This document proposes a new multilevel inverter topology for driving a four-pole induction motor. The topology uses four two-level inverters connected to the separated windings of the motor's stator to generate five voltage levels. This is done using a single DC link shared between the inverters. Sine triangular pulse width modulation is used to generate switching signals while avoiding common mode currents. Simulation results show the output voltage waveform for different modulation indices as well as the speed-torque characteristics of the induction motor driven by the proposed inverter topology. The topology reduces harmonics and improves efficiency compared to traditional multilevel inverter configurations.

1. A New Multilevel Inverter Topology for Four
pole Induction motor Drive
P.JAYANTH
PG Scholar
Department of Electrical and Electronics Engineering,
Gudlavalleru Engineering college, JNTU K
Gudlavalleru, A.P, India.
e-mail: mr.p.jayanth@gmail.com
D.SRINIVASA RAO
Associate Professor
Department of Electrical & Electronics Engineering,
Gudlavalleru Engineering college, JNTU K
Gudlavalleru, A.P, India.
e-mail: dsrinivasarao1993@gmail.com
Abstract- In this paper a new multilevel inverter topology
is proposed for the control of the induction motor. This
multilevel inverter uses a single DC link. The identical
voltage profile windings in induction motor stator are
disconnected and each winding is separated into two
parts. These parts are fed with two two-level inverters
from both sides. In order to generate five level voltages
in induction motor stator windings two two two-level
inverters are required. All the inverters are fed with
single DC link with magnitude . The problem with
the use of common DC link for all the inverters is,
common mode currents will pass through the induction
motor stator windings. To avoid this problem sine
triangular pulse width modulation is applied to all two
level inverters. All the harmonics are shifted to
switching frequency which will have less impact on the
motor stator windings. Total harmonic distortion is also
reduced by using this multilevel inverter topology. Due
to fewer harmonics, it gives near sinusoidal output
voltage. The efficiency of the system is also improved.
In this paper the proposed multilevel inverter, its operation,
modulation method, simulation results with
SIMULINK/MATLAB are shown.
Keywords- Sine triangular pulse width modulation method,
Total harmonic distortion (THD).
I. INTRODUCTION
From the past few decades multilevel inverters are widely
used for the control of the induction motor drives. Because
of the lower harmonics, reduced stresses on the power
electronic devices, improved efficiency of the drive and etc
[1]-[3].conventional multilevel inverter configurations are
diode clamped [4], flying capacitor [5] and cascade H-bridge
[6]. The disadvantages of using these conventional structures
are, more number of diodes are required in diode clamped
inverter, voltage balancing problem in flying capacitor
inverter and more number of DC sources in cascade H-
bridge structure. These structures are advantageous up to 3-
levels only. Increasing the number of levels increase circuit
complexity, cost and size. The alternative for this is open
end winding induction motor fed with 2 level inverters [7].
In the open end winding scheme of induction motor,
windings are fed with two level inverters to get three level
inverter topology. In the open end winding scheme as the
number of levels increase, conventional multilevel inverters
have to be used or the inverters have to be cascaded on both
sides of the induction motor drive [8].
In this paper a new multilevel inverter topology is
proposed for the control of the induction motor. This
topology produces five level voltages at the output by using
four conventional two level inverters. In this topology the
windings are separated and each part of the winding is fed
with two two-level inverters from both sides of the parts. So
in order to generate five level voltages on motor phase
windings four two level inverters are used. These inverters
use common DC link voltage with magnitude 4. Sine
triangular pulse width modulation is used in this inverter
topology.
II. INDUCTION MOTOR
The three phase induction motor is the most widely used
alternating current (AC) motor in industry. Induction motors
are popular because of their simplicity, good power factor,
rugged construction, reliability and simple operation. Its
characteristics are also similar to direct current shunt motor.
For a given output rating the physical size of the induction
motor is relatively small as compared to other types of
motors.
The efficiency of the induction motor is also high
as there are no frictional losses and maintenance is simple.
Induction motor mainly consists of stator and rotor. Even
though it is equipped with stator and rotor it is a singly
excited machine. In this machine there is no electrical
connection between stator and rotor.
Construction:
In this proposed scheme the three phase stator winding is
connected and each phase winding is separated into two
halves. This scheme is known as open ended winding of
induction motor. These separated windings are supplied
from the multilevel inverters to produce rotating magnetic
field.
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2. The rotor can be either squirrel cage or slip ring. The rotor
consists of a cylindrical laminated iron core with slots
around the core. These slots carry the rotor bars. Most of the
motors uses squirrel cage rotor because of its robust and
simple construction. In the proposed scheme stator three
phase winding is disconnected and each winding is
separated into two parts. But operating principle of this
motor is same as that of the conventional induction motor.
