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Power Electronics Sessional Sheets Expriment 01 2 Expriment 02 12 Expriment 03 18 Expriment 04 30 Expriment 05...

Power Electronics Sessional Sheets Expriment 01 2 Expriment 02 12 Expriment 03 18 Expriment 04 30 Expriment 05 40 Expriment 06 48 Expriment 07 54 Expriment 08 62 Expriment 09 74 Expriment 10 88  Chittagong University of Engineering & Technology (CUET) Department of Electrical & Electronic Engineering Course Title -Power Electronics Sessional Course No.-EEE484 Experiment no.l : The Power Thyristor OBJECTIVE To demonstrate the use of the power thyristor for switching do and ac. To observe the signal waveforms in power thyristor circuits, C THEORY Thyristor operation (dc switching) The thyristor or SCR (silicon controlled rectifier) is a diode which can be turned on by a current pulse on the gate. The symbol for this device is shown in Figure 1. It has three electrodes: the anode A, the cathode K, and the gate G. The anode and the cathode play the same role as in an ordinary diode. The gate electrode provides the means for turning on the thyristor. A K G > Figure-1: The symbol for the thyristor As in the case for the power diode, the power thyristor operates as a high-speed switch except that it is more sophisticated. The thyristor circuits examined in this exercise also belong to the class of half-wave rectifier circuits. The following set of rules apply to the operation of the power thyristor: Rule 1. There is no applied voltage. When there is no voltage applied between the anode and the cathode, the thyristor operates as an open switch and does not let current flow from A to K 1  E>0 A V* G K o Figure-2: Rule 1 for the operation of a thyristor. Rule 2. A reverse voltage ER is applied. When a reverse voltage is applied and the voltage at the anode is less than the voltage at the cathode, the thyristor also operates as an open switch. In this case we say that the thyristor is reverse biased. r 7 iC * c > A K — o G Figure-3: Rule 2 for the operation of a thyristor. 7 £r 3 A K O G tL £ fr + " A A K o \— 1« t G Figure-4: Rule 3 for the operation of a thyristor. 2  Rule 3. A forward voltage EF is applied. When a forward voltage is applied and the voltage at the anode is greater than the voltage at the cathode, the thyristor operates again as an open switch. This case is termed forward bias. However, when a current pulse IG flows from the gate to the cathode, the thyristor turns on and current IA flows from the anode to the cathode. Rule 4. The current IA drops to zero. As long as current flows between the anode and the cathode, the thyristor operates as a closed switch. However, as soon as IA drops below a certain value called the holding current IH the thyristor turns off. It must be retriggered with a current pulse applied to the gate electrode in order to conduct again. The value of the holding current is very small compared to the nominal thyristor current. '* < ' H A K G $ o Figure-5: Rule 4 for the operation of a thyristor. In summary, two conditions are necessary before a thyristor can turn on: 1. The voltage at the anode must be positive with respect to the cathode. V | 'v 2. A current pulse must flow from the gate to the cathode. MW Z The control gate current IG flows from the gate to the cathode. This current is very small compared to that flowing between the anode and the cathode. It is not necessary that continue IG to flow for the thyristor to stay turned on. The power gain (power output/control power applied to the gate) is very high for a power thyristor. It can be as much as 100 000. 3  AC switching using two thyristors Figure 6 shows schematically how a load is manually controlled using a magnetic contactor. It consists of: A - the magnetic contactor composed of a coil and NO (normally open) contacts, PB1, PB2 - Push Button switches. When PB1 is depressed, the coil of A is energized and the two sets of NO contacts close. One set maintains the coil ’s Held current and the other connects the load to the power source. When PB2 is depressed, the contactor’s coil is de-energized thus causing the contacts to open. This disconnects both the load and the coil from the line. CONTROL SECTION POWER SECTION Q CONTROL SOURCE Q PB A i C.. P8 2 o U A POWER SOURCE LOAD A o 1| Figure-6: Manual control of load using a magnetic contactors AC switching can also be done electronically using two power thyristors in an inverse-parallel (back-to-back) connection. The anode and cathode of the thyristor replace the NO contacts of the contactor. It is through the thyristor that the load is connected to the power source. The gate pulse tiring signals take the place of the manual control signal. Figure 7 shows schematically how the circuit is set- up. ps po I * Cl AC T 1 C2 E o (AC) SOURCE LOAD 1 Figure-7: