Detailed Notes: Phase Controlled Rectifiers - 1 | Power Electronics - Electrical Engineering (EE) PDF Download

 Page 1


  
 
  
PHASE CONTROLLED CONVERTERS SINGLE PHASE 
(RMS VOLTAGE CONTROLLERS) 
 
AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value 
of the alternating voltage applied to a load circuit by introducing Thyristors between the load and 
a constant voltage ac source. The RMS value of alternating voltage applied to a load circuit is 
controlled by controlling the triggering angle of the Thyristors in the ac voltage controller 
circuits. 
 
In brief, 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 ‘a 
 
 
There are two different types of thyristor control used in practice to control the acpower flow 
• On-Off control 
• Phase control 
These are the two ac output voltage control techniques. 
In On-Off control technique Thyristors are used as switches to connect the loadcircuit to the ac 
supply (source) for a few cycles of the input ac supply and then todisconnect it for few input 
cycles. The Thyristors thus act as a high speed contactor(or high speed ac switch). 
PHASE CONTROL 
In phase control the Thyristors are used as switches to connect the load circuitto the input ac 
supply, for a part of every input cycle. That is the ac supply voltage ischopped using Thyristors 
Page 2


  
 
  
PHASE CONTROLLED CONVERTERS SINGLE PHASE 
(RMS VOLTAGE CONTROLLERS) 
 
AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value 
of the alternating voltage applied to a load circuit by introducing Thyristors between the load and 
a constant voltage ac source. The RMS value of alternating voltage applied to a load circuit is 
controlled by controlling the triggering angle of the Thyristors in the ac voltage controller 
circuits. 
 
In brief, 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 ‘a 
 
 
There are two different types of thyristor control used in practice to control the acpower flow 
• On-Off control 
• Phase control 
These are the two ac output voltage control techniques. 
In On-Off control technique Thyristors are used as switches to connect the loadcircuit to the ac 
supply (source) for a few cycles of the input ac supply and then todisconnect it for few input 
cycles. The Thyristors thus act as a high speed contactor(or high speed ac switch). 
PHASE CONTROL 
In phase control the Thyristors are used as switches to connect the load circuitto the input ac 
supply, for a part of every input cycle. That is the ac supply voltage ischopped using Thyristors 
  
 
during a part of each input cycle.The thyristor switch is turned on for a part of every half cycle, 
so that inputsupply 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 ‘a’ (delay angle), theoutput RMS voltage across the load can be controlled. 
The trigger delay angle ‘a’ is defined as the phase angle (the value of ?t) atwhich the thyristor 
turns on and the load current begins to flow.Thyristor ac voltage controllers use ac line 
commutation or ac phasecommutation. Thyristors in ac voltage controllers are line commutated 
(phasecommutated) since the input supply is ac. When the input ac voltage reverses andbecomes 
negative during the negative half cycle the current flowing through theconducting thyristor 
decreases and falls to zero. Thus the ON thyristor naturally turnsoff, when the device current 
falls to zero.Phase control Thyristors which are relatively inexpensive, converter grade 
Thyristors which are slower than fast switching inverter grade Thyristors are normallyused.For 
applications upto 400Hz, if Triacs are available to meet the voltage andcurrent ratings of a 
particular application, Triacs are more commonly used.Due to ac line commutation or natural 
commutation, there is no need of extracommutation circuitry or components and the circuits for 
ac voltage controllers arevery simple. 
Due to the nature of the output waveforms, the analysis, derivations ofexpressions for 
performance parameters are not simple, especially for the phasecontrolled ac voltage controllers 
with RL load. But however most of the practicalloads are of the RL type and hence RL load 
should be considered in the analysis anddesign of ac voltage controller circuits. 
TYPE OF AC VOLTAGE CONTROLLERS 
The ac voltage controllers are classified into two types based on the type ofinput 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 of230V 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. 
Page 3


  
 
  
PHASE CONTROLLED CONVERTERS SINGLE PHASE 
(RMS VOLTAGE CONTROLLERS) 
 
AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value 
of the alternating voltage applied to a load circuit by introducing Thyristors between the load and 
a constant voltage ac source. The RMS value of alternating voltage applied to a load circuit is 
controlled by controlling the triggering angle of the Thyristors in the ac voltage controller 
circuits. 
 
