PWM Simetrical Regular Sampling Control Circuit for a Two

8/13/2019 PWM Simetrical Regular Sampling Control Circuit for a Two

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Abstract

Three-phase inverters are used for varia5le-frequenc drive applications and for highpower applications. 7 5asic three-phase inverter consists of three single-phase inverter switches

each connected to one of the three load terminals. &or the most 5asic control scheme3 the

operation of the three switches is coordinated so that one switch operates at each 8 degree point

of the fundamental output waveform. This creates a line-to-line output waveform that has si2steps. The si2-step waveform has a ero-voltage step 5etween the positive and negative sections

of the square-wave such that the harmonics that are multiples of three are eliminated as descri5ed

a5ove. When carrier-5ased PWM techniques are applied to si2-step waveforms3 the 5asic overallshape3 or envelope3 of the waveform is retained so that the ,rd harmonic and its multiples are

cancelled.

The three-phase PWM rectifier has the characteristic of drawing nearl sinusoidal currentwith sta5le switching frequenc and tight control of the position of modulation pulses. The

simulation results show that the designed technique can improve three-phase PWM rectifier

performance noticea5l.
http://en.wikipedia.org/wiki/Three-phase_electric_powerhttp://en.wikipedia.org/wiki/Variable-frequency_drivehttp://en.wikipedia.org/wiki/Three-phase_electric_powerhttp://en.wikipedia.org/wiki/Variable-frequency_drive


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Introduction

Pulse width modulation 9PWM: is the method of choice to control modern power

electronics circuits. The 5asic idea is to control the dut ccle of a switch such that a load sees acontrolla5le average voltage. To achieve this3 the switching frequenc 9repetition frequenc for

the PWM signal: is chosen high enough that the load cannot follow the individual switching

events. "witching3 rather than linear operation of the power semiconductors3 is of course done toma2imie the efficienc 5ecause the power dissipation in a switch is ideall ero in 5oth states.

In a tpical case3 the switching events are ;ust a s digital output. PWM is emploed in a wide variet of

applications3 ranging from measurement and communications to power control and conversion.

Pulse-width modulation3 or pulse-duration modulation 9P#M:3 is a commonl usedtechnique for controlling power to inertial electrical devices3 made practical 5 modern

electronic power switches.

PWM technique is 5ased on comparison of carrier signal with control signal. Intersection

points of signals shows commutation time of semiconductor power electronic switching

components. There are two PWM signals as smmetric and asmmetric. Pulses of smmetric

PWM signal are alwas smmetric according to center of each PWM period. Pulses of

asmmetric PWM signal are aligned with one side of PWM period. "mmetric PWM signals

produce less output current and voltage harmonic


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?asic purpose of PWM technique is to reduce inverter output harmonic level3 to increase

voltage magnitude3 to reduce switching losses. The most important characteristic of PWM

method is that as inverter input #C voltage is constant3 inverter output voltage and frequenc can

5e changed. '2istence of harmonics in energ sstem is understood from corruptions of current

and voltage waveforms that have sinusoidal form. %ne of the *nown harmonic sources is inverter

too. 'ffects of inverter output current harmonics cause that increment of voltage droop 5ecause

of current harmonic components3 overheating at induction motors 5ecause of occurred

oscillations3 fault measurements and decrement of life of equipments which are connected to

out of inverter. "o decrement of harmonics is desired. Inverter power devices are switched as

on-off man times in half period to produce output voltage which has less harmonic. "PWM is

used widespread from of old 5ecause of application easiness of PWM techniques. &undamental

output voltage magnitude of PWM techniques is provided 5 ad;usting of modulation inde2. If

modulation inde2 is less than one3 onl side 5ands of fundamental wave frequenc.

Theory

"ampling techniques

The smmetric and asmmetric PWM methods are e2plained with the following figure$

&igure !. "mmetric and asmmetric PWM

In smmetric PWM3 the positive 9or negative: pulse of ever PWM ccle is located in the middle

of theccle period3 while in the asmmetric PWM3 the pulses are usuall aligned to the start or end of

the PWM

ccle.


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Practicall3 asmmetric methods are relativel easier to realise3 5ut smmetric methods evo*e

fewer

harmonic interferences. Therefore3 smmetric PWM should 5e used when possi5le.

a: "imetrical regular sampling

Computation of the switching times in case of simetrical regular sampling

( )( )stuTT

+= 1!

11


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5: 7simetrical regular sampling

Power M%"&'T is a universall popular device for low-voltage3 low-power3 high-

frequenc applications3 such as switching mode power suppl3 porta5le 5rush and 5rushless #C

drives3 has no fear of competition 5 other devices in the future at that power level.

