1. Chapter 6 Sensorless Control for BLDC Motor Drives
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

2. Increase the volume of the system
Why?
Increase the volume of the system
Disadvantages of position sensors
Easy to be interfered
Low reliability
High precision demand for installation
High reliability
Sensorless control
Strong anti-disturbance capability
Low torque ripple
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd. 2

3. Indirect detecting methods for the rotor
Back EMF-based method
Flux linkage-based method
Inductance-based method
Artificial-intelligence-based method
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

4. Principle Back EMF-based method
Commutation with 30°electrical angle’s delay of zero crossing point of back EMF.
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

5. Back EMF signal at low speed
High speed
Low speed
Difficult to detect directly, weak signal at low speed, not easy to delay 30°.
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd. 5

6. Terminal Voltage Sensing Method
Back EMF-based method
Terminal Voltage Sensing Method
Back EMF Integration Method
Third Harmonic Back EMF Method
Freewheeling Diode Method
Line Back EMF Method
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

7. 1. Terminal Voltage Sensing Method
When phase A and B are conducted and phase C is inactive, there exists
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

8. 1. Terminal Voltage Sensing Method
When phase A and B are conducted and phase C is inactive, there exists
Phase C inactive
Phase B inactive
Phase A inactive
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

9. Signal is easy to obtain but difficult to delay.
1. Terminal Voltage Sensing Method
Waveform of terminal voltage of phase A
Zero crossing point detector
Signal is easy to obtain but difficult to delay.
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

10. 2. Back EMF Integration Method
Advantages: No need for rotor speed information during the control process; lagging or leading commutation of the motors could be done by regulating the threshold; insensitive to the switch signal.
Disadvantages: Integration accumulated errors and threshold setup problems.
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

11. 3. Third Harmonic Back EMF Method
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

12. 3. Third Harmonic Back EMF Method
Wider range of speed, smaller delay of the phase.
However, due to the continuous accumulation of noise signal at low speed, errors will be produced during the integration process, causing inaccurate commutation
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

13. Condition for D2 conducting
Phase A and B conducted, phase C inactive
4. Freewheeling Diode Method
Condition for D2 conducting
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

14. 4. Freewheeling Diode Method
Detection circuit
High sensibility.
Detection circuit is complicated due to the requirement of
six independent sources.
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

15. 5. Line Back EMF Method
No need to use the previous commutation instants, wider speed range of applications;
Complicated calculation of line back EMF
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

16. Flux linkage-based method
Large integral calculation;
Accumulative error will be produced at low speed;
Easily affected by the motor parameters.
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

17. Inductance-based method
Due to the saturation effect of stator core, the flux will change while injecting current pulses in different directions, leading to different inductance.
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

18. Inductance-based method
Well suitable for the rotor initial position detection at stand still;
Relies on high-precision current sensing due to the small difference between the inductances with different rotor positions.
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

19. Starting process for sensorless control
Determination of initial rotor position at standstill
Inductance method
Rotor locating method
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.

20. Three-step starting method
Starting methods for sensorless control
Easily influenced by factors such as load torque, applied voltage, acceleration curve, moment of inertia, etc.
Three-step starting method
Permanent Magnet Brushless DC Motor Drives and Controls, First Edition. Chang-liang Xia.
© 2012 Science Press. Published 2012 by John Wiley & Sons Singapore Pte. Ltd.