Harmonics Reduction of Multilevel Inverter Drive Using Sine Carrier Pulse Width Modulation Techniques

S. Ebanezar Pravin. Int. Journal of Engineering Research and Application www.ijera.com
ISSN : 2248-9622, Vol. 6, Issue 11, ( Part -2) November 2016, pp.66-73
www.ijera.com 66 | P a g e
Harmonics Reduction of Multilevel Inverter Drive Using Sine
Carrier Pulse Width Modulation Techniques
S. Ebanezar Pravin, S. P. Umayal
ABSTRACT
The main objective of this paper is to control the speed of an induction motor by using seven level diode
clamped multilevel inverter and improve the high quality sinusoidal output voltage with reduced harmonics. The
presented scheme for diode clamped multilevel inverter is sine carrier Pulse Width Modulation control. An open
loop speed control can be achieved by using V/ƒ method. This method can be implemented by changing the
supply voltage and frequency applied to the three phase induction motor at constant ratio. The presented system
is an effective replacement for the conventional method which has high switching losses, its result ends in a
poor drive performance. The simulation result portrays the effective control in the motor speed and an enhanced
drive performance through reduction in total harmonic distortion (THD). The effectiveness of the system is
verified through simulation using PSIM6.1 Simulink package.
Keywords: Diode clamped multilevel inverter; Induction motor; Sine carrier PWM technique; THD; V/ƒ
method.
I. INTRODUCTION
AN induction motor being rugged, reliable,
and relatively inexpensive makes it more preferable in
most of the industrial drives. They are mainly used for
constant speed applications due to unavailability of
the variable-frequency supply voltage [1]. But, many
applications are in need of variable speed operations.
In early times, mechanical gear systems were used to
obtain variable speed. Recently, power electronics and
control systems have matured to allow these
components to be used for motor control in place of
mechanical gears. These electronics not only control
the motor’s speed, but can improve the motor’s
dynamic and steady state characteristics. Adjustable
speed ac machine is equipped with an adjustable
frequency drive.
It is a Multi level inverter drive for speed
control of an induction motor. It controls the speed of
the electric machine by converting the fixed voltage
and frequency to adjustable values on the machine
side. High power induction motor drives using
classical three-phase converters have the
disadvantages of poor voltage and current qualities.
To improve these values, the switching frequency has
to be raised which causes additional switching losses.
Another possibility is to put a motor input filter
between the converter and motor, which causes
additional weight. The diode clamp method can be
applied to higher level converters. As the number of
level increases, the synthesized output waveform adds
more steps, producing a staircase waveform. A zero
harmonic distortion of the output wave can be
obtained by an infinite number of levels.
In this paper, a three-phase diode clamped
multilevel inverter fed induction motor is described.
The diode clamped inverter provides multiple voltage
levels from a five level unidirectional voltage
balancing method of diode clamped inverter [2]. The
voltage across the switches has only half of dc bus
voltage. These features effectively double the power
rating of voltage source inverter for a given
semiconductor device [3]. The presented inverter can
reduce the harmonic contents by using multicarrier
SPWM technique. It generates motor currents of high
quality. V/ƒ is an efficient method for speed control in
open loop. In this scheme, the speed of induction
machine is controlled by the adjustable magnitude of
stator voltages and its frequency in such a way that the
air gap flux is always maintained at the desired value
at the steady-state. Here, the speed of an induction
motor is precisely controlled by using seven level
diode clamped multilevel inverter.
II. CONVENTIONAL METHOD
The voltage source inverter produces an
output voltage or a current with levels either zero or
±Vdc. They are known as two level inverter. To
obtain a quality output voltage or a current waveform
with a minimum amount of ripple content, they
require high switching frequency along with various
pulse-width modulation strategies. In high power and
high-voltage applications, these five-level inverters
have some limitations in operating at high frequency
mainly due to switching losses and constraints of
device rating. The dc link voltage of a Five-level
inverter is limited by voltage ratings of switching
devices, the problematic series connection of
switching devices is required to raise the dc link
voltage. By series connection, the maximum
allowable switching frequency has to be more
lowered; hence the harmonic reduction becomes more
difficult [4]. In addition, the five level inverters
RESEARCH ARTICLE OPEN ACCESS

