Controlled rectifiers are line commutated ac to dc power converters which are used to convert a fixed voltage, fixed frequency ac power supply into variable dc output voltage.
Type of input: Fixed voltage, fixed frequency ac power supply.
Type of output: Variable dc output voltage
The input supply fed to a controlled rectifier is ac supply at a fixed rms voltage and at a fixed frequency. We can obtain variable dc output voltage by using controlled rectifiers. By employing phase controlled thyristors in the controlled rectifier circuits we can obtain variable dc output voltage and variable dc (average) output current by varying the trigger angle (phase angle) at which the thyristors are triggered. We obtain a uni-directional and pulsating load current waveform, which has a specific average value.
The thyristors are forward biased during the positive half cycle of input supply and can be turned ON by applying suitable gate trigger pulses at the thyristor gate leads. The thyristor current and the load current begin to flow once the thyristors are triggered (turned ON) say at ωt= α. The load current flows when the thyristors conduct from ωt= α to β . The output voltage across the load follows the input supply voltage through the conducting thyristor. At ωt= β, when the load current falls to zero, the thyristors turn off due to AC line (natural) commutation.
In some bridge controlled rectifier circuits the conducting thyristor turns off, when the other thyristor is (other group of thyristors are) turned ON.
The thyristor remains reverse biased during the negative half cycle of input supply. The type of commutation used in controlled rectifier circuits is referred to AC line commutation or Natural commutation or AC phase commutation.
When the input ac supply voltage reverses and becomes negative during the negative half cycle, the thyristor becomes reverse biased and hence turns off. There are several types of power converters which use ac line commutation. These are referred to as line commutated converters.
Single Phase Controlled Rectifiers are further subdivided into different types
Three Phase Controlled Rectifiers are of different types
The circuit diagram of a single phase fully controlled bridge converter is shown in the figure with a highly inductive load and a dc source in the load circuit so that the load current is continuous and ripple free (constant load current operation).
The fully controlled bridge converter consists of four thyristors T1 ,T2 ,T3 and T4 connected in the form of full wave bridge configuration as shown in the figure. Each thyristor is controlled and turned on by its gating signal and naturally turns off when a reverse voltage appears across it. During the positive half cycle when the upper line of the transformer secondary winding is at a positive potential with respect to the lower end the thyristors T1 and T2 are forward biased during the time interval ωt= 0 to π . The thyristors T1 and T2 are triggered simultaneously , the load is connected to the input supply through the conducting thyristors T1 and T2 . The output voltage across the load follows the input supply voltage and hence output voltage V0=Vm sin ωt . Due to the inductive load T1 and T2 will continue to conduct beyond ωt= π, even though the input voltage becomes negative. T1 and T2 conduct together during the time period α to (π + α ), for a time duration of radians (conduction angle of each thyristor =180º )
During the negative half cycle of input supply voltage for ωt= π to 2π, the thyristors T3 and T4 are forward biased. T3 and T4 are triggered at ωt= (π + α ) . As soon as the thyristors T3 and T4 are triggered a reverse voltage appears across the thyristors T1 and T2 and they naturally turn-off and the load current is transferred from T1 and T2 to the thyristors T3 and T4 . The output voltage across the load follows the supply voltage and V0=-Vm sin ωt during the time period ωt= (π + α ) to (2π + α ) . In the next positive half cycle when T1 and T2 are triggered, T3 and T4 are reverse biased and they turn-off. The figure shows the waveforms of the input supply voltage, the output load voltage, the constant load current with negligible ripple and the input supply current.
During the time period ωt= α to π, the input supply voltage Vs and the input supply current Is are both positive and the power flows from the supply to the load. The converter operates in the rectification mode during ωt= α to π .
During the time period ωt= π to (π + α ) , the input supply voltage Vs is negative and the input supply current Is is positive and there will be reverse power flow from the load circuit to the input supply. The converter operates in the inversion mode during the time period ωt= π to (π + α ) and the load energy is fed back to the input source.
The single phase full converter is extensively used in industrial applications up to about 15kW of output power. Depending on the value of trigger angle α , the average output voltage may be either positive or negative and two quadrant operations is possible.
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