SCR Firing angle control by RC phase shift control
The firing angle control of SCR is done by RC phase shift control. The working of RL phase shift control is explained here.
RC phase shift control: Working
Figure
A shows circuit for the RC phase shift control. It consists of one step
down transformer which
consists of center taped secondary
winding. The RC circuit is connected in series with transformer secondary
winding. The voltage drop
across R is ad = IR and voltage drop across capacitor is bd = IXC. 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 RC phase shift circuit.
RC phase shift control circuit diagram
ab = Voltage drop across transformer secondary
winding
bd = Voltage drop across capacitor
ad = Voltage drop across resistor
As the centre tapping transformer ac = cb
tan α = ad / bd
= IR / IXC
α =
tan-1 ( ωRC )
As ac = cb, the angle Ф = 2α
Ф = 2 tan-1 ( ωRC )
As the value of resistance increases, the voltage
drop across R increases resulting Ф increases.
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