Motor

Motors

As you can imagine the motors are one of the more important elements of a multirotor, because - well - they are an the only parts that keep it in the air. Since it doesn't have a wing, you have no surface to glide after all...

Motors have different characteristics you have to look for. First of all, it should be a brushless one. Those are more efficient than brushed ones. Then of course the size is an important factor, because bigger motors will usually be heavier and since there's usually more than one (multirotors, remember? ;-)) the weight multiplies too.

For brushless motors you have two kinds: Inrunners where only the shaft moves (usually used for very high RPM motors) and outrunners where the whole bell moves around the stator (lower RPM motors).

From the electrical point of view there is one important number that everybody talks and asks about: The kv rating. It is measured in rpm/V (revolutions per minute per volt). So if you put 1 V on an 1000 KV motor it will spin with 1000 rpm, 2 V means 2000 rpm and so on. This is directly related to the battery you have in your model. Most common ones are 3 S (11,1 V) or 4 S (14,8 V), so the maximum revolutions per minute of our 1000 KV motor would be 11 100 rpm respectively 14 800 rpm.

How fast a motor has to be depends on what you want to do with your model and is directly related to the propeller size as well. For example - if you want to build an agile multicopter you need a fast motor and smaller props but if you want to lift a heavy weight like a camera it needs to be the other way around. What to choose here is a difficult decision and deserves it's own post.

Select a proven design and motor setup. For beginners it helps a lot to ask experienced guys, look for build videos and plans that are similar to what you want to create and go for a similar setup.

Important ratings for a motor are the maximum allowed Voltage (V), the maximum current in Ampere (A) and the amount of power it can provide in Watt (W). The maximum current is an important value, because the ESC has to be able to handle it - adding 20% for security is always a good idea.

A good quality motor has good bearings, equal spacing between the magnets and is well balanced, so it doesn't put vibration on your frame.