Brushless Motor

First, how brushed motors work

 
To know why brushless motors are so efficient and powerful, it helps to know how standard brushed motors work.

In a conventional RC electric motor, such as what you find in the Sprint 2 car or E-Firestorm truck, you'll find two wires (positive and negative) that connect to specific points in the endbell of the motor, two curved permanent magnets inside the case or 'can' of the motor, and a spinning shaft with wires wrapped around it that goes down the centre of the motor can. The shaft and the wires together are known as the 'armature' of the motor, and at one end is where the motor pinion gear is attached - at the other end is a copper section, this is called the 'commutator'.

Touching the commutator to transfer electrical power to the armature are two 'brushes'. As you can see from the diagram above, the wires that come from the speedo bring power to the brushes, which physically contact the commutator, turning the armature into a basic electromagnet when electricity is applied.

How a standard motor spins

 
When an electromagnet has power applied to it, one end becomes the north 'pole' and the other becomes the south pole. Because the north pole of any magnet is automatically repelled from the north pole of another magnet, the motor armature will want to spin so its north pole is facing the south pole of the permanent curved magnets mounted inside the motor can. As the armature spins around to make the north/south poles meet, the electrical charge applied to the armature flips, so the poles are again repelled from each other and they make the armature spin, turning the pinion gear and your car or truck's transmission. Most electric motors have three poles instead of two - this prevents the battery from shorting out, lowering efficiency, and it also prevents the motor from getting stuck in one position.

The limitations of standard motors

 
The restrictions of brushed motors are made clear when you need to get huge amounts of power and speed from them. Because the brushes must remain in physical contact with the commutator at all times, there is significant friction from them, especially at high speeds. Any imperfection in the commutator makes the brushes bounce and lose contact, making the motor less efficient. This is why racers true the commutator of their race motors after nearly every run, and change the motor brushes almost as often. There is also significant electrical noise generated by the inefficient circuits, and the commutator and brushes eventually wear out, requiring replacement of parts of the motor, or the entire thing.

Now, how brushless motors work

 
The basic explanation of a brushless motor's construction is that it is similar to a brushed motor, except everything is 'inside out' and there are no brushes at all. The permanent magnets that would wrap around the armature in a normal motor are instead placed around the motor shaft, and this assembly is called the rotor. The wire coils are around the inside of the motor can, making several different magnetic poles. In a sensored brushless motor, there are sensors on the rotor that send signals back to the electonic speed control.

Why brushless motors are so much more efficient than brushed motors

Other than the ball bearings the rotor spins on, there is no physical connection at all, automatically making the brushless motor more efficient and longer-lasting because there is no friction of the brushes and commutator. Having a computer (the speedo) control the rotation of the rotor also vastly increases efficiency. There's also no sparking from brushes to commutator so electrical interference is drastically reduced, and finally the coils are much easier to keep cool, boosting efficiency even further.