In brief, 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 ‘a 
 
 
There are two different types of thyristor control used in practice to control the acpower flow 
• On-Off control 
• Phase control 
These are the two ac output voltage control techniques. 
In On-Off control technique Thyristors are used as switches to connect the loadcircuit to the ac 
supply (source) for a few cycles of the input ac supply and then todisconnect it for few input 
cycles. The Thyristors thus act as a high speed contactor(or high speed ac switch). 
PHASE CONTROL 
In phase control the Thyristors are used as switches to connect the load circuitto the input ac 
supply, for a part of every input cycle. That is the ac supply voltage ischopped using Thyristors 
  
 
during a part of each input cycle.The thyristor switch is turned on for a part of every half cycle, 
so that inputsupply 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 ‘a’ (delay angle), theoutput RMS voltage across the load can be controlled. 
The trigger delay angle ‘a’ is defined as the phase angle (the value of ?t) atwhich the thyristor 
turns on and the load current begins to flow.Thyristor ac voltage controllers use ac line 
commutation or ac phasecommutation. Thyristors in ac voltage controllers are line commutated 
(phasecommutated) since the input supply is ac. When the input ac voltage reverses andbecomes 
negative during the negative half cycle the current flowing through theconducting thyristor 
decreases and falls to zero. Thus the ON thyristor naturally turnsoff, when the device current 
falls to zero.Phase control Thyristors which are relatively inexpensive, converter grade 
Thyristors which are slower than fast switching inverter grade Thyristors are normallyused.For 
applications upto 400Hz, if Triacs are available to meet the voltage andcurrent ratings of a 
particular application, Triacs are more commonly used.Due to ac line commutation or natural 
commutation, there is no need of extracommutation circuitry or components and the circuits for 
ac voltage controllers arevery simple. 
Due to the nature of the output waveforms, the analysis, derivations ofexpressions for 
performance parameters are not simple, especially for the phasecontrolled ac voltage controllers 
with RL load. But however most of the practicalloads are of the RL type and hence RL load 
should be considered in the analysis anddesign of ac voltage controller circuits. 
TYPE OF AC VOLTAGE CONTROLLERS 
The ac voltage controllers are classified into two types based on the type ofinput 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 of230V 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-directionalcontroller). 
• Single phase full wave ac voltage controller (bi-directional controller). 
• Three phase half wave ac voltage controller (uni-directionalcontroller). 
• Three phase full wave ac voltage controller (bi-directional controller). 
APPLICATIONS OF AC VOLTAGE CONTROLLERS 
• Lighting / Illumination control in ac power circuits. 
• Induction heating. 
• Industrial heating & Domestic heating. 
• Transformer tap changing (on load transformer tap changing). 
• Speed control of induction motors (single phase and poly phase ac induction 
motor control). 
• AC magnet controls. 
PRINCIPLE OF ON-OFF CONTROL TECHNIQUE (INTEGRAL CYCLECONTROL) 
The basic principle of on-off control technique is explained with reference to asingle phase full 
wave ac voltage controller circuit shown below. The thyristorswitches T1 and T2 are turned on 
by applying appropriate gate trigger pulses toconnect the input ac supply to the load for ‘n’ 
number of input cycles during the timeinterval T ON  . The thyristor switches T1 and T2 are 
turned off by blocking the gatetrigger pulses for ‘m’ number of input cycles during the time 
interval  T OFF  . The accontroller ON time ON t usually consists of an integral number of input 
cycles. 
 
Page 4


  
 
  
PHASE CONTROLLED CONVERTERS SINGLE PHASE 
(RMS VOLTAGE CONTROLLERS) 
 
AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value 
of the alternating voltage applied to a load circuit by introducing Thyristors between the load and 
a constant voltage ac source. The RMS value of alternating voltage applied to a load circuit is 
controlled by controlling the triggering angle of the Thyristors in the ac voltage controller 
circuits. 
 
