RC Phase Shift Control

• 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.

ab = Voltage drop across transformer secondary winding

bd = Voltage drop across capacitor

ad = Voltage drop across resistor

As the center 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.

RC PHASE SHIFT CONTROL FIGURE. PDF

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