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Multiple choice question for engineering

Set 1

1. For the below given circuit,
power-electronics-interview-questions-answers-experienced-q1
a) T2 and T3 are gated together
b) T1 and T4 are gated together
c) T1 and T3 are gated together
d) T1 and T2 are gated together

View Answer

Answer: d [Reason:] T1-T2 are gated together at a time and T3-T4 are gated together at a time.

2. With respect to the below given circuit, from ωt = 0 to α, ____ thyristor(s) block the forward voltage
power-electronics-interview-questions-answers-experienced-q2
a) T1 and T2
b) T2
c) T3 and T4
d) T4

View Answer

Answer: a [Reason:] T1 and T2 are not yet gated, hence they block the forward voltage.

3. With respect to the below given circuit, from ωt = 0 to α,
power-electronics-interview-questions-answers-experienced-q2
a) voltage across all the thyristors is zero
b) negative voltage appears across T1 and T2
c) load voltage is Vm/2
d) voltage across T3 and T4 is zero

View Answer

Answer: d [Reason:] T1 and T2 are not yet gated, hence they block the forward voltage where as no voltage exists across T3 and T4.

4. For the below given circuit, when the devices T1 and T2 are conducting from ωt = α till they are naturally commutated at π voltage across the other two pair of devices is
power-electronics-interview-questions-answers-experienced-q1
a) zero
b) positive (forward voltage)
c) negative (reverse voltage)
d) none of the mentioned

View Answer

Answer: c [Reason:] When T1 and T2 are conducting they act like S.C’s and the other pair of devices i.e. T3 and T4 has to take care of the reverse voltage.

5. A single-phase full converter B-2 type connection has a RLE type of motor load connected. The minimum requirement to turn-on the device is
a) α > 30°
b) Vm sinα > E
c) Vm sinα < E
d) α < 30°

View Answer

Answer: b [Reason:] Device will not turn on until the supply voltage exceeds the motor back emf E.

6. For the below shown circuit with continuous load current.
power-electronics-interview-questions-answers-experienced-q2
Each thyristor pair conducts for
a) π radians
b) 2π radians
c) < π radians
d) > π radians

View Answer

Answer: a [Reason:] As the load current is continuous each pair conducts from π to π+α i.e π radians.

7. For the below given circuit
power-electronics-interview-questions-answers-experienced-q2
a) all the devices are gated together
b) T3 and T4 are gated together
c) T3 and T1 are gated together
d) T3 and T2 are gated together

View Answer

Answer: d [Reason:] T1-T2 are gated together at a time and T3-T4 are gated together at a time.

8. For the below shown circuit has dis-continuous load current waveform.
power-electronics-interview-questions-answers-experienced-q1
Each thyristor pair conducts for
a) π radians
b) 2π radians
c) < π radians
d) > π radians

View Answer

Answer: c [Reason:] Load current is discontinues than SCR pair conducts for less than 90° depending on the value of the inductor L.

9. A single-phase full converter with B-2 type of connection has a continuous load current waveform. The thyristor pairs T3, T4 is triggered at ωt =
a) 0
b) α
c) π+α
d) π-α

View Answer

Answer: c [Reason:] First, at α T1-T2 conduct, then after π+α the next pair is excited i.e. T3-T4 and likewise.

10. The PIV for each diode in a single-phase, full converter with B-2 type of controlled rectifier is
Consider supply voltage as Vs = Vm sinωt.
a) 2Vm
b) Vm
c) Vm/2
d) Vm/√2

View Answer

Answer: b [Reason:] The peak inverse voltage handled by each device is Vm in case of B-2 type of configuration.

Set 2

1. A single-phase half-wave rectifier with a FD is supplied by Vs = 240 V, 50 Hz, AC source with a load R = 10 Ω, L = 0.8 mH. The firing angle is so adjusted such that the output voltage obtained is 100 V. Find the firing angle.
a) 62°
b) 102°
c) 31°
d) 47°

View Answer

Answer: c [Reason:] Vo = Vm/2π cos(firing angle).

2. For the below shown circuit, a motor load (RLE) is connected to the SCR through supply Vs. The minimum value of the firing angle to turn on the SCR would be
power-electronics-interview-questions-answers-freshers-q2
a) Sin-1(E/Vm)
b) Sin-1(Vm/E)
c) Cos-1(E/Vm)
d) Cos-1(E/Vm)

View Answer

Answer: a [Reason:] The firing angle should be such that the value of V exceeds E. So SCR turns on at excatlly Vm sinωt = E Therefore ωt = min. angle = Sin-1(E/Vm).

