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By:

Kashan Ahmed Memon 14EL-02

Faraz Ahmed Jat 14EL-42

Abdul Shakoor Khuhro 14EL-26

Sajjad Ali Khaskheli 14EL-04

M.Ammar Ali Memon 14EL-86

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A controlled rectifier is a circuit that is used for converting AC supply into unidirectional DC supply & can control the power fed to the load. This process of converting alternating current (AC) to direct current (DC) is also called as controlled rectification.

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A silicon controlled rectifier is asemiconductor device that acts as a trueelectronic switch

It can change AC into DC and at the sametime can control the amount of power fed tothe load.

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Leakage Current

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Because it is silicon device and is used asrectifier and that rectification can becontrolled.

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4 layers

3 terminals◦ Anode, Cathode, Gate

Combination of rectifier (pn) and transistor (npn)

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SCR is solid state equivalent of Thyratron◦ 3 terminals (Grid, plate & cathode)

Gate equivalent to grid

Anode equivalent to plate

Cathode equivalent to cathode of thyratron

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When gate is open

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When gate is positive with respect to cathode

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Forward Characterstics

Reverse Characterstics

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1. Single Phase Half Wave Controlled Rectifier with R Load

2. Single Phase Half Wave Controlled Rectifier with RL Load

3. Single Phase Full Wave Controlled Rectifier with R Load

4. Single Phase Full Wave Controlled Rectifier with RL Load

5. Three Phase Controlled Rectifier

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The basic function of single phase controlledrectifier is to convert a single phase AC inputvoltage to controllable DC output voltage

Types◦ Single Phase Half Wave Controlled Rectifier

◦ Single Phase full wave/Bridge Controlled Rectifier

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Resistive R Load

Resistive+Inductive RL Load

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Consists of:-o AC Supply

o Thyristor

o Load

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The circuit consist of a thyristor T, a voltage source Vs and aresistive load R.

During the positive half cycle of the input voltage, the thyristor T isforward biased but it does not conduct until a gate signal is appliedto it.

When a gate pulse is given to the thyristor T at ωt = α, it getsturned ON and begins to conduct.

When the thyristor is ON, the input voltage is applied to the load.

During the negative half cycle, the thyristor T gets reverse biasedand gets tuned OFF.

So the load receives voltage only during the positive half cycle only.The average value of output voltage can be varied by varying thefiring angle α.

The waveform shows the plot of input voltage, gate current, outputvoltage, output current and voltage across thyristor.

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The circuit consist of a thyristor T, a voltage sourceVs, an inductive load L and a resistive load R.

During the positive half cycle of the input voltage, thethyristor T is forward biased but it does not conductuntil a gate signal is applied to it.

When a gate pulse is given to the thyristor T at ωt =α, it gets turned ON and begins to conduct.

When the thyristor is ON, the input voltage is appliedto the load but due to the inductor present in theload, the current through the load builds up slowly.

During the negative half cycle, the thyristor T getsreverse biased but the current through the thyristorsis not zero due to the inductor.

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The current through the inductor slowly decays tozero and when the load current (i.e the currentthrough the thyristor) falls below holding current, itgets turned off.

So here the thyristor will conduct for a few durationin the negative half cycle and turns off at ωt = β. Theangle β is called extinction angle.

The duration from α to β is called conduction angle. So the load receives voltage only during the positive

half cycle and for a small duration in negative half cycle. The average value of output voltage can be varied by

varying the firing angle α. The waveform shows the plot of input voltage, gate

current, output voltage, output current and voltageacross thyristor.

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Resistive R Load

Resistive Inductive (R-L) Load

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INTRODUCTION:-

3-Phase converters are 3-Phase controlled rectifiers.

Used to convert AC input power into dc output poweracross the load.

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Operate from 3-Phase AC supply voltage.

Provide Higher DC output voltage and Power.

