Torque Ripple Minimization of a BLDC Motor Drive by Using Electronic Commutation and Speed, Current Controllers

International journal of Engineering, Business and Management (IJEBM) [Vol-1, Issue-1, May-Jun, 2017]
AI Publications ISSN: 2456-7817
www.aipublications.com Page | 19
Torque Ripple Minimization of a BLDC Motor
Drive by Using Electronic Commutation and
Speed, Current Controllers
Manoranjan. Soora, A.Vandhana, V.Srujana
Assistant Professor, KITS, Warangal, India
M.Tech, Power Electronics, KITS, Warangal, India
Assistant Professor, KITS, Warangal, India
Abstract— Brushless DC motors are having a major
problem with harmonics in torque. The variations in speed
and production of noise should be minimized by using proper
topologies. BLDC motors have been gaining attention from
different Industrial and domestic appliance manufacturers,
because of their high efficiency, high power density and easy
maintenance and low cost. This paper presents a three phase
BLDC motor with low cost drive to be driven without DC link
capacitor. The proposed technique uses an electronic
commutation and operates the machine exclusive of the
intermediate DC link capacitor. The designing of Brushless
DC motor drive system along with control system for torque
ripple minimization, speed controller and current controllers
are presented using MATLAB / SIMULINK and results are
evaluated.
Keywords— Brushless machines, Torque ripple
compensation, six-phase voltage source inverter (VSI),
harmonics suppression, pulse width modulation (PWM).
I. INTRODUCTION
Permanent Magnet Synchronous (PMS) motors and
Brushless DC (BLDC) motors are becoming more useful in
industrial applications and home appliance because of their
high reliability, efficiency and low cost and maintenance
compared to other motors. By using rotor position BLDC
motors are commutated electronically and the rotor position
information can be obtained by using position sensors.
BLDC and PMS motors are now designed with high power
densities, these causes the increasing their popularity in
applications such as airspace applications and mobile
coolers.
PMS motors needs continuous rotor position information for
their operation and a significant computational time is
required to improve the motor performance by controlling
the rotor.
Hall Effect sensors or back emf sensing technique is used to
obtain the rotor position of BLDC motor for every 60
electrical degrees. Therefore, BLDC motors have becoming
more popular for industrial applications where efficiency,
compact and cost effective factors are considered.
.
The motor drive consists of a diode bridge rectifier and a
large electrolytic capacitor with a converter fed rotor for
rotor position information. The main function includes, bus
voltage stabilization, ripple current conduction due to
switching events, etc. In automotive applications, one of the

major problems is the exuberant and barbarous temperatures
they have to withstand, under hood, which during the
summer months would reduce their life.
The intermediate DC link capacitor used in indirect
conversion topologies, requires a large space for its
installation which results in increasing its weight and
occupying place. Usually, a large electrolytic capacitor is
employed to support the intermediate DC link voltage. The
lifetime and properties associated with the capacitor are
affected by the ambient temperature. Furthermore, the type
of dielectric material, the ambient temperature and the
storage temperature are the most significant aging factors for
an electrolytic capacitor mainly in hot or cold environments
viz. heating, ventilation and air conditioning applications. So
the inclusion of the capacitor in the circuit decreases the
overall converter reliability, as it is the most vulnerable
component amongst the other in the circuit.
Without the DC link capacitor, the rectified mains supply is
directly applied to the drive. The absence of DC link
capacitor causes to reduce the overall cost of the motor drive
but at the expense of harmonics in torque, which are
inevitable and expected to be around zero crossing points of
the supply.
II. MATHEMATICAL MODEL
In three-phase BLDC motor drives trapezoidal back emf can
be obtained by energizing only two phases and the third one
remains in off state at any instant of time. As such, only two
switches of the inverter are in conduction at any instant.
Usually, these two switches are controlled using PWM or a
micro controller is used to generate the hysteretic control
signals, either in torque control or speed control mode. This
switching approach is not suitable for motor drives with no
DC link capacitor because there is no continuation path for
the phase current when the controlled switches are in off
state. In the projected BLDC motor drive, a switching
algorithm, which is based on solitary switch control while
keeping the other switch in ON state for the complete
switching interval, is employed. The switch that remains in
ON state provides a freewheeling path to the inductive
current whereas the controlled switch in OFF state.
In a position sensor less BLDC motor drive, the commutation
points of the inverter can be obtained by knowing the back
EMF zero-cross-points (ZCP) and a period of time delay of
speed dependent. The phase back-EMF is trapezoidal which
is induced in the stator windings of a BLDC motor, so that
the Zero Crossing Point of the back-EMF can be detected by
monitoring the terminal voltage waveform of a silent phase.
The case in point when the terminal voltage of the silent
phase equivalent with the half dc link voltage, during which
point switching devices are turned ON is the zero crossing
point of the back-EMF. The estimation processes of
commutated points are shown in fig.
Fig.2
Two different methods for reducing the commutation torque
ripple when a single current sensor is used in the dc link of
the inverter to regulate the current flowing through two
phases in series. These methods are inappropriate to apply to
various trapezoidal BLDC motors, because they are open
loop in nature.
The introduction of independent current sensors in the motor
phases provides a great approach for reducing commutation
torque ripple, because the current regulation of each phase is
possible during commutation periods. The commutation

