CHAPTER-1

INTRODUCTION

1

1.1 WHAT IS AC TO DC CONVERTOR?

A AC TO DC CONVERTOR IS A DEVICE WHICH CONVERT

THE ALTERNATING CURRENT (AC) FROM THE MAINS TO A

DIRECT CURRENT (DC) .

1.2 WHY THE CONVERTOR IS NEEDED .

IN MANY ELECTRICAL APPLIANCES WE NEED OFTENLY DC

SUPPLY,HENCE THE DEVICE CONVERTOR IS USED THAT

CONVERTS REGULATED AC SUPPLY IN TO DC.

THIS DC OUTPUT ARE USED IN A GREAT VARIETY OF

APPLICATIONS, FOR EXAMPLE, SUCH AS CONTROLLING DC

MOTORS FOR HOUSEHOLD OR INDUSTRIAL USE (E.G., IN

WASHING MACHINES, REFRIGERATORS, DISHWASHERS,

INDUSTRIAL MACHINES). SUCH CONVERTERS ARE ALSO

KNOWN AS "SWITCH MODE POWER SUPPLY (SMPS) ”.

AC TO DC CONVERTERS GENERALLY COMPRISE A

RECTIFIER BRIDGE TO RECTIFY THE AC CURRENT OF THE

INPUT LINE AND A REGULATING DEVICE SUPPLYING ON

OUTPUT OF ONE OR MORE REGULATED DC VOLTAGES.

2

IN CONVERTERS WITHOUT ISOLATION BETWEEN THE

CONVERTERS INPUT AND OUTPUT, A NEUTRAL CONDUCTOR

OF THE INPUT LINE CAN BE PLACED DIRECTLY ON THE

OUTPUT, AND WILL ACT AS VOLTAGE A REFERENCE FOR

THE WHOLE CONVERTER.

AC-TO-DC CONVERTERS WHICH RECEIVE POWER FROM

AC POWER MAINS OFTEN RECTIFY THE SINEWAVE

(AC) MAINS VOLTAGE AND STORE ENERGY IN A

CAPACITOR. THE CAPACITOR GENERALLY CHARGES

TO THE PEAK MAINS VOLTAGE SUCH THAT CURRENT

ONLY FLOWS INTO THE POWER SUPPLY AROUND

THE PEAKS OF THE INPUT VOLTAGE.

MANY AC-DC POWER CONVERTERS EMPLOY POWER

FACTOR CORRECTION. THIS IS OFTEN

ACCOMPLISHED WITH TWO STAGES IN SERIES, A

BOOST CONVERTER INPUT STAGE AND A BUCK

CONVERTER SECOND STAGE.

3

THE POWER FACTOR CORRECTION (PFC)

TECHNIQUES CAN BE USED TO REDUCE THE

HARMONIC CONTENT OF THE INPUT CURRENT BY

REFORMING THE INPUT CURRENT INTO WHAT

APPROXIMATES A SINEWAVE. SUCH POWER FACTOR

CIRCUITS ARE, HOWEVER, GENERALLY COMPLEX.

AC TO DC CONVERTERS NEED POWER FACTOR

CORRECTION IN ORDER TO FULFILL

INTERNATIONAL STANDARDS OF LOW INPUT

HARMONIC CURRENT CONTENT. A FRONT-END

BOOST PFC CONVERTER IS ONE WAY TO OBTAIN

GOOD INPUT HARMONIC CURRENT TO MEET THESE

INTERNATIONAL STANDARDS. ANOTHER DC TO DC

CONVERTER IS GENERALLY CASCADED FROM THE

FRONT-END BOOST PFC CONVERTER TO PROVIDE A

STEADY OUTPUT VOLTAGE.

4

1.3 HOW THE CONVERTOR WORKS?

THE CONVERTOR MAY ALSO BE OF FOLLOWING FORMS:

AC TO DC(ALSO CALLED RECTIFIER)

DC TO AC(ALSO CALLED INVERTOR)

AC TO AC(VARIABLE FREQUENCY DRIVES)

DC TO DC(ALSO CALLED CHOPPER)

THE BASIC WORKING PRINCIPLE OF A AC TO DC CONVERTOR IS

SIMILAR TO THAT OF A DIODE RECTIFIER.