3. Choose the correct statement with respect to the below given circuit.
power-electronics-interview-questions-answers-freshers-q2
a) The load current can never be zero
b) The source current can never be zero
c) The load voltage can never be zero
d) All the given statements are false

View Answer

Answer: c [Reason:] Even if no current is flowing through the load when the SCR is off, voltage = E will exists at the load terminals at all times.

4. By using a freewheeling diode (FD) in a rectifier with RL load, the power consumed by the load
a) increases
b) decreases
c) is not affected
d) decreases to zero

View Answer

Answer: a [Reason:] The FD feeds inductor current again to the load.

5. A 230 V, 50 Hz, one-pulse SCR controlled converter has extinction angle β = 210°. Find the circuit turn-off time
a) 10 m-sec
b) 0
c) 8.3 m-sec
d) 5.4 m-sec

View Answer

Answer: c t = 2π-β/ω Where, π = 90° ω = 2xπx50 β = 210°.

6. A 230V, 50Hz, single-pulse SCR is feeding a RL load with α = 40° and β = 210°. Find the value of average output voltage
a) 54 V
b) 106 V
c) 84 V
d) 32 V

View Answer

Answer: c [Reason:] Vo = Vm/2π x (cosα-cosβ) Where Vm = √2Vs.

7. A single-pulse transformer with secondary voltage of 230 V, 50 Hz, delivers power to bulb of R = 10 Ω through a half-wave controlled rectifier circuit. For α = 60°, find the average current in the bulb
a) 2.5 A
b) 7.7 A
c) 9.6 A
d) 3 A

View Answer

Answer: b Explaantion: First find the average voltage, than Io = Vo/R Vo = (Vm/2π) x (1+cosα).

8. A single-pulse transformer with secondary voltage of 230 V, 50 Hz, delivers power to bulb of R = 10 Ω through a half-wave controlled rectifier circuit. For α = 60° and output AC power of 2127 Watts, find the rectification efficiency
a) 98.6 %
b) 42 %
c) 28 %
d) 19 %

View Answer

Answer: c [Reason:] Vo = (Vm/2π) x (1+cosα) = 77.64 V Pdc = Vo2xR = 602.8 W Rectification efficiency = Pdc/Pac = 28.32 %.

9. Find the PIV of the SCR
power-electronics-interview-questions-answers-freshers-q9
a) Vs
b) 2Vs
c) √2Vs
d) Vs/2

View Answer

Answer: c [Reason:] The maximum negative voltage faced by the SCR is –Vm which is nothing but √2Vs.

10. An SCR rectifier circuit is designed such that the average output voltage is 77.64 V & RMS value of output voltage is 145.873 V. Find the voltage ripple factor.
a) 1.879
b) 0.532
c) 1.590
d) 0.629

View Answer

Answer: c [Reason:] FF = 145.873/77.64 = 1.879 VRF = √(FF2-1) = 1.5908.

Set 3

1. For the following RC triggering circuit with R load and a firing angle of α, the voltage across the R load is zero for
power-electronics-question-bank-q1
a) ωt = 0 to α and ωt = π to 2π+α
b) ωt = 0 to α
c) ωt = π to 2π+α
d) ωt = α to 2π

View Answer

Answer: a [Reason:] The SCR is triggered at α therefore voltage appears across the load from ωt = α to π only as in case of a half wave firing circuit the SCR is naturally commuted at π.

2. For the following circuit, the SCR (thy2) is
power-electronics-question-bank-q1
a) forced commutated at ωt = π
b) forced commutated at ωt = 2π
c) naturally commutated at ωt = π
d) naturally commutated at ωt = 2π

View Answer

Answer: c [Reason:] Circuit is that of a half-wave RC firing circuit where natural commutation takes place at π due to the sinusoidal voltage source.

3. For the following RC triggering circuit with R load and a firing angle of α, the voltage across the SCR (thy2) will be zero for
power-electronics-question-bank-q1
a) ωt = 0 to α & ωt = π to 2π+α
b) ωt = 0 to π
c) ωt = α to 2π
d) ωt = α to π

View Answer

Answer: d [Reason:] The SCR is triggered at α therefore voltage appears across the load from ωt = α to π only as in case of a half wave firing circuit the SCR is naturally commuted at π.

