experiment no-1 theory and concept - dbit.ac.in · prepared by dev bhoomi institute of technology...

Dev Bhoomi Institute Of Technology Department of Electrical and Electronics

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PRACTICAL INSTRUCTION SHEET

EXPERIMENT NO. ISSUE NO. : ISSUE DATE:

10-July-2010

REV. NO. : REV. DATE :

PAGE: 1

LABORATORY Name & Code: PEE653:Power Electronics Lab SEMESTER: VI

Prepared By- Mr.Shubham Chamoli Approved By- Mr. Rohit Kumar

Experiment No-1

Theory and Concept Objective: To study V-I characteristics of SCR and measure latching and holding currents.

APPARATUSREQUIRED:

THEORY:

Fig (a) Elemantry circuit for obtaining thyrsistor V-I characteristics


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Fig (b) Static V-I characteristics of thyrsistor

An elementary circuit diagram for obtaining static V-I characteristics of a thyristor is shown in Fig. 4.2

(a). The anode and cathode are connected to main source through the load. The gate and cathode are fed

from a source Es which provides positive gate current from gate to cathode.

Fig. 4.2 (b) shows static V-I characteristics of a thyristor. Here Va is the anode voltage across thyristor

terminals A, K and Ia is the anode current. Typical SCR V-I characteristic


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shown in Fig. 4.2 (b) reveals that thyrsistor has three basic modes of operation; namely, reverse blocking

mode, forward blocking (off-state) mode and forward conduction (on-state) mode. These three modes of

operation are now discussed below:

Reverse Blocking Mode: When cathode is made positive with respect to anode with switch S open, Fig.

4.2 (a), thyristor is reverse biased as shown in Fig. 4.3 (a). Junctions J1 J3 are seen to be reverse biased

whereas junction J2 is forward biased. The device behaves as if two diodes are connected in series with

reverse voltage applied across them. A small leakage current of the order of a few milliamperes (or a few

microamperes depending upon the SCR rating) flows. This is reverse blocking mode, called the off-state,

of the thyristor. If the reverse voltage is increased, then at a critical breakdown level, called reverse break-

down voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rap-idly. A large

current associated with VBR gives rise to more losses in the SCR. This may lead to thyristor damage as

the junction temperature may exceed its permissible tempera-ture rise. It should, therefore, be ensured that

maximum working reverse voltage across a thyristor does not exceed VBR. When reverse voltage applied

across a thyristor is less than VBR, the device offers a high impedance in the reverse direction. The SCR

in the reverse blocking mode may therefore be treated as an open switch.

Note that V-I characteristic after avalanche breakdown during reverse blocking mode is applicable only

when load resistance is zero, Fig. 4.2 (b). In case load resistance is present, a large anode current

associated with avalanche breakdown at VBR would cause substantial voltage drop across load and as a

result, V-I characteristic in third quadrant would bend to the right of vertical line drawn at VBR.

Forward Blocking Mode: When anode is positive with respect to the cathode, with gate circuit open,

thyristor is said to be forward biased as shown in Fig. 4.3 (b). It is seen from this figure that junctions J1,

J3 are forward biased but junction J2 is reverse biased. In this mode, a small current, called forward

leakage current, flows as shown in Figs. 4.2 (b) and 4.3 (b). In case the forward voltage is increased, then

the reverse biased junction J2 will have an avalanche breakdown at a voltage called forward break over

voltage VB0. When forward voltage is less than VBO, SCR offers high impedance. Therefore, a thyristor

can be treated as an open switch even in the forward blocking mode.

Forward Conduction Mode: In this mode, thyristor conducts currents from anode to cathode with a very

small voltage drop across it. A thyristor is brought from forward blocking mode to forward conduction

mode by turning it on by exceeding the forward breakover voltage or by applying a gate pulse between

gate and cathode. In this mode, thy-ristor is in on-state and behaves like a closed switch. Voltage drop

across thyristor in the on state is of the order of 1 to 2 V depending on the rating of SCR. It may be seen

from Fig. 4.2 (b) that this voltage drop increases slightly with an increase in anode current. In conduction

mode, anode current is limited by load impedance alone as voltage drop across SCR is quite small. This

small voltage drop v across the device is due to ohmic drop in the four layers.