In brief, 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 ‘a 
 
 
There are two different types of thyristor control used in practice to control the acpower flow 
• On-Off control 
• Phase control 
These are the two ac output voltage control techniques. 
In On-Off control technique Thyristors are used as switches to connect the loadcircuit to the ac 
supply (source) for a few cycles of the input ac supply and then todisconnect it for few input 
cycles. The Thyristors thus act as a high speed contactor(or high speed ac switch). 
PHASE CONTROL 
In phase control the Thyristors are used as switches to connect the load circuitto the input ac 
supply, for a part of every input cycle. That is the ac supply voltage ischopped using Thyristors 
  
 
during a part of each input cycle.The thyristor switch is turned on for a part of every half cycle, 
so that inputsupply 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 ‘a’ (delay angle), theoutput RMS voltage across the load can be controlled. 
The trigger delay angle ‘a’ is defined as the phase angle (the value of ?t) atwhich the thyristor 
turns on and the load current begins to flow.Thyristor ac voltage controllers use ac line 
commutation or ac phasecommutation. Thyristors in ac voltage controllers are line commutated 
(phasecommutated) since the input supply is ac. When the input ac voltage reverses andbecomes 
negative during the negative half cycle the current flowing through theconducting thyristor 
decreases and falls to zero. Thus the ON thyristor naturally turnsoff, when the device current 
falls to zero.Phase control Thyristors which are relatively inexpensive, converter grade 
Thyristors which are slower than fast switching inverter grade Thyristors are normallyused.For 
applications upto 400Hz, if Triacs are available to meet the voltage andcurrent ratings of a 
particular application, Triacs are more commonly used.Due to ac line commutation or natural 
commutation, there is no need of extracommutation circuitry or components and the circuits for 
ac voltage controllers arevery simple. 
Due to the nature of the output waveforms, the analysis, derivations ofexpressions for 
performance parameters are not simple, especially for the phasecontrolled ac voltage controllers 
with RL load. But however most of the practicalloads are of the RL type and hence RL load 
should be considered in the analysis anddesign of ac voltage controller circuits. 
TYPE OF AC VOLTAGE CONTROLLERS 
The ac voltage controllers are classified into two types based on the type ofinput 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 of230V 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-directionalcontroller). 
• Single phase full wave ac voltage controller (bi-directional controller). 
• Three phase half wave ac voltage controller (uni-directionalcontroller). 
• Three phase full wave ac voltage controller (bi-directional controller). 
APPLICATIONS OF AC VOLTAGE CONTROLLERS 
• Lighting / Illumination control in ac power circuits. 
• Induction heating. 
• Industrial heating & Domestic heating. 
• Transformer tap changing (on load transformer tap changing). 
• Speed control of induction motors (single phase and poly phase ac induction 
motor control). 
• AC magnet controls. 
PRINCIPLE OF ON-OFF CONTROL TECHNIQUE (INTEGRAL CYCLECONTROL) 
The basic principle of on-off control technique is explained with reference to asingle phase full 
wave ac voltage controller circuit shown below. The thyristorswitches T1 and T2 are turned on 
by applying appropriate gate trigger pulses toconnect the input ac supply to the load for ‘n’ 
number of input cycles during the timeinterval T ON  . The thyristor switches T1 and T2 are 
turned off by blocking the gatetrigger pulses for ‘m’ number of input cycles during the time 
interval  T OFF  . The accontroller ON time ON t usually consists of an integral number of input 
cycles. 
 
  
 
Fig.1: Single phase full wave AC voltage controller circuit 
 
Fig.2: Waveforms 
Example 
Referring to the waveforms of ON-OFF control technique in the above diagram, 
n = Two input cycles. Thyristors are turned ON during ON t for two inputcycles. 
m = One input cycle. Thyristors are turned OFF during OFF t for one inputcycle 
 
Page 5


  
 
  
PHASE CONTROLLED CONVERTERS SINGLE PHASE 
(RMS VOLTAGE CONTROLLERS) 
 
AC voltage controllers (ac line voltage controllers) are employed to vary the RMS value 
of the alternating voltage applied to a load circuit by introducing Thyristors between the load and 
a constant voltage ac source. The RMS value of alternating voltage applied to a load circuit is 
controlled by controlling the triggering angle of the Thyristors in the ac voltage controller 
circuits. 
 
