ayan final project

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FABRICATION OF RIPPLEFREE VARIABLE DC

SOURCE FROM AC SUPPLY WITH

THYRISTOR CIRCUITA Project Work Submitted in Partial Fulfillment

Of the requirements for the Degree of

BACHELOR OF TECHNOLOGY OF WEST BENGAL

UNIVERSITY OF TECHNOLOGY

In

ELECTRICAL ENGINEERING DEPARTMENT

Under the supervision of Prof. Chinmay Kanti Roy

DEPARTMENT OF ELECTRICAL ENGINEERING

COLLEGE OF ENGINEERING & MANAGEMENT, KOLAGHAT

(Affiliated to West Bengal University Of Technology)

Purba Medinipur-721171, West Bengal, India

(Affiliated to West Bengal University Of Technology)



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ACKNOWLEDEGEMENTS

We would like to acknowledge our deep sense of gratitude towards Prof. Chinmay Kanti Roy,

for his constant support and invaluable guidance in the course of the project. We are indebted

to him for his timely suggestions and utmost care in spite of his heavy schedule. Our discussions

on a wide range of topics have been instrumental in giving our thought process a sense of

direction.

We would also like to thank Prof. A. K. Chakraborty, Head Of The Department, Electrical

Engineering, College Of Engineering & Management, Kolaghat for allowing us to utilize the

indispensable laboratory facilities, without which the project could not have been completed.

Lastly, a word of thanks also goes out to all those who have directly or indirectly

contributed to the successful completion of the project.

DATE:

NAME-

UNIVERSITY ROLL NO-

NAME-

UNIVERSITY ROLL NO-

NAME-

UNIVERSITY ROLL NO-



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CERTIFICATE

COLLEGE OF ENGINEERING AND MANAGEMENT,

KOLAGHAT(UNDER WEST BENGAL UNIVETSITY OF TECHNOLOGY)

This is to certify that AYAN KUMAR PANJA (ROLL NO-10716061001), NAYAN MANNA

(ROLL NO-10716061046), SUBHASISH DUTTA (ROLL NO-10716061014) of Electrical

Engineering have successfully completed the project on FABRICATION OF RIPPLEFREE

VARIABLE DC SOURCE FROM AC SUPPLY WITH THYRISTOR CIRCUIT in fulfillment of

the requirements FOR THE DEGREE OF B.TECH IN 8th

SEMESTER of COLLEGE OF

ENGINEERING AND MANAGEMENT, KOLAGHAT under the supervision and guidance of

Prof. Chinmay Kanti Roy.

----------------------------------

Prof. Chinmay Kanti Roy

Department of Electrical Engineering

(Internal guide)

-----------------------------------

PROF. A. K. CHAKRABORTY

Department of Electrical Engineering

(HOD EE)



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CONTENT

TOPICS PAGE

INTRODUCTION 5

CIRCUIT FOR IMPLEMENTATION 6

THYRISTOR 7

SINGLE PHASE SEMICOMVERTER 12

THYRISTOR FIRING CIRCUIT 13

CALCULATION FOR TRIGGERING CIRCUIT 14

COMPLETE CIRCUIT DIAGRAM 15

UJT TRIGGERING CIRCUIT 16

FILTERING CIRCUIT 17

CALCULATION FOR FILTERING CIRCUIT 18

DESIGN OF INDUCTOR 19

FUTURE SCOPE 21

CONCLUSION 22

REFERENCE 23



Page | 5

INTRODUCTION

DC motors are generally fed from undulated dc. In normal case it does

not create any problem. But when fed to a high power motor, it creates

appreciable amount of noise & a high temperature rise. The reason is

as follows,-

Whenever supply to motor is given after rectifying an a.c of frequency

f, then the motor draws a dc current on which an a.c component of

2f frequency has been superimposed. This alternating component

creates unfavorable effect on commutation, heating etc. The effect of this alternating current can be reduced if the value of this alternating

component gets reduced compared to the d.c component.

So our intention is to-

Fabricate a d.c component within which the a.c component will be less

than 5%.

Simultaneously, we want to realize the R-C triggering circuit to obtain

a variable d.c voltage so that speed of motor can be varied by varying

the d.c supply.

So, in total, we may say the objective of our project is two folds-

1. To produce a variable d.c voltage by firing thyristors at different

firing angles with simultaneous realization of R-C firing circuits.

(the magnitude of this variable d.c would be 0-250V)

2. To ripple free this d.c up to 95%.



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CIRCUIT FOR IMPLEMENTETION

As our aim is to obtain variable d.c, so we choose semiconverter circuit

to achieve it & a R-C triggering circuit to fire the SCRs. And for our

second objective, we choose L-C filter circuit.

i) The Semiconverter circuit converts the a.c voltage into a fully

controlled d.c voltage.

ii) The triggering circuit is used to control thyristors at different firing

angles.

iii) The filtering circuit is used to reduce the ripples of undulated d.c.

