Comparative analysis of multilevel inverter

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 02 Issue: 11 | Nov-2013, Available @ http://www.ijret.org 235
COMPARATIVE ANALYSIS OF MULTILEVEL INVERTER
TOPOLOGIES FOR INDUCTION MOTOR DRIVE APPLICATIONS
V.S. Prasadarao K1
, Gandham Tabita2
1
Research Engineer, Naresh Engineering Coaching (NEC) Institute, Vijayawada, India
2
PG Scholar, Department of Electrical and Electronics Engineering, Gudlavalleru Engineering College,
Gudlavalleru, India
Abstract
Induction motors are widely used in industries, because they are rugged, reliable and economical and hence they are called as work
horse power of industry. Induction motor drive requires suitable converters to get the required speed and torque without or negligible
ripples. Multilevel inverters can do this job. But the conventional MLIs such as Diode Clamped MLIs requires extra diodes in
conjunction with the active switches, Flying capacitor MLIs requires extra Capacitors and control also difficult if the levels increases
and the Cascaded H-bridge MLIs requires separate dc sources which limits its use. This paper proposes a new type of multi level
Inverter which converts the dc into ac using less number of switches when compared to conventional multilevel Inverters. The
proposed Inverter uses only seven switches to get 13-level output voltage compared conventional cascaded H-bridge uses twelve
switches. The comparison between the conventional MLIs and proposed inverter is presented in this paper. Finally the induction
motor using proposed inverter with thirteen level is simulated using Matlab/Simulink environment and the corresponding results are
presented in this paper.
Keywords- Grid interconnection, PV system, MLI, Renewable energy sources (RES).
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1. INTRODUCTION
The AC induction motor is a rotating electric machine designed
to operate a three-phase source of alternating voltage. The AC
induction motor (ACIM) is the most popular motor used in
consumer and industrial applications, and represented the
"muscle" behind the industrial revolution. The induction motor
is always rotating at synchronous speed; hence to get the speed
control we need control the both voltage and frequency. Because
voltage control of an ACIM means controlling the stator voltage
by rheostat. But it gives more losses and less efficiency.
Frequency control of ACIM results in saturation of core. Hence
to get the speed control of induction motor, we need vary both
voltage and current. This technique is called as constant V/F [1]
method. By choosing the suitable inverter we can vary both
voltage and frequency of the induction motor to get the required
speed control. Normally the conventional H-bridge inverter
produces a square output, which contains infinite number of odd
harmonics and dv/dt stress is also high. Normal PWM inverter
[2] can reduces the THD, but switching losses are high and also
this inverter is restricted to low power applications. The
importance of multilevel inverters [MLI] has been increased
since last few decades [3], [4]. These new types of inverters are
suitable for high voltage and high power application due to their
ability to synthesize waveforms with better harmonic spectrum
and with less THD. Generally MLIs are classified into three
types: they are 1.Diode Clamped MLIs 2. Flying capacitor MLIs
3. Cascaded H-bridge MLIs. Diode clamped MLIs require large
number of clamping diodes [5] as the level increases. In flying
capacitor MLIs, Switching utilization and efficiency [6, 7] are
poor and also it requires large number of capacitors as the level
increases and cost is also high. Cascaded H-bridge MLIs are
mostly preferred [8] for high power applications as the
regulation of the DC bus is simple. But it requires separate dc
sources and also the complexity of the structure is increases as
the level predominantly increase. In order to address the above
concerns, this paper proposes a new type of multilevel inverter
which requires less number of DC sources and switches
compared to Cascaded H-bridge MLIs.
2. INDUCTION MOTOR DRIVE
Generally a Drive is nothing but systems employed for motion
control. Drives are classified into two types based on the nature
of supply, they are DC drives and AC drives. The task of a
variable speed electrical drive is to convert the electrical power
supplied by the mains into mechanical power with a minimum
loss. To achieve an optimum technological process, the drive
must be variable in speed. Induction motor drive gain most
importance in industries due to their rugged construction and
reliability. The block diagram of the induction motor drive
system is shown in figure 1.