A simple electronic contactor 4 Two thyristors, connected as in Figure 7 are required to assure complete control of the load. This is because a thyristor conducts only in one direction. Qi and Q2 therefore conduct each in their turn, once each half-cycle. This electronic contactor has no mobile contacts and is therefore completely silent in operation. Procedure summary In the first part of the exercise, you will set up the equipment. In the second part, you will demonstrate the operation of a thyristor in a dc circuit. In the third part, you will use two back-to-back thyristors for ac switching. EQUIPMENT REQUIRED 1. Mobile Workstation 2. Resistive Load 3. DC voltmeter/Ammeter 4. AC Ammeter 5. AC Voltmeter 6. Three phase Wattmeter/Varmeter 7. Power Supply 8. Enclosure/Power Supply 9. Power Thyristor 10. Connection Leads & Accessories 11. Thyristor Firing Unit 12. Current/Voltage Isolator PROCEDURE CAUTION! High voltages are present in this laboratory exercise! Do not make or modify any banana lack connections with the power on unless otherwise specified! Setting up the equipment 1. Install the Power Supply, the Enclosure / Power Supply, the Resistive Load, the DC Voltmeter/Ammeter, the AC Ammeter, the AC Voltmeter, the Three-Phase Wattmeter/Varmeter, and the Power Thyristors modules in the Mobile Workstation. 2. Install the Thyristor Firing Unit and the Current/Voltage Isolators in the Enclosure / Power Supply. 5  Note: Before installing the Thyristor Firing Unit make sure that switches SW 1 and SW2 (located on the printed circuit board ) are in the O position. 3. Make sure that the main power switch of the Power Supply is set to the O (OFF) position. Set the voltage control knob to O. Connect the Power Supply to a three-phase wall receptacle. 4. Plug the. Enclosure! Power Supply line cord into a wall receptacle. Set the rocker switch of the Enclosure / Power Supply to the I (ON) position. 5. On the Power Supply, set the 24-V ac power switch to the I (ON) position. 6. Make sure that the toggle switches on the Power Thyristors and the Resistive Load modules are all set to the O (open) position. Thyristor operation 7. Set up the circuit of Figure 8. Connect the 0 V terminal on the Enclosu / re Power Supply to the common terminal of the Power Thyristors module. However, do not connect the +5V jack of the Enclosure! Power Supply to FIRING CONTROL INPUT 1 of the Power L Q Thyristors module yet. Note that the thyristor is reverse biased in this circuit. 8. On the Power Supply, set the Voltage Selector to 7-N. Make sure that the voltage control knob is set to the 0 position then set the main power switch to I (ON). Slowly turn the voltage control knob to increase the dc voltage to 100(%). Does the thyristor turn ' on ? (Does current flow through the thyristor?) Explain. 9. Connect the +5V jack of the Enclosure / Power Supply to FIRING CONTROL INPUT 1 of the Power Thyristors module. Does the thyristor turn on? Explain. On the Power Supply, set the voltage control knob to 0 then set the main power switch to O (OFF). L ): 10. Disconnect the wire between the + 5 V jack of the Enclosure / Power Supply and FIRING CONTROL INPUT 1 of the Power Thyristors module. Interchange the leads at the terminals of the thyristor on the Power Thyristors module. This will reverse the polarity of the voltage applied to the thyristor. On the Power Supply, set the main power switch to 1 (ON). Slowly turn the voltage control knob to increase the dc voltage to 100(%). Does the thyristor turn on? Explain. 6 V; ± 0 POWER THYRISTORS r i " — o; 1 i DC POWER i \ 1 I n Ri SUPPLY I I I \ i i I FIRING 1 1„ CowmoL t WPUTS G DC [ +5 V V is POWER SUPPLY i O V UNE VOLTAGE (VoC). l I dc ( mA) E ,  ] 17. On the Power Supply, set the main power switch to 1 ( ON), and set the voltage control knob to 100( %). Vary the FIRING ANGLE on the Thyristor Firing Unit while observing the oscilloscope. Adjust the FIRING ANGLE to 120° and sketch the waveform in Figure 6. Note: Adjust the FIRING ANGLE as closely as possible to the value given. ' * On the Power Supply, set the voltage control knob to 0 then set the main power switch to 0 (OFF). Disconnect the voltage isolator from the circuit and connect it across the load as shown in Figure 7. C POWER THYRISTORS r °r n i ~' ~ i i t ± R< OUTPUT COM I 1 I I r - Figure-7: Circuit to observe the voltage across the load. ' - the Power Supply, set the main power switch to 1 ( ON ), and set the voltage control knob to > : :. Vary the FIRING ANGLE on the Thyristor Firing Unit while observing the : ;;.. escope... the FIRING ANGLE to 120° and sketch the waveform in Figure 6. - is the conduction angle of the thyristor when the firing angle is 120°? Conduction angle =  On the Power Supply, set the voltage control knob to 0 then set the main power switch to O : OFF ). Controlled rectifier supplying a passive load 19. Set up the circuit of Figure 8 using the resistive load Z \ (a). POWER THYRISTORS ~ 1 I Q I i , SSL.