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## 12/03/2017

### 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 = 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.
• 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
= IXL / 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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