Update 9/3/2016 – Added new motors and removed some obsolete ones.

Basic Brushless Motor Theory

Our quadcopters use brushless outrunner DC motors to drive our props. Brushless motors are specialized types of DC motors that have no major wear components. They come in two types: outrunners and inrunners. These two differ by where the magnets are located: in an outrunner, the magnets are located in the “out”er casing. In an inrunner, the magnets are located “in”side the core of the motor. Inrunner motors are generally used in high-RPM applications like model jets or high speed planes. With higher RPM operation, you get less torque at lower speeds. Outrunner motors, on the other hand, generally spin at lower RPM but generate more torque at those slower speeds. This is more appropriate for direct-drive propeller driven aircraft like our quadcopters.

All DC brushless motors technically only consists of 4 major components:

  • motorThe stators – The stators consist of three “circuits” of extremely long, thin wires that are wound in a circular pattern around the inner core of the motor (on an outrunner). This wire is coated in an enamel that protects it from shorting to itself as it is wound in circles over and over again. As you may (or may not) remember from your high school physics class, current flowing through a wire produces a magnetic field. When that wire is circled around itself, the magnetic field is intensified. The more current, the more magnetic field strength, and the more torque from your motor. However, more current also produces more heat, particularly in smaller wires. The enamel coating on the wire can melt under high temperatures, causing a short and a “burned” motor. Therefore, as you desire higher power, you will want to start looking for motors with windings that have thicker wires – able to carry more electrical current while producing less heat. All told, the way in which the stator is built is the principle factor in determining the performance of your motor.
  • Permanent magnets – The permanent magnets generate a fixed magnetic field generated by a set of rare earth magnets. Rare earth magnets are similar to the horseshoe magnets you undoubtedly played with as a kid, only they are much stronger. In outrunner motors, they are glued to the outside of the motor case with epoxy.
  • The casing of the motor protects the magnets and windings. It is generally made out of a lightweight metal like aluminum. More advanced motors have cases that are designed like miniature fans to flow air over the motor windings as the motor spins, cooling them off and allowing them to create more power.
  • The shaft of the motor is attached to the spinning part outer part of the case in an outrunner. It is the working component of the motor that transfers the torque generated by the motor to the propeller.

So how does the motor actually work? I won’t go too far into it here, but the basics explanation is that the permanent magnets and the windings together create the motive force of the motor using magnetic fields. This is accomplished by switching current which wires in the stators carry the electrical current at any given time, aiming to create a continuous situation where the magnetic fields inside of the motors are slightlyt out of alignment. Like your horseshoe magnet, these fields fight with great force to align themselves properly, generating torque and causing the motor to turn as a result. The ESC is the complicated computer that senses the rotational movement and controls the flow of electrical current to generate the desired magnetic fields.

How to pick a motor

Now that you know the basics of how the motors work, lets talk about how you should decide which ones to buy!’

Motor Size

The first thing you should pick is the motor size you will be using. Fortunately, most manufacturers in our hobby have settled on a standard naming scheme for their motors. It will normally be a 4-digit number that looks something like “2205”. The first two digits of this number is the diameter (in mm) of the stator, and the second two numbers is it’s height (also in mm). In general, the bigger either of these numbers are, the more power the motor can handle and the more torque it will generate. However, the tradeoff is that bigger numbers here means a heavier motor – mostly by virtue that it is simply “bigger”.

Here are the common motor sizes you will see for mini quadcopters:

  • 1806 – These are generally used in miniquads that swing 3″ or 4″ propellers.
  • 2204 – For a long time, the most popular motors used with 5″ propellers.
  • 2205 – With the widespread use of tri-blade propellers in use today, this type of motor has become slightly more favored for use with 5″ quads.
  • 2206 – These motors are more rare, but are perfect for use in slightly bigger miniquads – such as those flying with 6″ props.

RPM Constant (kV)

All motors you will buy will have a kV rating. This rating determines how fast the motor will attempt to turn your propeller. The maximum RPM the motor will turn can be determined by multiplying it’s kV value by the voltage of your battery. So, for instance, if you bought a 2300kV motor and fly with a 4S LiPo battery, the motor will attempt to spin at:

14.8 * 2300 = 34040RPM

When it comes to actually using this number to pick your motor, I would recommend using it as a reference for how efficient you can expect your motor to run with your chosen propeller. In general for 2204 and 2205 motors:

  • 4″ propellers are generally run on 2600kV and more motors.
  • 5″ propellers are generally run on 2300-2500kV motors.
  • 6″ propellers are generally run on 2300kV and less motors.

Note these are not hard set rules, just general guidelines. If you follow them, you will probably have a pretty nice flying miniquad. Once you have your first build done, you can experiment a little bit, and you will sometimes get some pretty surprising results. For instance, I am currently flying a Bolt with 5″ props that has a 2633kV motor and it flies great. In another interesting recent test, I found that a motor with a much lower kV rating produced more thrust on several DAL props than other motors with higher kV ratings – in other words even though the higher kV motors wanted to spin faster than the motor with the lower kV ratings, in actual practice they were not able to due to torque issues.

