  This paper provides a technical review of position and speed sensorless
methods for controlling Brushless Direct Current (BLDC) motor drives, including
the background analysis using sensors, limitations and advances. The
performance and reliability of BLDC motor drivers have been improved because
the conventional control and sensing techniques have been improved through
sensorless technology. Then, in this paper sensorless advances are reviewed and
recent developments in this area are introduced with their inherent advantages
and drawbacks, including the analysis of practical implementation issues and
applications. The study includes a deep overview of state-of-the-art back-EMF
sensing methods, which includes Terminal Voltage Sensing, Third Harmonic
Voltage Integration, Terminal Current Sensing, Back-EMF Integration and PWM
strategies. Also, the most relevant techniques based on estimation and models
are briefly analysed, such as Sliding-mode Observer, Extended Kalman Filter,
Model Reference Adaptive System, Adaptive observers (Full-order and
Pseudoreduced-order) and Artificial Neural Networks.
