12 March 2017

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 = IXC
  • 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 / IXC
       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

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