TPCT S College of Engineering, Osmanabad. Laboratory Manual. Power Electronics. For. Third Year Students. Manual Prepared by Prof. S. G.

Transcription

1 TPCT S College of Engineering, Osmanabad Laboratory Manual Power Electronics For Third Year Students Manual Prepared by Prof. S. G. Shinde Author COE, Osmanabad

2 TPCT s College of Engineering Solapur Road, Osmanabad Department of Electronics &Telecommunication Vision of the Department: To be recognized by the society at large as an excellent department offering quality higher educationin the Electronics & Telecommunication Engineering field with research focus catering to the needs of the public ind being in tune with the advancing technological revolution. To achieve the vision the department will Mission of the Department: Establish a unique learning environment to enable the student s face the challenges of the Electronics & Telecommunication Engineering field. Promote the establishment of centers of excellence in technology areas to nurture the spirit of innovation and creativity among the faculty & students. Provide ethical & value based education by promoting activities addressing the needs of the society. Enable the students to develop skill to solve complete technological problems of current times and also to provide a framework for promoting collaborative and multidisciplinary activities.

3 College of Engineering Technical Document This technical document is a series of Laboratory manuals of Electronics and Telecommunication Department and is a certified document of College of engineering, Osmanabad. The care has been taken to make the document errorfree. But still if any error is found, kindly bring it to the notice of subject teacher and HOD. Recommended by, HOD Approved by, Principal

4 FORWORD It is my great pleasure to present this laboratory manual for third year engineering students for the subject of Power Electronics to understand and visualize the basic concepts of various circuits using electronic components. Power Electronics cover basic concepts of electronics. This being a core subject, it becomes very essential to have clear theoretical and designing aspects. This lab manual provides a platform to the students for understanding the basic concepts of Power Electronics. This practical background will help students to gain confidence in qualitative and quantitative approach to electronic circuits. H.O.D ECT Dept

5 LABORATORY MANUAL CONTENTS This manual is intended for the Third Year students of ECT branches in the subject of Power Electronics. This manual typically contains practical/ Lab Sessions related to Power Electronics covering various aspects related to the subject for enhanced understanding. Students are advised to thoroughly go through this manual rather than only topics mentioned in the syllabus as practical aspects are the key to understanding and conceptual visualization of theoretical aspects covered in the books.

6 SUBJECT INDEX: 1. Do s & Don ts in Laboratory. 2. Lab Exercises 1. To plot static characteristics of a SCR. 2. To plot V-I characteristics of Diac. 3. To study half controlled full wave bridge (semiconverter) rectifier. 4. To study V-I characteristic of TRIAC. 5. To perform of SCR commutation circuits. 6. To study fan control using SCR, and lamp dimmer using TRIAC. 7. To study the operation of series inverter and to obtain variable AC from DC input. 8. To study the operation of resistance triggering circuits of SCR. 9. To study the operation of resistance capacitance triggering circuits of SCR. 3. Quiz 4. Conduction of viva voce examination 5. Evaluation & marking scheme

7 Dos and Don ts in Laboratory :- 1. Do not handle any equipment before reading the instructions /Instruction manuals. 2. Read carefully the power ratings of the equipment before it is switched ON, whether ratings 230 V/50 Hz or 115V/60 Hz. For Indian equipment, the power ratings are normally 230V/50Hz. If you have equipment with 115/60 Hz ratings, do not insert power plug, as our normal supply is 230V/50Hz., which will damage the equipment. 3. Observe type of sockets of equipment power to avoid mechanical damage. 4. Do not forcefully place connectors to avoid the damage. 5. Strictly observe the instructions given by the Teacher/ Lab Instructor. Instruction for Laboratory Teachers:- 1. Submission related to whatever lab work has been completed should be done during the next lab session. 2. Students should be instructed to switch on the power supply after getting the checked by the lab assistant / teacher. After the experiment is over, the students must hand over the circuit board, wires, CRO probe to the lab assistant/teacher. 3. The promptness of submission should be encouraged by way of marking and evaluation patterns that will benefit the sincere students.

