In regard to brushless motors there are a couple of specifications that you should know and understand.
Those specs will help you choose the right motor for your copter and will help you compare motors with one another.
In this article I will only be discussing brushless outrunner motors, since this is the type of brushless motor we are exclusively dealing with in the quadcopter hobby.
Brushless motors consist of two main parts, the rotor and stator. Depending on the position of those components you have either an out- or inrunner motor.
The stator is the part of the motor with the windings on it and is also the stationary part. The rotor is the part with the magnets and as the name suggests, the rotating part.
With outrunner motors, the stator is on the inside and the rotor (the bell) is on the outside. With inrunner motors it is exactly the other way around. Inrunner motor are commonly used in RC cars.
The most obvious specs are the ones regarding the motors’ physical appearance. Basically every motor is specified by stator size - diameter and height of the stator to be precise.
Lets take a look at some motor sizes: 0802, 1103 or 2207. The first two numbers indicate the diameter of the stator. The second pair of numbers gives the height of the stator.
In the case of 0802 the 08 indicates that the stator has a diameter of 8mm and a height of 2mm 0802. 1103 indicates that the stator diameter is 11mm and stator height is 3mm.
On most brushless motors you can find this number sequence somewhere on the bell, usually together with the KV rating.
There are also sequences that symbolize fractions of a mm. Here you have to guess from context how to interpret them. Take for example the motor with the following specification: 08028
The first two numbers are again the diameter, so 8mm the last three numbers the height, so 2.8mm. Usually manufacturers stick to full mm and simply round down the sizes, but since I know at least one motor that has such an “ugly” size, I thought I would mention it.
The shaft diameter is extremely important when it comes to whoop sized motors. Most of the 3, 4 and 5” Motors have a shaft diameter of 5mm - that is nice and easy.
Whoop sized motors come with shaft diameters of 0.8, 1.0 and 1.5mm - this measurement is extremely important because it limits the props you will be able to use on your motor.
Usually a bigger shaft diameter also indicates more strength, but this also depends on the material used for the shaft.
I highly recommend going for 1.0mm or 1.5mm shaft diameter. With 2S I would definitely go for 1.5mm, since you have more impact when crashing and can use the extra rigidity of the shaft.
This one is obvious - the weight of the motor. But lighter does not always mean better. Sometimes the heavier motor might be the better choice because its components might be more sturdy, for example the motor might have a thicker shaft.
Be careful when comparing the weight, some manufacturers state the weight without the motors wires, which can make up to 1.5g, depending on length and availability of motor connector.
Mounting pattern refers to how the holes on the bottom of the motor are spaced out and which screws are used. With bigger motors you usually have four mounting holes for M3 screws.
With smaller whoop styled motors it is very common to have three mounting holes. Most commonly M1.4 or M1.6 screws are used. Especially when having 3 mounting holes those holes will all be on the same diameter. With older, bigger motors it was common to have two holes on one diameter and two on another, just to be compatible with a wider range of frames, but nowadays all four are on the same diameter.
Usually, the manufacturer will provide a technically drawing of the motor, in this drawing you can see the diameter and screw size of the mounting pattern.
Ball Bearings or Bushings
This is not so much a topic with bigger motors, since they basically all have ball bearings. But with the smaller sized motors there are quite a lot that use bushings instead of ball bearings. This does not necessary mean that the motors are bad. In some cases a bushing will work equally well as a ball bearings and for some motor sizes there simply are not any ball bearings that would fit.
Usually motors with ball bearings are smoother and have a higher longevity, on the other hand a ball bearing can seize up, this cannot happen with a bushing. Bushings are typically made from bronze and should be lubricated now and then.
Per definition a bushing is a bearing but not all bearings are bushings.
Although most of the following specs are also derived from the physical appearance it might not be as easy to determine their value by just looking at the motor itself since they are also derived from the type and amount of windings on the stator for example.
A lot of people confuse this with kilo Volts, which does not even make sense in context of a brushless motor. The KV rating indicates the RPM/V or in other words:
Revolutions per minute per Volt applied to the motor without load.
When a Prop is added into the equation the actual RPM/V should decrease to about 75-80% of the specified value - this is then considered the appropriate load fro the motor.
For example: A motor with a rating of 15000KV will complete 15000 revolutions per Volt applied. When this Motor is powered from a 1S battery, it will complete 15000 * 3.7 = 55500 revolutions per minute without load.
So with the appropriate prop, powered from a 1S battery, the motor should complete somewhere between 42000 and 44000 revolutions per minute.
To choose the proper KV you have to first decide on how many cells you want to operate on, so how many Volts the motor will be powered with.
For low cell counts you go with higher KV and for high cell counts with lower KV. There are some KV ranges that will work well across multiple cell counts (for example with either 1S or 2S), but typically each KV rating has its sweet spot with a certain cell count.
A lot of this is personal preference and also highly depends on the chosen prop.
The same feel can be accomplished with different constellations, for example a motor with 1600KV on 6S will feel very similar to a 2400KV motor on 4S - they both result in the same RPM.
Thrust highly depends on the chosen prop and the applied voltage. Sometimes a motor comes with this specifications, if so, then usually for a couple of different prop configurations and throttle positions.
For example: A motor powered from a 2S battery with a 1220 prop has thrust of 20g at 50% throttle. This means that at 50% throttle the motor can move 20g.
Thrust typically increases with throttle, meaning, the faster the motor spins, the higher the thrust. Thrust is visualized as a curve, the y axis being the thrust and the x axis being the throttle position - this can be linear, but most of the time has more of a sine shape.
To get the total thrust for your copter you multiply the thrust of a single motor prop combination by four, since you have four motors, and you then know how much weight your motors can move in the best case.
Thrust to weight ratio
The trust to weight ratio is an indicator of how powerful your quadcopter is, you take the total thrust and divide it by the AUW (All Up Weight) of your quadcopter.
To continue the example from above, we have four motors each with a thrust of 20g at half throttle, so 80g of total thrust and the weight of the copter is 40g.
This means we have a thrust to weight ratio of 80/40 = 2. Since the thrust to weight ratio is dependent on the throttle position it moves relative to the throttle curve.
A rule of thumb is, that the thrust to weight ratio should be at least 2.
Selecting a motor
Motor selection can be done in different ways. You either already know the components you will be using and choose your motor based on that or you choose your motor first and then base all the other components around your motor choice.
Choice of motor depends on frame, intended operating voltage, ESC’s and props.
When building a new copter I usually decide on operating voltage and battery size first, then choose a frame and based on that select motor. After selecting a motor I move on with the ESC’s and the flight-controller. With whoops this will most of the time be an AIO flight controller that also contains the ESCs.
I know other people do it the other way around - some first like to choose the motor and go from there. There is not really a right or wrong way - I think it depends on which component you think is the most important for you and thus which one you choose first and base the rest of the components on.