Detailed Notes: Phase Controlled Rectifiers - 2 | Power Electronics - Electrical Engineering (EE) PDF Download

 Page 1


  
 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER USING ASINGLE 
THYRISTOR 
 
A single phase full wave ac controller can also be implemented with onethyristor and four diodes 
connected in a full wave bridge configuration as shown inthe above figure. The four diodes act 
as a bridge full wave rectifier. The voltageacross the thyristor T1 and current through thyristor 
T1 are always unidirectional.When T1 is triggered at ?t =a , during the positive half cycle (0 =a 
=p ) , the loadcurrent flows through D1, T1 , diode D2and through the load. With a resistive 
load,the thyristor current (flowing through the ON thyristor T1 ) , the load current falls tozero at 
?t =p , when the input supply voltage decreases to zero at ?t =p , thethyristor naturally turns 
OFF.In the negative half cycle, diodes 3 4 D &D are forward biased during?t =p to 2p radians. 
When T1 is triggered at ?t = (p +a ) , the load current flows inthe opposite direction (upward 
direction) through the load, through D3 , T1 and D4 .Thus D3 , D4 and T1 conduct together 
during the negative half cycle to supply the loadpower. When the input supply voltage becomes 
zero at ?t = 2p , the thyristor current(load current) falls to zero at ?t = 2p and the thyristor 1 T 
naturally turns OFF. Thewaveforms and the expression for the RMS output voltage are the same 
as discussedearlier for the single phase full wave ac controller.But however if there is a large 
inductance in the load circuit, thyristor 1 T maynot be turned OFF at the zero crossing points, in 
every half cycle of input voltage andthis may result in a loss of output control. This would 
require detection of the zerocrossing of the load current waveform in order to ensure guaranteed 
Page 2


  
 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER USING ASINGLE 
THYRISTOR 
 
A single phase full wave ac controller can also be implemented with onethyristor and four diodes 
connected in a full wave bridge configuration as shown inthe above figure. The four diodes act 
as a bridge full wave rectifier. The voltageacross the thyristor T1 and current through thyristor 
T1 are always unidirectional.When T1 is triggered at ?t =a , during the positive half cycle (0 =a 
=p ) , the loadcurrent flows through D1, T1 , diode D2and through the load. With a resistive 
load,the thyristor current (flowing through the ON thyristor T1 ) , the load current falls tozero at 
?t =p , when the input supply voltage decreases to zero at ?t =p , thethyristor naturally turns 
OFF.In the negative half cycle, diodes 3 4 D &D are forward biased during?t =p to 2p radians. 
When T1 is triggered at ?t = (p +a ) , the load current flows inthe opposite direction (upward 
direction) through the load, through D3 , T1 and D4 .Thus D3 , D4 and T1 conduct together 
during the negative half cycle to supply the loadpower. When the input supply voltage becomes 
zero at ?t = 2p , the thyristor current(load current) falls to zero at ?t = 2p and the thyristor 1 T 
naturally turns OFF. Thewaveforms and the expression for the RMS output voltage are the same 
as discussedearlier for the single phase full wave ac controller.But however if there is a large 
inductance in the load circuit, thyristor 1 T maynot be turned OFF at the zero crossing points, in 
every half cycle of input voltage andthis may result in a loss of output control. This would 
require detection of the zerocrossing of the load current waveform in order to ensure guaranteed 
  
