PE UNIT III.pptx
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This document discusses different types of thyristor chopper circuits used in power electronics. It describes three methods of forced commutation used to turn off a conducting thyristor: voltage commutation, current commutation, and load commutation. Voltage commutation uses a pulse of reverse voltage from a charged capacitor. Current commutation uses an external current pulse greater than the load current. Load commutation relies on the load current becoming zero or transferring to another device. Circuits for the voltage-commutated and current-commutated choppers are presented. Finally, multiphase choppers using two or more parallel choppers in either in-phase or phase-shifted operation modes are introduced.
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This document discusses different types of choppers used in power electronics. It describes the principle of operation of choppers and their commutation methods including voltage commutation, current commutation, and load commutation. Specific chopper circuits are explained such as the voltage commutated chopper and current commutated chopper. Multiphase choppers and their operating modes are also introduced. Finally, different types of switching mode regulators including buck, boost, buck-boost, and Cuk regulators are classified.
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Thyristors require commutation to turn off, which involves reducing the anode current to zero and then applying a reverse voltage for a time. There are natural and forced commutation methods. Forced methods include classes A through F, which use resonant circuits, auxiliary thyristors, or line voltage reversals to commutate the main thyristor. Turn off time has two stages - reverse recovery time to remove outer layer carriers, then gate recovery time for inner layer recombination. Proper commutation circuit design is needed to apply reverse voltage for longer than the thyristor's turn off time.
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this is all about engineering thyristor is a very important in engineering specially in electrical and electronics engineering.Introduction to scr thyristers
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Turn ON-OFF Methods of SCR.pptx
- Forward voltage triggering turns on an SCR by increasing the voltage across it until avalanche breakdown occurs at the inner junction J2, known as the forward breakover voltage VBO.
- Temperature triggering increases the junction temperature until breakdown, but causes thermal runaway and is not commonly used.
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The conventional line commutated ac-to-dc
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contains higher-order harmonics. Moreover, it requires a
costly and bulky dc inductor or choke. The line current with
the high harmonic contents generates EMI and therefore it
causes more heating of the core of distribution or power
transformers. Alternatively, PWM based inverters using
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handling capability and reliability of these devices are quite
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commutated inverters are not suitable for PWM applications
due to the problems of commutation circuits. A pure sinusoidal
voltage output or waveform with low harmonic contents is
most desirable in the conversion from dc to ac. In the present
work, a novel two pulse line commutated inverter is been
proposed with control signal generated from PIC 16F 877A. It
improves the wave shape hence it reduces the total harmonic
distortion (THD) of the grid interactive-inverter. The
simulation of the circuit is done using SIMULINK. Moreover,
the performance of the proposed circuitry is far better than
the conventional line-commutated inverter. It reduces THD,
number of thyristors and dispenses with the bulky dc inductor/
choke. A prototype model is developed for discontinuous line
current mode. The results are also compared with the
simulation results in SIMULINK/ MATLAB.
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the high harmonic contents generates EMI and therefore it
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This document discusses thyristors, which are power semiconductor devices that operate as bistable switches. It covers thyristor characteristics such as static and switching behavior, and triggering methods including gate triggering. Key points include that thyristors can be turned on through thermal, light, voltage, or gate current methods, and turn off when forward current decreases below the holding current. The document also examines thyristor circuits and protection considerations against high di/dt and dv/dt.
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This document discusses thyristors and their characteristics. It begins with an introduction to thyristors, which are bistable semiconductor switches that operate from a nonconducting to conducting state. The document then covers thyristor characteristics such as forward blocking, avalanche breakdown, latching behavior, and turn-on and turn-off mechanisms. Various triggering methods like thermal, optical, voltage, dv/dt, and gate current are explained. Finally, the document discusses thyristor circuits and applications, gate triggering circuits, and protection considerations against high di/dt and dv/dt.
