controlled rectifiers

SEMINAR ON AC – DC CONVERTERS

BY:ANKUR MAHAJAN

M.E. I&C REGULAR -2011ROLL NO. 112505

GOAL OF THE SEMINAR SINGLE PHASE HALF WAVE & FULL WAVE

CONVERTERS.

SEMICONDUCTOR DEVICES – THE

HEART OF INDUSTRIAL

ELECTRONICS

MY OWN QUOTES : “THE DEVELOPMENT OF EVERY NATION DEPEND UPON

ITS POWER SOURCES, & POWER ELECTRONICS IS A MAGIC WAND WHICH GENERATE POWER SOURCES”

“POWER ELECTRONICS IS A LIFE BLOOD FOR MODERN COMPUTERS, COMMUNICATION & ELECTRONIC SYSTEMS , THUS WITHOUT IT OUR LIFE COMES TO HALT”

WHAT IS THE MOST IMPORTANT REASON FOR RAPID DEVELOPMENT IN THE FIELD OF POWER

ELECTRONIC DEVICES ?

PIE CHART OF WORL’S ENERGY PRODUCTION LEVEL

FOSSIL FUELSNUCLEAR

RENEWABLES

IN INDIA 70% OF ELECTRICAL ENERGY COMES FROM COAL

ACCORDING TO IEEE JOURNAL ON

POWER DELIVERY: WE HAVE NO

FOSSIL & NUCLEAR FUEL BY THE

END OF 22ND CENTURY.

87%

6%

7%

(A)USE EE EFFICIENTLY.(B) IMPROVE CONVERSION EFFICIENCY.

(C) USE RENEWABLES

INTRODUCTION

WHAT IS RECTIFICATION ? RECTIFICATION REFERS TO PROCESS OF

CONVERTING AN A.C. VOLTAGE OR CURRENT TO D.C. VOLTAGE OR CURRENT.

RECTIFIERS – RECTIFIER REFER TO POWER ELECTRONIC CONVERTERS WHERE THE ELECTRIC POWER FLOWS FROM A.C. SIDE TO THE D.C. SIDE.

NOTE: IN MANY SITUATIONS THE SAME CONVERTER CARRIES ELECTRICAL POWER FROM D.C. SIDE TO A.C. SIDE - INVERTERS

RECTIFIERS CLASSIFICATION

BECAUSE ON & OFF STATE OF DIODE IS DETERMINED BY THE STATE OF THE CIRCUIT.

BECAUSE THYRISTOR CAN BE TURNED ON BY CONTROL TERMINAL. i.e. GATE

ASSUMPTIONS :

ALL THE DEVICES & CIRCUIT COMPONENTS ARE IDEAL i.e. Ton =Toff =zero

INPUT IS PURE SINE WAVE

CHARACTERISTICS OF AN IDEAL SWITCH

TERMINOLOGIES

LET ‘f’ BE THE INSTANTANEOUS VALUE OF ANY VOLTAGE OR CURRENT ASSOCIATED WITH RECTIFIER CIRCUIT, THEN

TERMINOLOGIES (CONT.)

Angle atWhich SCR starts

conductingAngle at which SCR stops conducting

SINGLE PHASE FULLY CONTROLLED H/W RECTIFIER

CAN BE TURNED ON BY

APPLYING GATE SIGNAL

WHEN SCR IS FORWARD

BIASED.

OPERATION IS ALMOST

SAME AS THAT OF DIODE

CIRCUIT

FOR PURELY RESISTIVE LOAD

CURRENT FLOWS FROM TO THROUGH LOAD BECAUSE SCR IS IN FPRWARD CONDUCTION MODE.

π-α

CALCULATIONS

Average value of output voltage

On solving

ttdvv iav sin22

1

)cos1(2

iav

vv

CALCULATION OF RMS VALUE OF VOLTAGE

tdvv orms

2

2

1

tdtvv irms

22 sin2

2

1

tdtv

v irms

)2cos1(

2

2

2

1

])2

2sin(

1[2

tt

vv irms

)2cos1(2

12 ttSin

CALCULATION OF RMS VALUE OF VOLTAGE (CONT.)

On solving

Form factor

2

1

)2

2sin1(

2

irms

vv

)cos1(2

iav

vv

cos1

)22sin

1( 2

1

av

rms

v

vformfactor

RESISTIVE – INDUCTIVE LOAD

α to π π to β

RESISTIVE – INDUCTIVE LOAD

Thyristor turns ON at ωt= α & voltage appears across Load &

current starts building up. Inductor does not changes

current instantaneously, so load current does not become zero at

ωt = π It continue to flow but the

negative supply voltage decreases its magnitude

At ωt= β current becomes zero and thyristor starts blocking until

again turned ON.

