basic power electronic

7/28/2019 basic power electronic

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GROUP MEMBERS

Muhammad Uzi Migzuan

Muhamad Hazuan Mustafa

Abdil Naim Mohd Razi

Siti Nur Adilah Md IshakNur Anati Zolkifly

Eng Kean Ming

Dhinakar a/l Harisanka Rao


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1.Introduction

2.Analyze control and uncontrolled circuitof rectifier

3. Principle operation of rectifier4.Analyze voltage and current waveformwith load

a. Resistive load

b. Inductive load5.Analyze chopper circuit

6.Function of chopper

7.Principle operation of chopper


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8.Principle operation of step downchopper operation

9.Principle operation step down

chopper operation

10.Calculation on outputvoltage,current,power and efficiency

11.Application of chopper

12.Chopper operation:

a. Impulsed commutatedb. Resonant pulse


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A DC converter is an electronic circuit

that

convert:

a source of DC from 1 voltage level to

another.

Convert unregulated DC input to acontrolled DC output with a desired

voltage level.


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Rectification process

Is the process of converting anAC voltage source DC voltage.

No energy is stored within a rectifier

so that there is a constant connection

between the current and voltage on DC

side and current and voltage at AC side.


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A chopper is basically a dc to dc

converter whose main function/usage is

to create adjustable dc voltage from

fixed dc voltage sources through the

use of semiconductors.


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This chopper is also known as :

Parallel capacitor turn-off chopper

Voltage commutated chopper

Classical chopper.


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To start the circuit, capacitor C is initially charged with polarity (with plate

a positive) by triggering the thyristor T2.

Capacitor C gets charged through VS, C, T2 and load.

As the charging current decays to zero thyristor T2 will be turned-off.

With capacitor charged with plate a positive the circuit is ready for

operation.

Assume that the load current remains constant during the commutation

process.


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Once of the types of controlled rectifier is fully controlled and

semiconductor rectifier. A fully-controlled circuit contains only thyristers

(semiconductor controlled rectifiers (SCR)), whereas a semi-controlled

rectifier circuit is made up of both SCR and diodes as shown in Fig.(1). Due

to presence of diodes, free-wheeling operation takes place without

allowing the bridge output voltage to become negative


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Single phase uncontrolled half wave rectifiers suffer from poor output

voltage and/or input current ripple factor. In addition, the input

current contains a dc component which may cause problem (e.g.

Transformer saturation etc) in the power supply system. The output dc

voltage is also relatively less


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Some of these problems can be addressed using a full

wave rectifier. They use more number of diodes but

provide higher average and rms output voltage.

There are two types of full wave uncontrolled rectifiers

commonly in use. If a split power supply is available (e.g.

output from a split secondary transformer) only two

diode will be required to produce a full wave rectifier.These are called split secondary rectifiers and are

commonly used as the input stage of a linear dc voltage

regulator. However, if no split supply is available the

bridge configuration of the full wave rectifier is used.

This is the more commonly used full wave uncontrolled

rectifier configuration. Both these configurations are

analyzed next.


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Step down chopper is also known asbuck chopper.

In step down chopper output voltage is

less or equal than input voltage.

In DC-DC conversion circuits, thyristorsare used as switching elements.

Thyristor converter offers greaterefficiency, faster response, lowermaintenance, smaller size and smoothcontrol.


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When chopper is ON, supply isconnected across load.

Current flows from supply to load.

When chopper is OFF, load current

continues to flow in the same

direction through FWD due to

energy stored in inductor L.


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Load current can be continuous or

discontinuous depending on thevalues of L and duty cycle d

For a continuous current operation,

load current varies between twolimits Imaxand Imin

When current becomes equal to

Imaxthe chopper is turned-off andit is turned-on when current

reduces to Imin.


17/3117

On State Off state


18/31

When S is on (D is off),capacitor energy supplies theload voltage.

