end of chapter 3.docx

8/14/2019 End of Chapter 3.docx

http://slidepdf.com/reader/full/end-of-chapter-3docx 1/10



capacitor should never be exceeded) : internal impedance is so low that

charge current at start up will destroy the tube (the same phenomenon canoccur will silicon diodes).

Notation We will use the following symbol for all rectifiers (except gazfilled ones), regardless of the technology :

For a mains supply, three basic schemas can be used (wave at load terminals

is indicated to the right) :

Monophased rectifying :

Biphased rectifying :

Graetz brige rectifying :



If we denote internal impedance of rectifier plus that of windings (see article

on transformers) by and if the load can be modelled as a pure

resistance , instantaneous value of voltage at load terminals is :

where is the unloaded transformer voltage. An elementary computation

shows that mean value of is :

Previous schemas can be generalised to multiphased current : here is an

example of triphased rectifying, used in high power applications :



In the following, the number of phases will be denoted by n, and this will be

a parameter of all formula. An amateur will probably not be using

hexaphased current, however, it will appear in the following that filtering is

easier when number of phases increases. Purist audiophile can try toreconstruct multiphase waves with an onduler.



Principle operation of chopper

Principle of chopper have two step which is down and up

Step-down choppers.

Step-up choppers.

In step down chopper output voltage is less than input voltage.

In step up chopper output voltage is more than input voltage.

Principle Of Step-down Chopper

• A step-down chopper with resistive load.

• The thyristor in the circuit acts as a switch.

• When thyristor is ON, supply voltage appears across the load

• When thyristor is OFF, the voltage across the load will be zero.

V

i0

V0

Chopper

R

+



2

0

Average Output Current

RMS value of output voltage

1 ON

dc

dc

ON

dc

t

O o

V I R

t V V I d

R T R

V v dt T

2

0

2

But during ,

Therefore RMS output voltage

1

.

.

ON

ON o

t

O

ON

O ON

O

t v V

V V dt T

t V

V t V T T

V d V

2

2

Output power

But

Output power

O O O

O

O

O

O

O

P V I

V I

R

V P

R

dV P

R



Principle Of Step-up Chopper

• Step-up chopper is used to obtain a load voltage higher than

the input voltage V .

• The values of L and C are chosen depending upon the

requirement of output voltage and current.

• When the chopper is ON, the inductor L is connected across

the supply.

• The inductor current ‘I’ rises and the inductor stores energy

during the ON time of the chopper, t ON .

• When the chopper is off, the inductor current I is forced to

flow through the diode D and load for a period, t OFF .

• The current tends to decrease resulting in reversing the polarityof induced EMF in L.

• Therefore voltage across load is given by:-

• A large capacitor ‘C’ connected across the load, will provide a

continuous output voltage

+

VOV

Chopper

CLOA

D

DLI

+

. .,O O

dI V V L i e V V

dt



• Diode D prevents any current flow from capacitor to the source.

• Step up choppers are used for regenerative braking of dc

motors

end of chapter 3.docx

Documents