generate high frequency common-mode voltage
within the motor windings which may result in motor
and drive application problems [5]. From the aspect of
harmonic reduction and high dc-link voltage level,
Five-level approachseems to be the most promising
alternative. The harmonic contents of a seven-level
inverter are less than that of a five-level inverter at
the same switching frequency and the blocking
voltage of the switching device is half of the dc-link
voltage [6]. A seven level inverter will not generate
common-mode voltages if the inverter output
voltages are limited within certain available switching
states. So, the seven-level inverter topology is
generally used in realizing the high performance, high
voltage ac drive systems [7].
Fig 1. Five level inverter based drive circuit.
III. DRIVE SYSYEM DISCRIPTION
In the conventional technique normal PWM
method is used. So that the voltage and current is of
poor qualities and the switching frequency causes
more amount of switching losses. Those drawbacks
are rectified using three phase diode clamped
multilevel inverter. The voltage and current quality are
better and the switching losses are reduced when
compared to the conventional technique. Also the
THD is found to be better.
A. Structure of Seven Level Diode
ClampedMultilevel Inverter
The seven-level neutral point-clamped
voltage source inverter is shown in Fig.2. It contains
36 unidirectional active switches and 30 neutral point
clamping diodes. The middle point of the 6 capacitors
―n” can be defined as the neutral point [9]. The major
benefit of this configuration is each switch must block
only one-half of the dc link voltage (Vdc/6). In order
to produce seven levels, only two of the twelve
switches in each phase leg should be turned on at any
time. The dc-bus voltage is split into three levels by
two series-connected bulk capacitors, Ca and Cb, via.
They are same in rating. The diodes are all same type
to provide equal voltage sharing and to clamp the
same voltage level across the switch, when the switch
is in off condition. Hence this structure provides less
voltage stress across the switch.
B. Principle of Operation
Table.1. shows the voltage levels and their
corresponding switch states. State condition 1 means
the switch is on, 0means the switch is off. There are
two complementary switch pairs in each phase. These
pairs for one leg of the inverter are
(A1,A1’),(A2,A2’),(A3,A3’),(A4,A4’),(A5,A5’),(A6,
A6’). If one of the complementary switch pairs is
turned on, the other of the same pair must be off [10]-
[14].
To produce a staircase-output voltage, consider one
leg of the three-level inverter, as shown in Fig.3. The
steps to synthesize the seven-level voltages are as
follows.
1) For an output voltage level Vao=Vdc, turn on all
upper-half switches A1,A2.A3,A4,A5 and A6
2) For an output voltage level Vao=5Vdc/6, turn on
upper switch A2.A3,A4,A5,A6 and one lower
switch A1’.
3) For an output voltage level Vao=4Vdc/6, turn on
all lower half switches A3, A4, A5, A6 and A1’,
A2’.
4) For an output voltage level Vao=Vdc/3, turn on
all lower half switches, A5, A6, and A1’, A2’.A3’,
A4’.
all lower half switches, A6 and A1’, A2’.A3’, A4’,
A5’.
6) For an output voltage level Vao=0, turn on all
lower half switches , A1’,A2’.A3’,A4’,A5’ and A6’
all lower half switches, A4, A5, A6, and A1’,
A2’.A3’.

Table I Output Voltage Levels And Their Switching States
Voltage Va0
Switch State
A
1
A
2
A
3
A
4
A
5
A
6
A
1’
A
2’
A
3’
A
4’
A
5’
A
6’
V7 1 1 1 1 1 1 0 0 0 0 0 0
V6 0 1 1 1 1 1 1 0 0 0 0 0
V5 0 0 1 1 1 1 1 1 0 0 0 0
V4 0 0 0 1 1 1 1 1 1 0 0 0
V3 0 0 0 0 1 1 1 1 1 1 0 0
V2 0 0 0 0 0 1 1 1 1 1 1 0
V1 0 0 0 0 0 0 1 1 1 1 1 1
The most attractive features of multilevel inverters
are as follows:
1) They can generate output voltage switch extremely
low distortion and lower dv/dt.
2) They draw input current with very low distortion
[15].
3) They generate smaller common mode (CM)
voltage, thus reducing the stress in the motor
bearings [16].
4) They can operate with a lower switching frequency
[17].
5)
IV. PROPOSED SCHEME
The block schematic of multilevel inverter
fed three phase induction motor is show in figure.15.
The complete system will consist of two sections; a
power circuit and a control circuit. The power section
consists of a power rectifier, filter capacitor, and three
phase diode clamped multilevel inverter. The motor is
connected to the multilevel inverter. An ac input
voltage is fed to a three phase diode bridge rectifier, in
order to produce dc output voltage across a capacitor
filter. A capacitor filter, removes the ripple contents
present in the dc output voltage [18]-[19].
The pure dc voltage is applied to the three
phase multilevel inverter through capacitor filter. The
multilevel inverter has 36 MOSFET switches that are
controlled in order to generate an ac output voltage
from the dc input voltage The control circuit of the
proposed system consists of three blocks namely
microcontroller, opto-coupler and gate driver circuit.
The microcontroller is used for generating gating
signals required to drive the power MOSFET switches
present in the multilevel inverter. The voltage
magnitude of the gate pulses generated by the
microcontroller is normally 5V. To drive the power
switches satisfactorily the opto-coupler and driver
circuit are necessary in between the controller and
multilevel inverter. The output ac voltage is obtained
fromthe multilevel inverter can be controlled in both
magnitude and frequency (V/ƒ open loop control).
Fig 2. Seven level inverter based drive circuit

