Chapter 6 presentsa detailed analysis of the shape of the current and back-emf waveforms ina trapezoidal brushless dc motor drive and their effects on the torque/speedperformance.
Chapter 3 elaborates on the analysis and simulation work by presentinga comprehensive analysis which aims to show that direct three-phaserepresentation can be used as an effective tool for performance assessementof brushless dc drive systems operating over a wide speed range.
X-NUCLEO-IHM07M1 - Three-phase brushless DC motor …
When sliding mode observer with signum function is implemented to estimate the back-EMF of BLDC motor the resulted graph has chattering due to the highly discontinuous nature of the signum function.
Induction Versus DC Brushless Motors | Tesla
The brushless motor, unlike the DC brushed motor, has the permanent magnets glued on the rotor. It has usually 4 magnets around the perimeter. The stator of the motor is composed by the electromagnets, usually 4 of them, placed in a cross pattern with 90o angle between them. The major advantage of the brushless motors is that, due to the fact that the rotor carries only the permanent magnets, it needs of NO power at all. No connection needs to be done with the rotor, thus, no brush-commutator pair needs to be made! This is how the brushless motors took their name from. This feature gives the brushless motor great increament in reliability, as the brushes wear off very fast. Moreover, brushless motors are more silent and more efficient in terms of power consumption.
Induction Versus DC Brushless Motors
Servo control of motors has important applications msuch areas as robotics, numerically controlled machinesand 'fly by wire* aircraft systems. The development ofhigh power high coercivity magnetic alloys, such as samarium cobalt, has led to the advent of the brushless dc machine, which offers a more advantageous alternative to the brush machine. The brushless DC machine eliminates the need for brush contacts, through the use of electronic commutation. It has better thermal characteristics, as the rotor does not carry any current, and the rotor moment of inertia tends to be smaller than an equivalent brush machine. To achieve servo performance with a brushless dc machine requires the control of the motors torque angle. This thesis proposes digital control schemes for torque angle control, presents a simulation of the motor system with the schemes implemented, and concludes that digital schemes are more advantageous than analog control schemes, in the context of torque angle control of a brushless DC system.
Brushless DC Motors--Part II: Control Principles | EDN
As you understand, this is a major drawback of brushless motors. They need of a controller circuitry to operate. Yet when reliability is required, this motor is the most suitable. The following video demonstrates exactly how a typical (and very popular type) of a brushless motor is made:
Brushless DC Motors – Part I: Construction and Operating Principles
The trick of operation in BLDC motors is the Hall sensor that is attached to the stator. It faces the magnets perpendicularly and can distinguish if the North or South pole is in front of it. The following image shows this Hall senor. The photo is taken from a PC fan (yes, PC fans do have BLDCs!):