High output voltage ripple frequency

APPLICATION:

Used in High Power Variable Speed Industrial DC Drives

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3-Phase Full Wave Controlled Rectifier

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The 3-phase half wave converter combines three single phase halfwave controlled rectifiers in one single circuit feeding a commonload.

The thyristor S1 in series with one of the supply phase windings ‘a-n’ acts as one half wave controlled rectifier. The second thyristor S2in series with the supply phase winding ‘b-n’ acts as the second halfwave controlled rectifier. The third thyristor S3 in series with thesupply phase winding acts as the third half wave controlled rectifier.Figure shows three phase fully controlled rectifier.

When thyristor S2 is triggered at ωt=(5π/6α), S1 becomes reversebiased and turns-off. The load current flows through the thyristorand through the supply phase winding ‘b-n’. When S2 conducts thephase voltage vbnappears across the load until the thyristor S3 istriggered.

The 3-phase input supply is applied through the star connectedsupply transformer as shown in the figure. The common neutralpoint of the supply is connected to one end of the load while theother end of the load connected to the common cathode point.

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When the thyristor S1 is triggered at ωt=(π/6 + α)=(30° + α) , the phasevoltage Van appears across the load when S1 conducts. The load currentflows through the supply phase winding ‘a-n’ and through thyristor S1 aslong as S1 conducts.

When the thyristor S3 is triggered at ωt=(3π/2 + α)=(270°+α) , S2 isreversed biased and hence S2 turns-off. The phase voltage Van appearsacross the load when S3 conducts.

When S1 is triggered again at the beginning of the next input cycle thethyristor S3 turns off as it is reverse biased naturally as soon as S1 istriggered. The figure shows the 3-phase input supply voltages, the outputvoltage which appears across the load, and the load current assuming aconstant and ripple free load current for a highly inductive load and thecurrent through the thyristor T1.

For a purely resistive load where the load inductance ‘L = 0’ and the triggerangle α >(π/6), the load current appears as discontinuous load current andeach thyristor is naturally commutated when the polarity of thecorresponding phase supply voltage reverses. The frequency of output ripplefrequency for a 3-phase half wave converter is fs, where fs is the inputsupply frequency.The 3-phase half wave converter is not normally used inpractical converter systems because of the disadvantage that the supplycurrent waveforms contain dc components.

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Power Control

Switching

Zero Voltage Switching

Over-Voltage Protection

Pulse Circuits

Battery Charging Regulator

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A commonly used circuit for controlling power in load RL using two SCRs is shown in figure. Potentiometer R controls the angle of conduction of the two SCRs. The greater the resistance of the pot, lesser will be the voltage across capacitors C1 and C2 and hence smaller will be the time duration of conduction of SCR1 and SCR2 during a cycle

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Figure shows a circuit in which two SCRs are used for making and breaking an ac circuit. The input voltage is alternating and the trigger pulses are applied to the gates of SCRs through the control switch S. Resistance R is provided in the gate circuit to limit the gate current while resistors R1 and R2 are to protect the diodes D1 and D2 respectively

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In some ac circuits it is necessary to apply the voltage to the load when the instantaneous value of this voltage is going through the zero value. This is to avoid a high rate of increase of current in case of purely resistive loads such as lighting and furnace loads, and thereby reduce the generation of radio noise and hot-spot temperatures in the device carrying the load current.

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SCRs can be employed for protecting other equipment from over-voltages owing to their fast switching action. The SCR employed for protection is connected in parallel with the load. Whenever the voltage exceeds a specified limit, the gate of the SCR will get energized and trigger the SCR.

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SCRs are used for producing high voltage/current pulses of desired waveform and duration. The capacitor C is charged during the positive half cycle of the input supply and the SCR is triggered during the nega-tive half-cycle

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Diodes D1 and D2 are to establish a full-wave rectified signal across SCR1

and the 12 V battery to be charged. When the battery is in discharged condition, SCR2 is in the off-state. When the full-wave rectified input is large enough to give the required turn-on gate current (controlled by resistor R1), SCR1 will turn on and the charging of the battery will commence

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