torque ripple can be minimized during low speed by the
introduction of direct current sensing with hysteresis current
controller.
The commutation torque ripple can be reduced by the
introduction of feed-forward term in current controller to
compensate for the average voltage variation of the non-
commutated phase due to commutation in bipolar pulse
width modulation (PWM) inverter-fed BLDC motor drives.
However, this method cannot be applied to unipolar PWM
which is an appropriate low cost application, because the
voltage between the neutral point of the inverter and the
neutral point of the machine varies with the ON-OFF
position of the switching device of the inverter.
COMMUTATION TORQUE RIPPLE REDUCTION
STRATEGY
A. Analysis of commutation Torque Ripple
Average voltage 1mV applied to a non-commutated phase
before commutation is
 1......
2
1 a
dc
m D
V
V 
   2...
33
12
2
cbabdc
m
eeeDV
V




B.Voltage Disturbance Rejection Method
In order to minimize the pulsating current, The PWM duty
ratio during commutation must be modified in order that the
average voltage of the non commutated phase maintains
constant value, that is mm VV 2
 3.......
24
3
2
1
dc
cba
ab
V
eee
DD


 4.......'
T
t
DD c
bb 
In the voltage disturbance rejection method, the input for
compensation should be applied to the inverter only during
the commutation. So that the voltage disturbance rejection
method uses a commutation interval that can be measured or
estimated from phase current. However, using current
sensors increases the cost of a motor driver.
Table.1: Switching Algorithm
Fig.3: Hall Sensor signals and Switching Signals
(C) MEASUREMENT OF COMMUTATION
INTERVAL
Step
Hall sensor
output
Ha Hb Hc
Switch is
on state
Controlled
switch
1 1 0 0 A1 C2
2 1 1 0 C2 B1
3 0 1 0 B1 A2
4 0 1 1 A2 C1
5 0 0 1 C1 B2
6 1 0 1 B2 A1

Fig.4: Step 2 of the switching algorithm: (a) B1 is on; (b) B1
is off.
Fig.5: A buck converter based model of the motor drive
If R is assumed to be small.
)5()()(2)( te
dt
di
MLtV m
in 
)6()()(20 te
dt
di
ML m

Let the aped of the rotor is ω in rads-1
. Then the torque
produced by the motor Te (Nm), is given as
    )7(

tite
T m
e 
)8()(
)(
tF
dt
td
JTT me 


Where J is the combined inertia (kgm2
) of the rotor and load
and F is the mechanical speed of the motor related by
 9
dt
d
P r

Where P is the number of pole pairs of the BLDC motor
Time T is the interval for Vin (t) to reach E from 0V
and Tm is the period of the input mains voltage.
 10sin
2
1 1






 
MV
E
f
T

Where Vm is the peak value (V) and f is the
frequency (Hz) of the supply.
 
 
 13
2
12
2
11
2
3
2
1





















T
T
tt
T
tt
T
T
tt
m
m
m
Fig.6: Controllable and uncontrollable regions of the motor
drive at steady state.

Fig.7: Proposed technique for torque ripple compensation
IV. IMPLEMENTATION
 
 14avg
in
DC I
dt
tdV
C 
By approximating the derivative, the minimum value of
DCC that is required to provide avgI from the DC link is
given by
 15
2
EV
IT
C
in
avg
DC


