Detailed Notes: AC Voltage Controllers | Power Electronics - Electrical Engineering (EE) PDF Download

 Page 1


EE6503 POWER EELCTRONICS 
AC VOLTAGE CONTROLLERS  
1. Introduction. 
 AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value 
of the alternating voltage applied to a load circuit.An ac voltage controller is a type of thyristor 
power converter which is used to convert a fixed voltage, fixed frequency ac input supply to 
obtain a variable voltage ac output. The RMS value of the ac output voltage and the ac power 
flow to the load is controlled by varying (adjusting) the trigger angle ‘ ?’ 
 
 
 
Fig.1 
 
2. TYPE OF AC VOLTAGE CONTROLLERS 
 The ac voltage controllers are classified into two types based on the type of input ac 
supply applied to the circuit. 
? Single Phase AC Controllers. 
? Three Phase AC Controllers. 
Single phase ac controllers operate with single phase ac supply voltage of 230V RMS at 
50Hz in our country. Three phase ac controllers operate with 3 phase ac supply of 400V RMS at 
50Hz supply frequency. 
Each type of controller may be sub divided into  
? Uni-directional or half wave ac controller. 
? Bi-directional or full wave ac controller. 
In brief different types of ac voltage controllers are 
? Single phase half wave ac voltage controller (uni-directional controller). 
? Single phase full wave ac voltage controller (bi-directional controller). 
AC
V oltage
Controller
V
0(R MS )
f
S
V ariable AC 
R MS O/P V oltage
AC
Input
V oltage
f
s
V
s
f
s
Page 2


EE6503 POWER EELCTRONICS 
AC VOLTAGE CONTROLLERS  
1. Introduction. 
 AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value 
of the alternating voltage applied to a load circuit.An ac voltage controller is a type of thyristor 
power converter which is used to convert a fixed voltage, fixed frequency ac input supply to 
obtain a variable voltage ac output. The RMS value of the ac output voltage and the ac power 
flow to the load is controlled by varying (adjusting) the trigger angle ‘ ?’ 
 
 
 
Fig.1 
 
2. TYPE OF AC VOLTAGE CONTROLLERS 
 The ac voltage controllers are classified into two types based on the type of input ac 
supply applied to the circuit. 
? Single Phase AC Controllers. 
? Three Phase AC Controllers. 
Single phase ac controllers operate with single phase ac supply voltage of 230V RMS at 
50Hz in our country. Three phase ac controllers operate with 3 phase ac supply of 400V RMS at 
50Hz supply frequency. 
Each type of controller may be sub divided into  
? Uni-directional or half wave ac controller. 
? Bi-directional or full wave ac controller. 
In brief different types of ac voltage controllers are 
? Single phase half wave ac voltage controller (uni-directional controller). 
? Single phase full wave ac voltage controller (bi-directional controller). 
AC
V oltage
Controller
V
0(R MS )
f
S
V ariable AC 
R MS O/P V oltage
AC
Input
V oltage
f
s
V
s
f
s
? Three phase half wave ac voltage controller (uni-directional controller). 
? Three phase full wave ac voltage controller (bi-directional controller). 
3. APPLICATIONS OF AC VOLTAGE CONTROLLERS 
? Lighting / Illumination control in ac power circuits. 
? Induction heating. 
? Industrial heating & Domestic heating. 
? Transformers tap changing (on load transformer tap changing). 
? Speed control of induction motors (single phase and poly phase ac induction 
motor control). 
? AC magnet controls. 
 
4. AC VOLTAGE CONTROL TECHNIQUES 
There are two different types of thyristor control used in practice to control the ac power flow 
 
1. Phase control 
2. On-Off control 
 
These are the two ac output voltage control techniques. 
In On-Off control technique Thyristors are used as switches to connect the load circuit to the 
ac supply (source) for a few cycles of the input ac supply and then to disconnect it for few input 
cycles. The Thyristors thus act as a high speed contactor (or high speed ac switch). 
 
