Power Electronics

Power Electronics, Department of Electrical Engineering, University of South Asia

Laboratory Manual

Experiments in

POWER ELECTRONICS

Engr. Khawar JavedAsstt. Prof of Electrical Engineering

1


U n i v e r s i t y o f S o u t h A s i aL a h o r e , C a n t t

2


University of South Asia

Certificate

This is certify that Mr. / Miss.................................................................................

Roll No . ................................................. of.................... Semester in Bachelors

in Electrical Engineering has completed the term work in Power Electronics

for the academic year 20........ – 20........ as prescribed the curriculum.

Date: ................................. Enrollment No. .................................

Lab Assistant Subject Teacher Head of the Department Dean

3


4


A c k n o w l e d g m e n tThis document has been prepared to serve as a laboratory manual for Power Electronics course for electrical engineering students. The manual consists of a set of experiments designed to allow students to build, and verify power electronic circuits. This set of experiments cover relevant topics prescribed in the syllabus and are designed to reinforce the theoretical concepts taught in the classroom with practical experience in the lab. By the end of the course, students are expected to have a good understanding of power electronic devices.I would like to thanks Mr. Mehmood Sadiq, Chairman University of South Asia and Professor Engr. Dr. Junaid Zafar, Associate Dean, Faculty of Electrical Engineering, University of South Asia, Lahore Cantt. For their encouragement to publish this lab manual Moreover, I would like to express my sincere thanks to Asstt. Prof Manzar Ahmad for these helpful suggestions on the organization and formulation of the lab experiments of the laboratory manual. Further more, I would like to express my thanks to all the instructors and laboratory staff especially Miss Nudrat and Mr. Abdul Qadeer Ahmad.who have collaborated with me in the preparation of the lab manual. Any suggestions /comments to improve the manual may be directed to me at my email address.

[email protected]

Engr. Muhammad Khawer. 10-March-2011

[email protected]

5

mailto:[email protected]


GUIDELINES FOR LABORATORY NOTEBOOK

All experiments in this manual have been tried and proven and should give you little trouble in normal laboratory circumstances. However, a few guidelines will help you conduct the experiments quickly and successfully.

1. Each experiment has been written so that you follow a structured logical sequence meant to lead you to a specific set of conclusions. Be sure to follow the procedural steps in the order which they are written.

2. Read the entire experiment and research any required theory beforehand. Many times an experiment takes longer than one class period simply because a student is not well prepared.

3. Once the circuit is connected, if it appears “dead’’, spend few moments checking for obvious faults. Some common simple errors are: power not applied, switch off, faulty components, lose connection, etc. Generally the problems are with the operator and not the equipment.

4. When making measurements, check for their sensibility. 5. It’s unethical to “fiddle” or alter your results to make them

appear exactly consistent with theoretical calculations. 1. Heading: The experiment identification (number) should be at The top of each page. Your name and date should be at the top of the first page of each day’s experimental work.2. Object: A brief but complete statement of what you intend to find out or verify in the experiment should be at the beginning of each experiment.3. Diagram: A circuit diagram should be drawn and labeled so that the actual experiment circuitry could be easily duplicated at any time in the future. Be especially careful to record all circuit changes made during the experiment. 4. Equipment List: list those items of equipment which have a direct

6


Effect on the accuracy of the data. It may be necessary later to locate specific items of equipment for rechecks if discrepancies develop in the results.5. Procedure: In general, lengthy explanations of procedures areUnnecessary. Be brief. Short commentaries alongside the corresponding data may be used. Keep in mind the fact that the experiment must be reproducible from the information given in your notebook.6. Data: Think carefully about what data is required and prepare Suitable data tables. Record instrument readings directly. Don’t use calculated results in place of direct data; however, calculated results may be recorded in the same table with the direct data. Data tables should be clearly indentified and each data column labeled and headed by the proper units of measure.

7. Calculations: Not always necessary but equations and sample calculations are often given to illustrate the treatment of the experimental data in obtaining the results.

8. Graphs: Graphs are used to present large amounts of data in aconcise visual form. Data to be presented in graphical form should plotted in the laboratory so that any questionable data points can be checked while the experiment is still set up. The grid lines in the notebook can be used for most graphs. If special graph paper required, affix the graph permanently into the notebook. Give all graphs a short descriptive title. Label and scale the axes. Use units of measure. Label each curve if more than one on a graph.

9. Results: The results should be presented in a form which makes the interpretation easy. Large amounts of numerical results are generally used for small amounts of results. Theoretical and experimental results should be on the same graph or arrange in the same table in a way for easy correlation of these results.

10. Conclusion: This is your interpretation of the results of the experiment as an engineer. Be brief and specific. Give reasons for important discrepancies.

7


Table of Contents

S.No Description Page No. Date ofCompletion

1 Static characteristics of SCR and DIAC.

2 Static characteristics of MOSFET and IGBT.

3 Controlled HWR and FWR using RC Triggering circuit.

4 UJT firing circuit for HWR and FWR circuits.

5 Generation of firing signals for Thyristors/Triacs using digital

Circuit/ Microprocessor.

6 AC voltage controller using Triac – Diac combination.

7 Single phase fully controlled bridge converter FWR with R and RL loads.

8 Voltage (Impulse) commutated chopper – both constant frequency and variable frequency operations.

9 Parallel/ Series Inverters.

8


10 12V Battery Charger

11Lamp Dimmer using Diac &

Triac

12Fan Motor Speed Control

Experiment – 1Static characteristics of SCR and DIAC.

OBJECTIVES:9


1. To obtain V-I characteristics and to find on-state forward resistance of given SCR. 2. To determine holding, latching current and break over voltage of given SCR.

APPARATUS:1. Trainer kit 2. Patch cards 3. Multimeters.

CIRCUIT DIAGRAM:

Fig 1.1(a) Circuit diagram for VI characteristics of SCR.