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PE UNIT III.pptx
- 1. UNIT – III CHOPPERS POWER ELECTRONICS 1
- 2. THYRISTOR CHOPPER CIRCUITS 2 Actually, a chopper consists of main power semiconductor device together with their turn-on and turn-off mechanisms. The process of opening, or turning-off, a conducting thyristor is called commutation. In dc choppers, it is essential to provide a separate commutation circuitry to commutate the main power SCR. It may be recalled that a conducting thyristor can be turned off by reducing its anode current below holding current value and then applying a reverse voltage across the device to enable it to regain its forward blocking capability.
- 3. 3 Forced Commutation: In forced commutation, external elements L and C which do not carry the load current continuously, are used to turn-off a conducting thyristor. Forced commutation can be achieved in the following two ways: (i) Voltage commutation (ii) Current Commutation. (iii) Load Commutation.
- 4. (i) Voltage commutation 4 In this scheme, a conducting thyristor is commutated by the application of a pulse of large reverse voltage, This reverse voltage is usually applied by switching a previously charged capacitor. The sudden application of reverse voltage across the conducting thyristor reduces the anode current to zero rapidly. Then the presence of reverse voltage across the SCR aids in the completion of its turn-off process (i.e. aids in gaining the forward blocking capability of SCR).
- 5. (ii) Current Commutation. 5 In this scheme, an external pulse of current greater than the load current is passed in the reversed direction through the conducting SCR. When the current pulse attains a value equal to the load current, net pulse current through thyristor becomes zero and the device is turned off. The current pulse is usually generated by an initially charged capacitor. An important feature of current commutation is the connection of a diode in antiparallel with the main thyristor so that voltage drop across the diode reverse biases the main SCR. Since this voltage drop is of the order of 1 volt, the commutation time in current commutation is more as compared to that in voltage commutation.
- 6. (iii) Load Commutation. 6 In load commutation, a conducting thyristor is turned off when load current flowing through a thyristor either (i) becomes zero due to the nature of load circuit parameters or (ii) is transferred to another device from the conducting thyristor.
- 7. Voltage commutated chopper 7 One of the earliest chopper circuits which has been in wide use is the voltage-commutated chopper. This chopper is generally used in high-power circuits where load fluctuation is not very large. This chopper is also known as parallel-capacitor turn-off chopper, impulse-commutated chopper or classical chopper.
- 8. Voltage commutated chopper 8 In this diagram, thyristor T1 is the main power switch. Commutation circuitry for this chopper is made up of an auxiliary thyristor TA, capacitor C, diode D and inductor L. FD is the freewheeling diode connected across the RLE
- 9. Voltage commutated chopper 9 •Working of this chopper can start only if the capacitor C is charged with polarities as marked in Fig. This can be achieved in one of the two ways as under: (i) Close switch S so that capacitor gets charged to voltage Vs through source Vs, C, S and charging resistor Rc. Switch S is then opened. (ii) Auxiliary thyristor TA is triggered so that C gets charged through source Vs, C, TA and the load. The charging current through capacitor C decays and as it reaches zero, Vc = Vs and TA is turned off.
- 10. 10
- 11. 11
- 13. 13 T1 is the main thyristor. The other components, namely, auxiliary thyristor TA, capacitor C, inductor L, diodes D1 and D2 constitute commutation circuitry. FD is the freewheeling diode and Rc is the charging resistor. Simplifying assumptions for the chopper are as follows: (i) Load current is constant. (ii) SCRS and diodes are ideal switches. (iii) Charging resistor Rc is so large that it can be treated as open circuit during the interval.
- 14. 14
- 15. 15
- 17. Multiphase Chopper 17 A multiphase chopper is one that consists of two or more choppers connected in parallel. Similarly, three choppers connected in parallel will constitute a 3-phase chopper. A multiphase chopper may be operated in two modes, viz. in-phase operation mode and the phase- shifted operation mode. In the in-phase operation mode, all the parallel connected choppers are on and off at the same instant. In the phase-shifted operation mode, different choppers are on and off at different instants of time.
- 18. 18