This chapter introduces ideas on phase controlled SCR operated rectifiers. i.e. controlled rectifiers.

1. Phase Controlled Rectifiers
ER. FARUK BIN POYEN
ASST. PROFESSOR
DEPT. OF AEIE, UIT, BU
FARUK.POYEN@GMAIL.COM

2. Contents: 2

3. 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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4. Classification of Rectifiers: 4

5. 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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6. 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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7. 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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8. Classification of Controlled Rectifiers:
 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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9. Classification of Controlled Rectifiers:
 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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10. 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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11. 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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12. 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.
 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 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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13. 1 – φ Half Wave Controlled Rectifier (RL Load)
 The waveform shows the plot of input voltage, gate current, output voltage, output current
and voltage across thyristor.
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14. 1 – φ Half Wave Controlled Rectifier (RL Load)
 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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15. 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.
 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 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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16. 1 – φ Half Wave Controlled Rectifier (RL with FD)
 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.
 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 voltage across thyristor.
 There are two modes in this circuit.
 (a) Conduction Mode (b) Freewheeling Mode.
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17. 1 – φ Half Wave Controlled Rectifier (RL with FD)
 The waveform shows the plot of input voltage, gate current, output voltage, output current
and voltage across thyristor.
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18. 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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19. 1 – φ Full Wave Controlled Rectifier (R Load)
 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.
 The average value of output voltage can be varied by varying the firing angle α.
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20. 1 – φ Full Wave Controlled Rectifier (R Load)
 The waveform shows the plot of input voltage, gate current, output voltage, output current
and voltage across thyristor.
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21. 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 it
does 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 to
conduct.
 When the thyristor T1 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 A-T1-Load-N-A.
 During the negative half cycle, T2 is forward biased, the thyristor T1 gets reverse biased
but the current through the thyristor T1 is not zero due to the inductor and T1 does not
turns OFF.
 The current through the inductor begins to decay to zero and T1 conducts for a small
duration in negative half cycle.
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22. 1 – φ Full Wave Controlled Rectifier (RL Load)
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.
 The average value of output voltage can be varied by varying the firing angle α.
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23. 1 – φ Full Wave Controlled Rectifier (RL Load)
Mid – Point Converter
 The circuit diagram is shown below.
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24. 1 – φ Full Wave Controlled Rectifier (RL Load)
Mid – Point Converter
 The waveform shows the plot of input voltage, gate current, output voltage, output current
and voltage across thyristor.
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25. 1 – φ Full Wave Controlled Rectifier (RL Load)
Bridge Converter
 The circuit consist of four thyristors T1, T2, T3 and T4, a voltage source Vs and a RL
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.
 During the negative half cycle, T3 & T4 is forward biased, the thyristor T1 & T2 gets
reverse biased but the current through them is not zero due to the inductor and does not
turns OFF.
 The current through the inductor begins to decay to zero and T1 & T2 conducts for a
small duration in negative half cycle..
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26. 1 – φ Full Wave Controlled Rectifier (RL Load)
Bridge Converter
 When a gate pulse is given to the thyristor T3 & T4 at ωt = π+α, it gets turned ON and
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.
 The load receives voltage during both the half cycles.
 The average value of output voltage can be varied by varying the firing angle α.
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27. 1 – φ Full Wave Controlled Rectifier (RL Load)
Bridge Converter
 The circuit diagram is show below.
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28. 1 – φ Full Wave Controlled Rectifier (RL Load)
Bridge Converter
 The waveform shows the plot of input voltage, gate current, output voltage, output current
and voltage across thyristor.
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29. 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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30. 1 – φ Full Wave Controlled Rectifier (RL with FD)
 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
 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 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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31. 1 – φ Full Wave Controlled Rectifier (RL with FD)
 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.
 The load receives voltage during both the half cycles.
 The average value of output voltage can be varied by varying the firing angle α.
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32. 1 – φ Full Wave Controlled Rectifier (RL with FD)
 The circuit diagram is shown below.
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33. 1 – φ Full Wave Controlled Rectifier (RL with FD)
 The waveform shows the plot of input voltage, gate current, output voltage, output current
and voltage across thyristor
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34. 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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35. 1 – φ Full Wave Half Controlled Rectifier –
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.
 The load receives voltage during both the half cycles.
 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 voltage across thyristor.
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36. 1 – φ Full Wave Half Controlled Rectifier –
Semi Converter
 The circuit diagram of a symmetric Semi Converter.
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37. 1 – φ Full Wave Half Controlled Rectifier –
Semi Converter
 The waveform of a symmetric Semi Converter.
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38. 1 – φ Full Wave Half Controlled Rectifier –
Semi Converter
 The circuit diagram of an asymmetric Semi Converter.
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39. 1 – φ Full Wave Half Controlled Rectifier –
Semi Converter
 The waveform of an asymmetric Semi Converter.
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40. 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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41. 3 – φ Half Controlled Rectifier – R Load
 The circuit diagram of 3 – φ Half Controlled Rectifier with R Load is given below.
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42. 3 – φ Half Controlled Rectifier – R Load
 The waveform of 3 – φ Half Controlled Rectifier with R Load is given below.
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43. 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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44. 3 – φ Half Controlled Rectifier – RL Load
 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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45. 3 – φ Half Controlled Rectifier – RL Load
 The circuit diagram of 3 – φ Half Controlled Rectifier with RL Load is shown below.

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46. 3 – φ Half Controlled Rectifier – RL Load
 The waveform of 3 – φ Half Controlled Rectifier with RL Load is shown below.
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47. 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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48. 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 as shown in the diagram.
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49. 3 – φ Full Controlled Rectifier – RL Load
 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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50. 3 – φ Full Controlled Rectifier – RL Load
 The waveform for the 3 – φ Full Controlled Rectifier with RL Load is shown below.
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51. 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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