Lnct Letter

8/6/2019 Lnct Letter

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Abstract — This paper describes on how to formulate a spacevector modulator for the two-level inverter topology typicallyused for Low power induction motor drives. Space vector Pulse

Width Modulation variable speed drives are increasingly appliedin Many new industrial applications that require superiorperformance. In this Scheme V/Hz (Scalar control) ratiomaintain constant in order to get constant Torque over the entireoperating range. In such scheme A constant voltage andfrequency (V/Hz) supply to a.c drives is invariably obtained froma three-phase two level voltage source inverter. There is an

increasing trend of using space vector PWM (SVPWM) because

of their easier digital realization and better dc bus utilization.

The model of a three-phase voltage source inverter is discussedbased on space vector theory. Simulation results are obtainedusing PSIM Software with a 45-KW (60 Hp) motor drive. Testresult are given for effectiveness of the study .

Index Terms — Two Level Inverter, space vector modulation(SVM), Scalar control (Constant V/Hz)

I. INTRODUCTION

HE TWO-Level inverter topology has attracted attention in

high power medium voltage drive applications. Fig. 1

shows Main circuit of Three phase voltage source

inverter.Single-phase VSI cover low-range power applications

and three-phase VSI cover the medium- to high-power

applications[1]. The Main purpose of these topologies is to

provide a three-phase voltage source, where the amplitude,

phase, and frequency of the voltages should always be

controllable. Although most of the applications require

sinusoidal voltage waveforms (e.g., ASDs, UPSs, FACTS,

VAR compensators), arbitrary voltages are also required insome emerging applications (e.g., active filters, voltage

compensators)[2]. The standard three-phase VSI topology is

shown in Fig. 1 it has = 8 eight switching states and aregiven in Table 1[3]. The switches of any leg of the inverter

(S1 and S4, S3 and S6, or S5 and S2) cannot be switched on

simultaneously because this would result in a short circuitacross the dc link voltage supply. Similarly, in order to avoid

undefined states in the VSI, and thus undefined ac output

line voltages, the switches of any leg of the inverter cannot be

switched off simultaneously as this will result in voltages that

will depend upon the respective line current polarity. Out of

the eight switching states, two of them (7 and 8 in Table 1)produce zero ac line voltages[2].

TABLE 1

SWITCHING VECTORS, PHASE VOLTAGES AND LINE VOLTAGES

In this case, the ac line currents freewheel through either theupper or lower components[9]. The remaining states (1 to 6 in

Table 1) produce non-zero ac output voltages. In order to

generate a given voltage waveform, the inverter moves from

one state to another. Thus the resulting ac output line voltages

consist of discrete values of voltages that are Vi , 0, and -Vifor the topology shown in Fig. 1. The selection of the states inorder to generate the given waveform is done by the

modulating technique that should ensure the use of only the

valid states[2]

II. TWO - LEVEL INVERTER SCHEME FOR AN

INDUCTION MOTOR DRIVE

The circuit model of a typical three-phase voltage source

PWM inverter is shown in Fig. 1 There are the six power

switches(IGBT) that shape the output, which are controlled bythe switching variables a, a‟, b, b‟, c and c‟[4]. When an upper

IGBT is switched on, i.e., when a, b or c is 1,the

corresponding lower IGBT is switched off, i.e .,thecorresponding a‟, b‟ or c‟ is 0.Therefore, the on and off states

of the upper IGBT S1, S3 and S5 can be used to determine theoutput voltage[5].The relationship between the switching

variable vector [a, b, c]t and the line-to-line voltage Vector[Vab Vbc Vca]t is given by (2.1) in the following:[5][6]

Simulation Of SVPWM Method For Two Level

Inverter Fed Induction Motor Drive

Manish G Prajapati1,Rohit D Prajapati

2and Prof. Pinkal J Patel

3

E-Mail Id :- [email protected], [email protected] and [email protected]

3Assis. Professor, Dept of Electrical Engineering, S.P.C.E,Visnagar, Gujarat, India

