
Research Article


Speed Control Simulation for Induction Motor by Multi Level VSIFed to Analyse Current Harmonics and Selective Harmonics Elimination


Bahram Ramezani


ABSTRACT

This study presented the simulation studies on closedloop speed control for 3phase induction motor by applying V/F method. This induction motor is fed by three and fivelevel diode clamped Voltage Source Inverter (VSI). Sinusoidal pulse with modulation technique is used to control inverter power switches. The harmonics of phase current is analysed. Then selective harmonics is eliminated by comparison of sine wave with modified triangle carrier. In this method, we could eliminate selective harmonics with no need to complex equations solving. In this study, by present the quasi triangle carrier Total Harmonic Distortion (THD) is decreased.





INTRODUCTION
Multilevel power conversion was first introduced 26 years ago (Corzine,
2002). The general concept involves utilizing a higher number of active
semiconductor switches to perform the power conversion in small voltage steps.
There are several advantages to this approach when compared with traditional
(twolevel) power conversion. The smaller voltage steps lead to the production
of higher power quality waveforms and also reduce the dv/dt stresses on the
load and reduce the electromagnetic compatibility (EMC) concerns. Another important
feature of multilevel converters is that the semiconductors are wired in a seriestype
connection, which allows operation at higher voltages. However, the series connection
is typically made with clamping diodes, which eliminates overvoltage concerns.
Furthermore, since the switches are not truly series connected, their switching
can be staggered, which reduces the switching frequency and thus the switching
losses.
Figure 1 shows the general structure of the multilevel converter
system. In this case, a threephase motor load is shown on the AC side of the
converter. Generally, variablespeed induction motor employs the inverter as
powervarying component (Tipsuwanporn et al., 2006;
Carbone and Scappatura, 2005). However, the converter
may interface to an electric utility or drive another type of load. The goal
of the multilevel PulseWidth Modulation (PWM) block is to switch the converter
GTOs in such a way that the phase voltages v_{as}, v_{bs} and
v_{cs} are equal to commanded voltages v*_{as}, v*_{bs }and
v*_{cs}. The commanded voltages are generated from an overall supervisory
control and may be expressed in a general form as:
 Fig. 1: 
Multi level inverter structure 
where, v*_{s} is a voltage amplitude and θ_{c} is an electrical angle.
The fundamental multilevel inverter topologies are diodeclamped, flying capacitor,
cascaded Hbridge and multilevel Hbridge (Aghdam and Fathi,
2006; Corzine and Baker, 2002). Diode clamped multilevel
inverter is a very general and widely used topology for real power flow control
and is considered for investigation
 Fig. 2: 
Threelevel diode clamped inverter 
purpose in this study. The threelevel diode clamped inverter is shown in Fig.
2. Comparing this topology with that of a standard twolevel converter,
it can be seen that there are twice as many GTOs as well as added diodes (Choi
et al., 1991; Corzine and Wielebski, 2003).
However, it should be pointed out that the voltage rating of the GTOs is half
that of the GTOs in a twolevel converter. In threelevel inverter, the GTO
blocking voltage is one half the DClink voltage. When compared with the twolevel
converter, the additional voltage level allows the production of linetoground
voltages with lower harmonic distortion. Selective harmonic elimination pulsewidth
modulation methods remain of great interest for the control of highvoltage
highpower voltagesource converters (Xu and Agelidis, 2007;
Dahidah and Agelidis, 2007). The main challenge associated with SHEPWM
techniques is to obtain the analytical solution of the resultant system of the
nonlinear transcendental equations that contain trigonometric terms, which
in turn provide multiple sets of solutions (Sahali and Fellah,
2006; Dahidah and Agelidis, 2006; Agelidis
et al., 2006). This study is presented a method that by applying
it, we could eliminate selective harmonics with no need to complex equations
solving.
SINUSOIDAL PULSE WITH MODULATION
The control principle of the SPWM is to use several triangular carrier signals
keeping only one modulating sinusoidal signal.
 Fig. 3: 
Threelevel sinetriangle modulation technique 
For the three level inverter, two triangular carriers are needed (Fig.
3) (Generally speaking, if a mlevel inverter is employed, (m1) carriers
will be needed) (Massoud et al., 2004). The carriers
have the same frequency f_{c} and the same peaktopeak amplitude A_{c}.
The zero reference is placed in the middle of the carrier set. The modulating
signal is a sinusoid of frequency f_{m} and amplitude A_{m}.
At every instant, each carrier is compared with the modulating signal. Each
comparison switches the switch on if the modulating signal is greater than the
triangular carrier assigned to that switch. Obviously, the actual driving signals
for the power devices can be derived from the results of the modulatingcarrier
