Background: Brushless DC motors are highly efficient motors due to their high torque to
weight ratio, compact design, high speed operating capability and higher power density. Conventional
Hall sensor based rotor position sensing is affected by the heating, vibration, interference and noise.
Objective: The innovative, cost-effective and easily implementable sensorless techniques are essential
in order to achieve high efficiency, reduced current and reduced torque pulsations. Furthermore,
a delay free, high load fast startup is also an important issue.
Methods: In this paper, an extensive review of various techniques based on the detection of freewheeling
diode current, phase back EMF zero crossing point detection, back EMF integration method
and third harmonic back EMF was done. The effect of various PWM strategies on back EMF
detection were studied. Later on, the sensorless schemes based on flux linkage estimation and flux
linkage increment were introduced. The load torque observers, unknown input observers, sliding
mode observers, L∞ − induced observers, H∞ − deconvolution filter for back EMF estimation were
also reviewed. As the brushless DC motors have no back EMF at starting and for back EMF based
commutation, minimum speed is required for sufficient back EMF. Therefore various strategies of
an open and close-loop reduced current startup have been studied to achieve effective commutation
without reverse torque. Initial Position Detection (IPD) schemes, which are mostly based on saliency
and current response to inductance variation, are effective where reverse torque is strictly prohibited.
A detailed review of these Initial Position Detection techniques (IPD) has also been presented.
Results: The detailed mathematical and graphical analysis has been presented here in order to understand
the working of the state-of-the-art sensorless techniques.
Conclusion: The back EMF detection using direct and indirect methods of terminal voltage filtering
has the problem of delay and attenuation. PWM noise, freewheeling diode spikes and disturbance in
detected back EMFs are also major drawbacks. The problems such as, parameter detuning, underestimation
and overestimation, offset problem, system noise and observer gain variation, etc. limit the
applicability of observer based technique. Therefore, a more robust and precise position estimation
scheme is essential.