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CIRCUIT DIAGRAM:

PROCEDURE:

1. Connections are made as shown in the circuit diagram.

2. The value of gate current IG, is set to convenient value by adjusting VGG.

3. By varying the anode cathode voltage VAA gradually in step by step, note down the cor-responding

values of VAK and IA . Note down VAK and IA at the instant of firing of SCR and after firing (by

reducing the voltmeter ranges and ammeter ranges) then increase the supply voltage VAA . Note down

corresponding values of VAK and IA.

4. The point at which SCR fires, gives the value of break over voltage VBO.

5. A graph of VAK V/S IA is to be plotted.

6. The on state resistance can be calculated from the graph by using a formula.

7. The gate supply voltage VGG is to be switched off.

8. Observe the ammeter reading by reducing the anode cathode supply voltage VAA .The point at which

ammeter reading suddenly goes to zero gives the value of holding current IH

9. Steps no.2, 3, 4, 5, 6, 7, 8 are repeated for another value of gate current IG .\


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OBSERVATION:

RESULTS AND DISCUSSION:

The V-I characteristics of silicon controlled rectifier is plotted on the graph which is true according to theory.

PRECAUTIONS: 1. Keep your hand away from main supply.

2. Do not switch on the power supply unless you have checked the circuit connections as per the circuit

diagram.


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Experiment No-2

Theory and Concept

Objective:- To study UJT trigger circuit for half wave and full wave control.

APPARATUS:

UJT Firing circuit Study unit

Theory:

UJT is highly efficient switch. The switching time is in the range of nanoseconds. Since UJT exhibits

negative resistance characteristics it can be used as relaxation oscillator. The circuit diagram is as shown

with R1 and R2 being small compared to RB1 and RB 2 of UJT.

When VBB is applied, capacitor ‘C’ begins to charge through resistor ‘R’ exponentially towards VBB.

During this charging emitter circuit of UJT is an open circuit. The rate of charging 1=RC When this

capacitor voltage, which is nothing but emitter voltage, VE reaches the peak pointVP = VBB+VD , the

emitter base junction is forward biased and UJT turns on. Capacitor ‘C’ rapidly discharges through load R

2=R1C resistance R1 with time constant When emitter voltage decreases to valley point Vv, UJT turns off.

Once again the capacitor will charge towards VBB and the cycle continues. The resistor R in the circuit

will determine the rate of charging of the capacitor. If R is small the capacitor charges faster towards VBB

and thus reaches VP faster and the SCR is triggered at a smaller firing angle. If R is large the capacitor

takes a longer time to charge towards VP the firing angle is delayed.

Circuit Diagrams:-

Fig(a): Single phase Half wave Rectifier:


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Fig(b): Single phase Full wave Rectifier

PROCEDURE:

I. Firing of SCR using UJT for half wave rectifier:-

1) Switch ON the mains supply observe and note down the wave forms at the different points in the circuit

and also the trigger O/ps – T1, & T1’.

2) Make sure that the pulse transformer O/ps T1 & T1’ are proper and synchronized.

3) Now make the connections as given the circuit diagram for Half wave rectifier using AC source,UJT

relaxation Oscillator, SCR(T1)and a suitable load (50Ohms/25wattResistor)

4) Now switch ON the mains supply, observe and note down the output waveforms across load And SCR.

5) Draw the wave forms at different firing angle – 120, 90 & 60 for both Half & full wave rectifier

6) In the UJT firing Circuit the firing angle can be varied from 150degree to 30degree approximately.

7) We cannot vary from exact 0degree to 180degree as we vary in single phase converter firing circuit.

8) This is one of the simplest method of SCR triggering.

9) We can also fire SCR’s in the different Power circuits like Single phase half controlled converter,

Single phase AC voltage controller using both SCR and Triac.