8 Experiment No.1 Experiment Title: CHARACTERISTICS OF A SCR Aim:- To plot static characteristics of a SCR. Objective: To plot static characteristics of a SCR Apparatus:- Circuit board 0-300V high voltage supply,0-300v low voltage supply, milli-ammeter0-100ma and 0-10mA. Voltmeter 1-100V, multi meter, connecting wires etc. Circuit Diagram:- Theory: SCR Modes of operation: There are three modes of operation for an SCR depending upon the biasing given to it: 1. Forward blocking mode (off state) 2. Forward conduction mode (on state) 3. Reverse blocking mode (off state) Forward blocking mode In this mode of operation, the anode is given a positive voltage while the cathode is given a negative voltage, keeping the gate at zero potential i.e. disconnected. In this case junction J1 and J3 are forward-biased, while J2 is reverse-biased, due to which only a small leakage current exists from the anode to the cathode until the applied voltage reaches its break over value, at which J2 undergoes avalanche breakdown, and at this break over voltage it starts conducting, but below break over voltage it offers very high resistance to the current and is said to be in the off state.

9 Forward conduction mode SCR can be brought from blocking mode to conduction mode in two ways: either by increasing the voltage across anode to cathode beyond break over voltage or by applying positive pulse at gate. Once SCR starts conducting, no more gate voltage is required to maintain it in the on state. There are two ways to turn it off: 1. Reduce the current through it below a minimal value called the holding current and 2. With the gate turned off, short out the anode and cathode momentarily with a push-button switch or transistor across the junction. Reverse blocking mode SCRs are available with reverse blocking capability, which adds to the forward voltage drop because of the need to have a long, low-doped P1 region. (If one cannot determine which region is P1, a labeled diagram of layers and junctions can help). Usually, the reverse blocking voltage rating and forward blocking voltage rating are the same. The typical application for reverse blocking SCR is in current-source inverters. SCRs incapable of blocking reverse voltage are known as asymmetrical SCR, abbreviated ASCR. They typically have a reverse breakdown rating in the tens of volts. ASCRs are used where either a reverse conducting diode is applied in parallel (for example, in voltage-source inverters) or where reverse voltage would never occur (for example, in switching power supplies or DC traction choppers). Procedure:- 1. For forward characteristics - Connect the circuit as per the circuit diagram. - Adjust the value of gate current at 4.5mA &keep it constant. - Slowly increase the value of anode voltage till firing takes place. - Note down the readings and plot the graph. 2. For reverse characteristics - Reverse the high voltage power supply polarities and also the voltmeter, ammeter polarities on the HV side. - Note the anode current for changing voltage of Vak taking care that Vak does not exceed the reverse break over voltage Vboof SCR. - Remove all connection after switching off the power supply.

10 Observations:- a. Forward characteristics Ig=3.5mA Ig=4mA Vak Ig Vak Ig b. Reverse characteristics Ig=3.5mA Ig=4mA Vak Ig Vak Ig Graphs/Formula (if any) Result: - The graph plotted as per the observations is theoretically matched Conclusion: The SCR is unidirectional device which conduct in forward bias & does not in reverse bias. The forward break over voltage can be reduced by increasing the value of gate current.

11 Experiment No.2 Experiment Title: CHARACTERISTICS OF DIAC Aim:- To plot V-I characteristics of Diac. Objective: To plot V-I characteristics of Diac. Apparatus:- Circuit board,0-300v high voltage supply,0-30v Voltage supply,voltmeter (0-100V),Millimeter (0-100mA and 0-10mA), Multimeter, Connecting wires. Circuit Diagram:- Theory:

12 The DIAC is a diode that conducts electrical current only after its break over voltage, VBO, has been reached momentarily. The term is an acronym of diode for alternating current.when breakdown occurs, the diode enters a region of negative dynamic resistance, leading to a decrease in the voltage drop across the diode and, usually, a sharp increase in current through the diode. The diode remains in conduction until the current through it drops below a value characteristic for the device, called the holding current, IH. Below this value, the diode switches back to its high-resistance, non-conducting state. This behavior is bidirectional, meaning typically the same for both directions of current. Most DIACs have a three-layer structure with breakover voltage of approximately 30 V. Their behavior is similar to that of a neon lamp, but it can be more precisely controlled and takes place at a lower voltage. DIACs have no gate electrode, unlike some other thyristors that they are commonly used to trigger, such as TRIACs. Some TRIACs, like Quadrac, contain a built-in DIAC in series with the TRIAC's gate terminal for this purpose. DIACs are also called symmetrical trigger diodes due to the symmetry of their characteristic curve. Because DIACs are bidirectional devices, their terminals are not labeled as anode and cathode but as A1 and A2 or main teminal MT1 and MT2. Procedure:- Diac Characteristics: a. The connections are made as shown in the circuit diagram. b. Intially DIAC is connected in forward direction c. The input supply is increased in step by step by varying the voltage of power supply. d. The corresponding ammeter and voltmeter readings are noted and tabulated. e. Then the DIAC is connected in reverse condition. f. The above process is repeated.

13 Observations:- DIAC Characteristics: Graph: Result: - Thus the V-I characteristics of DIAC was obtained and graph was drawn. Conclusion: Thus we have studied V-I characteristics of Diode.

14 Experiment No.3 Experiment Title: SEMICONVERTER RECTIFIER Aim:- To study half controlled full wave bridge (semiconverter) rectifier. Objective: To study half controlled full wave bridge (semiconverter) rectifier. Apparatus:- SCR converter kit, Oscilloscope,Voltmeter,Multimeter,Connecting wires. Circuit Diagram:- Theory: The circuit contains two SCRs and two diodes. When source Vin is positive, SCR T1 can be triggered at a firing angle called α and then current flows out of the source through SCR T1 first, then through the load and returns via diode D2. If Vin = E* Sin (wt) then SCR T1 and diode D2 conduct during α<wt< π. But here we have two diodes D 2 and D4 instead of two SCRs. When the output of the bridge tends to becomes negative just after wt exceeds π, diode D4 tends to get forward-biased and it starts conducting. Then diode D2 is reversebiased and it stops conducting. During π <wt< (π + α), the devices in conduction are SCR T1 and diode D2 and the output of the bridge is clamped at zero, assuming that the on-state drops across devices in conduction is zero. During (π + α) <wt<2π, the devices in conduction are SCR T3 and diode D4 and so on.. Procedure:- 1. Make connections as shown in circuit diagram 2. Select resistor and inductor from load bank 3. Observe input and output waveform.

15 Waveforms :- Conclusion: Using phase controlled semiconverter we can convert ac voltage into variable dc voltage.we do not get output in negative half cycle due to inductive load.

16 Experiment No.4 Experiment Title: CHARACTERISTIC OF TRIAC. Aim:- To study V-I characteristic of TRIAC. Objective: To study V-I characteristic of TRIAC Apparatus:- Circuit board 0-300V high voltage supply, 0-30V low voltage supply, millimeter(0-10ma,0-1a),voltmeter(0-250v),connecting wires Circuit Diagram:- Theory: TRIAC, from triode for alternating current, is a generalizedtrade name for an electronic component that can conduct current in either direction when it is triggered