The controlled ac output voltage is fed to the
induction motor drive. When the power switches are
on, current flows from the dc bus to the motor
winding. The motor windings are highly inductive in
nature; they hold electric energy in the form of
current. This current needs to be dissipated while
switches are off. Diodes are connected across the
switches give a path for the current to dissipate when
the switches are off. These diodes are also called
freewheeling diodes. The V/ƒ control method permits
the user to control the speed of an induction motor at
different rates. For continuously variable speed
operation, the output frequency ofmultilevel inverter
must be varied. The applied voltage to the motor must
also be varied in linear proportion to the supply
frequency to maintain constant motor flux.
Fig 3. Basic block diagram.
V. MODULATION STRATEGY
This Paper mainly focuses on multicarrier
SPWM method. This method is simple and more
flexible than SVM methods. The multicarrier SPWM
method uses several triangular carrier signals,
keeping only one modulating sinusoidal signal. If an
n-level inverter is employed, n-1 carriers will be
needed. The carriers have the same frequency WC
and the same peak to peak amplitude Ac and are
disposed so that the bands they occupy arecontiguous.
The zero reference is placed in the middle of the
carrier set. The modulating signal is a sinusoid of
frequency Wm and amplitude Am. At every instant
each carrier is compared with the modulating signal.
Each comparison gives 1(-1) if the modulating signal
is greater than (lower than) the triangular carrier in
the first (second) half of the fundamental period, 0
otherwise. The results are added to give the voltage
level, which is required at the output terminal of the
inverter. Multicarrier PWM method can be
categorized into two groups: 1) Carrier Disposition
(CD) method 2) Phase shifted PWM method.
1) Advantages of multicarrier PWM techniques
2) Easily extensible to high number of levels.
3) Easy to implement.
4) To distribute the switching signals correctly in
order to minimize the switching losses.
5) To compensate unbalanced dc sources.
6) Alternative Phase Opposition Disposition
(APOD), where each carrier band is shifted by
180° from the adjacent bands
7) Phase Opposition Disposition (POD), where the
carriers above the zero reference are in phase, but
shifted by 180° from those carriers below the
zero reference.
8) In-Phase Disposition (PD), where all the carriers
are in phase [8].
In this paper the gating pulses for IGBT switches are
generated by using In-phase disposition
technique
Modulation Index;
(1)
N=number of Levels, Am=Modulation signal
Amplitude, Ac=Carrier Signal Amplitude
a. V/Ƒ Control Theory
Figure.7. shows the relation between the
(voltage and torque) versus frequency. The voltage
and frequency being increased up to the base speed.
At base speed, the voltage and frequency reach the
rated values. We can drive the motor beyond base
speed by increasing the frequency further. But the
voltage applied cannot be increased beyond the rated
voltage. Therefore, only the frequency can be
increased, which results in the field weakening and
the torque available being reduced. Above base
speed, the factors governing torque become complex,
since friction and windage losses increase
significantly at higher speeds. Hence, the torque
curve becomes nonlinear with respect to speed or
frequency
Fig 4. Sine Carrier PW Technique
Fig 5. Speed-Torque characteristics with V/ƒ control