5

BASIC BLOCK DIAGRAM

REGULATED AC SUPPLY

DC OUTPUT

6

REGULATOR CIRCUIT

230 V AC SUPPLY

DIODE RECTIFIER CIRCUIT

REGULATED DC 230 VOLT

LOAD

CIRCUIT DIAGRAM

7

2.WORKING

THE WORKING OF AC TO DC CONVERTOR IS BASICALLY DEVIDED

INTO FOLLOWING STAGES.

1. SUPPLY

8

2. AC REGULATOR(AC TO AC CONVERSION)

3. DIODE RECTIFIER CIRCUIT (AC TO DC CONVERSION)

4. REGULATED DC OUTPUT

2.1 SUPPLY

THE INPUT IS 440 VOLT AC SUPPLY GIVEN TO THE 3 PHASE

CONVERTOR.

HENCE THE PHASE VOLTAGE IN EACH PHASE BECOMES:

(440 / 1.732) =254.02

THERE BY THE AVERAGE VALUE OF THE VOLTAGE

BECOMES:

AVG VALUE=RMS VALUE/FORM FACTOR

HENCE = 254.02/1.1=230 V

2.2.REGULATOR CIRCUIT

9

THE REGULATOR CIRCUIT IS ACTUALLY A VARIABLE

FREQUENCY DRIVE.

THIS AC TO AC CONVERSION DRIVE BASED ON THE

CONSTANT V/F RATIO.

WHERE,

V=VOLTAGE

F=FREQUENCY

AS THE VOLTAGE INCREASE THE FREQUENCY ALSO

INCRESAE AND VICE VERSA.

SO IN THE REGULATOR CIRCUIT WE REDUCE THE VOLTAGE

THE HELP OF TRANSISTOR FT-12.

2.3 PROPERTY OF THE FT-12

THE TRANSISTOR FT-12 IS WORKING AS HIGH

PERFORMANCE PNPN TRIAC.

THEY ARE GENERALLY USED FOR THE AC SWITICHING

APPLICATION WITH HIGHLY INDUCTIVE LOADS.

10

HENCE WITH THE HELP OF FT-12 WE PERFORM

SWITCHING OPERATION IN THE INPUT WAVEFORM,BY

WHICH WE GET REGULATED AC SUPPLY.

2.4.DIODE RECTIFIER CIRCUIT

AFTER GETTING THE REGULATED AC THROUGH THE

REGULATOR CIRCUIT IT BECOMES EASY TO GET THE DC O/P

FROM THE AC.

THE DC O/P IS OBTAINED WITH THE HELP OF BRIDGE

RECTIFIER.

THE TWO DIODES ARE CONDUCTING DURING THE POSITIVE

HALF CYCLE AND THEN NEXT DIODES ARE CONDUCTING

DURING NEGATIVE HALF CYCLE.

HENCE WE GET THE DC O/P IN RESPECTIVE CYCLES.

2.5 .REGULATED DC OUTPUT

11

THE OUTPUT OF THE DIODE RACTIFIER CIRCUIT IS NOT

PURE DC.IT HAS SOME IRREGULAR SHAPE AS WELL AS LOW

POWER FACTOR.

TO IMPROVE OUTPUT DC WAVESHAPE AS WELL AS THE

POWER FACTOR THE CONDENSER IS CONNECTED IN THE

OUTPUT,WHICH ALSO ACTS AS THE FILTER CIRCUIT.

SO IN ABOVE FOUR STAGES THE WORKING OF THE AC-AC-DC

CONVERTOR CAN BE UNDERSTAND.

12

3.COMPONENT

USED IN THE

CIRCUIT

13

3.1FOLLOWING COMPONENTS ARE USED IN THE AC-AC-DC

CONVERTOR:

1. RESISTORS

2. CAPACITORS

3. INDUCTOR COIL

4. TRANSISTOR FT-12

5. DIODES

6. MILI AMMETER

7. DC VOLTMETER

8. ZERO BOARD

14

3.2COMPONENT SPECIFICATION

1. RESISTOR

THERE ARE TWO TYPES OF TRANSISTORS ARE USED IN THE

CIRCUIT VIZ.

VARIABLE RESISTOR AND THE FIXED RESISTORS.