SCR Specifications: It (rms) - 16 A It (rms) - 10 A It (rms) - 160 A

10


VBO = Forward break over voltageVBR = Reverse break over voltage

Ig = Gate current

Characteristic curve:

Fig 1.2(a) Static characteristic of SCR.

PROCEDURE:

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

2. Set R1 and R2 to mid position and V1 and V2 to minimum.

11


3. Set the gate current IG = IG1 (such that forward break over voltage is between 15 to 20 V), by varying R2 and V2.

4. Slowly vary V1 in steps of 2V and note down VAK and IAk at each step till SCR conducts. (Note down maximum VAK, which is forward break over voltage just before SCR conducts).

FINDING LATCHING CURRENT:

1. Ensure that the SCR is in the state of conduction.

2. Start reducing (VAK) anode voltage in steps of 2V; simultaneously check the state of SCR by switching off gate supply V2. If SCR switches off just by removing gate terminal, and switches on by connecting gate supply, then the corresponding anode current IA is the latching current (IL) for the SCR.

FINDING HOLDING CURRENT:

1. Ensure that the SCR is in the state of conduction.

2. Switch off the gate supply permanently.3. Start reducing (VAK) anode voltage in steps of 2V;

simultaneously check the state of SCR. If SCR switches off. Note down the anode current (IA) just before it drops to zero, which will be IH. Reverse the anode voltage polarity.

4. Vary VAK in steps of 5V till 25V and note down VAK and IA values at each step Plot forward and reverse characteristics using the above-tabulated values. Find the SCR forward resistance using the graph.

5. Repeat the above procedure for the forward and reverse characteristics of SCR for a gate current Ig = Ig2.

12


RESULT:

The values of VAK and IAK are noted down, plotted and SCR forward resistance is found. The values obtained are verified.

Observations:

Gate current IG = IG1 =__________mA

VAK(Volts) IA(mA)

Viva Questions:

1. What is a Thyristor?Ans) Thyristor is derived from the properties of a Thyratron tube and a Transistor. It is used as another name for SCR’S. They are power Semiconductor devices used for power control applications.2. What are SCR’s?Ans) SCR’s is Silicon controlled Rectifiers. They are basically used as Rectifiers.

13


3. What are the different methods of turning on an SCR?

Ans) *Anode to cathode voltage is greater than break over voltage.*Gate triggering*When dv/dt exceeds permissible value.*The cathode junction is exposed to light.

4. What is Forward break over voltage?

Ans) The voltage Vak at which the SCR starts conducting is called as Forward Break over voltage Vbo. This happens when the junction J2 undergoes Avalanche breakdown due to high reverse bias on junction J2.5. What is Reverse break over voltage?

Ans) If the reverse voltage is increased more than a critical value, avalanche Breakdown will occur at J1 and J3 increasing the current sharply. This is Reverse break over voltage VBO.

6. Why is Vbo greater than VBR?

Ans) In SCR the inner two p-n regions are lightly doped due to which the thickness of the depletion region at junction J2 is higher during forward bias than that of J1 and J3 under reverse bias.

7. What are modes of working of an SCR?

Ans) Reverse blocking mode, forward blocking mode and Forward conduction mode are the modes of working of an SCR.

8. Why does high power dissipation occur in reverse blocking mode?

Ans) High power dissipation occurs because as voltage increases beyond Vbr current increases rapidly.

14


9. Explain reverse current Ire?Ans) when cathode voltage is positive, J2 is forward biased; J1 and J3 are reverse biased. The thyristors will be in reverse blocking state and reverse leakage current Ire flows.

10. Why should the gate signal be removed after turn on?Ans) this prevents power loss in the gate junction.

Conclusions:

Conclude your results with the date obtained.

Registered No _______________

Teacher’s Initial ______________

Experiment 2(a)CHARACTERISTICS OF MOSFET

OBJECTIVES:

15


To draw static characteristic of MOSFET and hence to determine the output resistance and Trans conductance.

APPARATUS:

1. MOSFET module 2. Patch cards 3. Multimeters.

CIRCUIT DIAGRAM:

Fig 2.1(a)

16


Fig 2.2 (a) Static Characteristics of MOSFET.

PROCEDURE:

(a)Transfer Characteristics:

1. Connect the circuit as shown in the fig 2.1 (a).

2. Set VDS = 10V by varying V1. Keep R1 slightly more than ¼ of the total value.

3. Vary VGS by varying V2 (keep R2 to minimum position) and note down IDS for every 0.5V variation of VGS till 5V of VGS.

4. Min VGS voltage that is required for conduction is “Threshold voltage” (VTH).

5. Repeat the above experiment for different values of VDS2 = 15V.

17


(b) Drain Characteristics:

1. Rig up the circuit as shown in the fig 2.1(a).

2. Adjust VG by varying V2 to VTH.

3. Vary VDS by varying V1 in steps of 0.5v and note down IDS (Till IDS is constant).

4. Repeat the above procedure for different values of VGS2 = VTH+0.1 V.

RESULT:

The transfer characteristics & collector characteristics are obtained and their respective graphs are plotted and output resistance and Tran’s conductance are found.

Observations:

V1=VDS1 = 10V V1 = VDS2 =15V or 12V

VGS (V) IDS (mA) VGS (V) IDS (mA)

18


VGS = VGS1 = VTH VGS = VGS2 = VTH+0.1 V.

VDS (V) IDS (mA) VDS (V) IDS (mA)

Graphs:

19


Graphs:

VIVA QUESTIONS:1. What are MOSFET’s?Ans) Metal oxide silicon di-oxide field effect transistor is a voltage-controlled device. The parts of MOSFET are gate, drain and source.2. Draw the symbol of MOSFET.