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Fig 1 Induction motor drive system
The frequency of the fundamental AC voltage will determine the
speed of IM. To vary the speed of IM, the inverter fundamental
frequency need to be changed. The inverter output frequency
must be kept close to the required motor speed.
The torque- speed characteristics of an induction motor is given
by the formula:
T =

The torque may be plotted as a function of‘s’ and is called the
torque-slip (or torque-speed, since slip indicates speed)
characteristic — a very important characteristic of the induction
machine. The torque- speed characteristics of induction motor is
shown in figure2.
Fig2: The torque- speed characteristics of induction motor
3. PROPOSED TECHNOLOGY
The general structure of proposed new multilevel inverter is
shown in the figure 3. It consists of a one H-bridge inverter and
‘N’ number of cascaded cells, which are having a dc rating of
Vdc. The number of levels can be given by the formula:
Number of Levels = [n (n+1) +1]
Where n= Number of cells excluding the H-bridge. For
generating +Vdc we need turned on switches S1 and S2, for –
Vdc, switches S3 and S4 has to be turned on, and for zero
voltage either switches S1 and S3 or switches S2 and S4 has to
be turned on.
Cell
S5
S6
Sn
S1
S4
S3
S2
Vdc
2Vdc
NVdc
Load
Fig3: General structure of proposed new multilevel inverter
3.1 Thirteen Level Proposed Multilevel Inverter
(Symmetrical Configuration)
Circuit diagram of proposed thirteen level multilevel inverter in
symmetrical configuration is shown in figure 4. The thirteen
level proposed inverter in symmetrical configuration uses ten
switches compared to cascaded H-bridge inverter in symmetrical
configuration which uses twenty four switches and six separate
dc sources. . But in proposed inverter, the requirement of
separate dc sources is only six and hence the switching losses are
also low. Using proper switching sequence proposed circuit
generates thirteen levels in output voltage.
S5
S1
S4
S3
S2
Vdc
Load
S6
Vdc
S7
Vdc
S8
Vdc
Vdc
S9
Vdc
S10
Fig4. Circuit diagram of thirteen level proposed inverter
(symmetrical configuration)

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Table 1 shows the switching sequence used for creating thirteen
levels for the output voltage in symmetrical configuration.
Table 1: Switching Sequence for proposed thirteen level
Inverter (symmetrical configuration)
3.2. Thirteen Level Proposed Multilevel Inverter:
(Asymmetrical Configuration)
The thirteen level proposed inverter symmetrical configuration
uses only seven switches compared to cascaded H-bridge
inverter which uses twenty four switches and six separate dc
sources. But in proposed inverter, the requirement of separate dc
sources is only three and the switching losses are also low.
Using proper switching sequence proposed circuit generates
seven levels in output voltage. Circuit diagram of proposed
thirteen level multilevel inverter is shown in figure 5.
S5
S6
S7
S1
S4
S3
S2
Vdc
2Vdc
3Vdc
Load
Fig5. Circuit diagram of thirteen level proposed inverter
(asymmetrical configuration)
Table 2 shows the switching sequence used for creating thirteen
levels for the output voltage.
Table 2: Switching Sequence for proposed thirteen level
Inverter (asymmetrical configuration)
The output waveform has 13 levels: 6Vdc, 5Vdc, 4Vdc
3Vdc, 2Vdc, Vdc and 0. The output voltage waveform of
the ideal thirteen level inverter is shown in fig6.
Vdc
2Vdc
3Vdc
-Vdc
-2Vdc
-3Vdc
4Vdc
5Vdc
6Vdc
-4Vdc
-5Vdc
-6Vdc
0
Time(sec)
Fig6. Ideal thirteen level output voltage waveform

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The following table 3 shows the comparison between proposed
MLI and conventional multilevel inverters.
Table 3: The comparison between proposed MLI and
conventional multilevel inverters
Type of Inverter Number of switches Number of Diodes
Cascaded H-
bridge MLI
(Symmetrical)
24 -
Cascaded H-
bridge MLI
(Asymmetrical)
12
Proposed MLI
(Symmetrical) 10 -
Proposed MLI
(Asymmetrical) 7 3
4. PROPOSED MULTILEVEL INVERTER FED
INDUCTION MOTOR DRIVE
The general block diagram of the proposed system is shown in
figure 7. It consists of a proposed inverter which uses less
number of switches is connected to a three phase induction
motor to control its speed. In this paper the inverter uses less
number of switches compared conventional cascaded H-bridge
inverter [9]. The proposed inverter is used control the speed and
torque of the induction motor.
Fig7. Induction motor fed by the proposed MLI
5. MATLAB MODELLING AND SIMULATION
RESULTS
The following figures8 and 9 shows the Matlab/Simulink
diagram of proposed thirteen level MLI in symmetrical
cinfiguration and its output voltage wave form.
Fig8: Matlab/Simulink diagram of proposed symmetrical
thirteen level MLI
Fig9: Proposed thirteen level inverter in symmetrical
configuration output voltage
From figure 9, it is observed that the output voltage of proposed
MLI has thirteen levels with ten switches and six diodes. The
following figure 10 shows the spectrum analysis of thirteen level
output voltage.