Vo=Vc (if capacitor ischarged)

During on-state of switch S,voltage across inductorinstantly becomes equal toinput supply voltage. Currentthrough it increases graduallyand stores energy in its

magnetic field.

For very first time, when S isclosed Vo=0, as capacitor isnot charged.

When S is off (D is on),inductor voltage reverses its

polarity and adds in input

voltage to provide output

voltage which is equal to:

V0=Vi+VL

During off state of S, capacitor

charges and voltage at it

gradually build up to Vi+VL

(This capacitor voltages serves

as load voltage when next

time S in on) If S is off forever, inductor acts

as short circuit. It does not

develop any voltage and

Vo= Vi

18


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Voltage and current

waveforms for duty cycle

50%

d= 0.5 means Switch is on andoff for equal time intervals.

Energy that inductor develops

during on-state is completely

dessipated during off-state.

If duty cycle increases above

0.5, inductor will not

dessipate its energy

completely in off-states.The remaining inductor

voltage (due to left-over

energy) adds up next time

when switch is off and more

increased voltage appears at

output.


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20

If duty cycle increases above 0.5, inductor will not dessipateits energy completely in off-states. The remaining inductorvoltage (due to left-over energy) adds up next time when

switch is off and more increased voltage appears at output.

Neglecting losses, energy transferred by inductance during

TOFF must equal the energy gained by it during period

TON

Final expression for output load voltage is:

Vo=Vi [1/(1-d)]

If switch is open (d=0), output voltage is equal to input

voltage. As d increases, output voltage becomes larger than

input voltage.

So output voltage is always higher than input voltage if

switch is operated at an appropriately high frequency.


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Example 1

Input to the step up chopper is 200 V.

The output required is 600 V. If the

conducting time of thyristor is 200

sec. Calculate:

Chopping frequency,

If the pulse width is halved for

constant frequency of operation,

find the new output voltage.


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6

200 , 200 , 600

600 200 200 10

Solving for

300

ON dc

dc

ON

V V t s V V

TV VT t

T

T

T

T s


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6

6

Chopping frequency

1

1 3.33300 10

Pulse width is halved

200 10100

2ON

fT

f KHz

t s


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6

6

Frequency is constant

3.33

1300

Output voltage =

300 10200 300 Volts

300 100 10

ON

f KHz

T sf

TV

T t


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Example 2

A dc chopper in figure has a resistive loadof R = 10 and input voltage of V = 200 V.

When chopper is ON, its voltage drop is 2 V

and the chopping frequency is 1 kHz. If the

duty cycle is 60%, determine:

Average output voltage

RMS value of output voltage

Effective input resistance of chopper

Chopper efficiency.


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V

i0Chopper

+

R v0

V= 200v

R = 10

Chopper voltage drop, Vch = 2V

d= 0.60

f = 1kHz


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Average output voltage

0.60 200 2 118.8 VoltsRMS value of output voltage

0.6 200 2 153.37 Volts

dc ch

dc

O ch

O

V d V V

V

V d V V

V


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22

0

0 0

Effective input resistance of chopper is

118.811.88 Amps

10200

16.8311.88

Output power is

1 1

i

S dc

dcdc

i

S dc

dT dT

ch

O

V VRI I

VI

RV V

RI I

V VvP dt dt

T R T R


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2

2

0

0

0.6 200 22352.24 watts

10

Input power,1

1

ch

O

O

dT

i O

dT

ch

O

d V VP

R

P

P Vi dtT

V V VP dtT R


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0.6 200 200 22376 watts

10

Chopper efficiency,

100

2352.24100 99%

2376

ch

O

O

O

i

dV V V P

R

P

P

P


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Essentially, a chopper is an electronic switch that is used to

interrupt one signal under the control of another. Most

modern uses also use alternative nomenclature which helps

to clarify which particular type of circuit is being

discussed. These include: switched mode power supplies, including DC to DC

converters.

Speed controllers for DC motors

Class D Electronic amplifiers

Switched capacitor filters

Variable Frequency Drive

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basic power electronic