3. What is the difference between MOSFET and BJT?

20


Ans) The MOSFET is a voltage controlled device where as BJT is a current controlled device.4. What is the difference between JFET and MOSFET?Ans) There is no direct contact between the gate terminal and the n-type channel of MOSFET.5. Draw the structure of MOSFET.6. What are the two types of MOSFET?Ans) Depletion MOSFET – N channel in p substrate. -P channel in n substrate.7. What is the difference between depletion and enhancement MOSFET?Ans) The channel in the center is absent for enhancement type MOSFET but the channel is present in depletion type MOSFET. The gate voltage can either be positive or negative in depletion type MOSFET’s but enhancement MOSFET responds only for positive gate voltage.8. How does n-drift region affect MOSFET?Ans) The n- drift region increases the onstage drop of MOSFET and also the thickness of this region determines the breakdown voltage of MOSFET.9. How are MOSFET’s suitable for low power high frequency applications?Ans) MOSFET’s have high on state resistances due to which losses increase with the increase in the power levels. Their switching time is low and hence suitable for low power high frequency applications.10. What are the requirements of gate drive in MOSFET?Ans) *The gate to source input capacitance should be charged quickly.*MOSFET turns on when gate source input capacitance is charged to sufficient level.

21


*The negative current should be high to turn off MOSFET.

Experiment 2(b)VI -CHARACTERISTICS OF IGBT

OBJECTIVES:To plot the VI Characteristics of IGBT.

APPARATUS:1. Characteristics Study Unit.2. 0-50V DC Voltmeter.3. 15V DC Voltmeter.4. 0-500mA DC Ammeter.

CIRCUIT DIAGRAM:

22


Fig 2.1(b)

Transfer Characteristics Curve

23


Collector Characteristics Curve

Fig 2.2(b) V-I characteristics of IGBT.

PROCEDURE:

24


Transfer Characteristics:1.Connect the circuit as shown in figure.

2.Initially Keep V1 & V2 to minimum. Set V1=VCE1 = 10V.

3.Slowly vary V2 (VGE) and note down Ic and VGE reading for every 0.5V. (V (GE) MAX < 8V)

4.The minimum gate Voltage VGE, which is required for conduction of IGBT is called threshold voltage V TH.

5.If VGE is less than VTH, very small leakage current flows from collector to emitter. If VGE is greater than VTH, then collector current depends on VCE magnitude.

6.Repeat the same for different values of VGE and draw the graph of VGE V/S Ic.

Collector Characteristics:1. Initially set V2 to VGE1 = 5V. (≥ VTH)

2. Slowly vary V1 and note down Ic and VGE. For a particular gate voltage there is a pinch off voltage (Vp) between collector and emitter.

3. If Vce is less than Vp the device works in constant gain region and Ic is directlyproportional to Vce.

4. If Vce is more than Vp, constant Ic flows from the device and this operating region is called as constant current region.

5. Repeat the above for different values of VGE and note down Ic v/s Vce.

6. Draw the graph Ic v/s VGE for different values of VGE.

RESULT:25


The transfer characteristics & collector characteristics are obtained and their respective graphs are plotted. The values of Ic, VGE, VCE are noted down in tabular columns and verified.

Observations:

Transfer Characteristics:

Graphs:

26


Transfer characteristics graphCollector Characteristics:

VGE1 (V) = 4.9 V VGE (V) = 5.1 V VGE3 (V) = 5 V

VCE (V) Ic (mA) VCE (V) Ic (mA) VCE (V) Ic (mA)

Graphs:

27


Collector characteristics graph

VIVA QUESTIONS:1. What is IGBT?Insulated gate bipolar transistor is the latest device in powerElectronics .It is obtained by combining the properties of BJTAnd MOSFET.2. In what way IGBT is more advantageous than BJT and MOSFET?*It has high input impedance of the MOSFET and has low on-state voltage drop.*The turn off time of an IGBT is greater than that of MOSFET.*It has low onstage conduction losses and there is no problem of second Breakdown as in case of BJT.*It is inherently faster than a BJT.3. Draw the symbol of IGBT.

28


4. Draw the equivalent circuit of IGBT.

5. What are on state conduction losses? How is it low in IGBT?

A high current is required to break the junctions in BJT. This results in on state conduction losses. The conduction losses in IGBT are proportional To duty cycle of the applied voltage. By reducing the duty cycle conduction losses can be reduced.

6. What is second breakdown phenomenon?As the collector voltage drops in BJT there is an increase in collector Current and this substantially increase the power dissipation. This Dissipation is not uniformly spread over the entire volume of the device but is concentrated in highly localized regions where the local temperature may grow and forms the black spots. This causes the destruction of BJT. This is second breakdown.7.What is switching speed?

The time taken to turn on or turn off a power device is called switching Speed.

29


8. How is IGBT turned off?

An IGBT can turn off by discharging the gate by means of shortCircuiting it to the emitter terminal.

9. What is the rating of IGBT??

The current rating can be up to 400A, 1200V with switching frequency of 20 KHz.

10. What is non-punch through IGBT?

The IGBT’S without n+ buffer layer are called non-punch throughIGBT’s. They have symmetric voltage blocking capabilities and are used for rectifier applications.

Conclusions:




30


Experiment 3Controlled HWR and FWR using RC Triggering

circuit.OBJECTIVES:

1. To plot firing angle v/s VDC using R, RC, firing circuits.

2. To plot input, trigger, load voltage waveforms in an R, RC Triggering circuit for half and full wave rectifier circuit.

APPARATUS:1. R & RC firing circuit module.2. Rectifier module.3. Multimeters 4. CRO 5. Patch chords.

CIRCUIT DIAGRAM:

Fig 3.1 R-Triggering Half wave circuit.31


Fig 3.2 Waveforms across SCR and Load.

Procedure:

1. Connect the circuit diagram as shown fig 3.1.

2. Connect the load voltage waveform to C.R.O.

3. Vary Resistance Rf and note down SCR firing angle θf and VDC.

4. Note down the waveform of load volt, input voltage.

5. Plot graph of load volt VDC & θf (θf v/s VDC).

32


Observations:R-Triggering: Time (m Sec)

Time (m Sec) Θf in Degrees VDC

Table 3.1

Circuit Diagram (RC Triggering Circuit):

33


Fig 3.3 RC Triggering Half wave rectifier Circuit.

