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Simple position sensorless starting method for
brushless DC motor
P. Damodharan, R. Sandeep and K. Vasudevan, IET Electr. Power Appl., Vol. 2, No. 1, January 2008
Student: Po-Jui Hsiao
Adviser: Ming-Shyan Wang
Date : 16th-Dec-2009
Department of Electrical Engineering
Southern Taiwan University
Outline
Abstract
Introduction
Proposed sensorless starting scheme
Simulation of the proposed sensorless starting method
Hardware implementation and test results
Starting on no-load
Starting on load
Conclusions
References
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Abstract
A simple method by which the motor is started from standstill up to a
speed wherein sensorless methods will be able to detect the correct
commutation instants is proposed.
The proposed method relies on a difference of line voltages measured
at the terminals of the motor.
It is shown that this difference of line voltages provides an amplified
version of an appropriate back-EMF at its zero crossings. It is further
demonstrated that this information can be used to trigger devices so as
to develop an accelerating torque from zero speed.
This method is simple to implement and it can reliably start the motor
even with load.
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Introduction
When the motor is at standstill, there is no back-EMF induced in the
coil and thus a startup algorithm or an initial rotor position detection
method is required to start the motor reliably from standstill up to a
minimum speed where the conventional position sensorless control
methods based on back-EMF information could take over.
This paper proposes a simple and reliable method to detect the backEMF zero crossings. It is further shown in the paper that this method
can be used to start the machine as well, once the initial rotational
movement is established.
In this work, the rotor is first brought to a known position through a
prepositioning step. Subsequent rotation of the rotor is achieved by a
120 electrical degree triggering followed by a sequential triggering of
the devices based on zero crossings of the back-EMF.
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Proposed sensorless starting scheme
Fig. 1 BLDC motor drive along with typical phase current and back-EMF
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Proposed sensorless starting scheme
dia
Van  Ra ia  La
 ean
dt
(1)
where Ra is the stator resistance of the ‘A’ phase, La the phase
inductance, ean the back-EMF and ia the phase current.
Similar equations can be written for the other two phases, as in (2) and (3)
dib
Vbn  Rbib  Lb
 ebn
dt
( 2)
dic
Vcn  Rc ic  Lc
 ecn
dt
(3)
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Proposed sensorless starting scheme
From this, the line voltage Vab may be determined as
Vab  Van  Vbn
d (ia  ib )
 R(ia  ib )  L
 ean  ebn
dt
( 4)
d (ib  ic )
Vbc  R(ib  ic )  L
 ebn  ecn
dt
(5)
d (ic  ia )
Vca  R(ic  ia )  L
 ecn  ean
dt
( 6)
These line voltages can, however, be estimated without the need for
star point by taking the difference of terminal voltages measured
with respect to the negative DC bus.
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Proposed sensorless starting scheme
Subtracting (5) from (4) gives
d (ia  2ib  ic )
Vabbc  R(ia  2ib  ic )  L
 ean
dt
- 2ebn  ecn
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(7 )
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Proposed sensorless starting scheme
Consider the interval when phases A and C are conducting and phase B is
open as indicated by the shaded region in Fig. 1. In this interval, phase A
winding is connected to the +ve of the DC supply, phase C to the -ve of the
DC supply and phase B is open. Therefore ia  ic and ib  0 . It can
be seen from Fig. 1 (shaded region) that the back-EMF in phases A and C
are equal and opposite. Therefore in that interval (7) may be simplified as
Vabbc  ean  2ebn  ecn  2ebn
(8)
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Proposed sensorless starting scheme
Fig. 2 Flow chart of the proposed
startup scheme
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Proposed sensorless starting scheme
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Simulation of the proposed sensorless starting
method
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Simulation of the proposed sensorless starting
method
From the sensed terminal voltages with respect to negative DC bus
( Va , Vb , Vc ), line voltages and subsequently their differences
( Vcaab ,Vabbc , Vbcca ) are determined.
Va
Fig. 3 Functional sequence of the operations in the proposed algorithm
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Simulation of the proposed sensorless starting
method
Fig. 4 shows the simulated back-EMF waveform of phase B and the line
voltage difference Vabbc during the first triggering of TC+ and TB-. It can be
seen that the plot validates (8) in the region of zero crossing of the backEMF.
