CHAPTER-1 INTRODUCTION 1
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