RL Phase Shift Control

SCR Firing angle control by RL phase shift control

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

RL phase shift control: Working

• 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. 

RL phase shift control circuit diagram

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. 

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