RC Phase Shift Control

SCR Firing angle control by RC phase shift control

The firing angle control of SCR is done by RC phase shift control. The working of RL phase shift control is explained here.

RC phase shift control: Working

Figure A shows circuit for the RC phase shift control. It consists of one step down transformer which consists of center taped secondary winding. The RC circuit is connected in series with transformer secondary winding. The voltage drop across R is ad = IR and voltage drop across capacitor is bd = IX_C. The voltage drop across cd is given to the gate – cathode terminal of the SCR. There should be phase difference Ф between anode voltage and gate voltage.  As the variable resistance changes, the voltage drop across bd changes. Figure B shows vector diagram for RC phase shift circuit.

RC phase shift control circuit diagram

ab = Voltage drop across transformer secondary winding

bd = Voltage drop across capacitor

ad = Voltage drop across resistor

As the centre tapping transformer ac = cb

  tan α = ad / bd

= IR / IX_C

       α = tan^-1 ( ωRC )

As ac = cb, the angle Ф = 2α

      Ф = 2 tan^-1 ( ωRC )

As the value of resistance increases, the voltage drop across R increases resulting Ф increases.

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