Procedure:

1. Connect the circuit as shown in fig 3.3.

2. Follow the same procedure as in R Triggering.

WAVEFORMS:

34


Fig 3.4 SCR Waveforms across SCR and Load.

Observations:RC Triggering:

Time(m Sec) Θf in Degrees VDC

RC Full Wave Triggering Circuit:

35


Fig 3.5(b) RC Triggering full wave rectifier Circuit

PROCEDURE:

1. Connect the circuit as shown in fig 3.5.

2. Voltage the same procedure as in previous experiment. Follow across Capacitor.

WAVEFORMS:

36


Fig 3.6(b) Waveforms across SCR and Load.

Observations:RC Full Wave Triggering:

Time(m Sec) Θf in Degrees VDC

Graphs:

37


Graphs:

Result:1. Half and full wave R & RC triggering circuit have been rigged up and out put waveforms have been plotted.

2. Graph of firing angle and Vdc for R & RC triggering circuit have been plotted.

VIVA QUESTIONS:1. What is the maximum firing angle of R-triggering circuit and why?Ans) The maximum firing angle is 90°. This is because the source voltage reaches maximum value of 90° point and the gate current

38


has to reach Ig(min) some where between 0-90°. This limitation

means that load voltage waveform can only be varied from α=

0° to α = 90°.

2. What are the disadvantages of R triggering?Ans)

Trigger angle α is greatly dependent on the SCR’s Ig(min) and this value varies between SCR’s and it is also temperature dependent.

Maximum triggering angle achievable is 90°.3. In R-triggering circuit why Rmin is is connected in series with variable resistor?Ans) The limiting resistor Rmin is placed between anode and gate so that the peak gate current of the thyristor Igm is not exceeded.4. What is the maximum firing angle of RC-triggering and why?Ans) Maximum firing angle is 180°. This is because capacitor voltage and AC line voltage differ in phase. By adjusting the value of R it is possible to vary the delay in turning on the SCR from 0 to 10 msec and hence vary the firing angle from 0° to 180°.

Conclusions:



39



Experiment 4

UJT firing circuit for HWR and FWR circuits.OBJECTIVES:

To rig up and verify the operation of the SCR firing circuit using UJT.

APPARATUS:1. CRO probes.2. Patch cords.3. UJT trainer kit 4. Digital Multimeters

CIRCUIT DIAGRAM:

40


Fig4.1 (a) UJT firing circuit diagram for SCR.Design:

SCR – TYN 612, 12A/600VLoad = 50Ω, 25 W

VBB = DC supply voltage

Assume C and find RC

Vp = Peak voltage of UJT

Vv = Valley voltage of UJT

Where Remix = (VBB – VP) / Imp)

T = Time of triggering pulse

RB = Internal resistance of UJT (5kΩ)

Rmin = (VBB – Vv) / Iv

η = Intrinsic standoff ratio

R1 = 0.7RBB / ηVBB

η = 0.65

Observations:

TMS θf Vdc

41


Waveforms:

42


Fig 4.2(a) Waveforms of UJT relaxation and waveforms across SCR and load.

PROCEDURE:

43


1. The trainer kit is switched on with an AC supply voltage of 230V and 50Hz. A probe is connected to the CRO and one point is connected to the ground of the pulse transformer primary.

2. The rectified o/p across the diode is measured at point ‘A’ and is displayed on the CRO.

3. The voltage across the sneer diode and the capacitor is found out at point B

4. Note down the waveforms across the capacitor at point ‘C’.5. Note down the trigger waveform across the primary of pulse

transformer.6. Now the ground is removed and it is connected to the ground

of the secondary and note down Vdc.7. The waveform across the SCR and at point ‘D’ is found and

plotted.NOTE: Isolation of primary and secondary sides of pulse transformer is to be strictly maintained while measurements are carried out.

RESULT:The waveforms are verified and plotted.

VIVA QUESTIONS:1. What is an UJT and draw its equivalent circuit?UJT-uni junction transistor. It has only one type of charge carriers. It has three terminals emitter, base 1 and base 2. (‘Duo base’ as it has 2 bases)2.Why is an UJT used in SCR firing circuit?The voltage at base 1 of UJT is smaller than the voltage needed to trigger the Scrim the voltage is high, then it will trigger the SCR as soon as the ac supply is on.

3. What is valley voltage?

44


It is the voltage at which the UJT turns off and the capacitor starts charging again.4. What is the discharging path if the capacitor?The capacitor discharges through emitter, base and primary of the pulse transformer.5. What are the features of pulse transformer?The primary magnetizing inductance is high, coupling efficiency is high, and inter winding capacitance is low and has greater insulation.6. What is relaxation oscillator?? When the capacitor discharges to a valley voltage, the UJT turns off and capacitor starts charging again. This mode of working of UJT is called relaxation oscillator.7. What is interring base resistance?Inter base resistance is the resistance between 2 bases.8. What is intrinsic stand off ratio?Intrinsic stand off ratio=Rb1/(Rb1+Rb2). Its value ranges between .52 to .81.9. Why are the capacitors CIF and C used?Capacitor CIF is used to minimize the ripples and C is used for charging and discharging so that the trigger is eventually formed.10. What is a firing circuit?It is a circuit, which is used to trigger a device at various instants of time.Conclusions:Conclude your results with the date obtained.



45


Experiment 5Generation of firing signals for Thyristors/Triacs using digital Circuit/ Microprocessor.OBJECTIVES:

To control firing angle / duty cycle using digital triggering

APPARATUS:1.Digital firing circuit.2. SCR’s (Single or any combination) loads3.C.R.O4.Probes5.Preset (N no of counting bits)

BLOCK DIAGRAM:

46


Digital Firing Circuit -DFC

PROCEDURE:1. Switch on the mains supply unit.2. Observe AC reference signal and compare it with ZCD o/p A

and reset output C.3. Observe the carrier frequency oscillator o/p 5 KHz.4. Now set the 180o (converter) mode. Observe the counter o/p

keep the firing angle at 179o.5. Adjust the potentiometer R in such a way that very small pulse at the counter O/p is obtained.6. Now vary the firing angle from 180o to 0o step by step and observe the variation in trigger o/p’s Tp and Tn.7. Connect Tp and Tn to 1 and 2 input of pulse transformer isolation circuit and we will get the pulse transformer isolated and amplified outputs at P1 & P1 T2 and T2 respectively.