17 (turned on), and is formally called a bidirectional triode thyristor or bilateral triode thyristor. TRIACs are a subset of thyristors and are closely related to silicon controlled rectifiers (SCR). However, unlike SCRs, which are unidirectional devices (that is, they can conduct current only in one direction), TRIACs are bidirectional and so allow current in either direction. Another difference from SCRs is that TRIAC current can be enabled by either a positive or negative current applied to its gate electrode, whereas SCRs can be triggered only by positive current into the gate. To create a triggering current, a positive or negative voltage has to be applied to the gate with respect to the MT1 terminal (otherwise known as A1). Once triggered, the device continues to conduct until the current drops below a certain threshold called the holding current. The directionality makes TRIACs very convenient switches for alternating-current (AC) circuits, also allowing them to control very large power flows with milliampere-scale gate currents. In addition, applying a trigger pulse at a controlled phase angle in an AC cycle allows control of the percentage of current that flows through the TRIAC to the load (phase control), which is commonly used, for example, in controlling the speed of low-power induction motors, in dimming lamps, and in controlling AC heating resistors. Procedure:- Mode 1 1 Connection made according to circuit diagram. 2 The value of Igis set to convenient value by adjusting Vgg. 3 By varying the supply voltage Vm gradually in step-by-step,note down the corresponding values of Vmt2t1and i1 at the instant of firing of TRIAC and after firing (by reducing the voltmeter rang and increasing ammeter range) then increase the supply voltage Vmt2t1&i1. 4 The point at which TRIAC fires gives the value of break over voltge Vbo1 5 A graph of Vmt2t1 v/s i1 is to be plotted. 6 7.The gate supply voltage.vgg to be switched off 7 Observe the ammeter reading by reducing the supply voltage Vmt.the point at which the ammeter reading suddenly goes to zero gives the value of holding current ih. 8 Mode 2 1 Connect are made as shown in circuit diagram. 2 Step no. s 2, 3,4,5,6 and 7are to be repeated as in mode 1.

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19 Result: - The values of V and I are noted down, plotted and Triac characteristics are studied. The values obtained are verified.

20 Experiment No.5 Experiment Title: SCR commutation circuits. Aim:- To perform of SCR commutation circuits. Objective: :- To perform of SCR commutation circuits. Apparatus:- Experimental kit, DC power supply. Circuit Diagram:- Theory: Commutation of SCR It is the process to turn OFF conducting SCR. There are two ways of commutation of an SCR. (a) By reducing the forward current through the SCR below the holding current called as current commutation. (b) By applying a large reverse voltage across the SCR called voltage commutation Types of commutation

21 Natural commutation: when thyristor is turned off due to its forward current going below holding current it is said to be naturally commutated. Natural commutation takes place in the circuits powered by alternating current. Force commutation: When the thyristor operate on a pure DC input voltage, the forward current cannot be reduced below holding current naturally,therefore the thyristor must be commutated forcibly by using additional circuitry. This is called force commutation. This external commutation circuit will turn off the SCR by either current or voltage commutation. Classification of force commutation Class -A: Self commutated by resonating load. Class B: Self commutated by LC circuit. Class C: C or LC switched by another load carrying SCR. Class D: C or LC switched by an auxiliary SCR. Class E: An external pulse source for commutation. Class F: AC line commutation. Procedure:- Connect the circuit as shown in Figure. Connect G1 and G2 signals to gate of SCR. Switch ON the power supply. Connect oscilloscope across load resistance and observe waveform. Draw the waveform on graph paper. Graph:

22 Result: - Waveform of class Commutation are determined Conclusion: Thus we have verified the class C

23 Experiment No.6 Experiment Title: Phase control using SCR and TRIAC. Aim:- To study fan control using SCR and lamp dimmer using TRIAC. (Phase control using SCR and TRIAC.) Objective: :- To study fan control using SCR and lamp dimmer using TRIAC. Apparatus:- Circuit board, Connecting wires, Isolation transformer, CRO, true value multimeter. Circuit Diagram:- Theory: This is the circuit diagram of the simplest lamp dimmer or fan regulator.the circuit is based on the principle of power control using a Triac.The circuit works by varying the firing angle of the Triac. Resistors R1, R2 and capacitor C2 are associated with this. The firing angle can be varied by varying the value of any of these components. Here R1 is selected as the variable element. By varying the value of R1 the firing angle of Triac changes (in simple words, how much time should Triac conduct) changes. This directly varies the load power, since load is driven by Triac. The firing pulses are given to the gate of Triac T1 using Diac D1 Procedure:- 1. Make connections as per Circuit diagram.

