Test: SCR Commutation Techniques
20 Questions MCQ Test Power Electronics | Test: SCR Commutation Techniques
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The thyristor turn-off requires that the anode current
For effective turn-off of the SCR the anode current must fall below the holding current value.
In case of class A type commutation or load commutation with low value of R load the
In case of Class A commutation the requirement is that the circuit should be an under-damped RLC circuit.
The class A commutation or load commutation is possible in case of
The nature of the circuit should be such that when energized from the source, current must a a natural tendency to decay to zero for load commutation to occur in a SCR circuit.
In case of class B commutation or resonant-pulse commutation with L = 5 μH and C = 20 μC with initial voltage across the capacitor (Vs) = 230 V. Find the peak value of resonant current.
Ip = Vs x √C/L.
In case of class B commutation or resonant-pulse commutation with L = 5 μH and C = 20 μC with the initial voltage across the capacitor (Vs) = 230 V. Find the conduction time for auxiliary thyristor.
Ip = Vs x √C/L
ω = 1/√LC
t = π/ω.
An SCR is connected in series with L = 5 mH and C = 20 μF. Find the resonant frequency of the circuit.
ω = 1/√LC.
The type of commutation when the load is commutated by transferring its load current to another incoming thyristor is
In the Class C type commutation also called as complementary commutation the load is commutated by transferring the current th another device.
The type of commutation in which the pulse to turn off the SCR is obtained by separate voltage source is
In class E commutation, another voltage source is used. It is also called as external pulse commutation.
The natural reversal of ac supply voltage commutates the SCR in case of
Both line and natural commuataion are used in converters.
___________ commutation technique is commonly employed in series inverters.
Load commuation is used in inverter in which L and C are connected in series with the load or C in parallel with the load such that overall load circuit is under damped.
For the circuit shown in the figure below,
C = 4 pF
L = 16 μH
Vs = 200V
The capacitor voltage after the SCR is self commutated is
It is simply the negative of Vs as it is a series circuit & after turn off the whole voltage appears across the thyristor.
For the circuit shown below,
The maximum value of current through thyristors T1 & TA can be given by
I1 = (Vs/R)
T2 = (Vs√C/L).
For the circuit shown in the figure below,
C = 4 pF
L = 16 μH
Vs = 200 V
The peak thyristor current is
The peak SCR current is Vs/2.
For the circuit shown in the figure below, ___ type of commutation would take place for the SCR (THY1)
The given figure is that of a class B commutation or resonant-pulse commutation, which is a type of forced commutation technique.
For the circuit shown in the figure below
R1 = 50 Ω, R2 = 100 Ω, Vs = 100 V. At the time of turn-on of the SCR, initial thyristor current is
Io = Vs x [(2/R1)+(1/R2)].
Anode current (load current) becomes zero and turns off the device, hence the name line commutation.
___________ commutation is usually used in phase-controlled rectifiers
Line commuataion is used in converters.
Parallel capacitor is another name for forced commutation.
As an external source is used it is a external-pulse commutation technique.
Below is the circuit of a resonant-pulse commutation, the maximum voltage to which the capacitor C charges is
The capacitor charges from 0 to Vs with the right side plate positive.
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