4. In case of an RC full wave firing circuit with R load, the voltage across the load is zero for____________
a) ωt = 0 to α and ωt = π to 2π+α
b) ωt = 0 to α
c) ωt = π to 2π+α
d) ωt = α to 2π

View Answer

Answer: b [Reason:] The SCR is not triggered(turned-on) until α, hence no current flows until α.

5. For an RC full wave firing circuit the empirical formula for calculating the value of RC is
a) RC = 157/ω
b) RC = 157 x ω
c) RC = ω/157
d) RC = 157 x ω2

View Answer

Answer: a [Reason:] RC = 157/ω, Where ω is the angular frequency.

6. Pulse triggering can be only used by the _____________ type of triggering circuit
a) R
b) RC
c) UJT
d) RLC

View Answer

Answer: c [Reason:] R & RC produce prolonged pulses which increases the gate power dissipation. There is no RLC firing circuit.

7. The UJT terminals are
a) E, B1 and B2
b) E1, E2 and B
c) E, G and C
d) G, S and D

View Answer

Answer: a [Reason:] The terminals are Emitter, Base1 & Base 2.

8. In case of the UJT firing circuit, when the UJT turns on
a) the capacitor starts to charge
b) the capacitor starts to discharge
c) the capacitor remains unaffected
d) there is no capacitor in a UJT firing circuit

View Answer

Answer: b [Reason:] When the UJT is fired, C starts to discharge.

9. In the UJT firing circuit, the pulses are generated while the
a) capacitor charges
b) capacitor discharges
c) capacitor voltage is zero
d) there is no capacitor in a UJT firing circuit

View Answer

Answer: b Answer: When the C starts to discharge through the resistance with time constant RC the UJT turns on & hence pulses are generated.

10. Find the value of the charging resistor in case of a UJT firing circuit with firing frequency of 2 kHz, C = 0.04 μF, η = 0.72
a) 5.62 kΩ
b) 37 kΩ
c) 4.23 kΩ
d) 9.82 kΩ

View Answer

Answer: d [Reason:] T = 1/f = 1/2 kHz R = T/C ln(1/1-η).

Set 4

1. The below shown single phase controlled rectifier configuration is a
power-electronics-questions-answers-1phase-fw-acdc-1-q1
a) two pulse M-2 Connection
b) two pulse B-2 Connection
c) single pulse M-2 Connection
d) single pulse B-2 Connection

View Answer

Answer: a [Reason:] It is a two pulse i.e full wave M-2 connection.

2. In the below shown circuit, considering the load to be purely R load, the thyristor T2 would conduct from ωt =
power-electronics-questions-answers-1phase-fw-acdc-1-q1
a) 0 to π
b) α to π
c) π+α to 2π
d) π to 2π+α

View Answer

Answer: c [Reason:] The bottom winding forward biases the SCR T2 at π, conduction starts after the pulse is given at α & stops when T2 is naturally commutated at 2π.

3. For a single pulse M-2 rectifier with RL load and firing angle α >0°, the value of the inductance L is such that the load current is continuous.
power-electronics-questions-answers-1phase-fw-acdc-1-q1
Choose the correct statement.
a) The load current can never be positive
b) The load current can never be negative
c) The load current can never be zero
d) The load current is a constant DC

View Answer

Answer: c [Reason:] As mentioned, L is such that the load current is continuous. Hence, it never falls below a predefined positive value.

4. For the circuit below, the transformer ratio is 1:1 and Vs = Vm sinωt. When T1 is forward biased and is triggered at α than T2 is
power-electronics-questions-answers-1phase-fw-acdc-1-q1
a) subjected to a forward voltage of Vm sinα
b) subjected to a forward voltage of 2Vm sinα
c) subjected to a reverse voltage of Vm sinα
d) subjected to a reverse voltage of 2Vm sinα

View Answer

Answer: d [Reason:] When T1 is gated, the other SCR T2 is reversed biased due to the windings so connected.

5. The circuit turn-off time of M-2 type connection is
power-electronics-questions-answers-1phase-fw-acdc-1-q1
a) π/ω
b) α/ω
c) π-α/ω
d) π+α/ω

View Answer

Answer: c [Reason:] When T1 is gated, SCR T2 is off from ωt = α to π Thus t = π-α(offtime)/ω Same can be said when T2 is gated and T2 is off.