Since thyristor does not conduct over the entire input so it is called

DISCONTINUOUS CONDUCTION MODE

CALCULATIONS t

tvtvvv imio sin2sin

otherwisev ,00

Average value of output voltage

ttdvv iav sin22

1

)cos(cos2

,

iav

vvonsolving

RMS VALUE OF OUTPUT VOLTAGE

2

122 )sin2

2

1( ttdvv irms

21

)2cos1(1

2tdt

vv irms

21

2

2sin

2

2sin(

1

2

irms

vv

21

2

2sin2sin(

2

irms

vv

)2cos1(2

12 ttSin

SINGLE PHASE HALF CONTROLLED BRIDGE

Assume, load is highly inductive Because 90% of loads are inductiveCASE I

T1

T4

D3

D2

LOA

D A B

DURING +VE HALF CYCLE T1 &

D2 ARE FORWARD BIASED.

AT ωt = α, T1 IS TRIGGERED AND ITS

STARTS CONDUCTING.

SINGLE PHASE HALF CONTROLLED BRIDGE

At ωt =π, D3 starts conducting and -ve voltage appear across T1 & it

starts blocking voltage. THIS PROCESS IS CALLED LINE

COMMUTATION

T1

D2

A B

α to π

SINCE LOAD IS HIGHLY INDUCTIVE FREEWHEELING CURRENT FLOWS

THROUGH D2-D3FROM ωt =π To π+α

D3

D2

SINGLE PHASE HALF CONTROLLED BRIDGE

At ωt = π, T4 is in forward blocking mode at ωt = π +α T4 is triggered & +ve voltage appears across D2 and it stops conducting.

At ωt = 2π, D2 starts conducting and T4 is OFF, this is called LINE COMMUTATION

D3

T4

A

B

From ωt = 2π to 2π+α, only D2-D3 conducts,

because of load

SINGLE PHASE HALF CONTROLLED BRIDGE

2π+α,α

π

π+α

2π

Since thyristor does not conduct over the entire input so it is called

DISCONTINUOUS CONDUCTION MODE

Avg. current rating of thyristor is < that of diode, which is not desirable

CALCULATIONS

cos1 mav

vV

Vav is +ve and Iav is also +ve so source is supplying power to the load or we can say that load is PASSIVE.

Vav

α

------

----

----

---

1

0.5

π/2

π

Vo

Io

Operation in 1st Quadrant

SINGLE PHASE HALF CONTROLLED BRIDGE

CASE II: CONTINUOUS CONDUCTION MODE

T1 T3

D4 D2

LOAD

DURING α to π OPRATION OS SAME AS THAT OF CASE I.

FROM π to π+α D4 STARTS CONDUCTING & T3 IS IN FORWARD BLOKING MODE. THUS NO CONDUCTION.T1 & D4 CONDUCTS.

A B

α - π

SINGLE PHASE HALF CONTROLLED BRIDGE

FROM π to π+α FROM π+α to 2π

T1

D4

At ωt = π+α, T3 IS TRIGGERED & T1

COMMUTATES ( LINE COMMUTATION)

T3

A B

D4

FROM 2π, D2 STARTS CONDUCTING & T1 IS IN FORWARD BLOKING MODE. THUS NO CONDUCTION.T3 & D2 CONDUCTS.

SINGLE PHASE HALF CONTROLLED BRIDGE

2π+α,α

2π π

π+α

Since thyristor conducts over the entire input so it is called

CONTINUOUS CONDUCTION MODE

SINGLE PHASE HALF CONTROLLED BRIDGECONTINUOUS CONDUCTION MODE

DISCONTINUOUS CONDUCTION MODE

FULLY CONTROLLED BRIDGE

ASSUMPTIONS:a) LOAD CURRENT IS CONSTT. & RIPPLE FREE.

b) Io IS CONTINUOUS.IN THE +VE HALF, T1 T2 ARE FORWARD

BIASED.

IN THE NEGATIVE HALF T3 T4 ARE FORWARD

BIASED.

TI & T2 CONTINUE TO CONDUCT TILL T3 & T4

ARE TRIGGERED.

FULLY CONTROLLED BRIDGE

SINCE THERE ARE TWO PULSESPER CYCLE SO IT ISCALLED :TWO PULSE CONVERTER

AVERAGE VALUE OF OUTPUT VOLTAGE

ttdvV mav

sin1

cos

2 mav

vV

onsolving

Thus, Vav is +ve from o ˂α ˂ π/2 Vav is –ve from π/2 ˂ α ˂ π

Vo

Io

-Vo

REFERENCES

M.H.RASHID,POWER ELECTRONICS: CIRCUITS, DEVICES & APPLICATIONS, PENTICE HALL OF INDIA,(IIIRD Ed.),2004

CYRIL LANDER, POWER ELECTRONICS, MCGRAW HILLS CO. ,(IIIRD Ed.), 1993

B.K. BOSE, MODERN POWER ELECTRONICS & A.C. DRIVES, PEARSON EDUCATION INC.,2002