We will illustrate the working of each of this part in detail in

subsequent sections.



Page | 7

THYRISTOR

SILICON CONTROLLED RECTIFIERS (SCR)

A silicon controlled rectifier is a semiconductor device that acts as

a true electronic switch. it can change alternating current and at the

same time can control the amount of power fed to the load. SCR

combines the features of a rectifier and a transistor.

CONSTRUCTION

When a pn junction is added to a junction transistor the resulting

three pn junction device is called a SCR. ordinary rectifier (pn) and a

junction transistor (npn) combined in one unit to form pnpn device.

three terminals are taken : one from the outer p- type material called

anode a second from the outer n- type material called cathode K and

the third from the base of transistor called Gate. GSCR is a solid state

equivalent of thyratron. the gate anode and cathode of SCR correspond

to the grid plate and cathode of thyratron SCR is called thyristor



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WORKING

Load is connected in series with anode the anode is always kept at

positive potential w.r.t cathode.

WHEN GATE IS OPEN

No voltage applied to the gate, j2 is reverse biased while j1 and j3

are forward biased . J1 and J3 is just in npn transistor with base open,no current flows through the load RL and SCR is cut off. if the applied

voltage is gradually increased a stage is reached when RB junction J2

breakdown .the SCR now conducts heavily and is said to be ON state.

the applied voltage at which SCR conducts heavily without gate voltage

is called Break Over Voltage.



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WHEN GATE IS POSITIVE W.R.T CATHODE.

The SCR can be made to conduct heavily at smaller applied

voltage by applying small positive potential to the gate.J3 is FB and J2 isRB the electron from n type material start moving across J3 towards left

holes from p type toward right. electrons from j3 are attracted across

junction J2 and gate current starts flowing. as soon as gate current

flows anode current increases. the increased anode current in turn

makes more electrons available at J2 breakdown and SCR starts

conducting heavily. the gate looses all control if the gate voltage is

removed anode current does not decrease at all. The only way to stopconduction is to reduce the applied voltage to zero.

BREAKOVER VOLTAGE

It is the minimum forward voltage gate being open at which SCR

starts conducting heavily i.e. turned on

PEAK REVERSE VOLTAGE( PRV)

It is the maximum reverse voltage applied to an SCR without

conducting in the reverse direction.



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HOLDING CURRENT

It is the maximum anode current gate being open at which SCR is

turned off from on conditions.

FORWARD CURRENT RATING

It is the maximum anode current that an SCR is capable of passing

without destruction

CIRCUIT FUSING RATING

It is the product of square of forward surge current and the time

of duration of the surge

VI CHARACTERISTICS OF SCR



Page | 12

SINGLE PHASE SEMICONVERTER

We know that semiconverter construction may be of two types,-

namely symmetrical & unsymmetrical. We prefer here the

unsymmetrical type of construction because in that case although in

this case, we have to provide an extra freewheeling diode, but we can

utilize the common cathode connection. By using the common cathode

connection, we will be able to trigger the two SCRs with the help of

single triggering circuit.

� A single phase semi converter bridge with two thyristor and threediodes is used here.

� The two thyristors are T1 and T2; two diodes are D1, D2

� The third diode D3 is connected across the load is acted as

freewheeling diode

� The load is a dc motor, equivalent to an R-L-E type.

The trigger pattern of the two SCRs will be as follows -

1

2

0

1 2

, 2 , . . .

, 3 , . . .

& 1 8 0

T h y r i s t o r T i s t r i g g e r e d a t

t a t t

T h y r i s t o r T i s t r i g g e r e d a t

t a t t

T h e t i m e d e l a y b e t w e e n t h e g a t i n g

s i g n a l s o f T T r a d i a n s o r

[ E [ T E

[ T E [ T E

T

! !

! !

!

1 1

2 2

T h y r i s t o r & c o n d u c t

f r o m

T h y r i s t o r & c o n d u c t

f r o m 2

F W D c o n d u c t s d u r i n g

0 t o , , . . .

T D

t t o

T D

t t o

t t o

[ E T

[ T E T

[ E T T E

!

!

!

vO

Vm

0E

iO

[ t

T T E T

( )TE

E

[ t0

E E

( )TE



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THYRISTOR FIRING CIRCUITS

As we have mentioned previously that, we will use R-C triggering to fire

the SCRs, we use the conventional R-C phase shifter circuit to first shift

the phase with respect to supply & then feed this phase shifted voltage

to the gate terminal of SCR. We have to keep in mind that, SCR only

triggers at positive half. So the perfect synchronization has to be made

between SCR and triggering circuit so that each SCR receives the gate

pulse only at their respective positive half.

In the phase shift method of firing the scr, an R-C or R-L network isused to delay the gate signal with respect to anode voltage. It offers a

simple method of timing the SCR gate pulse in reference to a.c power

line variations.