As supply is given to the stator windings currents will flow
through stator windings and alternating flux will be
produced. This alternating flux will revolve at synchronous
speed. As the rotor conductors are short circuited, this flux
will cut the short circuited rotor conductors and
electromagnetic force (EMF) will be induced. This EMF
causes currents will flow in the rotor. This induced currents
causes alternating flux is generated and lags the stator flux.
The interaction between two magnetic fields creates
mechanical which is necessary to run the rotor.
Speed-torque characteristics:
The speed-torque characteristics of the induction motor are
shown in fig 3. For any load torque there are two operating
points B and D. The operating point B is unstable.
Fig. 1. Speed-torque characteristics of induction motor
Because if there is any requirement of speed rise causes the
developed torque increases then the load torque which
further raises speed. So, at this point the operation of the
induction motor is not good. The operating point D is stable
because at this point any tendency of speed rise will be
opposed by decrease in developed torque. Similarly if there
is any fall in speed then there will be increase in developed
torque to bring the motor to operating point D.
The maximum torque at point D is called breakdown
torque or pull in torque. Thus the region AC is called
unstable region and CE is called as stable region. If the
mechanical load applied to the motor is increased, at full
load, then there will be drop in speed until the developed
torque matches with the load torque. And the motor runs at
constant speed when both the torque are equal. The motor
will be stop whenever the load torque exceeds the
breakdown torque of the motor. Depending upon the design
of the motor the breakdown torque varies but it is in the
range of 200 to 300 percent of full load torque in standard
squirrel cage motors. In the operating region the speed
torque characteristics of induction motors are similar to DC
shunt motors.
III. DYNAMIC MODELLING OF INDUCTION MOTOR
There are two sets of identical voltage profile windings in
the conventional four pole induction motor. These windings
are connected in series which is shown in fig. 2.
Fig. 2. Induction motor stator winding general arrangement
For the proposed inverter these windings are disconnected
and each winding is separated into two parts as shown the
fig. 3.
Fig. 3.Arrangement for the proposed inverter
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3. The stator resistance, magnetizing inductance and stator
leakage inductance of each part is equal to half of the stator
winding because of the separated equal windings.
The voltage equations for the stator winding is given by
− = ∗ + ∗ − ∗ ∗ −
∗ ∗ (1)
− = ∗ + ∗ − ∗ ∗ −
∗ ∗ (2)
The sum of the voltage across the two separated parts is
equal to the total voltage across the winding and is given
by
= ( − ) + ( − ) (3)
The motor voltage can be obtained by substituting
equations (1) and (2) in equation (3)
= ∗ + ∗ −
1
2
∗ ∗ −
1
2
∗ ∗
(4)
Similarly the voltage equations for the remaining two phases
of induction motor is given by
= ∗ + ∗ −
1
2
∗ ∗ −
1
2
∗ ∗
(5)
= ∗ + ∗ −
1
2
∗ ∗ −
1
2
∗ ∗
(6)
In order to solve the voltage equations in dq0 frame basic
equations of induction motor can be used.
= ∗ + ∗ + ∗ (7)
= ∗ + ∗ + ∗ (8)
= ∗ − ∗ + ∗ (9)
= ∗ + ∗ (10)
= ∗ + ( − ) ∗ + ∗ (11)
= ∗ − ( − ) ∗ + ∗ (12)
= ∗ + ∗ (13)
Flux linkage equations of induction motor are given by
= ∗ + ∗ (14)
= ∗ + ∗ (15)
= ∗ (16)
= ∗ + ∗ (17)
= ∗ + ∗ (18)
= ∗ (19)
In terms of dq0 axis currents the expression for the
electromagnetic torque is given by
= ∗ ∗ ∗ ∗ + ∗ (20)
In terms of torque rotor speed of the motor is given by
= ∗
∗ ( − ) (21)
Where
d : direct axis,
q : quadrature axis,
s : stator variable,
r : rotor variable,
Vqs,Vds : q and d-axis stator voltages,
Vqr,Vdr : q and d-axis rotor voltages,
rr : rotor resistance,
rs : stator resistance,
Lls : stator leakage inductance,
Llr : rotor leakage inductance,
iqs,ids : q and d-axis stator currents,
iqr,idr : q qnd d-axis rotor currents,
p : number of pole,
J : moment of inertia,
Te : electrical output torque,
TL : load torque.
There is no difference between normal induction motor and
disconnected induction motor which can be observed from
the equations (4), (5) and (6).
