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,
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