Here we use a basic phase shifter network for full wave phase

control with a d.c output to a load. To utilize both halves of a.c cycle,

the SCRs are connected in a bridge circuit and are fired by gate pulses

on alternate half cycles.

This circuit can shift the firing angle 1800, giving almost

complete control of the a.c power supply. Here the firing circuit is

supplied from the same a.c supply by using a step down, centre tap

transformer. Purpose for using this transformer is-

i)To trigger the scr in a sine wave, the same frequency as the power supply.

ii) This transformer acts as isolating transformer which isolates the firing

circuit i.e. the phase shift network.

iii) By using this transformer we use a single supply instead of using an extra

supply for the firing circuit. So it also reduces the cost.



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CALCULATION OF TRIGGERING CIRCUIT

Triggering angle =

��

! !

For maximum possible triggering angle = 1800

!

!!

These two values are the boundary condition but manipulating for various

obtainable practical values we got C = 3.2

�� for = 600

is 0.342k and for =140��



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COMPLETE CIRCUIT DIAGRAM



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TRIGGERING OF THYRISTOR USING UJT

A synchronized UJT triggering circuit is shown in figure bellow. Diodes D1-D4

rectifies ac to dc. Resistor R1 lowers Vdc to a suitable value for the zener diode andUJT. Zener diode Z functions to clip the rectified voltage to a standard level Vz is

applied to the charging circuit RC. Current i1 charges the capacitor C at a rate

determined by R. When voltage across capacitor Vc reaches the UJT threshold

voltage LVz, the E-B1 junction of UJT breaks down and the capacitor discharges

through primary of pulse transformer sending a current i2.

As the current i2 is in form of pulse, windings of the pulse transformer have

pulse voltages at their secondary terminals. This voltage is applied to the gate of

SCR which trigger the SCR. As soon as the capacitor discharges, it starts charging

and prepares for next pulse. The time of charging of capacitor is controlled byvarying the resistance R. The firing angle can be controlled up to about 150

0.



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FILTERING CIRCUIT

An inductor L in series with the load reduces the ac component, or ac

ripples, considerably. It is because L in series with load offers high

impedance to ac component but very low resistance to d.c. Thus ac

component gets attenuated considerably.

A capacitor C across load offers direct short circuit to ac component,

these are therefore not allowed to reach the load. However, dc gets

stored in the form of energy in C and this allows the maintenance of

almost constant dc output voltage across the load.



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CALCULATION OF FILTERING CIRCUIT

��

�

��

��

�



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DESIGN OF INDUCTOR

The a.c power capacity of inductor ,Q = 48 X 5 = 240 VA

Q/2 = 120 VA

Te = Turns per volt

m =

= 0.000563 wb

Maximum flux density to avoid saturation in the core 1.0 wb/m2

Area of core Ai =

m

2= 0.000563 m

2= 563 mm

2

Gross area of core Agi = = 625.55 mm2

Assuming square cross-section width of the central limb = mm 25

mm = 1 inch

Total no of turns ,T = V.Te = 48 X 4 = 192

J = 2 A/mm2, area of conductor =

= 2.5 mm

2

Required diameter of conductor = mm2

= 1.58 mm2

= 15 SWG

Diameters of conductor with enameled covering = (1.58 + 0.111) mm = 1.691 mm

Area of conductor with insulation (a) = mm

2= 2.24 mm

2

Space factor = Sf = 0.8 X

= 0.69

Window area Aw =

= = 747.965 mm

2= 1.16 inch

2



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A B C D E

1 3.3125 2.625 0.53 0.5

Window width Ww =

= 0.62

Window height Hw = C 2E = 1.625

Window area Aw = 1.0075

Bm = 1wb/m2, Gross ampere turn (ATg) = 800000 X

= 563000 A/m

2

No of air gaps in series = 2

Z =

=

2fL =

2 × 100 × 0.075 =

=

= 0.18 mm = 0.00727 inch

Total height of 192 conductors = 192 0.046 = 8.832

Now, taking the height of each layer of winding as 1.472, we get a total of =

= 6 layers

Now, considering the insulation between the layers as 5nil (0.005)

We get width of the winding in each window is = 6 0.046 + 6 0.005 = 0.306

So we can accommodate the winding in the window



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CONCLUSION

Our expectation from the designed circuit is

y i) The dc output voltage should be smoothly controllable.

y ii) Ripple should be less than 5%.

y iii) Almost full range of voltage (0-250V) should be

obtained.

y iv) The circuit should be able to carry the full load current(5 amp).

We have been able to achieve very smoothly controllable d.c

output & simultaneously almost full range of voltage has been covered.

But in the filtering circuit we have observed some transient

phenomenon at the time of switching. We have succeeded to eliminate

a major portion of it, but still there is opportunity for further work.



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REFERENCE:

i) POWER ELECTRONICS by P.S.BIMBHRA.

ii) POWER ELECTRONICS by Md.H RASID.

iii) www.wikipidia.com.

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