24 2. Observe the variation in light intensity in case of lamp dimmer and speed of fan in case of fan regulator. 3. Switch off the mains and remove connections Observations:- : A.C freq = 50 Hz, α = ωt =2πft SR.NO VOLTAGE ACROSS LOAD TIME PEROID FIRING ANGLE (V) (msec) (α) Plot graph in between V and α Waveforms: Result: - Thus we have studied phase control using SCR & TRIAC.

25 Experiment No.7 Experiment Title: AC from DC input. Aim:- To study the operation of series inverter and to obtain variable AC from DC input. Objective: To study the operation of series inverter and to obtain variable AC from DC input. Apparatus:- Series inverter module,loading rheostat - 50,CRO, Connection wire. Circuit Diagram:- Theory: This circuit which converts DC power into AC power is called inverter. If the thyristor commutation circuit of the inverter is in series with the Load, then the inverter is called Series are tightly coupled. In this circuit, it is possible to turn- on-thyristortp before the current through thyristortn has become zero and vice-versa. Therefore, the Modifed Series Inverter can be operated behond the resonance frequency (fr) of the circuit. Inverter is operated at the resonance frequency (fr) if the load current waveform has low frequency and should not have zero current interval. The inverter s resonance frequency depends on the values of L, R and C in the circuit. Procedure:- 1. To begin with switch on the power supply to the firing circuit check that Trigger pulses by varying the frequency.

26 2. Connections are made as shown in the circuit diagram. 3. Now connect trigger outputs from the firing circuits to gate and cathode of SCRs T1 & T2. 4. Connect DC input from a 30v/2A regulated power supply and switch on the input DC supply. 5. Now apply trigger pulses to SCRs and observe voltage waveform across the load. Measure Vrms& frequency of o/p voltage waveform Firing Circuit: This part generates two pairs of pulse transformer isolated trigger two SCR s connected as series inverter. ON/OFF switch is provided for the trigger pulses which can be used to switch ON the inverter. Frequency of the inverter can be varied from 100 Hz to 1 KHz approximately. Power Circuit: This part consists of two SCR s two diodes. A center tapped inductor with tappings and 4 capacitors. Input supply terminals with ON/OFF switch and a fuse is provided. All the devices in this unit mounted on a proper heat sink, snubber circuit for dv/dt protection and a fuse in series with each device for short circuit protection. All the points are brought out to front panel for inter connections. They have to be interconnected as shown in the circuit diagram. Fly wheeling diodes can be connected across SCR s and its effect can be observed. Graph Result: - Thus the operation of a series inverter is studied.

27 Experiment No.8 EXPERIMENT TITLE: RESISTANCE TRIGGERING CIRCUITS OF SCR Aim:- To study the operation of resistance triggering circuits of SCR. Objective: To study the operation of resistance triggering circuits of SCR Apparatus:- Circuit board, Connecting wires, Isolation transformer, CRO, true value multimeter. Circuit Diagram:- R Triggering Circuit: Theory: Resistance Triggering: Resistance trigger circuits are the simplest & most economical method. During the positive half cycle of the input voltage, SCR become forward biased but it will not conduct until its gate current exceeds Igmin. Diode D allows the flow of current during positive half cycle only. R2 is the variable resistance & R is the stabilizing resistance.r1 is used to limit the gate current. During the positive half cycle current Ig flows. Ig increases and when

