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 = 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.
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
= 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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