1. ___________ commutation is usually used in phase-controlled rectifiers
a) line
b) load
c) forced
d) external-pulse
Explanation: Line commuataion is used in converters.
2. Parallel-capacitor commutation is
a) line commutation
b) load commutation
c) forced commutation
d) external-pulse commutation
Explanation: Parallel capacitor is another name for forced commutation.
3. Class E commutation is a/an
a) line commutation technique
b) load commutation technique
c) forced commutation technique
d) external-pulse commutation technique
Explanation: As an external source is used it is a external-pulse commutation technique.
4. Below is the circuit of a resonant-pulse commutation, the maximum voltage to which the capacitor C charges is
a) Vs
b) Vs/2
c) 2
d) √2Vs
Explanation: The capacitor charges from 0 to Vs with the right side plate positive.
5. A voltage source Vs = Vm sinωt is connected in series with a resistance R and an SCR. At some firing angle delay of α a positive gate pulse is applied to the SCR which turns on the SCR. Considering ideal conditions, at the instant α the voltage at the resistor terminals Vo
a) falls to zero
b) falls to –Vm sin α
c) rises to Vm sin α
d) rises to Vm sin ωt
Explanation: At the firing angle, the SCR turns on and thus the resistor or load R is connected to the source.
6. The average output voltage is maximum when SCR is triggered at ωt =
a) π
b) 0
c) π/2
d) π/4
Explanation: The sooner the conduction starts higher the average power. Though practically a device cannot be triggered at exactly zero degrees..
7. In the method of phase control, the phase relationship between ___ & ___ is controlled by varying the firing angle
a) supply current, supply voltage
b) end of the load current, end of the load voltage
c) start of the load current, start of the load voltage
d) load current, load voltage
Explanation: By varying the firing angle, the load current and load voltage can be controlled.
8. In a single phase half-wave thyristor circuit with R load & Vs=Vm sinωt, the maximum value of the load current can be given by
a) 2Vm/R
b) Vs/R
c) Vm/2
d) Vs/2
Explanation: Vm is the peak value of the load as well as supply voltage. I = Vm/R.
9. For a single phase thyristor circuit with R load & firing angle α, the conduction angle can be given by
a) π+α
b) 2π+α
c) π-α
d) α
Explanation: SCR is fired at α, hence it conducts from α till it is commutated by the reversal of the AC supply i.e at π radians.
10. A single-phase half-wave thyristor circuit with R load is triggered at an angle of α = 0°. As such, the maximum value of the average output voltage would be given by
Consider Vs = Vm sinωt.
a) Vm
b) 2Vm/π
c) Vm/π
d) Vm/α
Explanation: For the whole cycle of 2π, the R load would be active only for half of the cycle. Hence, Vo = Vm/π.