VARIABLE RESISTORS ARE USED TO REGULATE THE AC

SUPPLY.THE SPECIFICATION ARE GIVEN BELOW:

FIXED RESISTOR R1-10 Kilo Ohm

FIXED RESISTOR R2-0.1 Kilo Ohm

VARIABLE RESISTOR R3-560 Kilo Ohm

2. CAPACITORS

THE CAPACITORS ARE USED FOR THE POWER FACTOR

IMPROVEMENT AS WELL AS TO GET THE REGULATED DC

OUTPUT.

THE SPECIFICATION ARE AS FOLLOWS:

PAPER CAPACITOR C1- 0.01 Kilo Farad

PAPER CAPACITOR C2- 0.1 Kilo Farad

PAPER CAPACITOR C2- 0.5 Kilo Farad

15

3.INDUCTOR COIL

4.TRANSISTOR FT-12

RATED INPUT - 230±10% V AC

5.AC VOLTMETER

FOR EACH PHASE RATING-(0-300 V)

6.AC AMMETER

RATING-(0-30 A)

7.DC AMMETER

RATING-(0-10 A)

8.DIODE

RATING- 6 A

9.ZERO BOARD

10.CONNECTING WIRES

16

4.INTRODUCTION

TO CONTROLLED

RECTIFIER

17

INTRODUCTION:-

THREE – PHASE CONTROLLED RECTIFIERS HAVE A WIDE

RANGE OF APPLICATIONS , FROM SMALL RECTIFIERS TO LARGE

HIGH VOLTAGE DIRECT CURRENT (HVDC) TRANSMISSION

SYSTEMS. THEY ARE USED FOR ELECTRO CHEMICAL

PROCESSES , MANY KINDS OF MOTOR DRIVES, TRACTION

EQUIPMENT, CONTROLLED POWER SUPPLIES, AND MANY OTHER

APPLICATIONS. FROM THE POINT OF VIEW OF THE

COMMUTATION PROCESS, THEY CAN BE CLASSIFIED INTO TWO

IMPORTANT CATEGORIES: LINECOMMUTATED CONTROLLED

RECTIFIERS (THYRISTOR RECTIFIERS); AND FORCE –

COMMUTATED PWM RECTIFIERS.

18

4.1LINE-COMMUTATED CONTROLLED RECTIFIERS

(FIG 1.)

FIG.1 IS THE THREE - PHASE HALF - WAVE TOPOLOGY . TO

CONTROL THE LOAD VOLTAGE, THE HALF-WAVE RECTIFIER

USES THREE COMMON-CATHODE THYRISTOR ARRANGEMENT

THEPOWER SUPPLY AND THE TRANSFORMER ARE ASSUMED

IDEAL. THE THYRISTOR WILL CONDUCT ( ON STATE ) , WHEN

THE ANODE- TO -CATHODE VOLTAGE NAK IS POSITIVE, AND A

FIRING CURRENT PULSE IG IS APPLIED TO THE GATE

TERMINAL. DELAYING THE FIRING PULSE BY AN ANGLE A

CONTROLS THE LOAD VOLTAGE. SHOW FIG ( 2) THE FIRING

ANGLE A IS MEASURED FROM THE CROSSING POINT BETWEEN

19

THE PHASE SUPPLY VOLTAGES. AT THAT POINT , THE ANODE –

TO – CATHODE THYRISTOR COLOURFUL RECEPTION IN EVERY

FIELD.THYRISTORS VOLTAGE NAK BEGINS TO BE POSITIVE.

THE POSSIBLE RANGE FOR GATING DELAY IS BETWEEN A . 0_ AND

A . 180_, BUT BECAUSE OF COMMUTATION

FIG (.2 )

PROBLEMS IN ACTUAL SITUATIONS, THE MAXIMUM FIRING

ANGLE IS LIMITED TO _160_. WHEN THE LOAD IS RESISTIVE,

CURRENT ID HAS THE SAME WAVEFORM AS THE LOAD VOLTAGE.

AS THE LOAD BECOMES MORE AND MORE INDUCTIVE, THE

CURRENT FLATTENS AND FINALLY BECOMES CONSTANT. THE

THYRISTOR GOES TO THE NONCONDUCTING CONDITION (OFF

20

STATE) WHEN THE FOLLOWING THYRISTOR IS SWITCHED ON, OR

THE CURRENT TRIES TO REACH A NEGATIVE VALUE. THE LOAD

AVERAGE VOLTAGE CAN BE EVALUATED AND IS GIVEN BY

.