—, OUTPUT tc POWER rv SUPPLY AT '' - N. + * i COM. OUTPUT & I I I I I T " fWC \ CONTPOC HPVTS $wc.* ' 1 rmc / 2 THYRISTOR com TO V0CTA3E OM OUTPUTS tSOUW OUTPUT FIRING TO CURRENT OSCILLOSCOPE UNIT - ISOLWO* OUTPUT CN 2 CONTROL OUTPUT KPUT :< SOURCE * LINE VOLTAGE. i t dc i I. E I dc eI Z|W Z , ( b) C: (Voc) W (A) (V) (V) o£T}o oQ ^fo - -- -- 120 220 240 2.5 1.5 1.5 10 5 5 150 300 300 300 600 600 -- R 60 C R 60 0 0.2 H - - 1 , R 220 0 R 220 0, L 0.8 H R 240 0 R 240 0, L 0.8 H Figure-8: Controlled rectifier supplying a passive load On the Power Supply, set the main power switch to I ( ON), and set the voltage control knob to 100(% ), Vary the FIRING ANGLE and observe the waveforms on the  oscilloscope. On the Thyristor Firing Unit, set the FIRING ANGLE to 45°. Sketch the voltage and current waveforms in Figure 9 REFERENCE WAVEFORM 0 to T tao 270 I TOO 490 540 «30 z 770 PH4SC mOLE {DGGMCC3] ,, T I T T T ** I T t 90 «90 & ISO 270 340 940 430 7 ZO must war pxoftces] 1 o v T I T T T T y 90 * 0 270 940 490 T 940 990 720 «ASt (oe«a$I.DO Z , ( b) I^ 0 T sOOCTIVE) T 90 i 1 0 T 270 «0 T l 1 T * 450 540 430 720 FKASC M CLZ (tCC**C£$] 0 T T r T T T T T 90 » 40 270 340 450 540 «30 720 PWASC 0XCROS] Figure-9: Voltage and current waveforms (a=45°)  1 the first row of Table 1 ' in i OUTPUT OUTPUT OUTPUT LOAD CONDUCTION Z, VOLTAGE CURRENT POWER E,dc l,dc P0 = E, x l, ANGLE V A W degrees (a) Resistive ! ( b) Inductive ?? Table-1: Measurements for controlled rectifier circuit (a=45 °) On the Power Supply, set the voltage control knob to 0 then set the main power switch to 0 ( OFF). 21. Change the load in the circuit to the inductive load Zib). On the Power Supply, set the main power switch to 1 (ON), and set the voltage control knob to 100(%). Sketch the voltage and current waveforms in Figure 9. Fill in the second row of Table 1. On the Power Supply, set the voltage control knob to 0 then set the main power switch to O (OFF). Why is the conduction angle greater with an inductive load than with a purely resistive load’? What effect does the inductive load have on the average output voltage and current? The presence of an inductive load has the effect of reducing the average output voltage and increasing the average output current compared to a purely resistive load. What is the difference between this rectifier circuit and the half- wave rectifier circuit seen in Exercise 1 ’9  Add a freewheeling diode to the circuit, as shown in Figure 10. On the Power Supply, set the main power switch to 1 (ON), and set the voltage control knob to 100(%). POWER THYRISTORS r «n ± :\ cow, I * I \ I OUTPUT i AC | | ROWER ROWER | \ WOOES , dc ci OUTPUT SUPPLY /V N 1 i COM, t I I T nwc \ mm INPUTS SrtC IWT THYRISTOR TO mua - CM t 2 FIRING mm OUTPUT OSCILLOSCOPE FWNG TO CURRENT ROi CONTROL UNIT CONTTW OUTPUTS. ISQUTOR OUTPUT - CH 2 OUTPUT NPUT SOURCE Figure-10: Rectifier circuit with free wheeling diode 4, r :n the Table 2. \ OUTPUT OUTPUT OUTPUT LOAD VOLTAGE CURRENT POWER A E,dc 1,dc P0 = E, x !t V A W (b) Inductive Table- 2: Measurements for controiled rectifier circuit with free wheeling diode (a=45°)  1 ’ \ ~hat effect does the freewheeling diode have on the operation of the circuit ? On the Power Supply, set the voltage control knob to 0 then set the main power switch to O ( OFF). ) 5 4 : l 1  ¥ Chittagong University of Engineering & Technology (CUET) Department of Electrical & Electronic Engineering Course Title -Power Electronics Sessional Course No.-EEE484 Experiment no.4: Thyristor Single-Phase Bridge Rectifier OBJECTIVE « To demonstrate the operation of a thyristor single-phase bridge C » To demonstrate the operation of a thyristor single-phase bridge with free wheeling diode THEORY Thyristor single-phase bridge - The thyristor single-phase bridge (Figure 1) operates on the same principle as the diode single- phase bridge rectifier seen in Exercise 1, except that each thyristor begins to conduct only when a current pulse is injected into the gate (providing that the thyristor is forward biased). Once a thyristor begins to conduct, it continues to conduct until the current flowing through it becomes zero. With a resistive load, the current becomes zero the instant the ac source voltage Es passes through zero volts. Therefore, the output is a full-wave rectified voltage which is always positive - (Figure 2(b)). * *o m- O , I * z LOAD Co Figure-1: Thyristor single Phase bridge  Since conduction can be initiated at any angle in the waveform between 0° and almost 180°, the * 4 ^ average output voltage E0, and therefore the average current, can be varied between 0 and 100%. 32. 0 5 o) Sourc« Voltage a I c UJ § 5 o § b) Output Voltage (Resistive Lood) a uJ