4.1 PHASE CONTROL TECHNIQUE 
 
 In phase control the Thyristors are used as switches to connect the load circuit to the 
input ac supply, for a part of every input cycle. That is the ac supply voltage is chopped using 
Thyristors during a part of each input cycle. 
 The thyristor switch is turned on for a part of every half cycle, so that input supply 
voltage appears across the load and then turned off during the remaining part of input half cycle 
to disconnect the ac supply from the load. 
 By controlling the phase angle or the trigger angle ‘ ?’ (delay angle), the output RMS 
voltage across the load can be controlled. 
Page 3


EE6503 POWER EELCTRONICS 
AC VOLTAGE CONTROLLERS  
1. Introduction. 
 AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value 
of the alternating voltage applied to a load circuit.An ac voltage controller is a type of thyristor 
power converter which is used to convert a fixed voltage, fixed frequency ac input supply to 
obtain a variable voltage ac output. The RMS value of the ac output voltage and the ac power 
flow to the load is controlled by varying (adjusting) the trigger angle ‘ ?’ 
 
 
 
Fig.1 
 
2. TYPE OF AC VOLTAGE CONTROLLERS 
 The ac voltage controllers are classified into two types based on the type of input ac 
supply applied to the circuit. 
? Single Phase AC Controllers. 
? Three Phase AC Controllers. 
Single phase ac controllers operate with single phase ac supply voltage of 230V RMS at 
50Hz in our country. Three phase ac controllers operate with 3 phase ac supply of 400V RMS at 
50Hz supply frequency. 
Each type of controller may be sub divided into  
? Uni-directional or half wave ac controller. 
? Bi-directional or full wave ac controller. 
In brief different types of ac voltage controllers are 
? Single phase half wave ac voltage controller (uni-directional controller). 
? Single phase full wave ac voltage controller (bi-directional controller). 
AC
V oltage
Controller
V
0(R MS )
f
S
V ariable AC 
R MS O/P V oltage
AC
Input
V oltage
f
s
V
s
f
s
? Three phase half wave ac voltage controller (uni-directional controller). 
? Three phase full wave ac voltage controller (bi-directional controller). 
3. APPLICATIONS OF AC VOLTAGE CONTROLLERS 
? Lighting / Illumination control in ac power circuits. 
? Induction heating. 
? Industrial heating & Domestic heating. 
? Transformers tap changing (on load transformer tap changing). 
? Speed control of induction motors (single phase and poly phase ac induction 
motor control). 
? AC magnet controls. 
 
4. AC VOLTAGE CONTROL TECHNIQUES 
There are two different types of thyristor control used in practice to control the ac power flow 
 
1. Phase control 
2. On-Off control 
 
These are the two ac output voltage control techniques. 
In On-Off control technique Thyristors are used as switches to connect the load circuit to the 
ac supply (source) for a few cycles of the input ac supply and then to disconnect it for few input 
cycles. The Thyristors thus act as a high speed contactor (or high speed ac switch). 
 
4.1 PHASE CONTROL TECHNIQUE 
 
 In phase control the Thyristors are used as switches to connect the load circuit to the 
input ac supply, for a part of every input cycle. That is the ac supply voltage is chopped using 
Thyristors during a part of each input cycle. 
 The thyristor switch is turned on for a part of every half cycle, so that input supply 
voltage appears across the load and then turned off during the remaining part of input half cycle 
to disconnect the ac supply from the load. 
 By controlling the phase angle or the trigger angle ‘ ?’ (delay angle), the output RMS 
voltage across the load can be controlled. 
 The trigger delay angle ‘ ?’ is defined as the phase angle (the value of ?t) at which the 
thyristor turns on and the load current begins to flow. 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER (AC REGULATOR) OR 
RMS VOLTAGE CONTROLLER WITH RESISTIVE LOAD 
A single phase full wave ac voltage controller with a resistive load is shown in the figure below. 
It is possible to control the ac power flow to the load in both the half cycles by adjusting the 
trigger angle ' ' ? . Hence the full wave ac voltage controller is also referred to as to a 
bidirectional controller. 
Circuit Diagram: 
 
