Transcript A. Commutation Torque Ripple in the PAO Method

Robot and Servo Drive Lab.
Commutation Control for the Low-Commutation Torque
Ripple in the Position Sensorless Drive of the LowVoltage Brushless DC Motor
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 11, NOVEMBER 2014
Sang-Yong Jung, Member, IEEE, Yong-Jae Kim, Member, IEEE, Jungmoon Jae, and Jaehong
Kim, Member, IEEE
Teacher: Prof. Ming-Syhan Wang
Student (presented): Ika Noer Syamsiana
2016/4/9
Department of Electrical Engineering
Southern Taiwan University of Science and
Technology
OUTLINE :
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Abstract
Introduction
Phase advancing and overlapping control
Analysis of the torque ripple in the PAO method
Analysis of the commutation torque ripple with various
commutation control methods
Simulation and experimental results
Conclusion
Reference
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Abstract
This Journal discusses a commutation control method aimed :
 Reducing the commutation torque ripple in sensorless drive of brushless
direct current motors
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BLDC motors are generally used for low-cost applications because of
relatively high efficiency and low manufacturing cost
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On the other hand, they show a high torque ripple characteristic caused by
nonideal commutation currents
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In order to minimize torque ripple for the entire speed range, a
comprehensive analysis of commutation torque ripple was made according
to three commutation control methods whereupon an optimal current vector
trajectory for low torque ripple was devised
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INTRODUCTION
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The Advantages of BLDC Motor :
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High reliability
Simple frame
Straightforward control
Low friction
Compared to (PMSMs) :high-speed adjusting performance and power
density
The most important part of BLDC drive : Commutation Control
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The previous research :
No
Proposed
Disadvantage
1
Six-step commutation with sensorless  cost
effective solution that requires a simple control
frame and robust
High torque ripple caused by non ideal commutation current that
limits wider use
2
A BLDC motor with trapezoidal back EMF fed by
ideal rectangular current generates smooth
instantaneous torque
Ideal rectangular current shapes cannot be realized in practice
due to the phase inductance and finite inverter voltage
3
Using PWM
When it has different PWM patterns, occurs commutation torque
ripples
4
A Hysterisis and deadbeat current control to
minimize commutation torque ripple by using
inner current loop. In order to keep incoming and
outgoing phase currents changing at the same rate
during commutation, the duty cycle is regulated at
low speed and the deadbeat current control is
adopted at high speed.
PWM duty ratio was modified to compensate voltage
disturbance caused by commutation current in the sensorless
drive of the BLDC motor , an overlapping technique, which
extends the phase conduction period over 120 electrical degree,
was adopted to reduce the torque spike by exciting a new
conducting phase in advance
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The previous research :
No
Proposed
Disadvantage
5
The direct torque control (DTC) scheme
It needs arithmetic calculations for the extracting torque and flux
compensation term that can add further computational overload
to low-cost CPUs
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The duty ratio compensating torque fluctuation in
It needs real-time measurements and calculation of phase
current, angular position, and speed
PWM_ON _PWM method
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A buck converter was used with a new modulation
pattern to reduce the commutation torque ripple
The bandwidth of the buck converter was not considered, so this
structure can only handle torque pulsation at the low speed
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A super-lift Luo topology and SEPIC converter
These structures need complex control or additional power
switches
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The proposed method of this Journal
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The proposed method uses three commutation control methods
for full speed range operation :
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Conventional six-step and phase-advancing (PA)
methods are adopted below the base speed
The phase-advancing with overlapping (PAO)
method is used for over the base speed to obtain
higher speed operation with low torque ripple
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PHASE ADVANCING AND OVERLAPPING CONTROL
A. System Configuration
•13V sensorless BLDC fan motor drive system.
•The BLDC motor is driven by a conventional three-phase inverter.
•DC power is supplied by the battery
Fig 1. Power Circuit Configuration
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Single loop control configuration of the sensorless
BLDC motor drive system
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Speed control output
is directly fed to the
PWM module as the
duty ratio
The commutation
detection block
enables sensorless
operation of the
motor, comparing the
measured back EMF
with half dc-link
voltage
Mode number m is
transferred to the
PWM module to
determine
commutation status,
and actual speed is
measured by
comparing the mode
change instance with
internal timer register
value, tm , of the
digital signal
processor (DSP)
Fig 2a. Whole system configuration
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Fig 2b. phase-to-phase model of BLDC motor drive system
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During two-phase conduction the inverter phase-to-phase
output voltage is directly proportional to the duty ratio.
vab  DVdc
The system transfer function is calculated as :
J m  inertia
K t  torque constant
Bm  friction coefficien t λm  back emf constant
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PI gains :
ζ  damping ratio
ωn  frequency natural
In this case  = 1.11 and n = 2
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B. Phase-Advancing and Overlapping Control of the LowVoltage Sensorless BLDC Motor Drive
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A three-phase BLDC motor is generally driven by 120° two-phase conduction
switching, which is called the six-step commutation method.
