A New Approach to Sensorless Control Method for Brushless DC Motors Wind Energy Projects


This paper proposes a new position sensorless drive for brushless DC (BLDC) motors. Typical sensorless control methods such as the scheme with the back-EMF detection method show high performance only at a high speed range because the magnitude of the back-EMF is dependent upon the rotor speed. This paper presents a new solution that estimates the rotor position by using an unknown input observer over a full speed range. In the proposed method, a trapezoidal back-EMF is modelled as an unknown input and the proposed unknown input observer estimating a line-to-line back-EMF in real time makes it possible to detect the rotor position. In particular, this observer has high performance at a low speed range in that the information of a rotor position is calculated independently of the rotor speed without an additional circuit or complicated operation process. Simulations and experiments have been carried out for the verification of the proposed control scheme.


  1. BLDC motor
  2. Full speed range
  3. Sensorless control
  4. Unknown input observer



Fig. 1. Overall structure of the proposed sensorless drive system.


 Fig. 2. Response waveforms at under step change of load torque. (Speed reference: 50 rpm, Load: 0.2 → 0.5 Nm).


Fig. 3. Response waveforms under step change of load torque. (Speed reference: 1650 rpm, Load: 0.75 → 1.5 Nm).


Fig. 4. Response waveforms under step change of speed reference. (Load: 0.75 Nm, Speed reference: 50 → 1650 → 50 rpm).


This paper presented a new approach to the sensorless control of the BLDC motor drives using the unknown input observer. This observer can be obtained effectively by using the equation of augmented system and an estimated line-to-line back- EMF that is modelled as an unknown input. As a result, the actual rotor position as well as the machine speed can be estimated strictly even in the transient state from the estimated line-to-line back-EMF.

The novel sensorless method using an unknown input observer can

  • be achieved without additional circuits.
  • estimate a rotor speed in real time for precise control.
  • make a precise commutation pulse even in transient state as well as in steady state.
  • detect the rotor position effectively over a full speed range, especially at a low speed range.
  • calculate commutation function with a noise insensitive.
  • be easily realized for industry application by simple control algorithm.

The simulation and experimental results successfully confirmed the validity of the developed sensorless drive technique using the commutation function.


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[2] K. Xin, Q. Zhan, and J. Luo, “A new simple sensorless control method for switched reluctance motor drives,” KIEE J. Electr. Eng. Technol., vol. 1, no. 1, pp. 52-57, 2006.

[3] S. Ogasawara and H. Akagi, “An approach to position sensorless drive for brushless DC motors,” IEEE Trans. on Industry Applications, vol. 27, no. 5, pp. 928-933, 1991.

[4] J. C. Moreira, “Indirect sensing for rotor flux position of permanent magnet AC motors operating over a wide speed range,” IEEE Trans. on Industry Applications, vol. 32, no. 6, pp. 1394-1401, 1996.

[5] J. X. Shen, Z. Q. Zhu, and D. Howe, “Sensorless flux-weakening control of permanent-magnet brushless machines using third harmonic back EMF,” IEEE Trans. on Industry Applications, vol. 40, no. 6, pp. 1629-1636, 2004.

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