are thus referred to as coreless DC motors. The design looks like that of a shiny spray paint can or even a teargas canister. If your drone has a smaller multirotor that uses a gear mechanism to multiply mechanical power to the propeller blades, chances are that it is powered by acoreless DC motor. The main advantage of the coreless DCMs is the cost. They are generally very cheap to manufacture which translates a to a lower retail price. If they burn out, you can replace them easily without feeling the pinch.
They are also relatively lightweight and are easy to install on your multicopter. However, they have several disadvantages including the following:-
- They have a very short life span. This is especially true for the much smaller motors. If you fly them for a longer duration of time, they will burn out relatively quickly. Larger motors are longer lasting due to various design advantages. The coreless brushed DCMs tend to be smaller in size.
- The much smaller coreless DCMs used in the toy-grade drones will have a flight time of less than 6 hours before they burn out and you are forced to shop for new motors. You will also be forced to fly them over very short time intervals of only a few minutes to prevent them from overheating.
- Coreless DC motors do not have as much power as the brushless DC motors so you cannot use them to power larger drones or achieve a bigger and consistent thrust. They are best suited for the smaller and lighter toy-grade multi-copters.
Tips on Increasing the Lifespan of Coreless DC motors: To achieve more durability and slightly better performance with the coreless DCMs, try to “burn them in”. This simply means flying your quadcopter on low throttle during the first few flights before you gradually increase the thrust to a sustainable level. Also, most of them burn out due to the heat generated by the motor so try to fly the copter for shorter intervals of less than 6 minutes at a time in order to lengthen their lifespan.
Brushless DC Motors
If you are serious about your multicopter performance and want to get longer flight times or versatile flight maneuvers, then you will have to go with the brushless DC motors or BLDCs. These are the motors that you are most likely to find in every high performing multirotor such as photography quadcopters, videographty drones and quadcopter racing .
Unlike the brushed DC motors that have almost the same “can-like” appearance, the appearance of the brushless DC motors generally depends on the manufacturer and the type of the brushless drone motor. In general, they tend to be short and also stout in design. Also, unlike the brushed motors that transmit power to the propeller blades via a gear mechanism in order to maximize power output, in the case of BLDCs, the propellers are always mounted directly onto the shaft of the motor for direct power transmission which increases the efficiency of the motor.
The brushless DC motors have several advantages including the following:-
- They are very powerful motors and can deliver excellent torque to the propeller blades without the use of a gear mechanism.
- They generally have a longer lifespan, especially if they are used with care.
- Unlike the brushed DC motors that run for only a short duration of time, the brushless DC motors can run for a longer duration of time more consistently without enduring heat damage. You can therefore enjoy better performance, do lots of quadcopter tricks and have longer flight times. While heat may slow performance a bit, they will be able to regain their peak flight performance once they have cooled down.
- BLDCs have a high power density: This allows them to generate a lot of power with a very low weight, a good quality for any flying object.
- BLDCs have a good efficiency and will generate very little heat even on extended flights. As we have seen, excessive heat is the leading cause of motor burn outs. Low levels of heat in the brushless motors help in extending their life span.
- There are no contact parts like in brushed motors so the brushless motors do not undergo a lot of tear and wear, another factor that helps in lengthening their life span.
- Thanks largely to the above three properties, the brushless motors can spin very fast allowing you to obtain very high speeds with your multicopters.
- Precision in control: You can control the speeds of the BLDCs with a high degree of precision. You can also make quick adjustments in the operation of the motor thanks to the versatility of the brushless motor. With the quick adjustments, you are able to maneuver the motor and copter very easily.
The brushless DC motors can in turn be divided into two main types, namely:-
- Disc Type Motors
- Barrel Type Motors
Disc Type Motors
The disc type motors are high power motors that are shaped like a disc. They are much lighter in weight and will deliver a higher amount of torque to your quadcopter due to the large number of the motor windings.
The motors also have a higher pole count and a lower kV rating, factors which contribute to the higher torque in the motor. Because of this, they are generally matched with larger sized propellers. Using the disc type motors will assure you of stable flights with lower noise and turbulence. The drawback is that they are also the most expensive quadcopter motors.
Barrel Type Motors
As the name suggests, the barrel type motors are shaped like a barrel and generate less torque compared to the disc type barrels. They are heavier in weight and their mechanism of operation is just the opposite of that of the disc type motors. The pole counts for these motors are lower so they have a higher kV rating. They are better suited for more agile flights although they are less stable than disc type motors.