47


8. Connect these trigger o/p’s to gate and cathode of SCR’s for different power circuit’s as given in the table.9. Now set the 180o –100o switch to 100% mode (chopper) keep the duty cycle at 99%.10. Adjust the potentiometer ‘R’ in such a way that a very small pulse output is obtained.11. Now vary the duty cycle in steps from 99% to 1% and observe the counter o/p and also observe the time variation between main pulse Pm and auxiliary pulse Pa.12. Connect Pm and Pa to input 1 and 2 of pulse transformer isolator.RESULT: Control of firing angle /duty cycle using digital triggering is found.

Conclusions:




48


Experiment 6AC voltage controller using Triac – Diac combination.OBJECTIVES:i) To observe variation of intensity of light with reference to firing angle.

ii) To plot delay angle α V/S VL Load voltage and Conduction

angle β V/S IL Load current.APPARATUS:

1. Patch cords2. Multimeters3. Isolation Transformer, 10:14. Probes5. Lamp6. Triac Module

CIRCUIT DIAGRAM: (Diac Circuit)

Fig 6.1 Circuit diagram for AC voltage controller

49


Observations:

Experiment TRIGGER I/P’S TRIGGER O/P’S

TP TN TM TA T1 T1(1) T2 T2(2)1)Single Ph-half wave

converter

2)1-Ph-full wave converter.

3)1-Ph- half controlled bridge.

4) 1-Ph-Fully controlled bridge.

5)1-Ph.AC phase control.

6) Triac (short T1-T2 +ve -ve)

7) Complimentary commutation.

8) Auxillary commutation.

AC input through TransformerDIAC- DB-3Breakdown Voltage-32VTRIAC – BT 139

50


Use isolation transformer with 230V AC primary, 40-0-40 secondary.WAVEFORMS:

Fig .6.2 Expected input output waveform

PROCEDURE:

1. Connect circuit as shown in fig 6.1.

2. Connect diac-firing circuit as the triggering source.

3. Vary firing angle and note down the waveform; Vac, IL4. Use 10:1 probe, which is connected to oscilloscope for

measurement.

5. Note the change in brightness of lamp and plot the relevant characteristics.

6. Repeat the experiment with UJT firing circuit.

51


Observations:DIAC FIRING CIRCUIT: Rmin to max.

α(firing angle)

VL (V) IL (A) π - α(Conduction

angle) β

UJT FIRING CIRCUIT:This firing circuit is based on UJT relaxation oscillator. It generates pulses in synchronization with the AC supply. A pulse transformer is used to isolate the firing pulses.PROCEDURE:

1. Connect circuit as shown in diagram.

2. Vary firing angle and note down the waveform; Vac, IL

3. Use 10:1 probe, which is connected to oscilloscope for measurement.

52


4. Note the change in brightness of lamp and plot the relevant characteristics.

Observations:UJT FIRING CIRCUIT: Rmin to max.

α(firing angle)

VL (V) IL (A) π - α(Conduction angle) β

Graphs:

53


Graphs:

RESULT:The values of load voltage, firing angle, load current and conduction angle are found and verified for both Diac firing circuit and UJT firing circuit. Required graphs are plotted.VIVA QUESTIONS:1. What is ac voltage controller?If a Thyristor switch is connected between ac supply and load, the power flow can be controlled by varying the rms value of ac voltage applied to the load and this type of power circuit is known as an ac voltage regulator2. What are the applications of ac voltage controllers?The most common applications of ac voltage controllers are: industrial heating, on-load transformer taps changing, light

54


controls, speed control of polyphase induction motors and ac

magnet controls.

3. What do you mean by sequence control?The use of two or more stages voltages controllers in parallel for the regulation of output voltage. 4.Give the classification of ac voltage regulators. They are classified as: 1. Single phase controllers 2. Three phase controllersEach type can be subdivided into unidirectional and bi-directional control.5. What are the two types of control? *on off control: Here Thyristor switches connect the load to the ac source for a few cycles of input voltage and then disconnect it for another few cycles. *phase angle control: Here Thyristor switches connect the load to the ac source for a portion of each cycle of input voltage.6. Why are extra commutation components not required?The ac voltage controllers have main supply as input. The SCR’s in these controllers are turned off by natural commutation. Hence extra commutation components are not required. Therefore ac voltage controllers are simple and easy to implement if SCR’s are used.Conclusions:




55


Experiment 7SINGLE PHASE FULL CONTROLLED BRIDGE RECTIFIER FOR R & R-L LOADOBJECTIVES:

1. To plot Vdc v/s firing angle for R load.

2. To plot Vdc v/s conduction angle (π - α R-L load.

3. To observe load voltage on CRO.APPARATUS:

1. Trainer module 2. Multimeters3. CRO 4. Patch cords 5. Rheostat 6. Inductor

CIRCUIT DIAGRAM:

56


Fig 7.1(a) circuit diagram for full controlled, full wave rectifier

PROCEDURE:

1. Rig up the circuit and connect the triggering circuit as shown in the fig 7.1(a).

2. First connect the circuit for 40V AC tapping as shown in fig7.1 (a)

3. Adjust the triggering angle using variable resistance on∝ triggering circuit to observe the waveform on the CRO.

4. Connect 0-300Ω rheostats as load resistance.5. Repeat the experiment for various conditions of the load

with different tapping of Vp6. AC voltage (max 120V).7. Pure R load.8. R-L load (R load in series with L load).9. R-L loads with free wheeling Diode.10. Plot Vdc v/s Ω, Idc v/s conducting angle.EXPECTED WAVE FORMS:

57


Fig 7.2(b) Output waveforms across R-L load without freewheeling diode.

