RL Phase Shift Control

• Figure A shows circuit for the RL phase shift control. 
• It consists of one step down transformer which consists of center taped secondary winding. 
• The RL circuit is connected in series with transformer secondary winding. 
• The voltage drop across R is bd = IR and voltage drop across inductor is ad = IX_L. 
• 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 RL phase shift circuit.

ab = Voltage drop across transformer secondary winding

bd = Voltage drop across resistance

ad = Voltage drop across inductor

As the centre tapping transformer ac = cb

 tan α = ad / bd

          = IX_L / IR

       α = tan^-1 ( ωL / R )

As ac = cb, the angle Ф = 2α

     Ф = 2 tan^-1 ( ωL / R )

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

RL PHASE SHIFT CONTROL FIGURE.PDF

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