 
OPERATION 
Mode-1- From 0 to p 
The thyristor T1 is forward biased during the positive half cycle of the input 
supply voltage. The thyristor T1 is triggered at a delay angle of ?(0 ? ? ? ? ? . Considering 
the ON thyristor T1 as an ideal closed switch the input supply voltage appears across the 
load resistor RL and the output voltage vo=vs. 
During ?t ? ? to ? radians. The load current flows through the ON thyristor T1 
and through the load resistor RL in the downward direction during the conduction time of 
T1 from ? ? t ? to ? radians 
Mode-2- From 0 to p 
At ?t ? ? , when the input voltage falls to zero the thyristor current (which is 
flowing through the load resistor R L ) falls to zero and hence T1 naturally turns off . No 
current flows in the circuit during ?t ? ? to ? ? ? ? ?. The thyristor T2 is forward biased 
Page 4


EE6503 POWER EELCTRONICS 
AC VOLTAGE CONTROLLERS  
1. Introduction. 
 AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value 
of the alternating voltage applied to a load circuit.An ac voltage controller is a type of thyristor 
power converter which is used to convert a fixed voltage, fixed frequency ac input supply to 
obtain a variable voltage ac output. The RMS value of the ac output voltage and the ac power 
flow to the load is controlled by varying (adjusting) the trigger angle ‘ ?’ 
 
 
 
Fig.1 
 
2. TYPE OF AC VOLTAGE CONTROLLERS 
 The ac voltage controllers are classified into two types based on the type of input ac 
supply applied to the circuit. 
? Single Phase AC Controllers. 
? Three Phase AC Controllers. 
Single phase ac controllers operate with single phase ac supply voltage of 230V RMS at 
50Hz in our country. Three phase ac controllers operate with 3 phase ac supply of 400V RMS at 
50Hz supply frequency. 
Each type of controller may be sub divided into  
? Uni-directional or half wave ac controller. 
? Bi-directional or full wave ac controller. 
In brief different types of ac voltage controllers are 
? Single phase half wave ac voltage controller (uni-directional controller). 
? Single phase full wave ac voltage controller (bi-directional controller). 
AC
V oltage
Controller
V
0(R MS )
f
S
V ariable AC 
R MS O/P V oltage
AC
Input
V oltage
f
s
V
s
f
s
? Three phase half wave ac voltage controller (uni-directional controller). 
? Three phase full wave ac voltage controller (bi-directional controller). 
3. APPLICATIONS OF AC VOLTAGE CONTROLLERS 
? Lighting / Illumination control in ac power circuits. 
? Induction heating. 
? Industrial heating & Domestic heating. 
? Transformers tap changing (on load transformer tap changing). 
? Speed control of induction motors (single phase and poly phase ac induction 
motor control). 
? AC magnet controls. 
 
4. AC VOLTAGE CONTROL TECHNIQUES 
There are two different types of thyristor control used in practice to control the ac power flow 
 
1. Phase control 
2. On-Off control 
 
These are the two ac output voltage control techniques. 
In On-Off control technique Thyristors are used as switches to connect the load circuit to the 
ac supply (source) for a few cycles of the input ac supply and then to disconnect it for few input 
cycles. The Thyristors thus act as a high speed contactor (or high speed ac switch). 
 
4.1 PHASE CONTROL TECHNIQUE 
 
 In phase control the Thyristors are used as switches to connect the load circuit to the 
input ac supply, for a part of every input cycle. That is the ac supply voltage is chopped using 
Thyristors during a part of each input cycle. 
 The thyristor switch is turned on for a part of every half cycle, so that input supply 
voltage appears across the load and then turned off during the remaining part of input half cycle 
to disconnect the ac supply from the load. 
 By controlling the phase angle or the trigger angle ‘ ?’ (delay angle), the output RMS 
voltage across the load can be controlled. 
 The trigger delay angle ‘ ?’ is defined as the phase angle (the value of ?t) at which the 
thyristor turns on and the load current begins to flow. 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER (AC REGULATOR) OR 
RMS VOLTAGE CONTROLLER WITH RESISTIVE LOAD 
A single phase full wave ac voltage controller with a resistive load is shown in the figure below. 
It is possible to control the ac power flow to the load in both the half cycles by adjusting the 
trigger angle ' ' ? . Hence the full wave ac voltage controller is also referred to as to a 
bidirectional controller. 
Circuit Diagram: 
 