58


Fig 7.3(a) Output waveforms across R-L load with free wheeling diode.Observations:Ac Vin = 30V (AC), R = 135ΩT (ms) α(th) Vdc Idc (A) α (prac) Vdc TH

(V)Idc TH (A)

59


R-L load:

R-L LOAD without Dm R-L LOAD with Dm

α(th) T Α(P) Vdc Idc Vdc Idc

Calculations:

Vdc = (Vm/π ) * (1 + cosα )Idc = (Vm/π R) * (1 + cosα )(Vm(1 + cosα )) / π = VdcVm = (Vdc*π) / (1 + cosα )

Graphs:

60


Graphs:

61


RESULT: The values of Vdc, Idc & are found out, plotted and verified with expected waveforms.Viva questions:

1. What is a full controlled rectifier?It is a two-quadrant ac to dc converter. It has 4 thyristors and hence all of them can be controlled for rectification purpose. In a full converter the polarity of the output voltage can be either positive or negative but the output current has only one polarity.2. What is a semi converter?A semi converter is a one-quadrant converter and it has only one polarity of output voltage and current.3. What is a dual converter?A dual converter can operate in all 4 quadrants and both output voltage and current can be either positive or negative.4. How can we control the output voltage of a single-phase full converter?By varying the trigger angle.5.What is MCB?MCB-Miniature circuit breaker. This is used as switch, which opens or switches off when the voltage or current is abovethe rated value of that of MCB.6.How many lines are there in single-phase system?Two lines- 1line 1neutral7. What is the type of commutation used?Line commutation.8. Where is full bridge converter used?It is mainly used for speed control of dc motors.

Conclusions:

62





Experiment – 8Voltage (Impulse) commutated chopper – both constant frequency and variable frequency operations.OBJECTIVES: 1. To rig up DC Jones chopper and to measure the value of load

voltage (VLDC). 2. To plot the graphs of Frequency V/S VLDC and Duty Cycle

V/S VLDC.APPARATUS:

1. DC chopper power module-SDCP 2. Triggering circuit (DC chopper)3. Multimeters.4. Load 50Ω rheostat.

CIRCUIT DIAGRAM:

63


Fig 8.1(a) Circuit diagram of Jones ChopperT1, T2 – TYN 616D1 – BYQ 28200C – Commutation Capacitor10µ F / 100VL1- 0 –L2 - Commutation Inductor500-0-500 H / 2A.PROCEDURE:

a) For R – Load:1. Connections are made as shown in the figure 8.1(a). Use 50Ώ

Rheostat for R- Load (Freewheeling diode (DM) is to be connected only for RL load).2. Adjust VRPS output to 10v and connect to DC chopper module.3. Switch on DC toggle switch of chopper module.4. Switch on the trigger input by pushing- in pulsar switch.5. Observe the out put waveform across load on CRO.6. Keep the duty cycle at mid position and vary the frequency from minimum to maximum and record the output voltage VLDC readings.

64


7. Keep the frequency at mid position; vary duty cycle from minimum to maximum and output voltage VLDC readings.8. Note down the output waveform for mid value of frequency and duty cycle.b) R-L Load:1. Connections are made as shown in fig 9.1 (a). Load is 50Ώ Rheostat in series withinductor L =25mH or 50mH.2. Follow the same procedure as listed in steps 2 to 8 above.3. Readings and output waveform is to be recorded with and without free wheeling diode. NOTE: In both switching on / switching off of the equipment. First use DC toggle switch and then the pulsar.

Observations:Constant Duty cycle

Duty cycle: 50%, VIN= 10 to 15V

Slink Freq (Hz) VO (volts)

Constant Frequency, freq control at mid positVIN = 10V to 15V

65


Slink TON (sec) TOFF(sec) Duty cycle VO (volts)

EXPECTED PLOTS:

Fig 8.2(a) Showing the plots of Load voltage vs frequency and Load voltage vs. duty cycle.

Expected Waveforms:

66


Fig 8.3(a) Expected theoretical waveforms.RESULT:

The waveforms are plotted and verified.Note:The free wheeling diode DM clips the voltage, which occurs during TOFF state.VIVA QUESTIONS:1. What are choppers?A dc chopper converts directly from dc to dc and is also known as dc-dc converter.2. What does a chopper consist of?It can be a power transistor, SCR, GTO, power MOSFET, IGBT or a switching device.3. On what basis choppers are classified in quadrant configurations?The choppers are classified depending upon the directions of current and voltage flows. These choppers operate in different

67


quadrants of V-I plane. There are broadly following types of choppers: class a chopper (first quadrant); class B (second quadrant) Class C and class D (two quadrant choppers), class C in II quadrant and I whereas class Din IV quadrants, and I class E is four quadrant operators.4. What is different control strategies found in choppers?The different control strategies are pulse width modulation, frequency modulation and current limit control, variable pulse width and frequency.5. Explain the principle of operation of a chopper?A chopper acts as a switch, which connects and disconnects the load, hence producingVariable voltage.6.What are the advantages of DC choppers?* High ripple frequency, so small filters are required.*Power factor is better.*Efficiency is better.*Small and compact.*The dynamic response of choppers is fast due to switching nature of the device.7. Define duty cycle?Between 0 and 1 and is given by Ton /(Ton+Toff).8. How can ripple current be controlled?Ripple current is inversely proportional to the frequency and hence can be controlled by having higher frequency.9. What is step up chopper?If the output average voltage is greater than the supply voltage, then the chopper is called step up chopper.10.On what does the commutating capacitor value depend on?It depends on the load current.

Conclusions:


68




Experiment 9(a)Parallel/ Series Inverters.

