Phase Angle Control | Principle Of Phase Angle Control
In integral cycle control method, the output voltage can e controlled by opening and closing
the switch for one or several half cycles of the ac input voltage.
Whereas in phase controlled switching, the output voltage is controlled by opening and
closing the switch between a cycle.
i.e., switching device is so operated that the source and load gets connected for a part of each half cycle of the input voltage.
The principle of operation of phase control can be explained by considering the circuit
diagram as shown in below figure
Thermistor T is forward biased during positive half cycle, s turned ON at firing angle alpha and will remain ON till wt=π for resistive load.
During the period of thyristor load voltage becomes Vm sin (alpha) and load current becomes Vm sin (alpha) / R as shown in below Figure.
During negative half cycle, from wt = π to 2π, thyristor gets turned OFF and diode
D becomes forward based.
As a result diode D starts conducting during negative half cycle. It can be noted that only
positive cycle can be controlled whereas negative half cycle cannot.
Therefore, this circuit configuration is referred as single phase half wave voltage controller or 1 phase unidirectional voltage controller.
The undesirable de component is introduced in the load and supply circuits because the
positive and negative cycles are not identical for voltage and current waveforms as shown
in upper figure.
the switch for one or several half cycles of the ac input voltage.
 |
| Phase Angle Control | Principle Of Phase Angle Control |
Whereas in phase controlled switching, the output voltage is controlled by opening and
closing the switch between a cycle.
i.e., switching device is so operated that the source and load gets connected for a part of each half cycle of the input voltage.
The principle of operation of phase control can be explained by considering the circuit
diagram as shown in below figure
 |
| Principle of phase angle Control |
Thermistor T is forward biased during positive half cycle, s turned ON at firing angle alpha and will remain ON till wt=π for resistive load.
During the period of thyristor load voltage becomes Vm sin (alpha) and load current becomes Vm sin (alpha) / R as shown in below Figure.
 |
| Waveforms of single phase half wave ac voltage controller |
During negative half cycle, from wt = π to 2π, thyristor gets turned OFF and diode
D becomes forward based.
As a result diode D starts conducting during negative half cycle. It can be noted that only
positive cycle can be controlled whereas negative half cycle cannot.
Therefore, this circuit configuration is referred as single phase half wave voltage controller or 1 phase unidirectional voltage controller.
The undesirable de component is introduced in the load and supply circuits because the
positive and negative cycles are not identical for voltage and current waveforms as shown
in upper figure.
Advantages of phase angle control
Here only one source is required so control is very simple.
Disadvantages of Phase Angle Control
The output voltage waveform is not fully controlled.
Supply current, output current and voltage are in dc component.
Application of Phase Angle Control
Used in lighting and heating application.
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