 
turn off of theconducting thyristor before triggering the thyristor in the next half cycle, so that 
wegain control on the output voltage. 
In this full wave ac controller circuit using a single thyristor, as there are threepower devices 
conducting together at the same time there is more conduction voltagedrop and an increase in the 
ON state conduction losses and hence efficiency is alsoreduced.The diode bridge rectifier and 
thyristor (or a power transistor) act together as abidirectional switch which is commercially 
available as a single device module and it has relatively low ON state conduction loss. It can be 
used for bidirectional loadcurrent control and for controlling the RMS output voltage. 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER(BIDIRECTIONAL 
CONTROLLER) WITH RL LOAD 
In this section we will discuss the operation and performance of a single phasefull wave ac 
voltage controller with RL load. In practice most of the loads are of RLtype. For example if we 
consider a single phase full wave ac voltage controllercontrolling the speed of a single phase ac 
induction motor, the load which is theinduction motor winding is an RL type of load, where R 
represents the motor windingresistance and L represents the motor winding inductance.A single 
phase full wave ac voltage controller circuit (bidirectional controller)with an RL load using two 
thyristors T1 and T2 ( T1 and T2 are two SCRs) connected inparallel is shown in the figure 
below. In place of two thyristors a single Triac can beused to implement a full wave ac 
controller, if a suitable Traic is available for thedesired RMS load current and the RMS output 
voltage ratings. 
 
Fig.10 : Single phase full wave ac voltage controller with RL load 
Page 3


  
 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER USING ASINGLE 
THYRISTOR 
 
A single phase full wave ac controller can also be implemented with onethyristor and four diodes 
connected in a full wave bridge configuration as shown inthe above figure. The four diodes act 
as a bridge full wave rectifier. The voltageacross the thyristor T1 and current through thyristor 
T1 are always unidirectional.When T1 is triggered at ?t =a , during the positive half cycle (0 =a 
=p ) , the loadcurrent flows through D1, T1 , diode D2and through the load. With a resistive 
load,the thyristor current (flowing through the ON thyristor T1 ) , the load current falls tozero at 
?t =p , when the input supply voltage decreases to zero at ?t =p , thethyristor naturally turns 
OFF.In the negative half cycle, diodes 3 4 D &D are forward biased during?t =p to 2p radians. 
When T1 is triggered at ?t = (p +a ) , the load current flows inthe opposite direction (upward 
direction) through the load, through D3 , T1 and D4 .Thus D3 , D4 and T1 conduct together 
during the negative half cycle to supply the loadpower. When the input supply voltage becomes 
zero at ?t = 2p , the thyristor current(load current) falls to zero at ?t = 2p and the thyristor 1 T 
naturally turns OFF. Thewaveforms and the expression for the RMS output voltage are the same 
as discussedearlier for the single phase full wave ac controller.But however if there is a large 
inductance in the load circuit, thyristor 1 T maynot be turned OFF at the zero crossing points, in 
every half cycle of input voltage andthis may result in a loss of output control. This would 
require detection of the zerocrossing of the load current waveform in order to ensure guaranteed 
  
 
turn off of theconducting thyristor before triggering the thyristor in the next half cycle, so that 
wegain control on the output voltage. 
In this full wave ac controller circuit using a single thyristor, as there are threepower devices 
conducting together at the same time there is more conduction voltagedrop and an increase in the 
ON state conduction losses and hence efficiency is alsoreduced.The diode bridge rectifier and 
thyristor (or a power transistor) act together as abidirectional switch which is commercially 
available as a single device module and it has relatively low ON state conduction loss. It can be 
used for bidirectional loadcurrent control and for controlling the RMS output voltage. 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER(BIDIRECTIONAL 
CONTROLLER) WITH RL LOAD 
In this section we will discuss the operation and performance of a single phasefull wave ac 
voltage controller with RL load. In practice most of the loads are of RLtype. For example if we 
consider a single phase full wave ac voltage controllercontrolling the speed of a single phase ac 
induction motor, the load which is theinduction motor winding is an RL type of load, where R 
represents the motor windingresistance and L represents the motor winding inductance.A single 
phase full wave ac voltage controller circuit (bidirectional controller)with an RL load using two 
thyristors T1 and T2 ( T1 and T2 are two SCRs) connected inparallel is shown in the figure 
below. In place of two thyristors a single Triac can beused to implement a full wave ac 
controller, if a suitable Traic is available for thedesired RMS load current and the RMS output 
voltage ratings. 
 