OBJECTIVES: To rig up and verify the operation of parallel inverter using SCR. APPARATUS:

1. Trainer kit 2. Probes 3. Patch cords4. Rheostat5. CRO

CIRCUIT DIAGRAM:

69


Fig 9.1(a) Circuit diagram of Parallel inverter.

RL =100Ω /50Ω RheostatSCR’s – 10A/600V.Diodes – IN 4007, 10A/600V.WAVEFORMS: Case: Half (Frequency of firing circuit)

70


PULSE TRIGGERING:

Fig 9.2(a) Expected output waveforms.

PROCEDURE:

1. Connect the circuit as shown in the circuit diagram.2. Ensure following switching on procedure is strictly followed

so that commutation failure is avoided.

71


3. Switch on the DC input voltage and adjust it to 10V-15V using external VRPS. (Specific to inverter)

4. Switch on the parallel inverter main power supply.5. Switch on the DC switch, and then pulsar push button switch

to be pushed in.6. Observe the waveforms (Square wave o/p across the load)7. Set Rc potentiometer for different values and note the output

frequency.8. Follow the following switch off procedure.9. Switch off DC (Equipment), pulsar, and equipment power

supply.10. Disconnect gate connection of TP and TN.11. Monitor only pulsar output waveforms.12. Plot all the waveforms.

Observations:

Frequency FiringCircuit

Amplitude Time (msec) Frequency (Hz)

RESULT:Parallel inverter circuit is rigged up and output waveforms is plotted and verified with expected waveforms.

72


VIVA QUESTIONS:1.What are inverters and what are its applications?DC to AC converters is known as inverters. The function of an inverter is to change a DC input voltage into AC output voltage of desired magnitude and frequency. Inverters arewidely used in industrial applications like variable speed AC motor drives, induction heating, stand-by power supplies and uninterrupted power supplies.

2. Why is the circuit called parallel inverter?The circuit is called parallel inverter because the commutating capacitor is in parallel with the primary winding of the output transformer whose secondary is fed to the load.

3. What is the main classification of inverters?Inverters can be broadly classified into two types namely, Single-phase inverters and three phase inverters. Each type can use controlled turn-on and controlled turn-off devices (eg.BJT’s and MOSFET’s etc) or forced commutation thyristers depending on application.

4. What is VFI and CFI?An inverter is called a Voltage Fed Inverter (VFI) if the input voltage remains constant, a Current Fed Inverter (CFI) if the input current is maintained constant.

5. What is variable DC linked inverter?An inverter is called variable DC linked inverter if the input voltage is controllable.

6. What is inverter gain?The inverter gain may be defined as the ratio of the AC output to DC input voltage.7. Why the output voltage of an inverter is to be controlled?The output voltage of the inverter is to be varied as per the load requirement. When ever the input DC varies the output voltage can

73


change. Hence, these variations need to be compensated. The output voltage and frequency of an inverter is adjusted to keep voltage and frequency constant. Thus, the output voltage of an inverter is to be controlled.

8. What are the advantages and disadvantages of variable DC linked inverter?Advantages:

1. Harmonic content does not change with output voltage. 2. Control circuit of an inverter is simple.Disadvantages: 1. Additional chopper or control rectifier is required. 2. Efficiency of a circuit is reduced due to double conversion. 3. Transistors have to handle variable input voltages.

Experiment 9(b)Series Inverters.

74


OBJECTIVES: 1. To rig up a Series Inverter using SCR and note down the

waveforms. 2. To record the frequency of operation.

APPARATUS:1. Trainer kit 2. Probes 3. Patch cords4. Rheostat5. CRO

CIRCUIT DIAGRAM:

Fig 9.1(b) Circuit diagram of a Series inverter.

WAVE FORMS:

75


Fig 9.2(b) Expected output waveforms.

PROCEDURE:

1. Connect the circuit diagram as shown in the figure 9.1(b).

2. Connect L & C values as per the following details.

3. Connect C1 & C1’; L1 & L1’ for one set of readings.

4. Connect C2 & C2’; L2 & L2’ for second set of readings.

5. Note the output waveforms.

6. Note the waveforms across capacitors.

76


Observations:

Trigger pot

Position

L1 L1’ C1 C1’ O/PFREQUENCY

Minimum

¼½

Maximum

Trigger pot

Position

L2 L2’ C2 C2’ O/PFREQUENCY

Minimum

¼½

Maximum

RESULT:

1) Series Inverter using SCR is rigged up and output waveforms are noted.

2) The frequency of operation is ________.

77


VIVA QUESTIONS:1. What are series inverters?Inverters in which the commutating elements are permanently connected in series with theload resistance.2. What are the commutating elements in the above circuit?L and C are the commutating elements.3. What is the condition for selecting commutating element?They are selected in such a way that the current flow through series connected elements R, L, C is under damped4. What are the drawbacks of a basic series inverter? *If the inverter frequency exceeds the circuit ringing frequency the dc source will be short-circuited. *For output frequencies much smaller than the circuit ringing frequency, the load voltage is di started. *The source current flows only during the period when the Thyristor T1 is conducting. This results in large ripple in the source current and peak current rating of the source inverters.5. What are the applications of series inverters?*Induction heating*Fluorescent lighting*Variable speed ac motor drives*Aircraft power supplies*UPS*High voltage dc transmission linesConclusions:Conclude your results with the date obtained.



78


Experiment – 1012V Battery Charger

OBJECTIVES: To make a circuitry of Battery charger APPARATUS:

1. Resistors.2. Capacitors.3. Fuse (1amp).4. Toggle Switch.5. SCR (SC 3228).6. Power Transformer,primary 230V,Secondry

20V-2A.7. Diodes.8. Potentiometer 2W.9. Lamp 12V, 15A.

CIRCUIT DIAGRAM:

79


Procedure:

This battery charger can be used to recharge run-down batteries in automobiles etc.at a maximum charging rate of 2 amperes without removing them from their original mounting and without need for constant attention. When the battery is fully charged, the circuit automatically switches from charging current to “tickle” charge and an indicator lamp lights to provide a visual indication of this condition.it is also ideally suited for emergency lights.

