A phase controlled converter converts AC to DC energy (line commutated). In other words, it is used in the conversion of fixed-frequency and fixed-voltage AC power into variable DC voltage output. It is expressed as
Fixed Input − Voltage, frequency and AC power
Variable output − DC voltage output
The AC input voltage that goes into a converter is normally at fixed RMS (root mean square) and fixed frequency. The inclusion of phase-controlled thyristors in the converter ensures that a variable DC output voltage is obtained. This is made possible by altering the phase angle at which the thyristors are triggered. As a result, a pulsating waveform of the load current is obtained.
During the input supply half cycle, the thyristor is in forward bias and is switched ON via the application of sufficient gate pulse (trigger). Current starts to flow once the thyristor has been switched ON, that is, at a point ωt=α to point ωt=β. The moment the load current drops to zero, the thyristor switches OFF as a result of line (natural) commutation.
There are a number of power converters that utilize natural commutation. These include −
AC to DC converters
AC to AC converters
AC voltage controllers
Cycloconverters
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