In brief, 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 ‘a 
 
 
There are two different types of thyristor control used in practice to control the acpower flow 
• On-Off control 
• Phase control 
These are the two ac output voltage control techniques. 
In On-Off control technique Thyristors are used as switches to connect the loadcircuit to the ac 
supply (source) for a few cycles of the input ac supply and then todisconnect it for few input 
cycles. The Thyristors thus act as a high speed contactor(or high speed ac switch). 
PHASE CONTROL 
In phase control the Thyristors are used as switches to connect the load circuitto the input ac 
supply, for a part of every input cycle. That is the ac supply voltage ischopped using Thyristors 
  
 
during a part of each input cycle.The thyristor switch is turned on for a part of every half cycle, 
so that inputsupply 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 ‘a’ (delay angle), theoutput RMS voltage across the load can be controlled. 
The trigger delay angle ‘a’ is defined as the phase angle (the value of ?t) atwhich the thyristor 
turns on and the load current begins to flow.Thyristor ac voltage controllers use ac line 
commutation or ac phasecommutation. Thyristors in ac voltage controllers are line commutated 
(phasecommutated) since the input supply is ac. When the input ac voltage reverses andbecomes 
negative during the negative half cycle the current flowing through theconducting thyristor 
decreases and falls to zero. Thus the ON thyristor naturally turnsoff, when the device current 
falls to zero.Phase control Thyristors which are relatively inexpensive, converter grade 
Thyristors which are slower than fast switching inverter grade Thyristors are normallyused.For 
applications upto 400Hz, if Triacs are available to meet the voltage andcurrent ratings of a 
particular application, Triacs are more commonly used.Due to ac line commutation or natural 
commutation, there is no need of extracommutation circuitry or components and the circuits for 
ac voltage controllers arevery simple. 
Due to the nature of the output waveforms, the analysis, derivations ofexpressions for 
performance parameters are not simple, especially for the phasecontrolled ac voltage controllers 
with RL load. But however most of the practicalloads are of the RL type and hence RL load 
should be considered in the analysis anddesign of ac voltage controller circuits. 
TYPE OF AC VOLTAGE CONTROLLERS 
The ac voltage controllers are classified into two types based on the type ofinput 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 of230V 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-directionalcontroller). 
• Single phase full wave ac voltage controller (bi-directional controller). 
• Three phase half wave ac voltage controller (uni-directionalcontroller). 
• Three phase full wave ac voltage controller (bi-directional controller). 
APPLICATIONS OF AC VOLTAGE CONTROLLERS 
• Lighting / Illumination control in ac power circuits. 
• Induction heating. 
• Industrial heating & Domestic heating. 
• Transformer tap changing (on load transformer tap changing). 
• Speed control of induction motors (single phase and poly phase ac induction 
motor control). 
• AC magnet controls. 
PRINCIPLE OF ON-OFF CONTROL TECHNIQUE (INTEGRAL CYCLECONTROL) 
The basic principle of on-off control technique is explained with reference to asingle phase full 
wave ac voltage controller circuit shown below. The thyristorswitches T1 and T2 are turned on 
by applying appropriate gate trigger pulses toconnect the input ac supply to the load for ‘n’ 
number of input cycles during the timeinterval T ON  . The thyristor switches T1 and T2 are 
turned off by blocking the gatetrigger pulses for ‘m’ number of input cycles during the time 
interval  T OFF  . The accontroller ON time ON t usually consists of an integral number of input 
cycles. 
 
  
 
Fig.1: Single phase full wave AC voltage controller circuit 
 
Fig.2: Waveforms 
Example 
Referring to the waveforms of ON-OFF control technique in the above diagram, 
n = Two input cycles. Thyristors are turned ON during ON t for two inputcycles. 
m = One input cycle. Thyristors are turned OFF during OFF t for one inputcycle 
 
  
 
Fig.3: Power Factor 
Thyristors are turned ON precisely at the zero voltage crossings of the inputsupply. The thyristor 
T1is turned on at the beginning of each positive half cycle byapplying the gate trigger pulses to 
T1 as shown, during the ON time ON t . The loadcurrent flows in the positive direction, which is 
the downward direction as shown inthe circuit diagram when T1 conducts. The thyristor T2 is 
turned on at the beginningof each negative half cycle, by applying gating signal to the gate of T2 
, during TON  .The load current flows in the reverse direction, which is the upward direction 
whenT2 conducts. Thus we obtain a bi-directional load current flow (alternating loadcurrent 
flow) in a ac voltage controller circuit, by triggering the thyristors alternately.This type of control 
is used in applications which have high mechanical inertiaand high thermal time constant 
(Industrial heating and speed control of ac motors).Due to zero voltage and zero current 
switching of Thyristors, the harmonics generatedby switching actions are reduced. 
For a sine wave input supply voltage,