When switch S1 is closed, the full wave rectified output of the diodes D1,D2 and D3 changes capacitor C1 through R1 ,R2 and lamp L1 ant the voltage across it rapidly rises high enough to make D3 conduct, thereby triggering the SCR into conduction. The SCR and the battery under-charge then form essentially the full load on the rectifier and charging current flows through the battery that is proportional to the difference in potential between the battery voltage and the rectifier output. Resistor R1 limits the current to a safe value to protect the rectifier diodes in the event the load is a “dead” battery. The energy stored in C1 assures that the SCR conducts practically full 180º of each successive half cycle.

When the battery is fully charged, the transistor regenerative switch (see fig) into conduction(at a point preset by R6) and provides a low impedance discharge path for C1,which discharges through these transistors and R3 to about 1 volt. This value is too low to sustain conduction of SCR gate through D3 and the SCR stops conducting. A small current keeps flowing through the battery via lamp L1 and the regenerative switch providing a trickle charge of about 150mA to the battery. Glow of L1 indicates that the battery is on trickle charge.

80


Observations:

Conclusions:




81


Experiment – 11Lamp Dimmer

OBJECTIVES: To make a circuitry of Lamp Dimmer using Diac & Triac

APPARATUS:1. Resistors.2. Capacitors.3. Diac (SS 32D).4. Triac(ST 024 for 150W load)

(ST 044 for 500W load).5. RF Choke (60-80 µh).

CIRCUIT DIAGRAM:

82


Procedure:The lamp dimmer described here bases a Triac

and controls the light intensity from almost zero to full brilliancy without expensive and bulky rheostats and without wastage of power. The intensity of light is controlled by phase control.

The full schematic of dimmer is shown in Fig. The firing signal to Triac gate is given thorough a Diac.Resistance R1 and R2 and R3 and capacitor C2 and C3 provide a phase shifted ac signal to the Diac.When the voltage on C3 exceeds the break-over voltage of the Diac,the Diac switches to the On-state and triggers the Triac.If R2 is set at the maximum setting R3 will provide a smooth continuous control from 5% to over 95% of full power to the lamp load.

83


Capacitor C3 and inductance L1 from a low pass filter substantially reduce the radio-frequency,(RFI) because all phase control circuits generate radio frequency interface. Resistor R3 may be mounted at a remote place if required. The circuit uses a 4amp.Triac and can control power up to 500 watts.

As a rule of thumb,Triacs dissipate about 1W of power per 100W of controlled power and should, therefore be mounted on heat sink. In most cases the case of the Triac is connected to the main terminal No.2, therefore the heat sink should be well isolated from chasis.

The same circuit can be used to control speed of small motors like fans, pumps etc.

Observations:

84


Conclusions:




Experiment – 12Fan Motor Speed Control

OBJECTIVES: To make a circuitry to control fan motor speed using Diac & Triac APPARATUS:

1. Resistors.2. Capacitors.3. Diac (SS 32D).4. Triac(ST 024)

CIRCUIT DIAGRAM:

85


Procedure:A simple and effective circuit for smooth speed

control of small fan motors is described here. Using all solid-state components the circuit controls speed by controlling the power input to the motor coils and does not dissipate heat as with rheostat controls.

As shown in Fig the fan motor speed is governed by the phase control action of triac T1 which is connected in series with the field coils. This arrangement removes the need for a separate radio frequency interference filter. The circuit operates almost the same way as in fig. Potentiometer R2 is the control for speed and is mounted externally while potmeter R4 is used to set minimum speed of the fan and is fitted internally.R1 and R3 protect D1 when potmeters are adjusted for minimum

86


resistance.Triac ST 024 can handle small fan motors upto 100watts.Observations:

Conclusions:Conclude your results with the date obtained.



Appendix-A(LABORATORY REGULATIONS AND SAFETY RULES)

The following Regulations and Safety Rules must be observed in all concerned laboratory location.

1. It is the duty of all concerned who use any electrical laboratory to take all reasonable steps to safeguard the HEALTH and SAFETY of themselves and all other users and visitors.

2. Be sure that all equipment is properly working before using them for laboratory exercises. Any defective equipment must

87


be reported immediately to the Lab. Instructors or Lab. Technical Staff.

3. Students are allowed to use only the equipment provided in the experiment manual or equipment used for senior project laboratory.

4. Power supply terminals connected to any circuit are only energized with the presence of the Instructor or Lab. Staff.

5. Students should keep a safe distance from the circuit breakers, electric circuits or any moving parts during the experiment.

6. Avoid any part of your body to be connected to the energized circuit and ground.

7. Switch off the equipment and disconnect the power supplies from the circuit before leaving the laboratory.

8. Observe cleanliness and proper laboratory house keeping of the equipment and other related accessories.

9. Wear proper clothes and safety gloves or goggles required in working areas that involves fabrications of printed circuit boards, chemicals process control system, antenna communication equipment and laser facility laboratories.

10. Double check your circuit connections specifically in handling electrical power machines, AC motors and generators before switching “ON” the power supply.

11. Make sure that the last connection to be made in your circuit is the power supply and first thing to be disconnected is also the power supply.

12. Equipment should not be removed, transferred to any location without permission from the laboratory staff.

13. Software installation in any computer laboratory is not allowed without the permission from the Laboratory Staff.

88


14. Computer games are strictly prohibited in the computer laboratory.

15. Students are not allowed to use any equipment without proper orientation and actual hands on equipment operation.

16. Smoking and drinking in the laboratory are not permitted.

All these rules and regulations are necessary precaution in Electrical Laboratory to safeguard the students, laboratory staff, the equipment and other laboratory users.

Note: Dear students,

All data sheets and components details and software’s are available in Power Electronics Lab.

89

Power Electronics

Documents

lab manual

lab experiments

set of experiments

power electronics course

power electronicsfor

laboratory staff

laboratory notebook

power electronic circuits