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The six-step commutation in phase with the trapezoidal back EMF is the best choice
for high efficiency and the low torque ripple.
This commutation method is quite similar to the maximum torque per ampere
control in the SM-PMSM drive below the base speed because the stator flux always
leads the rotor flux by 90 electrical degrees
The PA technique, which is quite similar to the conventional field-weakening
control in the SM-PMSM drive above the base speed, is a good solution to obtain a
wider speed range
Another approach to obtain a wider speed range is the so-called overlapping
method that increases the conduction period over 120°, i.e., 150° or 180°. That is the
case of ou > 0 and ol = 0.
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That is the cases ou = ol > 0 :
Fig 3. Voltage and
current waveforms
according to the
commutation control
method:
(a)three-phase backEMFs and
(b)a-phase currents
This can reach 50%
of the average torque
with conventional
six-step commutation
method
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ANALYSIS OF THE TORQUE RIPPLE IN THE PAO METHOD
A. Modeling of the Commutation Torque Ripple
From the assumptions of symmetrical windings, general voltage equations for the
three-phase BLDC motor are :
rsy  stator resistance
L sy  stator leagake inductance
  differenti al operator
vsn  virtual neutral point
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Commutation torque ripples are presented in the three-phase
conduction states as :
The electrical
angular speed c is
assumed to be
constant during the
commutation period
t1-t2, due to
relatively large
mechanical time
constant. During t1t2, eb = Vep and ec =
-Vep, where Vep is
the peak value od
the phase back EMF
Fig 4. Waveforms of phase currents, electric torque, and phase back-EMFs when PAO is applied near the
maximum speed.
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The rate of change of torque is given by conventional power equation as
is assumed :
 to be constant during the commutation
period
1.
2.
Large mechanical time constant
The torque equation changes to :
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During the commutation period are represented in the
state space form as :
And :
The offset voltage 
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Fig. 5. Equivalent circuit of period t1-t2 in Fig. 4.
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The duty ratio making :
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B. Torque Ripple's Dependence on the Load Current
In the middle- and high-speed operations, ohmic voltage drop in the stator
winding is negligible
Commutation time :
Thus, the total torque variation caused by the commutation
current is calculated as
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At steady state :
 the commutation torque ripple mostly
depends on Ia1
Vep is obviously a speed dependent variable
(DVcd-Vep) is fixed when the load torque is not varying.
It should be pointed out that just a little variation in magnitude of the commutation
torque ripple is shown with respect to the Vep variation
Fig. 6. Magnitude of the commutation torque ripple
calculated from
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a. When Vep varies a (DVcd-Vep) is fixed
b. When Ia1 varies and Dvdc and Vep are fixed
c. When Vep varies Ia1 and Dvdc are fixed
ANALYSIS OF THE COMMUTATION TORQUE RIPPLE WITH
VARIOUS COMMUTATION CONTROL METHODS
A. Commutation Torque Ripple in the PAO Method
The offset voltage :
a-phase current during the overlapping period is directly calculated
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B. Commutation Torque Ripple in the PA Method
During commutation period , the offset voltage :
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Fig. 8. Variation of the commutation torque
ripple with respect to eb variation.
When eb (t1) at the start of
commutation reduces from Vep,
corresponding magnitude.
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C. Commutation Torque Ripple in the Conventional Overlapping Control
In this case, the time derivative of the commutation torque ripple becomes :
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DESIRED CURRENT TRAJECTORY FOR
THELOW COMMUTATION TORQUE RIPPLE
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The speed is controlled by
the PWM duty ratio at this
region (first trajectory)
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Fig 9. Theoretical full trajectory of the current vector for lowcommutation torque ripple
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Above the base speed, the d-axis reactive current should be injected to
weaken the air-gap field, and higher average voltage should be applied to
overcome the large back EMF
The PAO, which increases both current angle and average voltage applied,
would be a more suitable choice here.