To apply the voltage disturbance signal to the system, the
commutation duration must be known. To know the
commutation duration current sensors have been used.
The position senseless controller already uses the terminal
voltages for obtaining the ZCP of phase back-EMF. For
voltage disturbance signal, if the terminal voltage gives the
information related to commutation interval, then the current
sensors are not required. When using the out-going phase
unipolar PWM scheme, the silent phase terminal voltages
remains positive(or zero) dc bus voltage during commutation
and has a different (lower or higher) value than half the dc
bus voltage until silent phase back EMF reaches to zero. By
measuring the comparator output transition time after starting
the commutation,, the commutation duration can be known.
Fig.8: Configuration of the experimental BLDC motor drive
Torque ripple, defined as the difference between the least and
highest torque divided by the greatest torque, consists of two
components: the electromagnetic torque fluctuation and the
reluctance torque. Torque ripple in brushless dc motors is
mostly due to the fluctuations of the field distribution and the
armature MMF which depends on the motor structure and
feed current waveform. At high speeds, torque ripple is
usually filtered by system inertia. On the other hand at small
speeds torque ripple produces noticeable effects that may not
be tolerable in applications such as positioning and robotics.
An idealized brushless DC motor has a trapezoidal back
electromotive force (EMF) waveform. Zero torque ripples is
produced for this waveform when the motor is fed by
rectangular current. However for practical reasons, non-
uniformity of magnetic material and design tradeoffs it is
hard to produce the trapezoidal wave shape it a desired
manner. Therefore torque ripple appears even though
rectangular current is fed in conventional control. In addition,
since the motor windings are inductive, the current controller
has often no ability to produce the required di/dt in the
commutation period due to finite
DC bus supply voltage, the resulting torque ripple is called
commutation torque ripple. Torque ripple can create
undesirable noise and vibration in speed applications, and
can cause inaccuracies in motion control.

V. SIMULATION RESULTS
This paper proposes a method of minimization of torque
ripple of the BLDC motor with un-ideal back EMF
waveforms. The duty cycle of the pulses used to operate the
inverter gates is calculated in the torque controller in normal
conduction period and commutation period. Simulation
results show that compared with conventional control; there
is an apparent reduction in ripple.
Fig.10: Case 3 with M1 for E=65V, (a) Vm(t) and E; (b)
Im(t) by theoretical analysis; (c) Im(t) by simulation; and (d)
Im(t) by experiment
Fig.11: A comparison between the comprehensive model and
the simple model; (a) case 1 with M1; (b) case 2 with M2;
and (c) cas e3 with M1
Fig.12: Current drawn from the DC link for case 1

Fig.13: Proposed compensation for case 1: (a) simulated
)(tim
without a capacitor and with a 150 F capacitor, (b)
simulated )(tim
with proposed compensation; (c)
experimental )(tim
and (d) DC link voltage with proposed
commutation
The motor drive reliability expected to be improved by the
removal of DC link capacitor. The periodic torque ripples are
predictable by removing the DC link capacitor and the motor
drive applicability can be reduced. The compensation
capacitor is approximately around 3% of the innovative
capacitor.
The switching loss of the additional switch is not dominant
due to lower switching frequency and current in comparison
to the IGBTs in the inverter. So that the total harmonic
distortions are reduced and the overall performance of the
device is improved due to the absence of the DC link
capacitor. As the torque ripple compensation capacitance is
low the magnitude s of the capacitor charging currents and
transient problems associated are low and hence this method
is preferred for compensation and to improve the
performance of the drive.
Fig.14: Proposed commutation for case 3: (a) simulated )(tim
without a capacitor and with 150 F capacitor; (b)
simulated )(tim
with proposed compensation; (c)
experimental )(tim
; and (d) DC link voltage with proposed
compensation.
However, the major disadvantage of this proposed method is
that the overall motor drive becomes complex. An additional
IGBT and driving stage is needed in terms of hardware
components.
 1694.0
502
325
95
sin 1
msT 










Fig.15: Torque speed characteristic curve with a 150µF DC
link capacitor

The current in IGBT is measured to estimate the loss
introduced by the additional IGBT, which is used in
compensation circuit.
 172
ONC RIP 
Turn ON and turn OFF losses can be found by using turn on
and turn off switching energy parameters in the datasheet.
The sum of turn on and turn off energies gives the total
switching energy.
Switching loss can be calculated by  18SSS fEP 
Where Sf is the switching frequency. For the additional
switch the switching frequency is 100Hz for a main supply.
VI. CONCLUSION
A simple mathematical model and a reparation technique for
inherent torque ripples of a BLDC motor drive, operated
without a DC link capacitor, have been proposed. The
simplicity of the model permits the controller to be
implemented on inexpensive microcontroller platforms with
very low resources. With the proposed technique for
compensating torque ripples, comparable performance to a
conventional BLDC motor drive with a large DC link
capacitor can be achieved. However, with the torque ripple
compensation technique, the overall complexity of the motor
drive has been increased, which is a major disadvantage.
Based on the application, major augmentations in both
hardware and firmware may be required. The good
agreement between the theoretical results, simulated results
and experimental results demonstrate the accuracy of the
simple buck model and the effectiveness of the proposed
compensation technique. The proposed compensation
technique is expected to be useful for manufacturing low cost
BLDC motor drives with comparable performance.
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