power electronics phase controlled rectifiers - scr

Phase Controlled Rectifiers

ER. FARUK BIN POYEN

ASST. PROFESSOR

DEPT. OF AEIE, UIT, BU

[email protected]

mailto:[email protected]

Contents: 2

Classification of Rectifiers based on Control:

The converter circuit which converts AC to DC is called a Rectifier.

The rectifier circuit using diodes only is called an Uncontrolled rectifier circuit.

All rectifiers are broadly categorized into three sections.

1. Controlled Rectifier - It has only thyristors. NO diodes

2. Half Controlled Rectifier - It has thyristor + diodes

3. Uncontrolled Rectifier - Only diodes

Control here means controlling when to start rectification and when to stop.

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Classification of Rectifiers: 4

Phase Controlled Rectifiers:

Unlike diode, an SCR does not become conducting immediately after its voltage has

become positive.

It requires triggering by means of pulse at the gate.

So it is possible to make the thyristor conduct at any point on the half wave which applies

positive voltage to its anode.

Thus the output voltage is controlled.

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Phase Controlled Rectifiers - Applications

Steel rolling mills, paper mills, textile mills where controlling of DC motor speed is

necessary.

Electric traction.

High voltage DC transmissions.

Electromagnet power supplies.

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Classification of Controlled Rectifiers:

1. Single Phase Half Wave Controlled Rectifier with R Load.

2. Single Phase Half Wave Controlled Rectifier with RL Load.

3. Single Phase Half Wave Controlled Rectifier with RL Load and Freewheeling Diode.

4. Single Phase Full Wave Controlled Rectifier with R Load.

5. Single Phase Full Wave Controlled Rectifier with RL Load.

6. Single Phase Full Wave Controlled Rectifier with RL Load and Freewheeling Diode.

7. Single Phase Full Wave Half Controlled Rectifier (Semi Converter).

8. Three Phase Half Wave Controlled Rectifier.

9. Three Phase Full Wave Controlled Rectifier.

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Depends upon the period of conduction during each cycle of ac input voltage, they are

classified into two groups namely

(i) Half wave Rectifiers

(ii) Full wave Rectifiers

There are two types of full wave Rectifiers namely

(i) Full wave Rectifier using centre tapped transformer

(ii) Full wave Rectifier using bridge configuration

Depending upon the number of phase in the supply network, converters are classified as

(i) Single phase Rectifiers

(ii) Three phase Rectifiers

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Depending upon the number of pulse on the dc side in one period of the input ac voltage,

Rectifiers are further classified as

(i) Single pulse Rectifiers

(ii) Two pulse Rectifiers

(iii) Three pulse Rectifiers

(iv) Six pulse Rectifiers etc.

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1 – φ Half Wave Controlled Rectifier (R Load)

The circuit consists of a thyristor T, a voltage source Vs and a resistive load R.

During the positive half cycle of the input voltage, the thyristor T is forward biased but it

does not conduct until 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 applied to the load.

During the negative half cycle, the thyristor T gets reverse biased and gets turned OFF.

So the load receives voltage only during the positive half cycle only.

The average value of output voltage can be varied by varying the firing angle 𝛼.

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1 – φ Half Wave Controlled Rectifier (R Load)

The waveform shows the plot of input voltage, output voltage, gate current, output current

and voltage across thyristor.

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1 – φ Half Wave Controlled Rectifier (RL Load)

The circuit consist of a thyristor T, a voltage source Vs, an inductive load L and a resistive

load R.




conduct.

When the thyristor is ON, the input voltage is applied to the load but due to the inductor

present in the load, the current through the load builds up slowly.

During the negative half cycle, the thyristor T gets reverse biased but the current through

the thyristors is not zero due to the inductor.

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The waveform shows the plot of input voltage, gate current, output voltage, output current


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The current through the inductor slowly decays to zero and when the load current (i.e. the

current through the thyristor) falls below holding current, it gets turned off.

So here the thyristor will conduct for a few duration in the negative half cycle and turns

off at ωt = β. The angle 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.

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1 – φ Half Wave Controlled Rectifier (RL with FD)

The circuit consist of a thyristor T, a voltage source Vs, a diode FD across the RL load, an

inductive load L and a resistive load R.




conduct.

When the thyristor is ON, the input voltage is applied to the load but due to the inductor

present in the load, the current through the load builds up slowly.

During the negative half cycle, the thyristor T gets reverse biased. At this instant i.e at ωt

= π, the load current shift its path from the thyristor to the freewheeling diode.

When the current is shifted from thyristor to freewheeling diode, the thyristor turns OFF.

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The current through the inductor slowly decays to zero through the loop R freewheeling

diode - L.

So here the thyristor will not conduct in the negative half cycle and turns off at ωt = π.

So the load receives voltage only during the positive half cycle.




There are two modes in this circuit.

(a) Conduction Mode (b) Freewheeling Mode.

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1 – φ Full Wave Controlled Rectifier (R Load)

The circuit consist of four thyristors T1, T2, T3 and T4, a voltage source Vs and a R Load.

