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RL Phase Shift Control

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Circuit Description

  • 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.

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  • The voltage drop across R is bd = IR and voltage drop across inductor is ad = IXL.
  • 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.

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  • 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

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  • As the center tapping transformer ac = cb

tan α = ad / bd

= IXL / IR

α = tan-1 ( ωL / R )

  • As ac = cb, the angle Ф = 2α

Ф = 2 tan-1 ( ωL / R )

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

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Thank You

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