. EQ NO …………..(1)

WHERE VMAX IS THE SECONDARY PHASE –TO -NEUTRAL

PEAK VOLTAGE, VRMS ITS ROOT MEAN SQUARE (RMS)

VALUE, AND O IS THE ANGULAR FREQUENCY OF THE MAIN

POWER SUPPLY. IT CAN BE SEEN FROM EQ ( 1 ) THAT THE LOAD

AVERAGE VOLTAGE VD IS MODIFIED BY CHANGING FIRING

ANGLE A. WHEN A IS <90_, VD IS POSITIVE AND WHEN A IS >90_,

21

THE AVERAGE DC VOLTAGE BECOMES NEGATIVE.

IN SUCH A CASE, THE RECTIFIER BEGINS TO WORK AS AN

INVERTER, AND THE LOAD NEEDS TO BE ABLE TO GENERATE

POWER REVERSAL BY REVERSING ITS DC VOLTAGE

22

THE AC CURRENTS OF THE HALF – WAVE RECTIFIER ARE .

THIS DRAWING ASSUMES THAT THE DC CURRENT IS

CONSTANT (VERY LARGE LD). DISREGARDING COMMUTATION 23

OVERLAP, EACH VALVE CONDUCTS DURING 120 _ PER PERIOD .

THE SECONDARY CURRENTS ( AND THYRISTOR CURRENTS )

PRESENT A DC COMPONENT THAT IS UNDESIRABLE , AND

MAKES THIS RECTIFIER NOT USEFUL FOR HIGH POWER

APPLICATIONS . THE PRIMARY CURRENTS SHOW THE SAME

WAVEFORM, BUT WITH THE DC COMPONENT REMOVED.

THIS VERY DISTORTED WAVEFORM REQUIRES AN INPUT FILTER

TO REDUCE HARMONICS CONTAMINATION. THE CURRENT

WAVEFORMS ARE USEFUL FOR DESIGNING THE POWER

TRANSFORMER. STARTING FROM

EQ NO………………….(2)

WHERE VAPRIM AND VASEC ARE THE RATINGS OF THE

TRANSFORMER FOR THE PRIMARY AND SECONDARY SIDE,

24

RESPECTIVELY. HERE PD IS THE POWER TRANSFERRED TO

THE DC SIDE. THE MAXIMUM POWER TRANSFER IS WITH A . 0_

(OR A . 180_). THEN, TO ESTABLISH A

RELATION BETWEEN AC AND DC VOLTAGES, EQ (1) FOR A . 0_

IS REQUIRED:

EQ NO………………….(3)

AND25

EQ NO ……………….(4)

WHERE A IS THE SECONDARY TO PRIMARY TURN RELATION

OF THE TRANSFORMER. ON THE OTHER HAND, A RELATION

BETWEEN THE CURRENTS IS ALSO POSSIBLE TO OBTAIN.

EQ NO ……………….(5) & (6)

COMBINATION OF EQ. (5) & (6)

EQ NO…………(7)

EQUATION (7) SHOWS THAT THE POWER TRANSFORMER HAS

TO BE OVERSIZED 21% AT THE PRIMARY SIDE, AND 48% AT THE

SECONDARY SIDE. THEN A SPECIAL TRANSFORMER HAS TO BE

BUILT FOR THIS RECTIFIER. IN TERMS OF AVERAGE VA, THE

TRANSFORMER NEEDS TO BE 35% LARGER THAT THE RATING OF

THE DC LOAD. THE LARGER RATING OF THE SECONDARY

26

RESPECT TO PRIMARY IS BECAUSE THE SECONDARY CARRIES A

DC COMPONENT INSIDE THE WINDINGS. FURTHERMORE, THE

TRANSFORMER IS OVERSIZED BECAUSE THE CIRCULATION OF

CURRENT HARMONICS DOES NOT GENERATE ACTIVE POWER.

CORE SATURATION, DUE TO THE DC COMPONENTS INSIDE THE

SECONDARY WINDINGS, ALSO NEEDS TO BE TAKEN INTO

ACCOUNT FOR IRON OVERSIZING.