1 & 2 Student,Dept. of Electrical Engineering, S.P.C.E, Visnagar, Gujarat, India

Voltage

Vector

Switching

Vector

Line to Neutral

Voltage

Line to Line

Voltage

a b c Van Vbn Vcn Vab Vbc Vca

V0 0 0 0 0 0 0 0 0 0

V1 1 0 0 2/3 -1/3 -1/3 1 0 -1

V2 1 1 0 1/3 1/3 -2/3 0 1 -1

V3 0 1 0 -1/3 2/3 -1/3 -1 1 0

V4 0 1 1 -2/3 1/3 1/3 -1 0 1

V5 0 0 1 -1/3 -1/3 2/3 0 -1 1

V6 1 0 1 1/3 -2/3 1/3 1 -1 0

V7 1 1 1 0 0 0 0 0 0

mailto:[email protected]








Fig. 1. Three-phase voltage source PWM Inverter

= Vdc (2.1)

Also, the relationship between the switching variable vector

[a, b, c]t and the phase voltage Vector [Va Vb Vc]t can be

expressed below[5][6].

=3

Vdc(2.2)

As illustrated in Fig. 1, there are eight possible

combinations of on and off patterns for the three upper power

switches[6]. The on and off states of the lower power devices

are opposite to the upper one and so are easily determined

once the states of the upper IGBT are determined[2].

According to equations (2.1) and (2.2), the eight switching

vectors, output line to neutral voltage (phase voltage), andoutput line-to-line voltages in terms of DC-link Vdc, are

given in Table 1 [5]. Space Vector PWM (SVPWM) refers to

a special switching sequence of the upper three power igbt of a three-phase inverter [6]. It has been shown to generate

less harmonic distortion in the output voltages and orcurrents applied to the phases of an AC motor and to provide

more efficient use of supply voltage compared with sinusoidal

modulation technique [4]. Fig 2 Show the basic Switching

Vector of SVPWM Technique. In this schemes there are eightvalid switching state out of six active switching space vector

are evenly distributed 60° interval with 2 Vdc/3 length and

form a hexagon also two zero space vector are located at the

center of hexagon in the complex plane as shown in fig 2.[7]For a given Magnitude (length) and position, Vref can be

synthesized by three near by stationary vectors, based on

which the switching states of the inverter can be selected and

gate signals for the active switches can be generated [8]. WhenVref passes through sectors one by one, different sets of

switches will be turned on or off. As a result, when Vref

rotates one revolution in space, the inverter output voltagevaries one cycle over time. The inverter output frequency

corresponds to the rotating speed of Vref, while its output

Fig 2 Basic switching vectors and sectors

voltage can be adjusted by the Magnitude of Vref. „The Vref

(called the reference vector) vector rotates counter clockwise

with the angle θ varying between 0 and 360 degrees.[9]

III. PRINCIPLE OF CONSTANT V/HZ CONTROL

FOR INDUCTION MOTORS

In this type of control, the motor is fed with variable

frequency supply generated by the PWM control of an

inverter[10]. At steady state the machine terminal voltage isgiven by

V = jώA (3.1)

where V and A are the phasors of stator voltage and stator flux,

2ππ

1VVA (3.2)

It follows that if the ratio V/f remains constant with the change

of f , then A also remains constant and the torque is

independent of the supply frequency[3]. So in order to get

constant torque over the entire operating range The V/f ratio is

maintain constant. Since only magnitudes of the input

variables frequency and voltage are controlled, this isknown as “scalar control”[10] In actual implementation the

ratio between the Magnitude and frequency of the statorvoltage is usually based on the rated values of these variables,also known as motor ratings.[6] The realistic control limits

that are placed on the applied voltage and frequency are

illustrated in Fig. 3 [4]

A disadvantage of such a control is that the torque

developed is load dependent as it is not controlled directly.

Also, the transient response of such a control is not fast due to

the predefined switching pattern of the Inverter[10].



Fig. 3. V/f limits on frequency and voltage.