During the positive half cycle of the input voltage, the thyristors T1 & T2 is forward

biased but it does not conduct until a gate signal is applied to it.

When a gate pulse is given to the thyristors T1 & T2 at ωt = α, it gets turned ON and

begins to conduct.

When the T1 & T2 is ON, the input voltage is applied to the load through the path Vs- T1-

Load-T2-Vs.

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During the negative half cycle, T3 & T4 is forward biased, the thyristor T1 & T2 gets

reverse biased and turns OFF

When a gate pulse is given to the thyristor T3 & T4 at ωt = π+α, it gets turned ON and

begins to conduct.

When T3 & T4 is ON, the input voltage is applied to the load Vs-T3-Load-T4-Vs.

Here the load receives voltage during both the half cycles.


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1 – φ Full Wave Controlled Rectifier (RL Load)

Mid – Point Converter The circuit consist of two thyristors T1 and T2, a centre tap transformer, a voltage source

Vs and a RL Load.

During the positive half cycle of the input voltage, the thyristor T1 is forward biased but itdoes not conduct until a gate signal is applied to it.

When a gate pulse is given to the thyristor T1 at ωt = α, it gets turned ON and begins toconduct.

When the thyristor T1 is ON, the input voltage is applied to the load but due to theinductor present in the load, the current through the load builds up slowly through thepath A-T1-Load-N-A.

During the negative half cycle, T2 is forward biased, the thyristor T1 gets reverse biasedbut the current through the thyristor T1 is not zero due to the inductor and T1 does notturns OFF.

The current through the inductor begins to decay to zero and T1 conducts for a smallduration in negative half cycle.

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Mid – Point Converter

When a gate pulse is given to the thyristor T2 at ωt = π+α, it gets turned ON and begins to

conduct.

When the thyristor T2 is ON, the load current shifts its path from the T1 to T2 and

thyristor T1 turns OFF at ωt = π+α.

When T2 is ON, the current through the load builds up slowly through the path B-T2-

Load-N-B.

So here both the thyristor will conduct for a few duration in the negative half cycle.

The load receives voltage during both the half cycles.


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The circuit diagram is shown below.

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Bridge Converter

The circuit consist of four thyristors T1, T2, T3 and T4, a voltage source Vs and a RLLoad.

During the positive half cycle of the input voltage, the thyristors T1 & T2 is forwardbiased but it does not conduct until a gate signal is applied to it.

When a gate pulse is given to the thyristors T1 & T2 at ωt = α, it gets turned ON andbegins to conduct.

When the T1 & T2 is ON, the input voltage is applied to the load but due to the inductorpresent in the load, the current through the load builds up slowly through the path Vs-T1-Load-T2-Vs.

During the negative half cycle, T3 & T4 is forward biased, the thyristor T1 & T2 getsreverse biased but the current through them is not zero due to the inductor and does notturns OFF.

The current through the inductor begins to decay to zero and T1 & T2 conducts for asmall duration in negative half cycle..

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Bridge Converter


begins to conduct.

When the thyristor T3 & T4 is ON, the load current shifts its path to T3 & T4 and turns

OFF T1 & T2 at ωt = π+α.

When T3 & T4 is ON, the current through the load builds up slowly through the path Vs-

T3-Load-T4-Vs.

So here all the thyristor will conduct for a few duration in the negative half cycle.



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Bridge Converter

The circuit diagram is show below.

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Bridge Converter



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1 – φ Full Wave Controlled Rectifier (RL with FD)

The circuit consist of four thyristors T1, T2, T3 and T4, a voltage source Vs, a RL Load

and a freewheeling diode across the load.

During the positive half cycle of the input voltage, the thyristors T1 & T2 is forward

biased but it does not conduct until a gate signal is applied to it.

When a gate pulse is given to the thyristors T1 & T2 at ωt = α, it gets turned ON and

begins to conduct.

When the T1 & T2 is ON, the input voltage is applied to the load but due to the inductor

present in the load, the current through the load builds up slowly through the path Vs-T1-

Load-T2-Vs.

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During the negative half cycle (at ωt = π), T3 & T4 is forward biased, the thyristor T1 &

T2 gets reverse biased.

The current shifts its path to the freewheeling diode and circulates through the loop FD-R-

L-FD.

Thus T1 & T2 turns off at ωt = π


begins to conduct.

When T3 & T4 is ON, the current through the load builds up slowly through the path Vs-

T3-Load-T4-Vs.

During the next positive half cycle (at ωt = 2π), T1 & T2 is forward biased, the thyristor

T3 & T4 gets reverse biased.

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The current shifts its path to the freewheeling diode and circulates through the loop FD-R-

L-FD.

Thus T3 & T4 turns off at ωt = 2π

So here all the thyristor will conduct only in the positive half cycle.



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The circuit diagram is shown below.

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and voltage across thyristor

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1 – φ Full Wave Half Controlled Rectifier –

Semi Converter

The circuit consist of two thyristors T1 & T2, two diodes D1 and D2, a voltage source Vs,

a RL Load.