4.2 SIX-PULSE OR DOUBLE STAR RECTIFIER

THE THYRISTOR SIDE WINDINGS OF THE TRANSFORMER FORM A

SIX-PHASE SYSTEM, RESULTING IN A 6-PULSE STARPOINT

(MIDPOINT CONNECTION). DISREGARDING COMMUTATION

OVERLAP,

EACH VALVE CONDUCTS ONLY DURING 60_ PER PERIOD. THE

DIRECT VOLTAGE IS HIGHER THAN THAT FROM THE HALF-WAVE

RECTIFIER, AND ITS AVERAGE VALUE IS GIVEN BY

27

EQ NO

…………………(8)

THE DC VOLTAGE RIPPLE IS ALSO SMALLER THAN THE ONE

GENERATED BY THE HALF-WAVE RECTIFIER, DUE TO THE

ABSENCE OF THE THIRD HARMONIC WITH ITS INHERENTLY HIGH

AMPLITUDE. THE SMOOTHING REACTOR LD IS ALSO

CONSIDERABLY SMALLER THAN THE ONE NEEDED FOR A 3-PULSE

(HALF-WAVE) RECTIFIER. THE AC CURRENTS OF THE 6-PULSE

RECTIFIER ARE SHOWN IN FIG. 7. THE CURRENTS IN THE

SECONDARY WINDINGS PRESENT A DC COMPONENT, BUT THE

MAGNETIC FLUX IS COMPENSATED BY THE DOUBLE STAR. AS CAN

BE OBSERVED, ONLY ONE VALVE IS FIRED AT A TIME, AND THEN

THIS CONNECTION IN NO WAY CORRESPONDS TO A PARALLEL

CONNECTION. THE CURRENTS INSIDE THE DELTA SHOW A

SYMMETRICAL WAVEFORM, WITH 60_ CONDUCTION. FINALLY,

DUE TO THE PARTICULAR TRANSFORMER CONNECTION SHOWN

IN FIG. 12.6, THE SOURCE CURRENTS ALSO SHOW A

28

SYMMETRICAL WAVEFORM, BUT WITH 120_ CONDUCTION.

EVALUATION OF THE RATING OF THE TRANSFORMER IS DONE

INSIMILAR FASHION TO THE WAY THE HALF-WAVE RECTIFIER IS

EVALUATED

EQ NO ……………(9)

THUS, THE TRANSFORMER MUST BE OVERSIZED 28% AT THE

PRIMARY SIDE, AND 81% AT THE SECONDARY SIDE. IN TERMS OF

SIZE IT HAS AN AVERAGE APPARENT POWER OF 1.55 TIMES THE

POWER PD (55% OVERSIZED). BECAUSE OF THE SHORT

CONDUCTING PERIOD OF THE VALVES, THE TRANSFORMER IS

NOT PARTICULARLY WELL UTILIZED

29

30

4.3 FULL-WAVE RECTIFIERS

FIG. 2

31

THE WAVEFORMS FOR THE CIRCUIT OF FIG.1ARE SHOWN IN FIG.2.

THE VOLTAGE ACROSS THE LOAD RESISTOR IS A FULL-WAVE

RECTIFIED VOLTAGE. THE CURRENT HAS SUBTLE

DISCONTINUITIES BUT CAN BE IMPROVED BY EMPLOYING

SMALLER SIZE FILTER COMPONENTS. A TYPICAL FILTER FOR

THE CIRCUIT OF FIG.1MAY INCLUDE ONLY A CAPACITOR. THE

WAVEFORMS OBTAINED ARE SHOWN IN FIG.3. YET ANOTHER

WAY OF REDUCING THE SIZE OF THE FILTER COMPONENTS IS TO

INCREASE THE FREQUENCY OF THE SUPPLY. IN MANY POWER

SUPPLY APPLICATIONS SIMILAR TO THE ONE USED IN

COMPUTERS, A HIGH FREQUENCY AC SUPPLY IS ACHIEVED BY

MEANS OF SWITCHING. THE HIGH FREQUENCY AC IS THEN LEVEL

TRANSLATED VIA A FERRITE CORE TRANSFORMER WITH

MULTIPLE SECONDARY WINDINGS.

