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Multirotor Tech and DIY

How to choose motors for your quadcopter?

The motors are one of the most important parts of your quadcopter. Make the wrong choice and your quad will fly like a brick. In this post, I will look at the various specifications of a brushless motor you need to consider to make your choice. I will also mention some good motors available today. This may get a bit technical, but I suggest you read through it so that you can make an informed choice. No TL;DR here 😉  So let’s get to it:

  • Stator dimensions – You may often have seen motors listed as 2212, or 2204, or 1806, or even 1105. So what do these numbers mean? The first 2 digits refer to the diameter of the motor stator and the last 2 digits refer to its height. These figures are in mm. Larger motors usually are powerful than smaller motors due to the greater torque they have available to spin propellers. One of the reasons the Cobra 2204 was so popular is that it actually is a 2205. However, the cost of using bigger motors is greater weight, so a compromise must be found. 
A 2205 and 2212 motor

A 2205 and 2212 motor

  • kv rating – kv is the voltage constant for a motor. It tells us the RPM the motor will spin at if powered at 1V. Thus actual RPM (without load) will ideally be equal to kv rating x input voltage. However, the higher the kv, the lower the torque a motor has so it will be limited to smaller props. The general trend is that higher kv motors are suited for smaller props and so are used on smaller quads (as they have less space for props). Lower kv motors and big props are usually more efficient. and so are used on bigger quads.
  • Thrust and amp draw – It is crucial to take a look at how much thrust the motors give with various combinations of props and batteries and at what current draw. Traditional advice was to choose motors that would give your quad roughly a 2:1 thrust to weight ratio but now the general approach now is to go for as much thrust as possible. However, you need to check that the propellers required to get this thrust will fit on your frame.
  • Weight – It is important to check how much a motor weighs and compare it to others in its class. You generally want to keep the weight on the arms low for snappy flips and rolls, but a few grams here and there shouldn’t make much of a difference, especially if this is your first quadcopter.

Here is a table of various frame sizes and the typical motor size and kv rating  you would run on them. Note that this is only an approximation, and that you will have to check the data for your motors and props and cell count. For example, the table says you can run 3 or 4 inch props on 160 size frames but a 4045BN prop on 4S will be too much for the recommended 1407 motors.

Some frame sizes and typically used motors

Some frame sizes and typically used motors

Here are some good motors available now:

  • EMAX RS2205 (usually called the red base)- These are one of the most hyped motors right now. They generate massive amounts of thrust and are supposed to be quite durable and have good QC as well. They are pretty well priced at 20$ each. I have not used these myself though. You can find more information and test data for these motors in Oscar Liang’s blog post as well as at Mini Quad Test Bench. They are available at a lot of retailers, including BanggoodFPV Model and Surveilzone among Chinese sellers as well as several other retailers in the USA/ Europe.
  • RCX H2205 – These are a pretty good balance between price and performance at 13$. I run the 2633v version on my ZMR and I find it has plenty of power. However, it is pretty much limited to 5 inch props (non bullnose). If you want to run 6 inch props as well there is a 2350v version too (which is more efficient too). These motors are quite light compared to others in their class. However, they are not as durable (though I haven’t managed to break any so far).  This is not as much of a concern though since replacement parts including shafts, bearings and even entire bells are available pretty cheap. You can get these motors from MyRCMart in 2350kv and 2633kv. Thrust data and reviews are available in this thread at RCGroups.
One of my RCX H2205-2633kv. Slightly beaten up from all the crashes

One of my RCX H2205-2633kv. Slightly beaten up from all the crashes

  • Cobra 2204 – Once the king of mini quad motors, Cobra had some major issues with quality control. They are supposed to be sorted by now and many still swear by Cobra. However, they are kind of expensive at 25$ each and there seem to be a lot of better options available now. However, as of now, these are the best mini quad motors being sold in India and are available at RCHyper.
  • ZMX 2205 – These are very similar to RCX motors but feature an improved bell design, making them stronger. They also generate more thrust. Thrust data is available at RCGroups as well as MQ Test Bench. They are sold by Silver Drone but appear to be out of stock as of 4/3/2016. I believe a V2 is coming out soon so that  should be good. UPDATE – The ZMX V2 are out! The colour scheme may not appeal to everyone but these seem like excellent motors. Powerful, cheap and light! 

This is not even close to an exhaustive list. There are several other motors such as XNova, Rotorgeeks, DYS, Multistar etc. that I have not mentioned. You can find a lot of test data in the MQTB motor shootout which should definitely help with your choices.

1 Comment

  1. I need help selecting motors. I am building an 186mm x-style racer with a small central pod to hold my components. I will be flying 1300mah 4s battery on 5×4.5×3 Dal props. I’m currently leaning toward ZMX motors, but there are sooo many options. Please give me your recommendation. Thanks!

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