Download Elimination of the motor neutral voltage

A New Cost Effective Sensorless Commutation
Method for Brushless DC Motors Without
Phase Shift Circuit and Neutral Voltage
Cheng-Hu Chen and Ming-Yang Cheng,
Member, IEEE
PTT製作100%
Adviser : Ying-Shieh Kung
Student : Chien-Hung Chen
陳建宏
M9920109
南台科大電機系
OUTLINE
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ABSTRACT
I. INTRODUCTION
II. MATHEMATICAL MODELS OF EACH COMMUTATION STATE
III. PROPOSED ZCP DETECTION APPROACH BY AVERAGE LINE
TO LINE VOLTAGE
IV. ANALYSIS OF THE COMMUTATION ERROR
V. EXPERIMENTAL EVALUATION
VI. CONCLUSION
REFERENCES
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Abstract(1/2)
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This paper presents the analysis, design, and
implementation of a high performance and cost effective
sensorless control scheme for the extensively used
brushless dc motors.
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In an effort to decrease cost and increase ease of
implementation, the commutation signals are obtained
without the motor neutral voltage,multistage analog filters,
A/D converters, or the complex digital phase shift (delay)
circuits which are indispensable in the conventional
sensorless control algorithms.
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Abstract(2/2)
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In the proposed method, instead of detecting the zero
crossing point of the nonexcited motor back
electromagnetic force (EMF) or the average motor
terminal to neutral voltage, the commutation signals are
extracted directly from the specific average line to line
voltages with simple RC circuits and comparators.
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Theoretical analysis and experiments are conducted over a
wide operating speed range and different back EMF
waveforms to justify the effectiveness of the proposed
method.
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I. INTRODUCTION(1/2)
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DURING the last two decades, a lot of research on sensorless control
techniques for brushless dc motors (BLDCMs) have been conducted. This
research can be divided into four categories.
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Detection of the zero crossing point (ZCP) of the motor terminal
to neutral voltage with a precise phase shift circuit.
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Back electromagnetic force (EMF) integration method.
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Sensing of the third harmonic of the back EMF.
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INTRODUCTION(2/2)
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The neutral voltage is required for comparison with the non-conducted
back EMF or the average terminal voltage, in which it will introduce a high
common-mode noise.
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Since the zero crossing points of the conventional back EMF method are
inherently leading 30 electric degrees of the ideal commutation points, a
precise velocity estimator and a phase shift circuit (algorithm) are needed
to process the zero crossing signals so that accurate commutation points
can be determined.
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II. MATHEMATICAL MODELS OF EACH
COMMUTATION STATE
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Fig. 1 shows the equivalent circuit of a BLDCM and the
inverter topology.
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Fig. 2 illustrates the relationship
among the back EMF waveform of an
ideal BLDCM, the armature current,
the commutation signals (H1–H3),
and the switching signals (S1–S6) for
the inverter.
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According to the polarity of the
armature current as illustrated in Fig. 2,
the terminal voltage of each phase can
be divided into three sub-sections, i.e.,
positive, negative, and nonconducted.
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If the switch of the upper leg is conducted (e.g., S3 is on), the neutral
voltage can be expressed as
According to (6) and (7), the neutral voltage can be written as
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If the switch of the upper leg is not conducted (e.g., S3 is off), the neutral
voltage can be expressed as
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III. PROPOSED ZCP DETECTION APPROACH BY
AVERAGE LINE TO LINE VOLTAGE
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The major problem of the conventional back EMF sensing techniques is
that they require noisy motor neutral voltage and a fixed phase shift circuit.
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Since the noisy motor neutral voltage will introduce the common mode
noise into the sensorless circuit ,a low pass filter is indispensable.
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On the other hand, the fixed phase shift function over a wide speed range is
hard to implement with analog circuits.
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In order to cope with the aforementioned problems, the proposed method
extracts the commutation points directly from the motor terminal voltages
with simple comparators and a single stage low pass filter.
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If the terminal voltages are expressed in the average form (i.e., duty ratio),
the switching states in (3), (4), (17), and (18) can be eliminated. The
terminal voltages are rewritten as follows.
States I and II: Armature Current is Positive:
States III and VI: Armature is Open (Nonconducted):
States III and VI: Armature is Open (Nonconducted):
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According to (19)–(21), the ideal average terminal voltages for all three
phases with different duty ratios are illustrated in Fig. 5.
Fig. 5. Ideal average terminal voltages under different duty ratios.
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The measured instantaneous (upper trace) and average (lower trace)
terminal voltages as the duty ratio is increased from 10%,to 50%, to 100%
are shown in Fig. 6.
