Brushless DC Motors are an important class of highly efficient electric motors. They are most frequently used in electric vehicles because of their high efficiency and high power density. They are also present in the small DC fans used in power supplies and CPU coolers.
In order to discuss the Brushless DC Motor it is important to first have a basic understanding of the more common Brushed DC Motor which is present in many household items. The brushed DC motor consists of a rotor (the part that rotates) containing several coils of wire. This is attached to the output shaft. The rotor is surrounded by a magnetic field (this is usually a permanent magnet, but can be an electro magnet). Electrical connection to the rotor is accomplished via a device called a commutator. A commutator is basically a conductive ring that rotates with the rotor shaft. This ring has been split into segments (there are as many segments as there are coils) that are insulated from one another, but which are connected to the leads of the coils. A pair of carbon or brass brushes are positioned on either side of the commutator rub against it making electrical connections. The commutator segments are oriented relative to the permanent magnets such that when one coil is energized it will generate an electro-magnetic field that will repel or attract the permanent magnets. This causes a torque on the rotor, and the shaft turns. As the shaft turns, different segments of the commutator come into contact with the brushes, and different coils are excited. The result is that the shaft will continually rotate.
Brushless DC Motors are composed of a rotor (the part that rotates) containing a series of permanent magnets (usually high energy rare earth varieties like Samarium Cobalt, or Neodymium Iron Boron). This rotor sits in a housing surrounded by the stator (the stationary part). The stator is composed of several coils of wire. The number of wire coils varies depending on the type of motor. A Brushless DC Motor also requires some sort of position sensor, usually an encoder. Since the permanent magnets are located on the rotor instead of on the stator, the Brushless DC Motor is sometimes called an "inside-out" motor.
Brushless DC Motors also require a controller, and will not function without one, unlike a standard Brushed DC Motor. The controller takes input DC power as input as well the output of the position sensor and control signals from the operator (human or computer). These control signals usually consist of a throttle signal and other information such as the level of regenerative braking desired. Given position information and control signals the controller knows which of the coil(s) in the motor to excite and how much to excite it(them). Rotation of the rotor is accomplished by energizing the coils of the motor in a specific order which causes the magnetic field generated by the motor coils to rotate (or appear to rotate anyway). The rotor has no choice but to follow along. The controller and sensor combination act as an electrical commutator (as opposed to a mechanical commutator as seen in the Brushed DC Motor).
Brushless DC Motors have many advantages over their conventional cousins. Brushed DC Motors often require service, because the brushes wear out and or become damaged in normal operation. In addition to this, there is a spark generated each time the brushes come into contact with a new set of commutator segments. This generates wide band radio interference, and is also hazardous if working near flammable materials. Brushless DC Motors also have a higher power density because of their "inside-out" design. The coils of the motor generate most of the losses associated with most if not all motor types. Brushless DC Motors have the advantage of coils which are attached to the motor housing rather than sealed inside the case. The result is that heat can be dissipated faster. This is especially important since the magnets most commonly used in these motors have a low Curie temperature. At temperatures above 100C high energy Neodymium Iron Boron magnets begin to lose their magnetic properties. If operated at elevated temperatures for long periods of time the magnets will no longer be permanent magnets.
It is not unusually to have a Brushless DC Motor capable of generating 10HP that is the size of a small coffee can. All of this does come at a price, as Brushless DC Motors are considerably more expensive than their brushed counterparts.