Fig.10 : Single phase full wave ac voltage controller with RL load 
  
 
The thyristor T1 is forward biased during the positive half cycle of inputsupply. Let us assume 
that T1 is triggered at ?t =a , by applying a suitable gatetrigger pulse to T1 during the positive 
half cycle of input supply. The output voltageacross the load follows the input supply voltage 
when T1 is ON. The load current IO  flows through the thyristor T1 and through the load in the 
downward direction. Thisload current pulse flowing through T1 can be considered as the positive 
current pulse.Due to the inductance in the load, the load current IO  flowing through T1 would 
notfall to zero at ?t =p , when the input supply voltage starts to become negative.The thyristor 
T1 will continue to conduct the load current until all the inductiveenergy stored in the load 
inductor L is completely utilized and the load current through T1 falls to zero at ?t = ß , where ß 
is referred to as the Extinction angle,(the value of ?t ) at which the load current falls to zero. The 
extinction angle ß ismeasured from the point of the beginning of the positive half cycle of input 
supply tothe point where the load current falls to zero.The thyristor T1 conducts from ?t =a to ß 
. The conduction angle of T1 isd = (ß -a ), which depends on the delay angle a and the load 
impedance angle f .The waveforms of the input supply voltage, the gate trigger pulses of T1 and 
T2 , thethyristor current, the load current and the load voltage waveforms appear as shown inthe 
figure below. 
 
Fig. 11 : Input supply voltage & Thyristor current waveforms 
Page 4


  
 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER USING ASINGLE 
THYRISTOR 
 
A single phase full wave ac controller can also be implemented with onethyristor and four diodes 
connected in a full wave bridge configuration as shown inthe above figure. The four diodes act 
as a bridge full wave rectifier. The voltageacross the thyristor T1 and current through thyristor 
T1 are always unidirectional.When T1 is triggered at ?t =a , during the positive half cycle (0 =a 
=p ) , the loadcurrent flows through D1, T1 , diode D2and through the load. With a resistive 
load,the thyristor current (flowing through the ON thyristor T1 ) , the load current falls tozero at 
?t =p , when the input supply voltage decreases to zero at ?t =p , thethyristor naturally turns 
OFF.In the negative half cycle, diodes 3 4 D &D are forward biased during?t =p to 2p radians. 
When T1 is triggered at ?t = (p +a ) , the load current flows inthe opposite direction (upward 
direction) through the load, through D3 , T1 and D4 .Thus D3 , D4 and T1 conduct together 
during the negative half cycle to supply the loadpower. When the input supply voltage becomes 
zero at ?t = 2p , the thyristor current(load current) falls to zero at ?t = 2p and the thyristor 1 T 
naturally turns OFF. Thewaveforms and the expression for the RMS output voltage are the same 
as discussedearlier for the single phase full wave ac controller.But however if there is a large 
inductance in the load circuit, thyristor 1 T maynot be turned OFF at the zero crossing points, in 
every half cycle of input voltage andthis may result in a loss of output control. This would 
require detection of the zerocrossing of the load current waveform in order to ensure guaranteed 
  
 
turn off of theconducting thyristor before triggering the thyristor in the next half cycle, so that 
wegain control on the output voltage. 
In this full wave ac controller circuit using a single thyristor, as there are threepower devices 
conducting together at the same time there is more conduction voltagedrop and an increase in the 
ON state conduction losses and hence efficiency is alsoreduced.The diode bridge rectifier and 
thyristor (or a power transistor) act together as abidirectional switch which is commercially 
available as a single device module and it has relatively low ON state conduction loss. It can be 
used for bidirectional loadcurrent control and for controlling the RMS output voltage. 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER(BIDIRECTIONAL 
CONTROLLER) WITH RL LOAD 
In this section we will discuss the operation and performance of a single phasefull wave ac 
voltage controller with RL load. In practice most of the loads are of RLtype. For example if we 
consider a single phase full wave ac voltage controllercontrolling the speed of a single phase ac 
induction motor, the load which is theinduction motor winding is an RL type of load, where R 
represents the motor windingresistance and L represents the motor winding inductance.A single 
phase full wave ac voltage controller circuit (bidirectional controller)with an RL load using two 
thyristors T1 and T2 ( T1 and T2 are two SCRs) connected inparallel is shown in the figure 
below. In place of two thyristors a single Triac can beused to implement a full wave ac 
controller, if a suitable Traic is available for thedesired RMS load current and the RMS output 
voltage ratings. 
 