The model simulates three phase inverter with smmetric PWM modulation in whichreference is sampled at the positive pea* of the carrier and is held constant till the ne2t positive

( )( )stuTTT

+= 1!

1!


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pea* of the carrier appears. The model demonstrates smmetric regular sampling at positive

pea*s of carrier and fourier analsis of three phase inverter with this modulation technique.

"mmetric modulation can 5e o5served on the scope used 5 ooming the waveforms to largescale on time a2is.

In inverter circuits3 the PWM is used in order to o5tain inverter output to 5e sinusoidal

with magnitude and frequenc controlla5le.PWM control technique is to that conclusion as the theoretical 5asis of the semiconductor

switching devices turn on and off control3 so that the output to 5e a series of unequal amplitude

equal to the width of the pulse3 with these pulses instead of sine waves or other of the requiredwaveform. 7ccording to certain rules of each pulse-width modulated inverter circuit output

voltage can change the sie3 5ut also can change the output frequenc.

Induction motor driver circuits are harmonic sources 5ecause of their semiconductor

switching power components. 0armonics are provided 5 inverter3 are inevita5le3 sooptimiation of these is made. &or this purpose3 more fast and had less switching losses ones of

used semiconductor power switching components are preferred3 also PWM 9Pulse Width

Modulation: techniques used in inverter are developed.

?ecause of advances in solid state power devices and microprocessors3 switching powerconverters are used in more and more modern motor drivers to convert and deliver the required

energ to the motor @1A. Tas* of inverter is convert #C input voltage to 7C voltage with desired

magnitude and frequenc. %utput voltage regulation is made as constant or varia5le frequenc.

(aria5le output voltage can 5e o5tained *eeping constant inverter gain and ad;usting #C input

voltage. 7nother method3 if #C input voltage is constant and not ad;usta5le3 varia5le output

voltage can 5e o5tained 5 ad;usting of inverter gain3 this is provided 5 PWM control of

inverter @!A.

PWM signals are o5tained putting related signal data to sin ta5le at software which

developed for carrier 5ased PWM signals. &or different PWM techniques3 inverter output currentharmonic analsis is made changing modulation inde2 and modulation ratio with harmonic

analer. %utput current of the sstem which controlled according to different PWM techniques

and output current harmonics are investigated comparing each other.

PWM control technique is to that conclusion as the theoretical basis of the

semiconductor switching devices turn on and off control, so that the output to be a series of

unequal amplitude equal to the width of the pulse, with these pulses instead of sine waves or

other of the required waveform. According to certain rules of each pulse-width modulated

inverter circuit output voltage can change the size, but also can change the output frequency.


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5.Theoretical calculations and simulation results

In this pro;ect we use the simetrical sampling sinusoidal PWM technique for simulatingthe inverter motor circuit.

"imetrical sampling calculations

Initial values$*0/+f

s ;

s1

1

+Ts1B/

1

+T 6,s ;

(,+(d ;

(!1+(out ;

4.+ma ;

01+f1 .

Computing of the switching times in case of simetrical regular sampling$

9 :9 :s

C

1 tuD1T

!

1+T ;

9 :9 :sC

! tu-1T!1DT+T ;

s1+1

1+

f

1+T

!-

ref

ref ;

01--+fref ;

(!1+Ema2 ;

*0/+1B/+f ,c ;


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(,+Ed ;

c T+T! ;

s1+1B1

1+T 6-

, ;

/+1

1B/

+f

f

+m !

,

ref

c

f 9the frequenc modulation ratio:

4.+ma 9the amplitude modulation ratio:

trtr

ma2

a E

!1+

E

E+m ;

A(@,.!,,+4.

!1+E

E

!1+4. tr

tr

;

9 : 9 :t1BF!sin!1+tGsinBE+u ma2 ;

9 :tGsinB!1+u ;

9 :1tGsinB!1+1u ;

.9 :

64tGsinB!1+64u ;

/TB+T ref ;

+T ;

/TB1+Tref1 ;

!.+1B/

1+

1B/

1+

/

1+

/

1B1+T

,!

!-!-

1 ;

6.+!.B!+/

1B!+T

!-

! ;

8.+!.B,+/

1

B,+T

!-

, ;

H.+!.B6+/

1B6+T

!-

6 ;

1.+T/ ;

1!.+T8 ;

16.+TI ;

..

4H.+T64 .

The model simulates three phase inverter with smmetric PWM modulation in which

reference is sampled at the positive pea* of the carrier and is held constant till the ne2t positivepea* of the carrier appears. The model demonstrates smmetric regular sampling at positivepea*s of carrier and fourier analsis of three phase inverter with this modulation technique.

"mmetric modulation can 5e o5served on the scope used 5 ooming the waveforms to large

scale on time a2is.

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PWM Simetrical Regular Sampling Control Circuit for a Two