28 Ig= Igmin the SCR turns ON.The firing angle can be varied from 0 90 by varying the resistance R. Procedure:- R Firing 1. Connections are made as shown in fig. 2. Switch on the power supply to the CRO. 3. Set the CRO to the line trigger mode. 4. Switch on power supply to the SCR trainer. 5. Observe the waveform on the CRO. 6. Study the waveforms for various firing angle by varying the pot in R trigger circuit. 7. Observe the range of firing angle control. 8. and the SCR. 9. Measure the average dc voltage across the load and rms value of the ac input voltage using a For any one particular firing angle plot the waveforms of the ac voltage, voltage across the load 10. Digital multimeter. 11. Calculate the dc output voltage using the equation. Graph V - Vrms value of ac input voltage Vm - \/2Vrms.And compare the measured value

29 Observation Table:- Voltage Input Input Resistance O/P Across S.No. Voltage Cycle Value Voltage (Anode- (V) Time (K) V rms (V) Cathode) (Ms) V rms (V) Result: - Thus the operation of resistance and UJT triggering circuits of SCR has been studied.

30 Experiment No.9 Experiment Title: resistance capacitance triggering circuits of SCR. Aim:- To study the operation of resistance capacitance triggering circuits of SCR. Objective: :- To study the operation of resistance capacitance triggering circuits of SCR. Apparatus:- Circuit board, Connecting wires, Isolation transformer, CRO, true value multimeter. Circuit Diagram:- Theory: By varying the variable resistance R, the firing angle can be varied from In the negative half cycle the capacitance C charges through the diode D2 with lower plate positive to, the peak supply voltage Emax.This Capacitor voltage remains constant at until supply voltage attains zero value. During the positive half cycle of the input voltage, C begins to charge through R. When the capacitor voltage reaches the minimum gate trigger voltage SCR will turn on. Procedure:- 1. Connections are made as shown in fig. 2. Switch on the power supply to the CRO. 3. Set the CRO to the line trigger mode.

31 4. Switch on power supply to the SCR trainer. 5. Observe the waveform on the CRO. 6. Study the waveforms for various firing angle by varying the pot in R trigger circuit. 7. Observe the range of firing angle control. 8. For any one particular firing angle plot the waveforms of the ac voltage, voltage across the load and the SCR. 9. Measure the average dc voltage across the load and rms value of the ac input voltage using g' a digital millimeter. 10. Calculate the dc output voltage using the equation. Graphs:

32 Observation Table: Voltage Input Input Resistance O/P Across S.No. Voltage Cycle Value Voltage (Anode- (V) Time (K _ ) V rms (V) Cathode) (Ms) V rms (V) Result: - Thus the operation of resistance capacitance and UJT triggering circuits of SCR has been studied.

33 3.Quiz on the subject:- 1. A silicon controlled rectifier (SCR) is a a)unijunction device. b)device with 3 junctions. c)device with 4 junctions. d)none of the above. Answer:b 2. Atriac is a a)2 terminal switch. b)2 terminal bilateral switch. c)3 terminal unilateral switch. d)3 terminal bidirectional switch. Answer:d 3.Which of the following finds application in speed control of a dcmotor a)fet b)npn transistor. C)SCR. D)none of the above. Answer:c 4. A half controlled converter has better output voltage compared to full. Answer: form factor 5. At the boundary between continuous and discontinuous conduction the value output current at wt=α is of the Answer: zero

34 4. Conduction of Viva-Voce Examinations: Teacher should conduct oral exams of the students with full preparation. Normally, the objective questions with guess are to be avoided. To make it meaningful, the questions should be such that depth of the students in the subject is tested. Oral examinations are to be conducted in cordial environment amongst the teachers taking the examination. Teachers taking such examinations should not have ill thoughts about each other and courtesies should be offered to each other in case of difference of opinion, which should be critically suppressed in front of the students. 5. Evaluation and marking system: Basic honesty in the evaluation and marking system is absolutely essential and in the process impartial nature of the evaluator is required in the examination system to become. It is a primary responsibility of the teacher to see that right students who are really putting up lot of hard work with right kind of intelligence are correctly awarded. The marking patterns should be justifiable to the students without any ambiguity and teacher should see that students are faced with just circumstances.

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