IV. ALGORITHAM FOR SIMULATING THE SPACE

VECTOR PWM

Step 1. Determine Vref, and angle θ (Theta)

Step 2. Determine time duration T1, T2, T0

Step 3. Determine the switching time of each

IGBT (s1 to s6)

A. Step 1. Determine Vref , and angle θ (Theta)

120

Poleref SpeedFreq (4.1)

Fbase

VbaseFreqVref (4.2)

Vdc

Vref 3Ma (4.3)

timeFrequencyπ2Theta (4.4)

B. Step 2. Determine time duration T 1 , T 2 ,T 0

From Fig. 4, the switching time duration in any Sector can

be calculated as follows:

Fs

1Ts (4.5)

Theta)3pi(sinMa)(TsT1 (4.6)

(Theta)si nMa)(TsT2 ) (4.7)

T2-T1-TsT0 (4.8)

Where, TS = Sampling Time

Ma = Modulation Index

Fs = Sampling Frequency

Fig. 4. Vector Diagram When Reference Vector in Sector 1

(Where α = 60°)

C. Step 3: Determine the switching time of each IGBT

(s1 to s6)

Fig.5 shows space vector PWM switching patterns at

each sector. Based on this Fig. the switching time at each

sector is summarized in Table 2, and it will be built in

Simulink model to simulate SVPWM [5]

SVPWM Switching Pattern at Sector 1 and 2





Fig 5 Space Vector PWM switching patterns at each sector

Table.5

SWITCHING TIME CALCULATION AT EACH SECTOR

SECTOR

UPPER SWITCHES

(S1,S2,S3)

UPPER SWITCHES

(S4,S6,S2)

1

S1 = T1+T2+T0/2

S3=T2+T0/2

S5=T0/2

S4 = T0/2

S6=T1+T0/2

S2=T1+T2+T0/2

2

S1 = T1+T0/2

S3=T1+T2+T0/2

S5=T0/2

S4 = T2+T0/2

S6=T0/2

S2=T1+T2+T0/2

3

S1 = T0/2

S3=T1+T2+T0/2

S5=T2+T0/2

S4 = T1+T2+T0/2

S6=T0/2

S2=T1+T0/2

4

S1 = T0/2

S3=T1+T0/2

S5=T1+T2+T0/2

S4 = T1+T2+T0/2

S6=T2+T0/2

S2=T0/2

5

S1 = T2+T0/2

S3=T0/2S5=T1+T2+T0/2

S4 = T1+T0/2

S6=T1+T2+T0/2S2=T0/2

6

S1 = T1+T2+T0/2

S3=T0/2

S5=T1+T0/2

S4 = T0/2

S6=T1+T2+T0/2

S2=T2+T0/2

V. RESULTS AND DISCUSSION

The proposed scheme is simulated on a 45 kW (60 Hp)

4-Pole,415 volts three-phase induction motor at no load with

open loop V/f control for different modulation indicescovering the entire speed range. The dc link voltage of around

585V is used. The carrier frequency used for PWM generationis limited to 5kHz. The simulation circuit for two-level

inverter is shown in fig. 7. The open loop dc link capacitor

voltage scheme is implemented using DLL (Dynamic Link

Library) block of PSIM simulation software package [1].

Output frequency can be varied according to the reference

speed input in DLL (Dynamic Link Library) block. Base

frequency and Base voltage input in DLL block is used to

Maintain V/f ratio constant. At low frequency the reference

vector rotate in region-1 so the two levels in voltage are

getting in inverter.

Fig 6 Simulation Circuit For Two level Inverter

The 2-level SVPWM scheme is used for the PWM signal

generation, based on the sampled amplitudes of reference

phase voltages. The line voltage, line current and Speed

waveforms for inverter operation are presented in Figs. Thesimulation results prove the ability of open loop SVPWM

scheme.