Weight

One thing to keep in mind with motors is that you need at least four of them for your miniquad. This means every gram you can save on your motor gets multiplied by four. However, as mentioned above – heavier motors generally produce more power and easily offset their own weight as a result.

Price

As with weight, price is four times more important when buying motors. You can expect to spend between $10 and $25 apeice for your motors.

Motor Shaft

The shaft on the Cobra motors is the perfect size and length. If you can't get exact measurements from the manufacturer, just compare the motor you are looking at to this picture.

The shaft on the Cobra motors is the perfect size and length. If you can’t get exact measurements from the manufacturer, just compare the motor you are looking at to this picture.

The length and size of your motor shaft is actually an important consideration that is often ignored. Ideally, you will want to buy a motor that has a 5mm shaft, because 5mm is the most common hole diameter for all miniquad props. If you have to get a motor with a smaller shaft, you will need to fit your props with a shaft size adapter. Likewise, a larger shaft will require you to drill out a larger hole on the prop.

You also want to make sure your shaft is long enough to accommodate your prop as well as a locknut. If the motor you are looking at offers shaft length, I would look for something at least 13mm long. Shorter will work, but Vesp can tell you that if you get too short you’ll need to really torque your nuts to keep them tight during flight, resulting in stripped threads.

Known Good Brands

I won’t do any bench tests on motors in this article. Instead, I leave that research up to you. However, I do want to provide a list of known good motor manufacturers that will get you started.

Brother Hobby Tornado
Price: $22
The Tornado has an interesting design that might improve its ability to self-cool while running. From bench tests it performs quite well and is stocked in the US.

BeeRotor race motors

BeeRotor race motors

Cobra
Price: $21-$30
Until recently considered the best of the best. These are still high quality motors sold by a manufacturer who has been in the business for years. A new line, called the “Champion” series, will be coming out in the next few weeks which uses N52 magnets and drastically improves performance.

DYS
Price: $15-$22
These guys have been around awhile and have a lot of models out there. The new SE2205 is claimed to be better than the current thrust-king, the EMax RS2205. Make sure you pick up the “SE” or “MR” motors.

EMax – Propwashed Best Value Pick
Price: $14-$25
The RS2205 motors by EMax were one of the first to market with the N52 magnets, and sparked a revolution in miniquad motor design. Most manufacturers offer N52 motors these days, but the RS2205 is still perhaps the most popular motor on the market. They are fondly called the “red bottoms” on many online forums. If you get these, make sure you pick up a bulk pack, which can drive the price down below $15/pp. There is a new “S” version out (called the “RS2205S”) which is incrementally better, but costs more at ~$25.

Luminier
Price: $20-$35
Good motors sold by GetFPV. They are famed for great balance and quality control, meaning they are possibly the smoothest-running motors on the market. Lots of pros swear by these motors. They are, however, in the mid-high range in pricing.

rebel

Rebel RS2205 Motor

RCX
Price $12-$16
Good motors at an amazing price. Myrcmart sells these, and has a combo deal with Littlebee ESCs that drops the combined price for 4 ESCs and motors to $85. The “V2” models are a great value, while the SE2205 motor uses N52 magnets and is comparable to the EMax RS2205 for a cheaper price.

Rebel Motors
Price: $30-$35
Rebel motors carries a lineup of high-end 2205 and 2206 motors for the racing market. These motors have a really interesting design feature: soldering pads mounted at the base of the motor instead of motor leads. This promises to clean up your build significantly. Unfortunately, these motors are likely the most expensive motors on the market.

RotorGeeks
Price: $21-$25
These guys have a great reputation in the hobby. Like their ESCs, their motors are top notch.

Tiger Motors
Price: $20-$26
Tiger RC has been in the hobby for decades. Their motor division produces some high quality products, albeit at a relatively high price. Check out their F40 and F60 motors for some insane thrust figures. Keep in mind that these motors are generally a bit heavier than your average 2205/2206 though.

Storm Drones
Price: $20
HeliPal.com carries the new Storm Drones motor design. This design has been getting rave reviews for being a true 2204-sized motor (with all the weight savings) while producing thrust figures that are comparable to most 2205 motors.

xnova

XNova Super Sonic Series

X-Nova
High quality, cool looking motors. Shaft, weight and price leaves a little to be desired.

ZMX
N/A
The ZMXv2 N52 motors sat as our “best value” for a long time due to their great price (at $13/pp) and amazing performance. Unfortunately due to problems with their manufacturer, SilverDrone had to discontinue the line. They have a new series of motors coming out soon that will reportedly be higher end. For now we are keeping this on the list as a manufacturer to watch.

 

More Research & References

RCGroups Multirotor Power Systems Forum
Fantastic forum dedicated to motors, ESCs, and batteries for motors. This serves as a great jumping off point for learning more about the motors you are choosing.

Miniquad Test Bench Motors Tests
QuadMcFly has done some fantastic bench testing of many of the hobbies most popular motors on this website. He breaks each motor down by its performance with different types of props. I’m eagerly awaiting a compilation article comparing each motor to the others.

Like this buyer’s guide? Check out some of other ones!

 

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