II. Firing of SCR using UJT for Full wave rectifier:- 1) Switch ON the mains supply observe and note down the wave forms at the different points in the circuit

and also the trigger O/ps – T1, & T1’.

2) Make sure that the pulse transformer O/ps T1 & T1’ are proper and synchronized.

Now make the connections as given the circuit diagram for Full wave rectifier

using AC source, UJT relaxation Oscillator, SCR’s(T1andT2)and a suitable


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load(50Ohms/25wattResistor)

3) Now switch ON the mains supply, observe and note down the output waveforms across load And SCR.

4) Draw the wave forms at different firing angle – 120, 90 & 60 for both Half & full wave rectifier

5) In the UJT firing Circuit the firing angle can be varied from 150degree to 30degree approximately.

6) We cannot vary from exact 0degree to 180degree as we vary in single phase converter firing circuit.

7) This is one of the simplest method of SCR triggering.

8) We can also fire SCR’s in the different Power circuits like Single phase half controlled converter

,Single phase AC voltage controller using both SCR and Triac.

a. UJT Relaxation Oscillator:

1) To Study oscillator using UJT, short CF to the diode bridge rectifier to get filtered DC output.

2) Now we will get the equidistant pulses at the o/p of pulse transformer.

3) The frequency of the pulses can be varied by varying the potentiometer RC.

4) Draw the wave forms at different points by varying RC.

Observation Table:

Table1:- Synchronized UJT triggering circuit for HWR:-

Table 2: Synchronized UJT triggering circuit for FWR:-


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NOTE:- a) At lower firing angles SCR may not turn on due to the fact that UJT takes some time (charging

time) to generate first firing pulse. b)At higher firing angles SCR may not turn on due to the fact that VAK

is insufficient to turn-on SCR.

RESULT:- UJT Relaxation oscillator and UJT HALF & FULL WAVE CONTROLLED CIRCUITS is

constructed and its performance is studied


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Experiment No-3

Theory and Concept

Objective:- . To study single-phase half wave controlled rectified with (i) resistive load (ii) inductive

load with and without freewheeling diode.

Apparatus required: 1. Power thyristors

2. Rheostat

3. CRO

4. Transformer (1-phase) 230V/24V

5. Connection wires

Theory:

A semi converter uses two diodes and two thyristor and there is a limited control over the level of dc

output voltage. A semi converter is one quadrant converter. A one-quadrant converter has same polarity of

dc output voltage and current at its output terminals and it is always positive. It is also known as two-pulse

converter. Figure shows half controlled rectifier with R load. This circuit consists of two SCRs T1 and T2,

two diodes D1 and D2. During the positive half cycle of the ac supply, SCR T1 and diode D2 are forward

biased when the SCR T1 is triggered at a firing angle ωt = α, the SCR T1 and diode D2 comes to the on

state. Now the load current flows through the path L - T1- R load –D2 - N. During this period, we output

voltage and current are positive. At ωt = π, the load voltage and load current reaches to zero, then SCR T1

and diode D2 comes to off state since supply voltage has been reversed. During the negative half cycle of

the ac supply, SCR T2 and diode D1 are forward biased. When SCR T2 is triggered at a firing angle ωt =

π + α, the SCR T2 and diode D1 comes to on state. Now the load current flows through the path N - T2- R

load – D1 -L. During this period, output voltage and output current will be positive. At ωt = 2π, the load

voltage and load current reaches to zero then SCR T2 and diode D1 comes to off state since the voltage

has been reversed. During the period (π + α to 2π) SCR T2 and diode D1 are conducting.

Vout=(√2Vs)(1+Cosα)/π


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Circuit Diagram:

Procedure:

1. Make the connections as per the circuit diagram.

2. Connect CRO and voltmeter across the load.

3. Keep the potentiometer at the minimum position.

4. Switch on the step down ac source.

5. Check the gate pulses at G1-K1 & G2-K2, respectively.

6. Observe the wave form on CRO and note the triggering angle ‘α’ and

7. Note the corresponding reading of the voltmeter. Also note the value of Maximum amplitude Vm from

the waveform.