(X- axis: 1 div = 50 ms, Y- axis: 1 div= 250 V)

(X- axis: 1 div = 50 ms, Y- axis: 1 div= 20 A)

(X- axis: 1 div = 1s, Y- axis: 1 div= 100 RPM)

Fig 7; Line Voltage, Line Current and Speed Waveform at 10Hz, Ma=0.34





(X- axis: 1 div = 1 s, Y- axis: 1 div= 200 RPM)

Fig 8; Line Voltage, Line Current and Speed Waveform at 20Hz,Ma=0.69




Fig 9; Line Voltage, Line Current and Speed Waveform at 30Hz , Ma=1.02

(X- axis: 1 div = 12.5 ms, Y- axis: 1 div= 250 V)


(X- axis: 1 div = 1 s, Y- axis: 1 div= 250 RPM )

Fig 10;Line Voltage, Line Current and Speed Waveform at 40Hz,Ma=1.3




Fig 11; Line Voltage, Line Current and Speed Waveform at 50Hz,Ma=1.73



VI. CONCLUSION

In this paper a space vector pulse width modulator is

proposed to generate switching patterns for a two-level

inverter from an input command voltage vector. It has been

shown that the SVPWM technique utilizes DC bus voltage

more efficiently and generates less harmonic distortion in a

three-phase voltage-source inverter. The switching patternavailable from the direct switching between positive andnegative half buses is violated all times and minimum

switching is assured. The feasibility of the proposed

method Further more, a low-cost implementation of thecontrol technique has been shown to provide satisfactory

performance. Therefore, the economical feasibility of thegeneral purpose two-level inverter in low voltage drive

applications has been strengthened and its application range.

The proposed method is based on standard two level inverter

and then uses a mapping process to achieve the SVM for

multilevel. Simulation results have been provided to prove the

viability of the scheme.

REFERENCES

[1] Prof P J Patel, Rakesh A Patel, Vinod Patel,Prof. P. N. Tekwani

“Implementation of Self Balancing Space Vector Switching

Modulator for Three-Level Inverter”, IEEE Region 10 Colloquium and

the Third International Conference on Industrial and Information

Systems, Kharagpur, INDIA December, 2008

[2] Sunil Panda, Anupam Mishra, B Srinivas“Control of Voltage Source

Inverters using PWM/ SVPWM for Adjustable Speed Drive

Applications” thesis of Department of Electrical Engineering National

Institute Of Technology Rourkela, Rourkela ,May- 2009

[3] Mr.Venkatara Mana Naik, Mrs. K.Sree Gowri “Space Vector Based

Hybrid Pulse Width Modulation for reduced Current Ripple” National

Conference on Intelligent Electrical Systems (NCIES‟09), April 2009

[4] Zhenyu Yu and David FigoliAC “Induction Motor Control UsingConstant V/Hz Principle and Space Vector PWM Technique with

TMS320C240” DSP Digital Control System Applications, April

1998,pp 10-18.

[5] jin-woo jung “space vector pwm inverter” mechatronic systems

laboratory department of electrical and computer engineering he ohio

state university, pp 4 - 14.

[6] Hamid A. Toliyat and Steven Campbell “dsp-based

electromechanical motion control”Texas A&M University Department

of Electrical Engineering College Station, Texas, pp 223,224 (v/f

control)

[7] Do-Hyun jang and duck yong yoon “Space vector technique for two

phase inverter fed single phase induction Motor “IEEE 1999

[8] B. Wu, High-Power Converters and AC Drives, IEEE Press and

Wiley,2006, pp 101-117.

[9] Krisztina Leban, Cristian Lascu, and Alin Argeseanu “Implementation

of Space Vector Modulation Strategies for Voltage Source Inverters for

Induction Motor Drives” University of Timisoara

[10] M. Lakshmi Swarupa, G. Tulasi Ram Das and P.V Raj Gopal

“Simulation and Analysis of SVPWM Based 2-Level and 3-Level

Inverters for Direct Torque of Induction Motor” International Journal of

Electronic Engineering Research ISSN 0975 - 6450 Vol- 1 No.3 (2009)

pp. 169 – 184

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Lnct Letter