Motor is a very important part of a drone, especial for DIY FPV drones. Most pilots are all build a FPV quad by themselves, because they know very much about it. For our newbies, you maybe buy a RTF FPV quad from online shop, if you want to DIY a FPV quad, you need to know the motors.
When you get a FPV quad brushless motors, you will also can get product specs, for example you get a Tattu 2305 2450KV motor; what’s the mean of these specs?
“2305” is the size of this brushless motor. “23” means the stator width is 23mm, while “05” means the stator height is 5mm. Basically, the taller stator the more power at higher RPM, while the wider stator the more torque at lower RPM.
（What is brushless motor stator?）
A stator is the stationary part of the motor in the middle, which is wrapped around by copper wires (windings). It’s made of many layers of thin metal plates that is laminated together with very thin insulation layer in between.
The size of propellers the motor is designed for determines the prop shaft size. Motors for 4″, 5″ and 6″ propellers have M5 motor shaft. Modern motors has the prop shaft integrated into the motor bell. Older generation motors might have separate prop adapters.
I have introduced the 2305’s mean of the Tattu 2305 2450KV motor, and now let me introduce the “2450KV” mean. The 2400 is an important parameter of brushless motors, which indicates the theoretical increase of motor RPM (rotation per minute) when voltage goes up by 1 volt without load (i.e. propeller). For example, when powering a 2450KV motors with a 3S LiPo battery (12.6V), the motor would spin at about 30870 RPM without props on (2450*12.6). Typically this just an estimation, or round-up number specified by manufacturer.
Once you mounted a propeller on the motor, the RPM decreases due to air resistance. Higher KV motors would attempt to spin the propeller faster, but lower KV motors normally generate higher torque. That’s why we tend to see larger props paired with low KV motors, and smaller props with high KV motors.
The KV value of a motor is determined by the number of copper wire winding in the motor stator, and the magnetic strength of the magnets. But generally the higher number of winds decreases the KV of the motor, while lower number of winds increases the KV of the motor.
By pairing high KV motors with excessively large propellers, the motor will attempt to spin it fast like they would do with smaller props, and therefore drawing too much current and generating too much heat. Eventually it could burn out the motor due to overheat and shorts in motor coils.
N and P Numbers
You might have seen something like “12N14P” printed on the box of a motor. The number before the letter N means the number of electromagnets in the stator, and the number before P means the number of permanent magnets in the motor.
Most motors in RC have the same 12N14P configuration. Some lower KV motors might have more electromagnets and permanent magnets to help increase torque more efficiently (and thus more expensive). While it’s good to know what this is, it’s not an essential piece of information when picking motors especially for mini quad.
Frame Size, Prop Size and Motor Size
How to choose frame, prop and motor? Most of the times by knowing frame size, we can estimate what motor size we should use. This is because frame size limits props size, and each propeller size requires a different motor RPM to generate thrust efficiently.
This table below is only a simplified example to give you some ideas. It assumes you are running 4S LiPo batteries. You might also see people using slightly higher or lower KV motors than this table suggests. Frame size is referring to wheelbase (aka diagonal motor to motor distance).
Voltage and Current Draw
It’s important to understand that voltage has a large impact on your motor and propeller choice too. Your motor will try to spin much harder when a higher voltage is applied, and thus drawing a higher current. Always check thrust data first.
Now you have got a good understanding of what motors you want to run and estimated current draw, you can select ESC of the required current rating.