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The position sensorless drive of the BLDC motor generally measures threephase voltages. The voltage of the nonconducting phase is detected to
calculate rotor position during two-phase conduction instance. Therefore,
two-phase conduction instances should appear six times during a period.
Otherwise, the controller fails to detect the rotor position. Thus, the angle
of the current vector cannot exceed 5/24 . Practically
is much less
than /3 for high-load conditions because the tail current is enlarged
depending on the load current and thus the commutation period extends.
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After the speed is saturated with 5/24current angle and full
duty, more average pole voltage can be applied by reducing ol
to zero
Though the commutation torque ripple is enlarged a little, the
maximum possible speed is obtained with ol. So, the final
current angle approaches /6 at the maximum speed operation
The magnitude of average current is defined as
where Tp denotes one electric period
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SIMULATION AND EXPERIMENTAL RESULTS
MATLAB Simulink was used for simulation, and a 6-channel PWM module
in dsPIC33FJ32MC204 for experiments
Parameters for Simulation and Experiment
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SIMULATION AND EXPERIMENTAL RESULTS
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Comparing the simulated current, and torque ripples in three commutation controls. The load
torque of 1Nm was applied when the motor was operating at 1900 r/min for all cases
a. Six-step
commutation
b. PA
ou = ol = /12
c. PAO
ou = /12, ol = 0
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Fig. 10. Current and torque waveforms according to the commutation control ( Vdc=13 V)
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d. PA
ou = ol = /4
e. PAO
ou = /4, ol = 0
Fig. 10. Current and torque waveforms according to the commutation control ( Vdc=13 V)
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A BLDC motor was built for
automotive fan application
experiments. The PWM
frequency was set to 20kHz,
the speed control loop
operated once in a mode, and
the sensorless position
estimation was implemented
with the back EMF-based
method.
The six-step commutation
method is adopted, i.e.,ou =
ol = 0 , at the start
Fig. 11. Waveforms of the phase currents at start operation
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The experimental waveforms of the pole voltages and phase currents with respect to the
commutation control are shown in figure
Fig 12. Pole voltage, and current waveforms according to the commutation control (Vdc=13 V, r =
2500 r/min, Te = 1 Nm): (a) line-to-line back-EMF, (b) six-step commutation, (c) PA, and (d) PAO.
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Fig 12. Pole voltage, and current waveforms according to the commutation control ( Vdc=13 V, r =
2500 r/min, Te = 1 Nm): (d) PAO.
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Fig 13. Measured waveforms of mechanical vibration
(Vdc = 13 V, r = 2500 r/min, Te =1 Nm): (a) six-step commutation, (b) PA, and (c) PAO
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Fig 13. Measured waveforms of mechanical vibration
(Vdc = 13 V, r = 2500 r/min, Te =1 Nm): (c) PAO
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Fig 14. Measured waveforms of mechanical vibration (Vdc = 13 V, r = 2500 r/min, Te =1 Nm): a. six-step
commutation, b. PA, c. PAO
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They are calculated from the measured voltage and current
waveforms. Note that ou = ol in PA, whereas ol = 0 in
PAO.
The commutation torque ripple reduces maximally 15% for
(ou = ol)  [0,/6] with PA
Over /6 the commutation torque ripple becomes severe with
PA and thus it is not suitable for this area. On the other hand
PAO slightly increases the commutation torque ripple,
maximally 13%, compared with six-step commutation.
Moreover the PAO reveals the highest maximum speed
available for the same ou
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Fig 15. The magnitude of commutation torque ripple (D= 0.95, Te=1.13 Nm, Vdc=13 V)
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Fig 16. Maximum speed measured (D=0.95, Te=1.13 Nm, Vdc =13 V)
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CONCLUSION
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A commutation control method aimed at reducing the
commutation torque ripple for low- to high-speed operation in
the low-voltage sensorless drive of the BLDC motor has been
discussed.
In order to minimize the torque ripple through the entire speed
range, a comprehensive analysis of the commutation torque
ripple was made depending on common three commutation
control methods.
An optimal current vector trajectory for the low torque ripple
in the entire speed range was designed based on the analysis.
The proposed sensorless drive method for the low torque
ripple was implemented for automotive fan applications
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Reference :
Department of Electrical Engineering
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Department of Electrical Engineering
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Department of Electrical Engineering
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Department of Electrical Engineering
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