32

FIG.2

THE SECONDARY VOLTAGES ARE THEN RECTIFIED EMPLOYING A

SIMPLE CIRCUIT AS SHOWN IN FIG. 4.4 OR FIG. 4.6 WITH MUCH

SMALLER FILTERS. THE RESULTING VOLTAGE ACROSS THE LOAD

RESISTOR IS THEN MAINTAINED TO HAVE A PEAK-PEAK

VOLTAGE RIPPLE OF LESS THAN 1%. FULL-WAVE RECTIFICATION

CAN BE ACHIEVED WITHOUT THE USE OF CENTER-TAP

TRANSFORMERS. SUCH CIRCUITS MAKE USE OF FOUR DIODES IN

SINGLE-PHASE CIRCUITS AND SIX DIODES IN THREE-PHASE

CIRCUITS.

THE CIRCUIT CONFIGURATION

33

FIG.3

IS TYPICALLY REFERRED TO AS THE H-BRIDGE CIRCUIT. A

SINGLE-PHASE FULL-WAVE H-BRIDGE TOPOLOGY IS SHOWN IN

FIG.4. THE MAIN DIFFERENCE BETWEEN THE CIRCUIT TOPOLOGY

SHOWN IN FIGS.1AND4 IS THAT THE HBRIDGE CIRCUIT EMPLOYS

FOUR DIODES WHILE THE TOPOLOGY OF FIG.1 UTILIZES ONLY

TWO DIODES. HOWEVER, A CENTER-TAP TRANSFORMER OF A

HIGHER POWER RATING IS NEEDED FOR THE CIRCUIT OF FIG. 1.

THE VOLTAGE AND CURRENT STRESSES IN THE DIODES IN

FIG.1ARE ALSO GREATER THAN THAT OCCURRING IN THE DIODES

OF FIG.4. IN ORDER TO COMPREHEND THE BASIC DIFFERENCE IN

34

THE TWO TOPOLOGIES, IT IS INTERESTING TO COMPARE THE

COMPONENT RATINGS FOR THE SAME POWER OUTPUT. TO MAKE

THE COMPARISON EASY, LET BOTH TOPOLOGIES EMPLOY

VERY LARGE FILTER INDUCTORS SUCH THAT THE CURRENT

THROUGH R IS CONSTANT AND RIPPLE-FREE.

LET THIS CURRENT THROUGH R BE DENOTED BY IDC. LET THE

POWER BEING SUPPLIED TO THE LOAD BE DENOTED BY PDC . THE

OUTPUTPOWER AND THE LOAD CURRENT ARE THEN RELATED BY

THE FOLLOWING EXPRESSION:

PDC = IDC2×R

FIG. 4

35

THE RMS CURRENT FLOWING THROUGH THE FIRST SECONDARY

WINDING IN THE TOPOLOGY IN FIG.1WILL BE THIS IS BECAUSE

THE CURRENT THROUGH A SECONDARY WINDING FLOWS ONLY

WHEN THE CORRESPONDING DIODE IS FORWARD-BIASED. THIS

MEANS THAT THE CURRENT THROUGH THE SECONDARY

WINDING WILL FLOW ONLY FOR ONE HALF CYCLE. IF THE

VOLTAGE AT THE SECONDARY IS ASSUMED TO BE V, THE VA

RATING OF THE SECONDARY WINDING OF THE

TRANSFORMER IN FIG.1WILL BE GIVEN BY:

VA1 = V × IDC/√2

VA2 = V × IDC/√2

VA = VA1 + VA2 = √2 × V × IDC

THIS IS THE SECONDARY-SIDE VA RATING FOR THE

TRANSFORMER SHOWN IN FIG. 1.

FOR THE ISOLATION TRANSFORMER SHOWN IN FIG.4, LET THE

SECONDARY VOLTAGE BE V AND THE LOAD CURRENT BE OF A

CONSTANT VALUE IDC. SINCE, IN THE TOPOLOGY OF FIG.4, THE

SECONDARY WINDING CARRIES THE CURRENT IDC WHEN DIODES 36

D1 AND D2 CONDUCT AND AS WELL AS WHEN DIODES D3 AND D4

CONDUCT, THE RMS VALUE OF THE SECONDARY WINDING

CURRENT IS IDC . HENCE, THE VA RATING OF THE SECONDARY

WINDING OF THE TRANSFORMER

SHOWN IN FIG.4 IS WHICH IS LESS THAN THAT NEEDED IN THE

TOPOLOGY OF FIG. 1. NOTE THAT THE PRIMARY VA RATING FOR

BOTH CASES REMAINS THE SAME SINCE IN BOTH CASES THE

POWER BEING TRANSFERRED FROM THE SOURCE TO THE LOAD

REMAINS THE SAME.