Fig. 4 Line-to-line voltage difference with back-EMF
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Simulation of the proposed sensorless starting
method
Fig. 5 Detection of back-EMF zero crossing
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Simulation of the proposed sensorless starting
method
Fig. 6 Inverter switching
signals with the back-EMF
zero crossings and speed
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Simulation of the proposed sensorless starting
method
Fig. 7 Speed and phase current waveform
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Simulation of the proposed sensorless starting
method
Fig. 8 Rotor prepositioning from different initial positions
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Hardware implementation and test results
Fig. 9 Block diagram of the experimental setup
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Starting on no-load
Fig. 10 Line-to-line voltage difference with back-EMF (experimental)
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Starting on no-load
Fig. 11 Phase current and speed waveform on no-load
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Starting on no-load
Fig. 12 Switching signals for inverter with 50% duty ratio PWM on no-load
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Starting on no-load
Fig. 13 Phase current and speed waveform on no-load with 50% duty ratio PWM
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Starting on load
Fig. 14 Phase current and speed waveform on load with 50% duty ratio PWM
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Conclusions
This method makes use of line-to-line voltage differences to detect and
amplify back-EMF signals so that even EMF zero crossings caused by
initial rotor rotation can be easily detected.
Subsequent device triggerings ensure acceleration and are based on
further zero crossing detections.
The motor is found to start smoothly from standstill and run up to a
speed where a sensorless scheme can take over.
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References
1 Kenjo, T., and Nagamori, S.: ‘Permanent-magnet and brushless DC
motors’ (Clarendon Press, Oxford, 1985)
2 Miller, T.J.E.: ‘Brushless permanent-magnet and reluctance motor
drives’ (Clarendon Press, Oxford, 1989)
3 Iizuka, K., Uzuhashi, H., Kano, M. et al.: ‘Microcomputer control for
sensorless brushless motor’, IEEE Trans. Ind. Appl., 1985, IA-21, (4),
pp. 595–601
4 Chen, H.-C., and Liaw, C.-M.: ‘Current-mode control for sensorless
BDCM drive with intelligent commutation tuning’, IEEE Trans.
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5 Cheng, K.-Y., and Tzou, Y.-Y.: ‘Design of a sensorless commutation
IC for BLDC motors’, IEEE Trans. Power Electron., 2003, 18, (6),
pp. 1365–1375
6 Su, G.-J., and McKeever, J.W.: ‘Low-cost sensorless control of
brushless DC motors with improved speed range’, IEEE Trans.
Power Electron., 2004, 19, (2), pp. 296–302
7 Jung, D.-H., and Ha, I.-J.: ‘Low-cost sensorless control of brushless DC
motors using a frequency-independent phase shifter’, IEEETrans.Power
Electron., 2000, 15, (4), pp. 744–752
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References
8 Moreira, J.C.: ‘Indirect sensing for rotor flux position of
permanent magnet AC motors operating over a wide speed range’,
IEEE Trans. Ind. Appl., 1996, 32, (6), pp. 1394–1401
9 Shao, J., Nolan, D., Teissier, M. et al.: ‘A novel microcontroller-based
sensorless brushless DC (BLDC) motor drive for automotive fuel
pumps’, IEEE Trans. Ind. Appl., 2003, 39, (6), pp. 1734–1740
10 Ogasawara, S., and Akagi, H.: ‘An approach to position sensorless
drive for brushless DC motors’, IEEE Trans. Ind. Appl., 1991, 27,
(5), pp. 928–933
11 Kim, T.-H., and Ehsani, M.: ‘Sensorless control of BLDC motors from
near-zero to high speeds’, IEEE Trans. Power Electron., 2004, 19, (6),
pp. 1635–1645
12 Tursini, M., Petrella, R., and Parasiliti, F.: ‘Initial rotor position
estimation method for PM motors’, IEEE Trans. Ind. Appl., 2003,
39, (6), pp. 1630–1640
13 Jang, G.H., Park, J.H., and Chang, J.H.: ‘Position detection and
start-up algorithm of a rotor in a sensorless BLDC motor utilising
inductance variation’, IEE Proc.,- Electr. Power Appl., 2002, 149,
(2), pp. 137–142
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References
14 Lee, W.-J., and Sul, S.-K.: ‘A new starting method of BLDC motors
without position sensor’, IEEE Trans. Ind. Appl., 2006, 42, (6),
pp. 1532–1538
15 Acarnley, P.P., and Watson, J.F.: ‘Review of position-sensorless
operation of brushless permanent-magnet machines’, IEEE Trans.
Ind. Electron., 2006, 53, (2), pp. 352–362
16 Fairchild Semiconductor: ‘Using the ML4425/ML4426 BLDC motor
controllers’, Application note 42004, June 1996
17 Micro Linear: ‘ML4435 Sensorless BLDC motor controller
datasheet’, May 2000
18 Allegro MicroSystems: ‘8904 3-Phase brushless DC motor controller/
driver with back-EMF sensing’, datasheet, 2003
19 Microchip: ‘Sensorless BLDC control with back-EMF filtering’,
Application note AN1083, 2007
20 STMicroelectronics: ‘BLDC motor start routine for the ST72141
microcontroller’, Application note AN1276, 2000
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