 
OPERATION 
Mode-1- From 0 to p 
The thyristor T1 is forward biased during the positive half cycle of the input 
supply voltage. The thyristor T1 is triggered at a delay angle of ?(0 ? ? ? ? ? . Considering 
the ON thyristor T1 as an ideal closed switch the input supply voltage appears across the 
load resistor RL and the output voltage vo=vs. 
During ?t ? ? to ? radians. The load current flows through the ON thyristor T1 
and through the load resistor RL in the downward direction during the conduction time of 
T1 from ? ? t ? to ? radians 
Mode-2- From 0 to p 
At ?t ? ? , when the input voltage falls to zero the thyristor current (which is 
flowing through the load resistor R L ) falls to zero and hence T1 naturally turns off . No 
current flows in the circuit during ?t ? ? to ? ? ? ? ?. The thyristor T2 is forward biased 
during the negative cycle of input supply and when thyristor T2 is triggered at a delay 
angle ? ? ? ? ?, the output voltage follows the negative halfcycle of input from ?t ? ? ? ? ?)  
to 2 ? . When T2 is ON, the load current flows in the reverse direction (upward direction) 
through T2 during ? t ? ? ? ? ? ? to 2 ? radians. The time interval (spacing) between the gate 
trigger pulses of T1 and T2 is kept at ? radians or 1800 . At ? t ?2 ?,  the input supply 
voltage falls to zero and hence the load current also falls to zero and thyristor T2 turn off 
naturally. 
Instead of using two SCR’s in parallel, a Triac can be used for full wave ac voltage 
controlFor applications up to 400Hz,. 
 
Phase control Thyristors which are relatively inexpensive, converter grade Thyristors 
which are slower than fast switching inverter grade Thyristors are normally used.  
 Due to ac line commutation or natural commutation, there is no need of extra 
commutation circuitry or components and the circuits for ac voltage controllers are very simple. 
WAVEFORMS 
Page 5


EE6503 POWER EELCTRONICS 
AC VOLTAGE CONTROLLERS  
1. Introduction. 
 AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value 
of the alternating voltage applied to a load circuit.An ac voltage controller is a type of thyristor 
power converter which is used to convert a fixed voltage, fixed frequency ac input supply to 
obtain a variable voltage ac output. The RMS value of the ac output voltage and the ac power 
flow to the load is controlled by varying (adjusting) the trigger angle ‘ ?’ 
 
 
 
Fig.1 
 
2. TYPE OF AC VOLTAGE CONTROLLERS 
 The ac voltage controllers are classified into two types based on the type of input ac 
supply applied to the circuit. 
? Single Phase AC Controllers. 
? Three Phase AC Controllers. 
Single phase ac controllers operate with single phase ac supply voltage of 230V RMS at 
50Hz in our country. Three phase ac controllers operate with 3 phase ac supply of 400V RMS at 
50Hz supply frequency. 
Each type of controller may be sub divided into  
? Uni-directional or half wave ac controller. 
? Bi-directional or full wave ac controller. 
In brief different types of ac voltage controllers are 
? Single phase half wave ac voltage controller (uni-directional controller). 
? Single phase full wave ac voltage controller (bi-directional controller). 
AC
V oltage
Controller
V
0(R MS )
f
S
V ariable AC 
R MS O/P V oltage
AC
Input
V oltage
f
s
V
s
f
s
? Three phase half wave ac voltage controller (uni-directional controller). 
? Three phase full wave ac voltage controller (bi-directional controller). 
3. APPLICATIONS OF AC VOLTAGE CONTROLLERS 
? Lighting / Illumination control in ac power circuits. 
? Induction heating. 
? Industrial heating & Domestic heating. 
? Transformers tap changing (on load transformer tap changing). 
? Speed control of induction motors (single phase and poly phase ac induction 
motor control). 
? AC magnet controls. 
 