WHEN DIODE D2 IN THE CIRCUIT OF FIG.1CONDUCTS, THE

SECONDARY VOLTAGE OF THE SECOND WINDING VSEC2(V)

APPEARS AT THE CATHODE OF DIODE D1. THE VOLTAGE BEING

BLOCKED BY DIODE D1 CAN THUS REACH TWO TIMES THE PEAK

SECONDARY VOLTAGE () (FIG.2). IN THE TOPOLOGY OF FIG.4,

WHEN DIODES D1 AND D2 CONDUCT, THE VOLTAGE VSEC (V),

WHICH IS SAME AS VSEC2 APPEARS ACROSS D3 AS WELL AS

ACROSS D4. THIS MEANS THAT THE DIODES HAVE TO WITHSTAND

ONLY ONE TIMES THE PEAK OF THE SECONDARY VOLTAGE, VPK.

37

FIG.5

THE RMS VALUE OF THE CURRENT FLOWING THROUGH THE

DIODES IN BOTH TOPOLOGIES IS THE SAME. HENCE, FROM THE

DIODE VOLTAGE RATING AS WELL AS FROM THE SECONDARY VA

RATING POINTS OF VIEW, THE TOPOLOGY OF FIG.4 IS BETTER

THAN THAT OF FIG. 1. FURTHER, THE TOPOLOGY IN FIG.4 CAN BE

DIRECTLY CONNECTED TO A SINGLE-PHASE AC SOURCE AND

DOES NOT NEED A CENTER-TOPPED TRANSFORMER. THE

VOLTAGE WAVEFORM ACROSS THE LOAD RESISTOR IS SIMILAR

TO THAT SHOWN IN FIGS.2 AND3. IN MANY INDUSTRIAL

APPLICATIONS, THE TOPOLOGY SHOWN IN FIG.4 IS USED ALONG

WITH A DC FILTER CAPACITOR TO SMOOTH THE RIPPLES ACROSS

THE LOAD RESISTOR. THE LOAD RESISTOR IS SIMPLY A 38

REPRESENTATIVE OF A LOAD. IT COULD BE AN INVERTER

SYSTEM OR A HIGH-FREQUENCY RESONANT LINK. IN ANY CASE,

THE DIODE RECTIFIER-BRIDGE WOULD SEE A REPRESENTATIVE

LOAD RESISTOR.

THE DC FILTER CAPACITOR WILL BE LARGE IN SIZE COMPARED

TO AN HBRIDGE

CONFIGURATION BASED ON THREE-PHASE SUPPLY SYSTEM.

WHEN THE RECTIFIED POWER IS LARGE, IT IS ADVISABLE TO ADD

A DC-LINK INDUCTOR. THIS CAN REDUCE THE SIZE OF THE

CAPACITOR TO SOME EXTENT AND REDUCE THE CURRENT

RIPPLE THROUGH THE LOAD. WHEN THE RECTIFIER IS TURNED

ON INITIALLY WITH THE CAPACITOR AT ZERO VOLTAGE, A

LARGE AMPLITUDE OF CHARGING CURRENT WILL FLOW INTO

THE FILTER CAPACITOR THROUGH A PAIR OF CONDUCTING

DIODES.

THE DIODES D1 ∼D4 SHOULD BE RATED TO HANDLE THIS LARGE

SURGE CURRENT. IN ORDER TO LIMIT THE HIGH INRUSH

CURRENT, IT IS A NORMAL PRACTICE TO ADD A CHARGING

RESISTOR IN SERIES WITH THE FILTER CAPACITOR. THE

CHARGING RESISTOR LIMITS THE INRUSH CURRENT BUT 39

CREATES A SIGNIFICANT POWER LOSS IF IT IS LEFT IN THE

CIRCUIT UNDER NORMAL

OPERATION. TYPICALLY, A CONTACTOR IS USED TO SHORT-

CIRCUIT THE CHARGING RESISTOR AFTER THE CAPACITOR IS

CHARGED TO A DESIRED LEVEL. THE RESISTOR IS THUS

ELECTRICALLY NONFUNCTIONAL DURING NORMAL OPERATING

CONDITIONS.