During the positive half cycle of the input voltage, the thyristors T1 & D1 is forward

biased but it does not conduct until a gate signal is applied to T1.

When a gate pulse is given to the thyristors T1 at ωt = α, it gets turned ON and begins to

conduct.

When the T1 & D1 is ON, the input voltage is applied to the load but due to the inductor

present in the load, the current through the load builds up.

During the negative half cycle (at ωt = π), T2 & D2 is forward biased, the thyristor T1 &

D1 gets reverse biased.

The current shifts its path to D2 and T1 in case of symmetrical converter (D1 & D2 in

case of asymmetical converter) and circulates through the load.

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Semi Converter

When a gate pulse is given to the thyristor T2 at ωt = π+α, it gets turned ON and begins to

conduct.

When T2 & D2 is ON, the current through the load builds up.

During the next positive half cycle (at ωt = 2π), T1 & D1 is forward biased, the thyristor

T2 & D2 gets reverse biased.

The current shifts its path to D1 and T2 in case of symmetrical converter (D1 & D2 in

case of asymmetrical converter) and circulates through the load.





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Semi Converter

The circuit diagram of a symmetric Semi Converter.

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Semi Converter

The waveform of a symmetric Semi Converter.

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Semi Converter

The circuit diagram of an asymmetric Semi Converter.

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Semi Converter

The waveform of an asymmetric Semi Converter.

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3 – φ Half Controlled Rectifier – R Load

The circuit consist of a delta star transformer and 3 thyristors T1, T2, T3 which are

connected on the secondary star connected winding and a resistive load.

When Va is positive, T1 becomes forward biased and conducts. During the negative cycle

of Va, T1 turns off.

Similarly T2 and T3 conducts only during the positive cycles of V b and V c respectively.

The average output voltage can be varied by varying the firing angles of the thyristors.

The waveforms shows the output voltage for various firing angles.

In the waveform, V a is denoted as V an, V b as V bn, V c as V cn.

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The circuit diagram of 3 – φ Half Controlled Rectifier with R Load is given below.

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The waveform of 3 – φ Half Controlled Rectifier with R Load is given below.

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3 – φ Half Controlled Rectifier – RL Load

The circuit consist of a delta star transformer and 3 thyristors T1, T2, T3 which are

connected on the secondary star connected winding and a RL load.

When V a is positive, T1 becomes forward biased and conducts. During the negative cycle

of V a, the current through T1 is not zero due to inductor present in the load.

So T1 will remain ON during the negative cycle of V a.

When V b is positive, T2 is triggered and the load current gets transferred from T1 to T2.

At this instant, T1 turns OFF.

During the negative cycle of V b, the current through T2 is not zero due to inductor

present in the load.

So T2 will remain ON during the negative cycle of V b.

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When T3 is triggered during positive cycle of Vc, the load current is transferred from T2

to T3.

At this instant, T2 turns OFF

Similarly T3 conducts during the negative cycle of Vc and turns OFF when T1 is

triggered.

The average output voltage can be varied by varying the firing angles of the thyristors.

The waveforms shows the output voltage for various firing angles.

In the waveform, V a is denoted as V an, V b as V bn, V c as V cn.

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The circuit diagram of 3 – φ Half Controlled Rectifier with RL Load is shown below.

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The waveform of 3 – φ Half Controlled Rectifier with RL Load is shown below.

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3 – φ Full Controlled Rectifier – RL Load

The circuit consist of 6 thyristors, T1, T2, T3, T4, T5, T6, a three phase supply and a RL

load.

The thyristors T1, T3, T5 form the positive group.

The thyristors T4, T6, T2 form the negative group.

Thyristors T1, T3, T4, T6 produces the full wave rectified output of V ab across the load.

Thyristors T3, T5, T6, T2 produces the full wave rectified output of V bc across the load.

Thyristors T1, T5, T4, T2 produces the full wave rectified output of V ca across the load.

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The circuit consist of 6 thyristors, T1, T2, T3, T4, T5, T6, a three phase supply and a RL

load as shown in the diagram.

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All these 3 outputs are given simultaneously to the same RL load. The effect is that all the

3 individual output mentioned above gets superimposed on each other to get the final

output.

The waveform of the output for different firing angles are shown below.

The average output voltage can be varied by varying the firing angle.

For firing angle < 90, the circuit works as rectifier.

For firing angle > 90, the circuit works as Line commutated inverter.

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The waveform for the 3 – φ Full Controlled Rectifier with RL Load is shown below.

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References:

Power Electronics, Dr. P S Bimbhra.

https://www.slideshare.net/maneesh001/phase-controlled-rectifiers?qid=24ce81bf-9c7e-

4685-8ff8-d123b7af60e5&v=&b=&from_search=1

NPTEL, Power Electronics, Module 2, AC to DC Converters, Version 2 EE IIT,

Kharagpur.

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https://www.slideshare.net/maneesh001/phase-controlled-rectifiers?qid=24ce81bf-9c7e-4685-8ff8-d123b7af60e5&v=&b=&from_search=1

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