8. Set the potentiometer at different positions and follow the step given in (6) for every position.

9. Tabulate the readings in the observation column.


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Waveform:

Observation Table:

Result: Thus the operation of single phase half controlled converter using R and RL load has studied and the

output waveforms has been observed.


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Experiment No-4

Theory and Concept

Objective:- . To study single phase (i) fully controlled (ii) half controlled bridge rectifiers with resistive

and inductive loads

Apparatus required: 1. Power thyristors

2. Rheostat

3. CRO

4. Transformer (1-phase) 230V/24V

5. Connection wires

Theory:-

Phase controlled AC-DC converters employing thyristor are extensively used for changing constant ac

input voltage to controlled dc output voltage. In phase-controlled rectifiers, a thyristor is tuned off as AC

supply voltage reverse biases it, provided anode current has fallen to level below the holding current.

Controlled rectifiers have a wide range of applications, from small rectifiers to large high voltage direct

current (HVDC) transmission systems. They are used for electrochemical processes, many kinds of motor

drives, traction equipment, controlled power supplies, and many other applications.

Single-Phase Full-Wave Controlled Rectifier

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.

In a semi-controlled rectifier, control is affected only for positive output voltage, and no control is

possible when its output voltage tends to become negative since it is clamped at zero volts.

As shown in Fig. (2), thyristor T1 can be fired into the ON state at any time provided that voltage vT1 > 0.

The firing pulses are delayed by an angle a with respect to the instant where diodes would conduct.

Thyristor T1 remains in the ON state until the load current tries to go to a negative value. Thyristor T2 is

fired into the ON state when vT2 > 0, which corresponds in Fig. (2) to the condition at which v2 > 0. The

mean value of the load voltage with resistive load is given by:


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Figure (3) shows the voltage and current waveforms of the fully controlled bridge rectifier for a resistive

load. Thyristors T1 and T2 must be fired simultaneously during the positive half-wave of the source

voltage Vs to allow conduction of current. Alternatively, thyristors T3 and T4 must be fired

simultaneously during the negative half wave of the source voltage. To ensure simultaneous firing,

thyristors T1 and T2 use the same firing signal

Circuit Diagram:


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Procedure:

1. Connect the single phase full wave controlled rectifier circuit shown in Fig.(1-a) on the power electronic trainer.

2. Turn on the power

3. Plot the input and output waveforms on the same graph paper.

4. Measure the average and RMS output voltage by connect the AVO meter across load resistance.

5. Turn off the power

Waveforms:


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The following tasks you can perform through alter table command.


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Observation Table:

.

Result: Thus the operation single phase (i) fully controlled (ii) half controlled bridge rectifiers with resistive and

inductive loads has studied and the output waveforms has been observed


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Experiment No-5

Theory & Concept

Objective:- To study three-phase fully/half controlled bridge rectifier with resistive and inductive loads

Apparatus Required: 1. Power electronic trainer.

2. Connection wires.

3. Oscilloscope.

Theory: Phase controlled AC-DC converters employing thyristor are extensively used for changing constant ac

input voltage to controlled dc output voltage. In phasecontrolled rectifiers, a thyristor is tuned off as AC

supply voltage reverse biases it, provided anode current has fallen to level below the holding current.

Controlled rectifiers have a wide range of applications, from small rectifiers to large high voltage direct

current (HVDC) transmission systems. They are used for electrochemical processes, many kinds of motor

drives, traction equipment, controlled power supplies, and many other applications Three- phase Full

wave controlled rectifier: Fig(1-a) shows the three-phase bridge rectifier. The configuration does not need

any special transformer, and works as a 6-pulse rectifier. The series characteristic of this rectifier produces

a dc voltage twice the value of the half-wave rectifier .

The load average voltage is given by:


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Fig(3) shows the voltages of each half-wave bridge of this topology and , the total instantaneous dc

voltage vD, and the anode-to-cathode voltage vAK in one of the bridge thyristors. The maximum value of

vak= √3 Vmax, which is the same as that of the half-wave converter

Circuit Diagram:


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Procedure:

1. Connect the three-phase full wave controlled rectifier circuit shown in Fig.(1-a) on the power electronic

trainer.