A TYPICAL ARRANGEMENT SHOWING A SINGLE-PHASE FULL-

WAVE H-BRIDGE RECTIFIER SYSTEM FOR AN INVERTER

APPLICATION IS SHOWN IN FIG.5. THE CHARGING CURRENT AT

TIME OF TURN-ON IS SHOWN IN A SIMULATED WAVEFORM IN

FIG.6. NOTE THAT THE CONTACTS ACROSS THE SOFT-CHARGE

RESISTOR ARE CLOSED UNDER NORMAL OPERATION. THE

CONTACTS ACROSS THE SOFTCHARGE RESISTOR ARE INITIATED

BY VARIOUS MEANS. THE COIL FOR THE CONTACTS COULD BE

POWERED FROM THE INPUT AC SUPPLY AND A TIMER OR IT

COULD BE POWERED ON BY A LOGIC CONTROLLER THAT SENSES

THE LEVEL OF VOLTAGE ACROSS THE DC BUS CAPACITOR OR

SENSES THE RATE OF CHANGE IN VOLTAGE ACROSS THE DC BUS

CAPACITOR. A SIMULATED WAVEFORM DEPICTING THE INRUSH 40

WITH AND WITHOUT A SOFT-CHARGE RESISTOR IS SHOWN IN

FIG.6 A AND B, RESPECTIVELY.

FIG.6 (A) & (B)41

CHAPTER 5

APPLICATION

THE BEST WAY TO MAKE USE OF OUR TECHNICAL KNOWLEDGE

TO WED OUR THEROTICAL CONCEPTS WITH PRACTICAL

APPLICATIONS. OUR PROJECT HAS NUMEROUS APPLICATIONS

THAT ARE :-

1. 230V DC AS PER REQUIREMENT.

2. 36V DC BATTERY CHARGER.

3. 3V DC MOBILE CHARGER.

OTHER AC TO DC CONVERTER APPLICATIONS ARE.

1. WASHING MACHINES.

2. REFRIGERATORS.

3. DISHWASHERS.

42

CHAPTER 6

CONCLUSION

THE CONVERTER CONVERTS A.C. INTO D.C. IT IS

USED WHEN A DC MOTOR IS TO BE USED WITH A.C.

SUPPLY. ALTHOUGH THE THYRISTOR CONVERTER IS

COSTLY, IT IS MORE EFFICIENT FOR ENTIRE SPEED

RANGE, LOAD RANGE AND THE TOTAL

INSTALLATION COST IS LESS. THE TYPE OF

CONVERTER DEPENDS ON THE RATED POWER. THE

CHOICE IS AS FOLLOWS : ---

LOW POWER,(BELOW 20KW) :SINGLE PHASE CONVERTER. ---HIGH POWER,(ABOVE 20KW):

THREE PHASE OR MULTI-PHASE CONVERTER. WITH THE THYRISTOR CONVERTER THE OUTPUT DC VOLTAGE CAN BE ADJUSTED.

43

THE METHOD OF PHASE CONTROL IS USED FOR THYRISTOR BRIDGE CONTROL.

THIS TYPE OF CONTROL IS USUALLY REQUIRED FOR ELECTRICAL TRACTION OF TRAINS USING SEPARATELY EXCITED DC MOTOR.

44

REFERENCES

BIBILOGRAPHY

1. R. KRISHNAN, “ELECTRIC MOTOR DRIVES, MODELING, ANALYSIS AND CONTROL”, 1ST ED. PEARSON EDUCATION, SINGAPORE, 2001.

2. “POWER ELECTRONICS” M.D. SINGH, KHANCHANDANI.

3. “POWER ELECTRONICS” RASHID.

4. “OPERATIONAL AMPLIFIERS” ARPAD BARNA , DAN I. PORAT

5. “DIGITAL ELECTRONICS” M. M. MANO

6. “INTEGRATED ELECTRONICS” MILLMAN , HALKIAS

7. “POWER ELECTRONICS” DR. BAHISHAM YUNUS

45