Electric Drives:

DC brushless reluctance (Description and Applications)

Brushless DC (BLDC) Motors The advent of inexpensive high power switching semiconductors has enabled radical new solutions to the problem of switching and much simpler mechanical designs. Reluctance motors and permanent magnet dependent switching systems Electronic Control which produce rotating magnetic fields to pull the rotor around.

Synchronous Brushless:

DC motors are not strictly DC motors. They use a pulsed DC fed to stator windings on the ground to create a rotating magnetic field and they operate at synchronous speed. Although they do not use mechanical switches, they do require, however, electronic switching to provide the rotating field that adds a little complexity.

Rotating field:

In the diagram below, a pair of poles is first fed with a pulse of current which magnetized A1 pole like a south pole and A2 as a north pole operation of the magnet in its original position. The other hubs are not powered. Then, the current pole pair A is off and the pair pole B is powered by a pulse of current causing B1 pole to be magnetized as a south pole and B2 to be a north pole. The magnet will then rotate clockwise to align pole pair B. Pulsating stator pole pairs in the sequence of the magnet will continue to rotate clockwise to keep joins the pair of poles on. In practice, the poles are fed with a polyphase waveform strengthened to create the smooth field rotation.
Rotational speed:

It is controlled by the pulse rate and torque by the current pulse.
Mechanical Construction No power is supplied, or induced, in the rotors which are constructed from permanent magnets or iron and are dragged by the rotating field. In the absence of currents in the rotors of these machines have rotor I2R losses.
Without the mechanical commutator and rotor windings, the motors have a rotor inertia low allowing much higher speeds to achieve and the elimination of this switch high current mechanics, sparks and RFI is also excreted .
The stator windings are easy to manufacture and install, winding bobbin.
Because all the circuits of heat are placed in the stator, heat dissipation is easier to control and higher currents and powers of the engine can also be achieved.

Permanent Magnet Motors:
The rotor is constructed from permanent magnets that are dragged by the rotating field. Characteristics of speed and torque are very similar to a shunt wound "brushed" (field voltage) DC motor with constant excitation. As with brushed motors spinning magnets from the stator poles create a back EMF in the stator windings. When the engine is fed through a period of three stepped waveform with positive and negative pulses with a duration ranging from 120 degrees, the back EMF wave or stream will be trapezoidal.
Depending on engine size, the magnets can be arranged as a full magnet ring, as rays, or incorporated into the core of the rotor.
A drawback of permanent magnet machines is that magnets are susceptible to complications at high temperature and loss of magnetization above the Curie temperature.
The permanent magnet motors are inherently more efficient than wound rotor machines because they lack the conduction losses associated with currents of the rotor.
Synchronous:

The motor speed is directly proportional to the pulse frequency of the inverter. If the supply frequency is fixed and the engine operates in open loop mode, then it will operate at a fixed synchronous speed. Changing supply frequency will change the motor speed accordingly.
Variable speed operation The brushless DC motor can be done to emulate the characteristics of its cousin brushed in which speed is controlled by changing the voltage applied, rather than changing the frequency of supply. Supply frequency always changes, but he did so as a result of the shift motor not the cause.
In this system, the converter pulses are triggered in a closed loop system by a signal representing the instantaneous angular position of the rotor. The frequency of feeding is controlled by motor speed. A Hall effect device is usually used to detect the rotor position.
In this configuration, increasing the supply voltage pulsed current of the inverter will increase the current in the stator windings thereby increasing the pressure to the rotor poles to the original engine speed, as in a dc motor brushed. Although the engine runs at variable speeds, it is still an application synced from the feedback loop triggers the inverter pulses in synchronism with engine rotation forcing the supply frequency to monitor the motor speed. This also means that the engine will self start.
Features:

High efficiency and power density.
N windings on the ground and therefore no conduction losses.
Compact designs:

The magnets are generally smaller than the detours necessary to provide the equivalent field.
Speeds up to 100,000 rpm possible. The torque is proportional to speed like a DC motor brushed.
Waveform trapezoidal shape.
No switch, so low maintenance and longevity.
Applications:

Applications traction low power wheelchairs and golf carts to use high-powered automobile.