IV. PROPOSED FIVE LEVEL INVERTER
For the four pole induction motor drive five level inverter
topology is proposed. By using four two-level inverters the
available four terminals which are the result of the
disconnection of two identical voltage profile winding coils
are supplied and it is shown in fig. 4. A single DC link with
magnitude 4 supplies all two-level inverters. In the fig 3
the first inverter are S11 to S16 , the second inverter switches
are S21 to S26, the third inverter switches are S31 to S36 and the
fourth inverter switches are S41 to S46. These switches have
blocking voltage of Vdc⁄4. The complementary switches of
the first inverter are (S11, S12), (S13, S14), (S15, S16). The
complementary switches of the second inverter are (S21, S22)
, (S23, S24), (S25, S26). The complementary switches of the
third inverter are (S31, S32), (S33, S34), (S35, S36). The
complementary switches of the fourth inverter are (S41, S42),
(S43, S44), (S45, S46). During the voltage levels , 0,−
in
order to isolate the middle inverters inverter-2 and inverter-3
the auxiliary switches S1 to S6 are required
Proceedings of International Conference On Current Innovations In Engineering And Technology
International Association Of Engineering & Technology For Skill Development
ISBN : 978 - 1502851550
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4. Fig. 4. Proposed inverter
The unequal voltage will be distributed across the motor
phase winding if the switches S1 to S6 are short circuited
,which results in unequal flux distribution. The table-I shows
the possible switching combinations which are used to
generate five voltage levels across motor phase-A winding.
The table-II shows the comparison between proposed and
conventional multilevel inverter topologies in terms of
power electronic switches, clamping diodes, isolated voltage
sources, capacitor banks and bi-directional switching
devices. Same numbers of switching devices are required for
all the topologies.
TABLE I
SWITCHING COMBINATIONS FOR GENERATING FIVE VOLTAGE LEVELS
TABLE II
COMPARISON BETWEEN CONVENTIONAL AND PROPOSED INVERTER
From the table II it is observed that clamping diodes are not
required in proposed inverter whereas 18 diodes are required
for neutral point converter (NPC). It requires single isolated
voltage source whereas cascade H-bridge requires 6 isolated
voltage source. 4 capacitor banks are required for NPC and
18 for flying capacitor (FC). The proposed converter does
not require a capacitor bank.
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5. V. MODULATION STRATEGY
Multi carrier based pulse width modulation techniques are
applied to multi level inverters. The most popular and
easiest multi carrier pulse width modulation technique is
sine triangular PWM. In this PWM triangular waves are
used as carrier waves and sinusoidal wave is used as
reference wave. The number of carrier waves depends upon
the number levels at the output. The relation between
voltage levels at the output and carrier waves is given by
No.of carrier waves= no.of levels at the output-1
In this proposed inverter five levels appear at the output. so
the number of carrier waves used are 4. Fig. 5 shows the
sine triangular PWM for the proposed multi level inverter
topology.
Fig. 5. Sine triangular PWM
The table-III shows the pattern which is followed for
generating five voltage levels at the output.
TABLE III
COMPARISON BETWEEN MODULATING AND CARRIER SIGNALS
VI. SIMULINK/MATLAB CIRCUITS
The proposed five level inverter is simulated using
MATLAB/SIMULINK. Sine triangular pulse width
modulation is used to generate gating pulses for the power
electronic switching devices which are used in the inverter.
Four triangular carrier waves of frequency 10 kilo hertz are
compared with sine wave of frequency 50hertz. Four
inverters are supplied with single DC link of magnitude .
The simulation circuit with open ended stator winding is
shown in fig 6.
Fig. 6.Simulation circuit of proposed five level inverter
with open ended winding
The proposed five level inverter output voltage
for different modulation index values is shown
in fig 7 and fig 8.
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6. Fig. 7. Output voltage for modulation index 0.2,0.4
Fig. 8. Output voltage for modulation index 0.6, 0.8
The simulation circuit of proposed inverter is
with induction motor is shown in fig 9.
Fig.9.Proposed inverter with induction motor.
The speed torque characteristics of the
induction motor are shown in fig 10. As speed
increases torque also increase upto break down
torque or maximum torque. Then torque decrease
with further increase in speed.
Fig. 10. Speed-torque characteristics of induction motor
VII. CONCLUSION
In this paper three phase induction motor which is fed from
the five level inverter is presented. The stator winding of
induction motor is disconnected and each phase is separated
into two halves. The identical voltage profile windings are
fed with four two-level inverters to generate five level
voltages in the phase windings. Sine triangular pulse width
modulation is applied to the proposed inverter to generate
gating pulses for switching devices. These four inverters are
fed with common DC link with magnitude . Speed torque
characteristics are observed. The important feature of this
topology is that, it will be operated as three level inverter if
the middle inverter switches are failed.
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