2. Turn on the power

3. By use oscilloscope, plot the input and output waveforms on the same graph paper" same axis".

4. Measure the average and RMS output voltage by connect the AVO meter across load resistance.

5. Turn off the power

6. Use an inductive load. With L=100mH measure the output voltage and plot the output waveform.

7. Repeat step 6 with L=100mH measure the output voltage and plot the output waveforms.

8. Repeat step 6 & 7 with connect the freewheeling diode across the load.

9. Connect the three-phase bridge half-control rectifier circuit shown in Fig.(1- b).

10. Repeat steps (2-7).

Observation Table:


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Waveforms:

Result: Thus the operation of three-phase fully/half controlled bridge rectifier with resistive and

inductive loads and waveforms are observed.


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Experiment No-6

Theory & Concept

Objective:- To study single-phase ac voltage regulator with resistive and inductive loads

Apparatus required: Trainer Kit

CRO

Patch chords

Theory:

R- Load An AC voltage regulator consists of two SCRs connected in anti parallel During positive half cycle, the

SCR2 is forward biased. The current flow is through terminal P – SCR2 – the load and the terminal N.

During the negative half cycle the SCR1 is forward biased. The current flow is through terminal N –

SCR2 – load –terminal P.

The firing angle of the SCRs is kept at 450 If tha delay angles of the two SCRs are equal, and the input

voltage is Vm sinωt, the RMS output voltage will be given by formula stated in model calculation.

Thus by varying α from 0 to π, the RMS value of output voltage can be controlled from RMS input

voltage to 0

R- L Load

During the positive half cycle SCR2 is triggered into a firing angle delay of α, the current rises slowly due

to the inductor. The current continues to flow even after the supply voltage reverses, due to the energy

stored in the inductor.

As long as the SCR2 conducts, the conduction drop across it will reverse bias SCR1 , hence it will not

conduct even if gating signal is applied. It can be triggered into conduction during the negative half cycle

after SCR2 turns OFF. The wave forms are shown for both

continuous and discontinuous current


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Circuit Diagram:


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Procedure: R –Load

1. Connect anode of SCR2 to the cathode of SCR1

2. Connect the 24V AC positive terminal to anode of SCR2

3. Connect R load terminal between cathode of SCR1 and 24V AC output.

4. Connect the CRO across the load

5. Connect the voltmeter across the load terminals

6. Connect G2 & K2 of firing circuit to G2 & K2 of SCR 2

7. Switch ON the trainer kit

8. Place the switch S2 in SCR mode

9. Switch ON the 24V AC supply

10. Switch ON the denounce switch.

11. Note down the peak value of voltage Vm , triggering angle α and conduction angle γ

12. By varying the firing angle the output can be varied

13. Plot the graph Vm versus α and γ

RL – Load

1. Connect R and L in series then connect the load terminals between cathode of SCR1 and 24V ac input.

2. Repeat the above steps

3. Observe the waveforms

Observation Table:


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Waveform:


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Results: The SCR based single phase AC voltage controller or regulator with R & RL load is studied and the

required graphs are plotted.


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Experiment No-7

Theory & Concept

Objective:- To study single phase cyclo-converter.

Apparatus Required: Cyclo-converter Kit, CRO & Patch cords.

Theory: In the cycloconverter one group of thyristers produce positive polarity of the load voltage and the other

group produces the other polarity. One group of SCRs are gated together. Depending on the polarity of the

input only one of them will conduct. When P is positive w.r.t O then SCR1 will conduct otherwise SCR2

will conduct. Thus in both half cycles of the input the load voltage will be positive. The SCRs get turned

OFF by natural commutation at the end of every half cycle.

Depending on the desired frequency, gating pulses to positive group of SCRs will be stopped and SCRs

3& 4 will be gated SCR 3 conducts when p is +ve and SCR4 conducts when P is –ve.

