What are Brushless RC Motors?

Brushless RC motors, along with LiPo batteries, have revolutionized our hobby. Only short while ago, RC electric flight was limited to small airplanes and short flight times. Not anymore!

Brushless motors have made it possible for electricity to power large model airplanes that only could be flown with nitro-powered engines in the past.

The significantly increased power to weight ratio of brushless RC motors has opened the door for an entire new breed of powerful electric RC airplanes. Micro brushless motors have also brought indoor flight to a new level!

Brushless RC Motors vs Canned Motors

To fully appreciate the advantages of a brushless RC motor, you need to have a basic understand of how motors work.

"How Stuff Work” has an in depth explanation of how motors work, and the graphics in the section were taken from that article. Below is a quick summary of how a canned motor works.

Standard "canned" Motor

A standard brushed motor is often referred as a canned motor. Every canned motor consists of the following parts:

Armature- The rotating portion of the motor. It consists of the poles, terminals, and the commutator.

Poles- Copper wires wound around a piece of medal forming an electromagnet. The poles are attached to the armature. Most motors have 3 or more poles

Terminal- Point at which the copper wire of a poll attaches.

Commutator- A switch on the armature that reverses the current to the poles every 1/2 rotation so that the magnetic fields of each will always maintain rotation.

Brushes- Tabs in the motor cap that are wired to the battery and make contact with the plates on the commutator as the armature rotates.

Magnets- The outer shell (or can) of the motor is lined with two permanent magnets, of opposite polarity. This non-rotating portion of the motor is also referred as the motor stator.

The battery is wired directly to the brushes. The brushes make contact with the plates of the commutator as the motor turns. There are the same number of plates on the commutator as there are poles on the armature.

When the brushes come in contact with the appropriate plates of the commutator, a particular pole (electromagnet) is charged. When a pole is charged, it is attracted to one of the magnets in the can and repelled by the other.

The commutator acts as a switch by switching the polarity of each pole every time the pole passes a magnet. When the polarity is switched, the pole is attracted to the next magnet in the can while being repelled by the one it just passed. This process repeats as long as power is supplied to the motor.

Disadvantages of Canned Motors

The brushes and commutator wear out
The brushes and commutator must be cleaned periodically
Friction from the brushes slow the motor down
Friction from the brushes lead to shorter flight times and battery life.
Friction from the brushes cause lower power to weight ratio

How brushless motors work

Brushless RC motors work on the same principle as brushed motors, except the electromagnets(poles) are stationary and the permanent magnets are on the spinning portion of the motor.

Since the electromagnets are stationary, there is no need for brushes! The electronic speed controller(ESC) takes care of switching the voltage of the electromagnets.

It's important to know that a brushless speed controller and a brushed speed controller can not be used interchangeably. They perform completely different tasks.

The quickest way to tell the difference between a canned motor and a brushless motor is to count the number of wire leads. All brushless RC motors have 3 wires. Canned motors have 2 wires. The third wire is used for feedback. Switching any two of these wires will change the rotation of the motor.

Inrunner vs Outrunner

There are two types of brushless RC motors. The permanent magnets of inrunner brushless motors are positioned on the inside of the electromagnets.

Inrunner brushless motors are set up very similar to the canned motor explained above, except the permanent magnets and electromagnets are in opposite positions.

An outrunner brushless motor has the permanent magnets on the outside of the electromagnets. You can see in the picture to the right that the outer hub holding the permanent magnets has the output shaft attached in the center.

Inrunner Brushless Model Aircraft Motors

The faster a motor spins, the more efficient it is. Inrunner motors turn very fast and are much more efficient than outrunner motors. Inrunner brushless RC motors require a reducing gearbox between the motor and propeller. For this reason, the output speed and torque of the propeller can easily be "tweaked" to facilitate different flying characteristics by using different size gears.

The downside is added parts that can and do fail. The gears get stripped, and the gearbox shafts are easily bent. It can also be an obstacle when mounting the gearbox motor combination neatly, especially under a cowling.

Outrunner Brushless Model Aircraft Motors

Outrunner RC motors spin much slower and provide much more torque than inrunner motors. The greatest benefit of an outrunner motor is the fact that a gearbox is not needed. This makes the airplane literally silent! Outrunner motors are much easier to mount.

The downside of an outrunner motor is that you have a very small window of propellers you can use. You pretty much have size the motor for the airplane. As mentioned above, you can easily tweak the output speed and torque of an inrunner motor by sizing the gears differently

Outrunner brushless RC motors are also less efficient that inrunner motors. This shouldn't be a huge factor when making your decision between the two.

Outrunners	Inrunners

Low RPM's, high torque Less efficient than inrunners No gearbox required Narrow prop selection Silent	High RPM's, low torque More efficient than outrunners Require a gearbox Wide prop selection Noisy

If you're in the market for a brushless motor, you'll be pleased to know that Red Rocket Hobby shop carries a wide selection of brushless RC motors at a very reasonable price. Red Rocket is a very reputable company. The owner also flies RC and will be glad to help your with sizing your motor.