comparison by means of a logic circuit. SPWM technique can be classified according
to carrier and modulating signals.
The main parameters of the modulation process are: • 
The frequency ratio k = f_{c}/f_{m}, where
f_{c} is the frequency of the carriers and f_{m} is the
frequency of the modulating signal 
• 
The modulation index M = A_{m}/(m' *A_{c}),
where A_{m} is the amplitude of the modulating signal, A_{c},
is the peaktopeak amplitude of the carriers and m' = (m1)/2, where m
is the number of level 
SPEED CONTROL FOR INDUCTION MOTOR
In recent years, application of three, four and five level VSI has become common
in speed control for Induction Motors in order to reduce torque variation and
accelerate dynamic response (Fang et al., 1995;
Singh et al., 1998; SongManguelle
and Rufer, 2003).
 Fig. 4: 
Schematic of speed control of induction motor 
 Fig. 5: 
(a) Phase voltage and (b) phase current of induction motor
fed by 3level diode clamped inverter 
An egregious problem of this method is balancing of capacitor voltage so use
of the isolated power supply is proposed. The isolated power supply is used
here. The induction motor is controlled by V/F method (Dubey,
1989; Ong, 1997). The schematic of speed control of
induction motor is shown in Fig. 4. The parameters of the
induction motor are given:
 Fig. 6: 
(a) Phase voltage and (b) phase current of Induction Motor
Fed by 5level Diode Clamped Inverter 
The phase voltage (V_{ag}) and stator phase current (i_{as})
wave forms are shown in follow Fig. 5 and 6.
In this study, the frequency ratio is 33(K = fc/fm = 33). The three and five
level voltage waveforms that feed the induction motor are shown in Fig.
5a and 6a. The current waveform of induction motor is
shown in Fig. 5b and 6b.
 Fig. 7: 
Phase current harmonic spectrum of induction motor fed by
3level diode clamped inverter 
 Fig. 8: 
Phase current harmonic spectrum of induction motor fed by
5level diode clamped inverter 
SELECTIVE HARMONIC ELIMINATION
Application of multilevel VSI in induction motor drive to cause to generation
of current and voltage harmonics (Dubey et al., 1986;
Corzine et al., 1998; Iturriz
and Ladoux, 2000; Krein et al., 2004). Selective
Harmonic Elimination (SHE) is normally a twostep digital process. First, the
switching angles are calculated offline, for several depths of modulation, by
solving many nonlinear equations simultaneously. Second, these angles are stored
in a look up table to be read in real time. Consider a quasitriangular waveform
to be used as the carrier signal in a PWM implementation. A PWM implementation
is a technique to control inverter power switches. By applying this technique,
we could generate a quasi Sinusoidal waveform with desired frequency. In principle,
the frequency and phase can be modulated. To represent this, consider a triangular
carrier function written as:
where, ω_{sw} is the base switching frequency, β(t) is a phasemodulation signal and ö is a static phase shift. The modulating signal will be represented as m(t) = m_{d}xsin(ωt) where m_{d} is the depth of modulation and ω is the desired output fundamental frequency.
where, J_{n} is a Bessel function of the first kind. The natural number
M is infinity in principle. The functions σ(m_{d}) and G(m_{d})
have been determined by curve fitting as:
Figure 7 and 8 show a magnitude spectrum for K = 33(ω_{sw }= 33ω). By applying this switching frequency ratio are eliminated second, 3th, ..., 32th harmonics. By using this method, we could eliminate the selective harmonics(second, 3th, ..., 32th Harmonics). In this method we don’t require to solve complex equations. By using the quasitriangular waveform (Eq. 2) to be used as the carrier signal in a PWM implementation, the current THD of Induction Motor by 3level Diode Clamped Inverter fed is 6% and for Induction Motor by 5level Inverter fed is 2.7%. If we use a common triangular waveform to be used as the carrier signal, the current THD of Induction Motor by 3level Diode Clamped Inverterfed would be 9% and for Induction Motor by 5level Inverter fed is 5%. CONCLUSIONS
In this study, the simulation studies on closedloop speed control for 3phase
Induction Motor by applying V/F method was presented. This induction motor was
fed by Multilevel diode clamped Voltage Source Inverter (VSI). Sinusoidal pulse
with modulation technique was used to control inverter power switches. Then
the harmonics of the phase current were analysed. The Selective harmonics were
eliminated by comparison of sine wave with modified triangle carrier (Eq.
2). In this study, the frequency ratio was 33(ω_{sw }= 33ω).
By applying this switching frequency ratio, second, 3th, ..., 32th current harmonics
were eliminated. In this method, we don’t require to solve complex equations.
In this study, by present the Novel triangle carrier Total Harmonic Distortion
(THD) was decreased.
Using the quasitriangular waveform (Eq. 2) as the carrier signal in a PWM implementation, the Phase current THD of Induction Motor fed by 3level Diode Clamped Inverter is 6% and for Induction Motor by 5level Diode Clamped Inverter 2.7%.

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