6. Find the expression for the average value of the output voltage for the below given circuit. Consider the load current to be continuous, firing angle = α, transformer ration 1:1 and Vs = Vm sinωt.
a) (Vm/π)cosα
b) (Vm/π)(1+cosα)
c) (2Vm/π)cosα
d) (2Vm/π)(1+cosα)

View Answer

Answer: c [Reason:] As the load current is continues, the voltage is positive from α to π and negative from π to π+α and so on. Thus, Vavg = 1/Period ∫ Vm sin⁡ωt d(ωt) Where period = π And the integral runs from α to α+π.

7. A fully controlled converter uses
a) diodes only
b) thyristors only
c) both diodes and thyristors
d) none of the mentioned

View Answer

Answer: b [Reason:] Fully controlled implys that all the elements are “fully controlled” hence, it uses SCRs only except the FD.

8. A single phase full-converter using R load is a _________ quadrant converter and that using an RL load without FD is a __________ quadrant converter
a) one, one
b) two, one
c) one, two
d) two, two

View Answer

Answer: c [Reason:] In R load both V and I are positive, in RL load the voltage can be negative but current is always positive.

9. A single phase full controlled bridge converter (B-2) uses
a) 4 SCRs and 2 diodes
b) 4 SCRs
c) 6 SCRs
d) 4 SCRs and 2 diodes

View Answer

Answer: b [Reason:] 4 SCR’s are connected in a bridge fashion.

10. In a B-2 type full controlled bridge converter
a) one SCR conducts at a time
b) two SCRs conduct at a time
c) three SCRs conduct at a time
d) four SCRs conduct at a time

View Answer

Answer: b [Reason:] B-2 is the bridge type controller, in which 2 devices conduct at a time. One acting as the current supplying path and other acts as a return path.

Set 5

1. 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

View Answer

Answer: c [Reason:] At the firing angle, the SCR turns on and thus the resistor or load R is connected to the source.

2. The average output voltage is maximum when SCR is triggered at ωt =
a) π
b) 0
c) π/2
d) π/4

View Answer

Answer: b [Reason:] The sooner the conduction starts higher the average power. Though practically a device cannot be triggered at exactly zero degrees.

3. In the below given circuit, the SCR is triggered at a certain firing angle α.
power-electronics-questions-answers-1phase-hw-acdc-1-q3
The load current falls to zero at ωt =
a) α
b) α/2
c) π
d) π/2

View Answer

Answer: c [Reason:] The SCR is naturally commutated at 90°.

4. 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

View Answer

Answer: c [Reason:] By varying the firing angle, the load current and load voltage can be controlled.

5. 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

View Answer

Answer: c [Reason:] Vm is the peak value of the load as well as supply voltage. I = Vm/R.

6. Below shown ideal circuit is triggered at a firing angle of α. The voltage across the SCR is zero from ωt =
power-electronics-questions-answers-1phase-hw-acdc-1-q3
a) 0 to α
b) 0 to π
c) α to 2π
d) α to π

View Answer

Answer: d [Reason:] Before the SCR is triggered all of the supply voltage appears across it that is from 0 to α, at α the device is gated and it starts to conduct acting like a S.C (ideally). Hence voltage across it is zero from α to π. At π the SCR is again naturally commutated due to the reversal of the AC supply & a negative voltage appears across it.

7. Below shown ideal circuit is triggered at a firing angle of α. From ωt = π to 2π+α
power-electronics-questions-answers-1phase-hw-acdc-1-q3
a) SCR voltage is zero
b) Load voltage is maximum
c) SCR voltage is –Vmsinα
d) SCR acts like a short circuit

View Answer

Answer: c [Reason:] From ωt = π to 2π+α, negative cycle is active and the SCR experiences revered biased voltage by Vs.

8. 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

View Answer

Answer: c [Reason:] 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.

9. For the below given circuit with firing angle α & angular frequency of ω, the circuit turn-off time in secs can be given by
power-electronics-questions-answers-1phase-hw-acdc-1-q3
a) α/π
b) π/α
c) ω/π
d) π/ω

View Answer

Answer: d [Reason:] The circuit turns of at ωt = π as it is a half wave circuit. Hence toff = π/ω.

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

View Answer

Answer: c [Reason:] For the whole cycle of 2π, the R load would be active only for half of the cycle. Hence, Vo = Vm/π.