b. Energy Saving In Variable Torque Load
Application
Many systems are used constant speed
motors and control process flow rates or pressures by
mechanically regulation using throttling valves,
dampers, fluid couplings or variable inlet vanes etc.
These devices generally do not control flow or
pressure efficiently because energy is dissipated
across the throttling device. Running a motor at full
speed while throttling the input or output is like
driving a car with one foot on the accelerator and the
other on the brake; a part of the produced output
immediately goes to waste. A variable speed drive
can save over 60% of the energy.Variable speed
drives and the loads they are applied to can generally
be divided into 3 groups
1) Constant power
2) Constant torque
3) Variable torque
In variable torque load applications, both
torque and power change with speed. Torque varies
with speed squared, and power varies with speed
cubed. This means that at half speed, the power
required is approximately one eighth of rated
maximum. Common examples of variable torque
loads are centrifugal fans, blowers and variable
discharge pressure pumps. The use of a variable
speed drive with a variable torque load often returns
significant energy savings. In these applications the
drive can be used to maintain various process flows
or pressures while minimizing power consumption. In
addition, a drive also offers the benefits of increased
process control, which often improves product quality
and reduces scrap. Effective speed ranges are from
50% to 100% of maximum speed and can result in
substantial energy savings.
(2)
(3)
(4)
P = Power in Watts; T = Torque in N-m; N = Motor
rotation speed in rpm;
A variable speed drive can also make it
possible to stop a motor completely when it is not
required as re-starting with a variable speed drive
causes far less stress than starting direct on line - soft
start is an inherent feature of the drive.
Regulating the motor speed has the added
benefit of easily accommodating capacity rises
without extra investment, as speed increases of 5-
20% is no problem with an AC variable speed drive
as long as there is enough spare capacity in the
system. Reduced maintenance compared to DC
systems (brushes and commutators) reduced
motor/application noise levels.
VI. SIMULATED CIRCUITS AND
WAVEFORMS
Figure 7. Shows the PWM circuit to
generate the gating signals for the multilevel inverter
switches. To control a three phase multilevel inverter
with an output voltage of seven levels; sixcarriers are
generated and compared at eachtimetoa set of three
sinusoidal reference waveforms. One carrier wave
above the zero reference and one carrier wave below
the reference. These carriers are same in frequency,
amplitude and phases; but they are just different in dc
offset to occupy contiguous bands. Phase disposition
technique has less harmonic distortion on line
voltages.Simulated model for entire circuit is shown
in Figure8.Induction Motor Drive using Seven Level
Inverter Simulation DiagramOutput voltage for 50 Hz
and 45Hz frequency are shown in be figure 8 and 11
Fig 6. Output Phase Voltage for 50Hz Frequency.

Fig 7. Simulation Circuit
Speed-Torque curves for 50 Hz and 45 Hz
frequencies are shown in figure 9 and 12.
Fig 8. N-T Curves for 50Hz Frequency
The FFT plot of the output voltage is shown in
Figure10. The plot shows that the harmonic content
present in the output voltage is very Low.
Fig 9. FFT for Output Voltage 50HZ. THD=0.4%
Fig10. Output phase-phase voltage for 45Hz
frequency
The frequency of reference signal
determines the inverter output frequency; and its peak
amplitude controls the modulation index. The
variation in modulation index changes the rms output
voltage of the multilevel inverter. By varying the
reference signal frequency as well as modulation
index, the speed of an induction motor gets
controlled.
Fig 11. N-T curves for 45Hz frequency

The speed-torque curves conclude that the voltage
and frequency applied to the motor gets decreased;
then the speed of an induction motor also decreases
simultaneously.
VII. CONCLUSION
In this paper a diode clamped multilevel
inverter has been presented for drive applications.
The sine multicarrier SPWM technique can be
implemented for producing low harmonic contents in
the output; hence the high quality output voltage was
obtained. This high quality output voltage is better
then the SPWM. The open loop speed control was
achieved by maintaining V/ƒ ratio at constant value.
The simulation results show that the proposed system
effectively controls the motor speed and enhances the
drive performance through reduction in THD. This
drive system can be used energy saving in variable
torque load applications like boiler feed pumps
conveyors, rolling mills, printing machines etc.
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S.Ebanezar Pravinreceived his Bachelor of
Engineering degree in Electrical and Electronics
Engineering from Anna University, Chennai at 2009

and his Master of Technologydegree in Power
Electronics and Drives from Karunya University,
Coimbatore at 2011. He is currently working towards
his Ph.D in the Department of Electrical Engineering,
Anna University, Chennai. He is also working as an
Assistant Professor with the Department of
Electricaland Electronic Engineering, at SCAD
college of Engineering and Technology,
Cheranmahadevi. His research interests are in the
area of power converterand drives.
SP. Umayal is a Professor and also the Dean of
Electrical and Electronics Engineering at
Muthayammal Engineering College, Rasipuram.
Shecompleted her Bachelor of Engineeringin
Electrical and Electronics Engineeringat Thiyagarajar
college of Engineeringat 1990 and also her Master of
Engineering in Power Systemat the year 1999. She
received her Ph.Dat 2008 inthe field of Power System
Optimization. Her areas ofresearch include Power
system optimization, FACTS andpower quality. She
has 35 publications in her research area. Currently
she is guiding 5 Ph.D. researchscholars. She is also a
life member of IE (I) and ISTE.

Harmonics Reduction of Multilevel Inverter Drive Using Sine Carrier Pulse Width Modulation Techniques

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Harmonics Reduction of Multilevel Inverter Drive Using Sine Carrier Pulse Width Modulation Techniques