Fig.10 : Single phase full wave ac voltage controller with RL load 
  
 
The thyristor T1 is forward biased during the positive half cycle of inputsupply. Let us assume 
that T1 is triggered at ?t =a , by applying a suitable gatetrigger pulse to T1 during the positive 
half cycle of input supply. The output voltageacross the load follows the input supply voltage 
when T1 is ON. The load current IO  flows through the thyristor T1 and through the load in the 
downward direction. Thisload current pulse flowing through T1 can be considered as the positive 
current pulse.Due to the inductance in the load, the load current IO  flowing through T1 would 
notfall to zero at ?t =p , when the input supply voltage starts to become negative.The thyristor 
T1 will continue to conduct the load current until all the inductiveenergy stored in the load 
inductor L is completely utilized and the load current through T1 falls to zero at ?t = ß , where ß 
is referred to as the Extinction angle,(the value of ?t ) at which the load current falls to zero. The 
extinction angle ß ismeasured from the point of the beginning of the positive half cycle of input 
supply tothe point where the load current falls to zero.The thyristor T1 conducts from ?t =a to ß 
. The conduction angle of T1 isd = (ß -a ), which depends on the delay angle a and the load 
impedance angle f .The waveforms of the input supply voltage, the gate trigger pulses of T1 and 
T2 , thethyristor current, the load current and the load voltage waveforms appear as shown inthe 
figure below. 
 
Fig. 11 : Input supply voltage & Thyristor current waveforms 
  
 
ß is the extinction angle which depends upon the load inductance value. 
 
Fig. 12 : Gating Signals 
Waveforms of single phase full wave ac voltage controller with RL load fora >f . 
Discontinuous load current operation occurs for a >f and ß < (p +a ) ;i.e., (ß -a ) <p , 
conduction angle <p . 
 
Page 5


  
 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER USING ASINGLE 
THYRISTOR 
 
A single phase full wave ac controller can also be implemented with onethyristor and four diodes 
connected in a full wave bridge configuration as shown inthe above figure. The four diodes act 
as a bridge full wave rectifier. The voltageacross the thyristor T1 and current through thyristor 
T1 are always unidirectional.When T1 is triggered at ?t =a , during the positive half cycle (0 =a 
=p ) , the loadcurrent flows through D1, T1 , diode D2and through the load. With a resistive 
load,the thyristor current (flowing through the ON thyristor T1 ) , the load current falls tozero at 
?t =p , when the input supply voltage decreases to zero at ?t =p , thethyristor naturally turns 
OFF.In the negative half cycle, diodes 3 4 D &D are forward biased during?t =p to 2p radians. 
When T1 is triggered at ?t = (p +a ) , the load current flows inthe opposite direction (upward 
direction) through the load, through D3 , T1 and D4 .Thus D3 , D4 and T1 conduct together 
during the negative half cycle to supply the loadpower. When the input supply voltage becomes 
zero at ?t = 2p , the thyristor current(load current) falls to zero at ?t = 2p and the thyristor 1 T 
naturally turns OFF. Thewaveforms and the expression for the RMS output voltage are the same 
as discussedearlier for the single phase full wave ac controller.But however if there is a large 
inductance in the load circuit, thyristor 1 T maynot be turned OFF at the zero crossing points, in 
every half cycle of input voltage andthis may result in a loss of output control. This would 
require detection of the zerocrossing of the load current waveform in order to ensure guaranteed 
  
