Motors in a multirotor

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Motors in a multirotor

Post by neclovek on Fri Oct 16, 2015 5:10 pm

This is part of new tutorial series which i plan on doing here so people will get more educated about multirotors.

Motors are one of the most basic parts of a multirotor flying vehicle (for simplicity i will call it drone). They spin the propellers which lift the drone.

How motors work (very basic principle): alternating current flows through motor coils. That creates magnetic field which attract magnets placed in the motor so they move - motor spins. Motor consists of stator (part that is stationary) and rotor (part that spins).

There are (mostly) two types of motors used in drones:
Brushed inrunners - Used mostly in toy grade drones
Brushless outrunners

Inrunner means the inside of the motor spins, outrunner means the outside of the motor spins.

Brushed motors have little brushes which make contact on pads (commutator) that are placed on the rotor and connected to the coil. In brushed motors the whole coil spins inside the motor while magnets are stationary. Since the coil inside is spinning it automatically reverses its own polarity of connection to the brushes so all we need to spin the motor is a DC (direct current).

Brushless motors have static coils and magnets spin outside the coil. Since the coils don't change polarity on their own we need to use AC (alternating current) to spin the motor but because of direct connection between input wires and coils they are able to handle bigger currents and because the magnets are placed outside the coils the magnetic field has more torque.

Rest of the post will be mostly about brushless motors.

Let's have a look at some basic parameters and ratings. Basic classification of brushless motors is by their size and speed. Often you will see stuff like Brushless outrunner 1806 2300KV or 2212 1000KV and so on. But what does it mean? The first 4 digit number is the motor armature size in milimeters - first two numbers are armature diameter and second two are armature height. And the KV? That's the speed - how much RPM (revolutions per minute) will the motor spin per 1 volt while it's not loaded. So unloaded 1000KV motor will spin at 15000RPM with 15V battery.
Then we have other parameters like maximum current (A), maximum voltage (V), no load current (A), maximum power (W), coil resistance (R)

Choosing a right motor:
Each motor should have some performance data available from the manufacturer. They look like THIS(link).
When building a drone, if you don't choose the motors first, you will need to know either your frame size, your prop size or your ESC/battey. For now let's ignore all the armature size and KV rating stuffs. For example:
You are building a 550mm quad and you want to use a 3S battery so you go check the performance tables. For each propeller size there should be a separate table. In those tables you look on the part that says 3S and check if there is enough lift (tutorial on that coming soon) and if the current or power doesn't exceed the maximum safe value. If everything is OK you can choose which configuration is best based on efficiency (g/W - grams per watt) and don't forget to check if the prop will fit your frame.
Now how do we use the size/kv stuff? Well, that's pretty hard but generally bigger motors are stronger and smaller ones spin faster so for 250mm quad you use 1806 or 2206 and similar sizes. For 600mm hexacopter you use 2212 or 2830 and so on. And of course the bigger the KV, the smaller prop you can put on, otherwise you will overload the motor so if you have a big frame, you can fit a big prop and that would mean you will want a slower motor.

This should be all for motors. I hope i didn't forget to mention anything but if i did, just comment below or PM me and I'll update it.

_________________
My quads:
- Eachine H8 Mini White (Great indoor flyer)
- DIY Cheapo-Quad v2 (More info soon)
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neclovek
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