Circuit Diagram:


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PROCEDURE:

1. Keep all the switches in the OFF position.

2. Connect the banana connector as given below

A1 to K3& 24V AC output

A2 to K4& 24V AC output

A3 to K1 & L1

A4 to K2& L3

R2 to L2

Out put of 0V to R1

G1 to G1

G2 to K2

G3 to K3

G4 to K4 and

K1 to K1

3. Select the output frequency level from the table given below

4. Switch ON the trainer kit using Power ON/OFF switch

5. Switch ON the pulse release switch

6. Switch ON the 24V AC output

7. Vary the control voltage (Vc) to vary the firing angle, Observe the Waveforms.


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Waveforms:


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Results: The output of the cyclo converter for f, f/2 , f/3 and f/4 have been studied.


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Experiment No-8

Theory and Concept

Objective:- To study triggering of (i) IGBT (ii) MOSFET (iii) power transistor

Apparatus Required:- (1) Triggering kit

(2) Oscilloscope

(3) Connecting wires.

THEORY & FORMULAE USED: The meaning of IGBT, MOSFET & Power Transistor is to switch on the concerning components. Switch

on means in the case of IGBT collector to emitter should fully internally shorted i.e. voltage between

emitter and collector should be almost zero or few milli volts and in the case of non triggering conditions

emitter to collector internally totally open circuit. In the field the requirement is to operate many or atleast

two IGBTs at a time. To make proper electrically isolation in between triggering pulses is required here

we will be discussed the same. Such type of triggering circuit is use for Mosfet and for power transistor

more current is required so driver amplifier is used to drive the power transistor.

Gate Frequency Generation: The Gate frequency generation circuit is given in Fig-‘1’. To operate Gate

wave circuit +5 V DC is developed through diode D1, D2, D3, D4, C3, C4 and IC 5. IC1 is a stable multi

vibrator VR1 to control its frequency. It will give the output from pin no. 3 and fed to IC2 that is JK flip-

flop and its output will come through pin no. 14 & pin no. 15.The frequency at 14 and 15 will half the a

stable multivibrator fre-quency. The output from pin no. 15 will feed to opto coupler IC3 and output of

opto cou-pler is fed to driver transistor T1. The output of T1 will fed to the base of power transistor T2,

the process will same as IC3 and T1. The difference between them pin no 15 of IC2 will at 0o and pin no

14 of IC 2 will at 180o.

For electric isolation every opto coupler is required separate isolated 12 V supply, so it also given in

diagram.

Triggering of IGBT:

To triggering of IGBT, its required emitter to gate voltage approximates 10 V DC. To op-erate at a time

many IGBTs, electrically isolation is required. So its necessary to use opto coupler for triggering of

IGBTs at a time with perfect electrically isolation. IGBT is volt-age operated device so voltage with

current is not required the gate current almost negligi-ble. The output voltage from opto coupler is

sufficient to trigger IGBT.


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The gate of IGBT should never hang in air. Because due to available voltage in air the IGBT may partially

ON and may heat & damage so its Gate to emitter a high ohmic resistance approximate 10K is hard

coupled. The IGBTs is the combination of MOSFET & power transistor to use at high voltage switching.

Triggering of MOSFET: All features to trigger the MOSFET is same as IGBT, only difference is MOSFET is used for low voltage

switching. Safety measures 10 K resistance is also required. Source to gate voltage approximate 10 V DC is

required. Same electrically isolation is required which is operate by opto coupler.

Triggering of Power Transistor: Transistor is current operated device to switching it the requirement is low voltage but few milli amps current

also. The output of opto coupler is fed to a driver transistor and transis-tor will provide sufficient power to a

drive a power transistor. Power transistor mostly used for high voltage and high frequency switching

Circuit Diagram:

Fig : IGBT Triggering Circuit


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Fig Pulse Transformer Triggerring circuit

Waveforms:

Result : Triggering of (i) IGBT (ii) MOSFET (iii) power transistor has been studied.

experiment no-1 theory and concept - dbit.ac.in · prepared by dev bhoomi institute of technology...

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