 
turn off of theconducting thyristor before triggering the thyristor in the next half cycle, so that 
wegain control on the output voltage. 
In this full wave ac controller circuit using a single thyristor, as there are threepower devices 
conducting together at the same time there is more conduction voltagedrop and an increase in the 
ON state conduction losses and hence efficiency is alsoreduced.The diode bridge rectifier and 
thyristor (or a power transistor) act together as abidirectional switch which is commercially 
available as a single device module and it has relatively low ON state conduction loss. It can be 
used for bidirectional loadcurrent control and for controlling the RMS output voltage. 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER(BIDIRECTIONAL 
CONTROLLER) WITH RL LOAD 
In this section we will discuss the operation and performance of a single phasefull wave ac 
voltage controller with RL load. In practice most of the loads are of RLtype. For example if we 
consider a single phase full wave ac voltage controllercontrolling the speed of a single phase ac 
induction motor, the load which is theinduction motor winding is an RL type of load, where R 
represents the motor windingresistance and L represents the motor winding inductance.A single 
phase full wave ac voltage controller circuit (bidirectional controller)with an RL load using two 
thyristors T1 and T2 ( T1 and T2 are two SCRs) connected inparallel is shown in the figure 
below. In place of two thyristors a single Triac can beused to implement a full wave ac 
controller, if a suitable Traic is available for thedesired RMS load current and the RMS output 
voltage ratings. 
 
Fig.10 : Single phase full wave ac voltage controller with RL load 
  
 
The thyristor T1 is forward biased during the positive half cycle of inputsupply. Let us assume 
that T1 is triggered at ?t =a , by applying a suitable gatetrigger pulse to T1 during the positive 
half cycle of input supply. The output voltageacross the load follows the input supply voltage 
when T1 is ON. The load current IO  flows through the thyristor T1 and through the load in the 
downward direction. Thisload current pulse flowing through T1 can be considered as the positive 
current pulse.Due to the inductance in the load, the load current IO  flowing through T1 would 
notfall to zero at ?t =p , when the input supply voltage starts to become negative.The thyristor 
T1 will continue to conduct the load current until all the inductiveenergy stored in the load 
inductor L is completely utilized and the load current through T1 falls to zero at ?t = ß , where ß 
is referred to as the Extinction angle,(the value of ?t ) at which the load current falls to zero. The 
extinction angle ß ismeasured from the point of the beginning of the positive half cycle of input 
supply tothe point where the load current falls to zero.The thyristor T1 conducts from ?t =a to ß 
. The conduction angle of T1 isd = (ß -a ), which depends on the delay angle a and the load 
impedance angle f .The waveforms of the input supply voltage, the gate trigger pulses of T1 and 
T2 , thethyristor current, the load current and the load voltage waveforms appear as shown inthe 
figure below. 
 
Fig. 11 : Input supply voltage & Thyristor current waveforms 
  
 
ß is the extinction angle which depends upon the load inductance value. 
 
Fig. 12 : Gating Signals 
Waveforms of single phase full wave ac voltage controller with RL load fora >f . 
Discontinuous load current operation occurs for a >f and ß < (p +a ) ;i.e., (ß -a ) <p , 
conduction angle <p . 
 
  
 
 
Fig. 13 : Waveforms of Input supply voltage, Load Current, Load Voltage andThyristor 
Voltage across T1  
Note 
? The RMS value of the output voltage and the load current may be varied byvarying the 
trigger angle a . 
? This circuit, AC RMS voltage controller can be used to regulate the RMSvoltage across 
the terminals of an ac motor (induction motor). It can be used tocontrol the temperature 
of a furnace by varying the RMS output voltage. 
? For very large load inductance ‘L’ the SCR may fail to commutate, after it istriggered and 
the load voltage will be a full sine wave (similar to the appliedinput supply voltage and 
the output control will be lost) as long as the gatingsignals are applied to the thyristors T1 
and T2 . The load current waveform willappear as a full continuous sine wave and the 
load current waveform lagsbehind the output sine wave by the load power factor angle f. 
 
TO DERIVE AN EXPRESSION FOR RMS OUTPUT VOLTAGE VO(RMS ) OF 
ASINGLE PHASE FULL-WAVE AC VOLTAGE CONTROLLER WITH RL LOAD.