Fig. 6. Measured instantaneous (first trace) and average
(second trace) terminal voltages under different duty ratios. (a)
Duty ratio = 10%. (b) Duty ratio = 50%. (c) Duty ratio = 100%.
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Equation (23) reveals that the zero
crossing points of the average line to
line voltage will occur at 30 and 210
electric degrees.
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According to (22) and (23), Fig. 7
shows the phase relationship among
the ideal back EMF, the average
terminal voltage, and the average line
to line voltage of phase “a ” and
phase “c .”
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It is clear to see that the average line
to line voltage Vac lags 30 electric
degrees compared with the back
EMFea , namely the zero crossing
points of the line to line voltage are
in phase with the ideal commutation
signals.
Fig. 7. Phase relationship among the back EMF,
the average terminal voltage,and the average
line to line voltage.
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• Fig. 8(a) illustrates the practical circuit for implementing the proposed
approach to obtain the commutation signals (namely the virtual Hall effect
signals H1~H3 ).
• Consequently, the circuit needed in the proposed approach is much
simplercompared with that needed in the conventional circuit shown in Fig.
8(b).
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Fig. 8. Proposed and conventional sensorless commutation circuits. (a)
Proposed cost effective sensorless commutation circuit. (b) Conventional
sensorlesscommutation circuit.
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IV. ANALYSIS OF THE COMMUTATION
ERROR
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A. Phase Delay by the Low Pass Filter and the Armature Impedance
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B. Voltage Spikes by the Residual Current
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A. Phase Delay by the Low Pass Filter and the
Armature Impedance
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The phase delay angles caused by the input low pass filter and the armature
impedance shown in Fig. 9(a) and (b) can be expressed as
Since the 30 (or 90 ) phase shift circuit shown in Fig. 8(b) is not required in the
proposed approach, the corner frequency fc of the input low pass filter can be easily
determined by the maximum motor speed RPMmax and the switching frequency fs ,
in which the value of fc can be chosen as
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B. Voltage Spikes by the Residual Current
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The voltage spikes shown in Figs. 4 and 6 are created by the residual
current when the armature current is blocked by the power switches. The
voltage spike is the main cause for the commutation error in the
conventional back EMF integration method and the window-captured back
EMF method (detecting back EMF during the silent period) .
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V. EXPERIMENTAL EVALUATION
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Fig. 10 shows the block diagram
of the proposed sensorless control
method. The system can be
divided into several subblocks,
including a velocity command
generator, an open loop starting
process, a line to line voltage
based virtual Hall effect signal
circuit, an electric commutation
table, and a PWM generator.
Fig. 10. Block diagram of the overall system.
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Fig. 11. Structure of the employed
BLDCMs. (a) Type I (segmented
magnet), trapezoidal back EMF. (b)
Type II (ring magnet), sinusoidal back
EMF.
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Fig. 12. Measured back EMF waveforms of employed BLDCMs.
(a) Type Imotor. (b) Type II motor.
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Fig. 13. Measured commutation signals under different duty ratios and back EMF waveforms
(from top to bottom: average terminal voltage V a, average terminal voltage Vc , average
line to line voltage Vac , estimated commutation signal, signal from Hall effect sensor). (a)
Duty ratio = 10%, type I motor. (b) Duty ratio= 50%, type I motor. (c) Duty ratio = 100%,
type I motor. (d) Duty ratio = 10%, type II motor.
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It can be seen that the signal
from the conventional
solution strongly depends on
the operating speed; the
mismatch angle is leading
21.8 in 10% full-speed .
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Fig. 14. (a) Duty ratio = 10%.
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Lagging 14.4 in 50%
full-speed.
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Fig. 14. (a) Duty ratio = 50%.
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lagging 22.8 in full speed.
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Fig. 14. (a) Duty ratio = 100%.
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VI. CONCLUSION
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Unlike conventional back EMF based sensorless commutation methods
which focus on detection of the ZCP of the motor terminal to neutral
voltage, a novel sensorless commutation method based on the average line
to line voltage is proposed in this study. Both theoretical analysis and
experimental results verify that satisfactory performance can be achieved
with the proposed sensorless commutation method. Compared with the
conventional solutions, the proposed method has several advantages,
including the following.
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Elimination of the motor neutral voltage:
 The neutral voltage is not required in the proposed method, only the
three motor terminal voltages need to be detected.
Elimination of the fixed phase shift circuit:
 The proposed specific average line to line voltage inherently lags 30
electric degrees compared with the phase back EMF. Moreover,
experimental results have revealed that the phase relationship is
insensitive to operating speed and load conditions.
Low starting speed:
 Since the amplitude of the line to line voltage is significantly larger
than the phase voltage, even a small back EMF can be effectively
detected. Namely, a lower open loop starting speed can be achieved.
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REFERENCES
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