In spite of these, the barrel type motors are the most widely used due to their low cost relative to that of the disc type motors. The disc type cost a lot of money. Consider that a typical quadcopter requires four motors and you can see why most hobbyists shy away from the costlier disc type motors. However, strong drone enthusiasts looking for a superior performance in flight tend to go with the disc type motors. You will find them widely deployed in professional drone photography and videography.
The main disadvantage with brushless motors in general is that they are relatively expensive. To function, the brushless DC motors may also need extra components such as the Electronic Speed Controllers or ESCs. However, if you hedge that cost against their key advantages such as durability and an incredible performance, it is easy to see why you get very good value for money with these motors.
The Power to Weight Ratio
We have mentioned that the power density of the motor is an important factor determining its performance and efficiency. Let’s see how.
When you are shopping for a quadcopter motor, you will want to buy one that has a high power to weight ratio. This has several implications. For example, a motor with a power to weight ratio of 4 can lift as much as four times its weight while a motor with a power/weight ratio of 1 is virtually useless as it will not lift even its own weight.
When buying quadcopter motors, go for lighter options with a lot of power. You have to put the lifting capacity of the motor into consideration. Look at how much combined weight it can hover at a particular thrust.
The motor power specifications should be such that it will be able to hover the multicopter together with the payload at 50% capacity. The manufacturer generally makes this process easier for you. They will specify the amount of payload that you can safely hover on your copter without stretching it beyond its design limits. If you will be using the copter for photography, for example, you have to pay close attention to the weight of the onboard gimbal and camera to ensure it lies within the specified payload design limit for the quadcopter. If a motor must run at its full capacity to hover the copter and the payload, you will be pushing its beyond its limit and it will give you shorter flight times, less durability and less than optimal performance.
The challenge, therefore, is in figuring out the right specs of the motor that will assure an optimal performance. It is not always an easy process, especially if you are starting out and are a bit intimidated with the having to go through “technical” data that determine the performance of the motor.
Calculating the power to weight ratio
We have seen that you have to ensure that your motor delivers 50% more thrust than the weight of the quadcopter, including the weight of the payload. This is necessary as it still leaves your motor with enough thrust that allows you to control the quadcopter in a windy environment. The extra thrust also allows you to use the drone for more aggressive flight maneuvers. If you need a more responsive drone and flight maneuvers, you need more thrust or less drone weight.
For example, if your quadcopter weighs 1000gm or 1Kg, the motors on your quadcopter will need to produce 2000g or 2Kg in thrust for optimal flying. This means that for a 1kg quadcopter, you will need a thrust of 500gm per motor or a weight to power ratio of 2:1.
If you are looking for a very agile quadcopter, then you need an even higher weight to power ratio of 3:1 or 4:1 that allows you to execute all those complex maneuvers. However, for most quadcopter applications, a weight to power ratio of 2:1 is generally sufficient. The extra thrust allows you to add extra weights such as a camera or even bigger and heavier batteries for longer flight times.
Consider the Propeller and Motor Combination
The propellers go together with the motors. For the best efficiency, you must have the right motor-propeller combination. In fact, if you choose the right propeller that suits your desired performance requirements first, you will have an easier time finding the matching motors that will go well with it.
If you are looking for more stable flights and better acceleration, go for the larger propellers. However, if you want some speed and are planning to execute some flight maneuvers or acrobatics, a shorter propeller would be more ideal.
For acrobatic maneuvers, choose shorter propellers with diameters of less than 8 inches. For speed, acceleration and larger payloads, go for larger propellers with a diameter of more than 12 inches. These are especially suited for stable flight applications such as drone photography as they can carry gimbals and cameras and still generate sufficient thrust with good flight times.
In summary, here are the main features to look for when picking a suitable propeller for your quadcopter:-
The diameter: A larger diameter gives you more thrust and is suitable for carrying larger payloads, stable flights and high speeds. A smaller diameter gives you less thrust and higher acceleration and is suited for acrobatic maneuvers.
The pitch of the propeller: The pitch of a propeller simply refers to the distance that it will travel in a single revolution. If it has a low pitch, the propeller will generate greater torque and will be impacted less by the turbulence. As a result, this kind propeller will support higher pay loads with less energy consumption. Higher pitch propellers are better for acrobatics and drone racing.
There are ways in which these propeller specifications might determine the kind of motor that you choose. Keep in mind that there are no hard and fast rules when it comes to matching propeller pitch to motor. In fact, unless you are a hardened hobbyist who pays attention to the finest details, you shouldn’t even worry about the propeller pitch.