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Fig10: THD of the 7-level proposed inverter
From figure 10, the THD of the proposed thirteen level MLI in
symmetrical configuration is 14.21%.
The following figures11 and 12 shows the Matlab/Simulink
diagram of proposed thirteen level MLI in asymmetrical
configuration and its output voltage wave form.
Fig11: Matlab/Simulink diagram of proposed thirteen level MLI
in asymmetrical configuration
Fig12: Proposed thirteen level inverter in asymmetrical
configuration output voltage
From figure 12, it is observed that the output voltage of
proposed MLI in asymmetrical configuration has thirteen levels
with seven switches and three diodes. The following figure 13
shows the spectrum analysis of seven level output voltage.
Fig13: THD of the 13-level proposed inverter
From figure 13, the THD of the proposed seven level inverter is
13.94%.
Matlab/simulink diagram of induction motor drive system based
on proposed inverter is shown figure 14.

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Fig 14: Matlab/Simulink diagram of proposed MLI fed
induction motor drive system
The following figure 15 shows the phase voltage waveforms of
the induction motor
Fig15: Phase voltages of the induction motor
The following figure 16 shows stator, rotor currents, speed and
electromagnetic torque of the induction motor.
Fig 16: Stator, rotor currents, speed and electromagnetic torque
of the induction motor
From above figure, the speed of the induction motor settles to
constant after transient period is over.
CONCLUSIONS
This paper proposes a induction motor drive system based on
new multilevel inverter with reduced number of switches. The
proposed multilevel inverter uses less number of switches, hence
the switching losses and cost of inverter is less compared to
conventional MLIs. Increasing the number of output voltage
levels reduces the lower order harmonics and the THD. It's
preferred that the output voltage has no lower order harmonics
because their filtering is so hard. From the results the speed and
torque have less ripples.
REFERENCES
[1] “Modern power electronics and ac drives” by bimal k bose.
[2] Muhammad H Rashid , “Power Electronics: circuits, Devices
and Applications", Pearson Education, Third Edition, 2004.
[3] J.S. Lai and F.Z. Peng, “Multilevel Converters - A new
breed of power converters,” Conference Record of the IEEE-IAS
Annual Meeting, 1995, pp. 2348-2356.
[4] M. Manjrekar and G. Venkataramanan, “Advanced
topologies and modulation strategies for multilevel inverters,”
Conference Record of the IEEE-PESC, 1996, pp. 1013-1018.
[5] Mohan M. Renge and Hiralal M. Suryawanshi, “Five-Level
Diode clamped Inverter to eliminate Common Mode Voltage
and Reduced dv/dt in Medium voltage rating Induction Motor
Drives, IEEE Transactions on Power Electronics, vol. 23, no.4,
pp. 1598–1607 July 2008.

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[6] Mostafa Khazraei, Hossein Sepahvand, Keith A. Corzine,
and Mehdi Ferdowsi, “Active Capacitor Voltage Balancing in
Single-Phase Flying-Capacitor Multilevel Power Converters”
IEEE Transactions on Industrial Electronics, VOL. 59, NO 2,
FEBRUARY 2012.
[7] Xu Lie, Jon C. Clare, Patrick W. Wheeler, Lee Empringham,
and Li Yongdong, “Capacitor Clamped Multilevel Matrix
Converter Space Vector Modulation” IEEE Transactions on
industrial electronics, VOL. 59, NO. 1, JANUARY 2012.
[8] Fernanda Carnielutti, Humberto Pinheiro, and Cassiano
Rech, “Generalized Carrier-Based Modulation Strategy for
Cascaded Multilevel Converters Operating Under Fault
Conditions” IEEE Transactions on Industrial Electronics, VOL.
59, NO. 2, FEBRUARY 2012.
[9] J. Rodriguez, J.-S. Lai, and F. Z. Peng, "Multilevel inverters:
a survey of topologies, controls, and applications," IEEE Trans.
Ind. Electron., vol. 49, pp. 724 -738, 2002.
BIOGRAPHIES
V S Prasadarao K Received the B.TECH
degree in Electrical and Electronics from
Lakireddy Balireddy College of
engineering, Mylavaram, Krishna (DT),
A.P, India, in 2011 and MTECH in power
Electronics and Electrical drives from
Gudlavalleru engineering college,
Gudlavalleru, Krishna (DT), A.P, India.
Currently he is working as Research
Engineer in Naresh Engineering Coaching (NEC) Institute,
Vijayawada, Krishna (DT), A.P, India. His interested areas are
in the field of renewable energies, power converters, and
electrical drives.
Gandham Tabita Received the B.TECH
degree in Electrical and Electronics
Engineering from Bapatla Engineering
College, Bapatla, Guntur (DT), A.P, India,
in 2005. Currently she is pursuing M.TECH
Gudlavalleru engineering college,
Gudlavalleru, Krishna (DT), A.P, India.
Her research interests include Power Converters, Electrical
Drives, and machines.

Comparative analysis of multilevel inverter

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Comparative analysis of multilevel inverter