Transcript Detection of back-EMF zero crossing

Robot and Servo Drive Lab.
Simple position sensorless starting
method for brushless DC motor
Electric Power Applications, VOL. 2, Page. 49~Page. 55,
Jan 2008, By Sandeep R, Vasudevan, K.
Professor: MING-SHYAN WANG
Student: CIH-HUEI SHIH
Department of Electrical Engineering
Southern Taiwan University of Science and Technology
2016/3/26
Outline
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Proposed sensorless starting scheme
Flow chart
Block Diagram
Switching sequence
BLDC motor parameters
Functional operations
Detection of back-EMF zero crossing
Line-to-line voltage difference with back-EMF
Inverter switching signals
Speed and phase current waveform
Result
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2016/3/26
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Abstract
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Position sensorless methods for brushless DC motors based on
back-EMF zero crossing suffer from a starting problem since
there is no back-EMF at standstill. A simple method by which the
motor is started from standstill up to a speed where in sensorless
methods will be able to detect the correct commutation instants is
proposed.
2016/3/26
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Introduction
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This paper proposes a simple and reliable method to detect the
back-EMF 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.
2016/3/26
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Proposed sensorless starting scheme
Consider a BLDC motor having three stator phase windings
connected in star. PMs are mounted on the rotor. The BLDC
motor is driven by a three-phase inverter in which the
devices are triggered with respect to the rotor position as
shown in Fig. 1. The phase A terminal voltage with respect
to the star point of the stator Van is given in as
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Southern Taiwan University of Science and Technology
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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)
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Department of Electrical Engineering
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Southern Taiwan University of Science and Technology
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where the symbols have their obvious meanings. From this,
the line voltage Vab may be determined as
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Robot and Servo Drive Lab.
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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.
Subtracting (5) from (4) gives
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Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Flow chart
Start
Commutate to
TC+ TB-
Excite Device
TB+ TCPre-position
No
No
End of
time
Tp?
Commutate to
TC+ TA-
No
2016/3/26
Detect the
+ve going
Zero
crossing of
vabbc ?
Detect the
-ve going
Zero
crossing of
vcaab ?
Commutate to
TA+ TB-
End of
time
Tp?
Exit Start-up
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
No
Continue
the Start-up
Sequence
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Block Diagram
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Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Switching sequence
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Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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BLDC motor parameters
2016/3/26
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Functional operations
2016/3/26
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Detection of back-EMF zero crossing
2016/3/26
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Line-to-line voltage difference with back-EMF
2016/3/26
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Inverter switching signals
2016/3/26
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Speed and phase current waveform
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Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Result
Rotor prepositioning from different initial positions
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Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Line-to-line voltage with Back-EMF
2016/3/26
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Phase current and speed waveform on no-load
Switching signals for inverter with 50% duty ratio
PWM on no-load
Phase current and speed waveform on no-load with 50%
duty ratio PWM
Phase current and speed waveform on load with 50%
duty ratio PWM
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Conclusion
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A simple technique to start the BLDC motor for position
sensorless schemes is proposed. This method makes use of
line-to-line voltage differences to detect and amplify backEMF 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. Simulation and experimental results are shown,
which validate the suitability of the proposed method.
2016/3/26
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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Thanks for your listening
2016/3/26
Department of Electrical Engineering
Robot and Servo Drive Lab.
Southern Taiwan University of Science and Technology
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