4. AC VOLTAGE CONTROL TECHNIQUES 
There are two different types of thyristor control used in practice to control the ac power flow 
 
1. Phase control 
2. On-Off control 
 
These are the two ac output voltage control techniques. 
In On-Off control technique Thyristors are used as switches to connect the load circuit to the 
ac supply (source) for a few cycles of the input ac supply and then to disconnect it for few input 
cycles. The Thyristors thus act as a high speed contactor (or high speed ac switch). 
 
4.1 PHASE CONTROL TECHNIQUE 
 
 In phase control the Thyristors are used as switches to connect the load circuit to the 
input ac supply, for a part of every input cycle. That is the ac supply voltage is chopped using 
Thyristors during a part of each input cycle. 
 The thyristor switch is turned on for a part of every half cycle, so that input supply 
voltage appears across the load and then turned off during the remaining part of input half cycle 
to disconnect the ac supply from the load. 
 By controlling the phase angle or the trigger angle ‘ ?’ (delay angle), the output RMS 
voltage across the load can be controlled. 
 The trigger delay angle ‘ ?’ is defined as the phase angle (the value of ?t) at which the 
thyristor turns on and the load current begins to flow. 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER (AC REGULATOR) OR 
RMS VOLTAGE CONTROLLER WITH RESISTIVE LOAD 
A single phase full wave ac voltage controller with a resistive load is shown in the figure below. 
It is possible to control the ac power flow to the load in both the half cycles by adjusting the 
trigger angle ' ' ? . Hence the full wave ac voltage controller is also referred to as to a 
bidirectional controller. 
Circuit Diagram: 
 
 
OPERATION 
Mode-1- From 0 to p 
The thyristor T1 is forward biased during the positive half cycle of the input 
supply voltage. The thyristor T1 is triggered at a delay angle of ?(0 ? ? ? ? ? . Considering 
the ON thyristor T1 as an ideal closed switch the input supply voltage appears across the 
load resistor RL and the output voltage vo=vs. 
During ?t ? ? to ? radians. The load current flows through the ON thyristor T1 
and through the load resistor RL in the downward direction during the conduction time of 
T1 from ? ? t ? to ? radians 
Mode-2- From 0 to p 
At ?t ? ? , when the input voltage falls to zero the thyristor current (which is 
flowing through the load resistor R L ) falls to zero and hence T1 naturally turns off . No 
current flows in the circuit during ?t ? ? to ? ? ? ? ?. The thyristor T2 is forward biased 
during the negative cycle of input supply and when thyristor T2 is triggered at a delay 
angle ? ? ? ? ?, the output voltage follows the negative halfcycle of input from ?t ? ? ? ? ?)  
to 2 ? . When T2 is ON, the load current flows in the reverse direction (upward direction) 
through T2 during ? t ? ? ? ? ? ? to 2 ? radians. The time interval (spacing) between the gate 
trigger pulses of T1 and T2 is kept at ? radians or 1800 . At ? t ?2 ?,  the input supply 
voltage falls to zero and hence the load current also falls to zero and thyristor T2 turn off 
naturally. 
Instead of using two SCR’s in parallel, a Triac can be used for full wave ac voltage 
controlFor applications up to 400Hz,. 
 
Phase control Thyristors which are relatively inexpensive, converter grade Thyristors 
which are slower than fast switching inverter grade Thyristors are normally used.  
 Due to ac line commutation or natural commutation, there is no need of extra 
commutation circuitry or components and the circuits for ac voltage controllers are very simple. 
WAVEFORMS 
  
 
 
SINGLE PHASE FULL WAVE AC VOLTAGE CONTROLLER (BIDIRECTIONAL 
CONTROLLER) WITH RL LOAD 
if we consider a single phase full wave ac voltage controller controlling the speed of a single 
phase ac induction motor, the load which is the induction motor winding is an RL type of load, 
where R represents the motor winding resistance 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 in parallel is shown below. 
 
 
The thyristor T1 is forward biased during the positive half cycle of input supply. Let us 
assume that T1 is triggered at ? ?t ? , by applying a suitable gate trigger pulse to T1 during the 
positive half cycle of input supply. The output voltage across the load follows the input supply 
voltage when T1 is ON. The load current io flows through the thyristor T1 and through the load