The propeller dimensions are generally quoted as AxC where “A” represents the length of the propeller and “B” the pitch. So a propeller with the dimensions 8x5” has a length of 8 inches and a pitch of 5 and would be ideal for executing acrobatic maneuvers.
The KV Rating
The KV rating of the motor is determined by the number of copper wire windings in the motor. In practical terms, it refers to the number of revolutions per minute that you will get for every volt of power that you provide. It is generally given in terms of RPMs per Volt. For example, a motor might have a KV rating of 1000rpm at 1V.
You don’t have to go into the complex electrical principles. Let’s see how this practically factors into your choice of a suitable motor for your quadcopter:-
The kV rating value will have an impact on the amount of payload that your motor can carry. A 1000kV motor on a 12V battery will spin at 12,000rpm with zero load.
Reading the numbers on a motor
Motor specifications are given in two four-digit numbers. For example, a motor may have the spec 1804-2204. The first part shows the stator width and stator height and the second four digits show the KV rating of the motor. An 1804-2204 motor has a stator width of 18 inches, a stator height of 4 inches and a KV rating of 2204 at zero load. If this connected to the commonly used 4S LiPo batteries with 16.6V when fully charged, it will spin at 2204 x 16.6 which is equivalent to 39906 RPM.
In practice, the motor will rarely achieve that “theoretical” RPM. There are some factors that will reduce the efficiency such as the air resistance, the pitch of the propeller and the length of the propeller. A shorter stator height gives better efficiency than a longer stator height.
The Pole Count of the Motor
There are more important considerations to look into when choosing a motor so don’t get too fussy about the pole count. However, if you are fastidious about the kind of motor which you will buy for your multicopter, you may give this a look.
There are two main selections here. Some quadcopters have a higher pole count which will in turn need more voltage and produce a higher torque. However, that comes at the price of lower revolutions per minute so these motors will need bigger blades.
A lower pole count gives you just the opposite. You get less voltage, less torque but higher revolutions per minute. In this case, the propeller blades needs to be smaller as the motors are incapable of driving the larger propellers. You can see the importance of choosing your propeller blades before shopping for the motor! It makes your choice of a motor a lot easier.
Generally, quadcopters or multicopters will use motors with a higher pole count. This eliminates the need to use a gear box system to multiply the toque thus simplifying the design of the drone.
Ideally, the motors you pick should be able to rotate in both clockwise and anticlockwise directions. With a quadcopter, this can give you very good stability especially when two motors move in a clockwise direction while the remaining two move in an anticlockwise direction.
While it is possible to fit motors that fly in the same direction onto your quadcopter, it makes the drone needlessly difficult to fly especially if you lack the experience. Motors that move in both clockwise and counter-clockwise directions make the drone flight smoother and contribute to the effectiveness of the drone.
There are many trade-offs that you have to make when buying a quadcopter or multicopter motor. In a lot of instances, you will have to make compromises either to save money or achieve a desired performance. How you plan to use the copter also has a bearing on the type of the motor that you will choose.
Before you begin shopping for a quadcopter motor, you need to know the features of the copter such as the total weight of the copter. Get an approximate weight by adding up the weights of all the components on the drone including the quadcopter frames, electronic speed controllers, the 4S LiPo battery, transmitters, FPV cameras and flight controller etc. You should also factor in the weight of the pay load you are planning to carry such as the gimbals and cameras. Once you have an idea of the weight of the copter and the pay load, it is easy to calculate the required thrust for your motors. Think also about the intended use of the copter as this also has a bearing on the type of motor that you need to attach to the device.
One of the most important factors to put in mind is the power to weight ratio. You have to pick a motor that will be able to power and lift the quadcopter from the ground and still have sufficient thrust to do other maneuvers. Unless you are planning to put your copter into more intensive or aggressive uses, you can choose a barrel-type motor with a power to weight ratio of 2:1. For aggressive acrobatic maneuvers or sharp adjustments, go for the higher power to weight ratios.
For performance and stability, look at factors such as the kV rating and a higher pole count for the motor. Higher kVs might give you faster copters but you will have to compromise on certain performance factors such as the efficiency and the stability of the flights.
When buying a quadcopter motor, try also to be clear on the purpose for which you plan deploy your copter right from the outset. That can give you a clear impression on the kind of motor you need to purchase for optimal performance.
There are also factors to keep in mind such as pricing and the brand of the motor. Don’t be too fussy about these. Try to get the motor specifications right first before considering factors such as the pricing and brand. Generally, the same class of motors with a defined set of specifications will lie in the same price range.