BLDC motor control
Sensorless BLDC control, sometimes called the sensorless trapezoidal controller of BLDC, uses back EMF (BEMF) to determine the location of the motor rotor relative to the motor stator.

A voltage applied across a motor winding forces the motor rotor to rotate. However, the movement of the rotor through the magnetic field of the motor is analogous to the behavior of a generator consequently the controller not only receives an applied voltage but also generates its voltage. This voltage is known as the back EMF or back electromotive force and is proportional to the rotational speed of the motor. The rear EMF can be used to determine the speed and position of a motor's rotor; no sensors are required. Controlling a motor via the rear EMF is not an easy task; Most sensorless BLDC motors are controlled by a microcontroller, digital signal processor, or dedicated controller IC. The following figure shows a typical sensorless BLDC controller.

Brushless DC motors offer significant advantages over standard brush motors. Brushless DC implementations can be sensorless or based on Hall-effect sensors built into the BLDC controller. Sensorless systems reduce costs and require fewer interconnections between the controller module and the BLDC controller; They can be somewhat complex, but high-performance ICs help simplify the design task. Although sensorless systems are often preferable, the use of Hall effect sensors may be a better option for low-speed applications.

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