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Multiple choice question for engineering

Set 1

1. For a step-down chopper shown in the figure below, dc source voltage = 230 V and the voltage drop across the chopper = 2 V. Find the value of average output voltage for a duty cycle of 0.4.
power-electronics-questions-answers-quiz-q1-q2-q3-q4
a) 230 V
b) 92 V
c) 90 V
d) 91.2 V

View Answer

Answer: d [Reason:] The voltage drop across the switch occurs only when it is on. Vo = (Vs – 2) x Duty cycle = 91.2 V.

2. For the step-down chopper shown in the figure below, dc source voltage = 230 V and the voltage drop across the chopper = 2 V. Find the value of rms output voltage for a duty cycle of 0.4.
power-electronics-questions-answers-quiz-q1-q2-q3-q4
a) 91.2 V
b) 230 V
c) 144.2 V
d) 145 V

View Answer

Answer: c [Reason:] The voltage drop across the switch will occurs only when it is on. Vo (rms) = √0.4 x (230 – 2) = 144.2 V.

3. For the step-down chopper shown in the figure below, dc source voltage = 230 V. Find the power delivered to the load of R = 10 Ω. Duty cycle = 40 %. Take voltage drop at the switch to be 2 V.
power-electronics-questions-answers-quiz-q1-q2-q3-q4
a) 1 kW
b) 2.08 kW
c) 569 W
d) 207 W

View Answer

Answer: b [Reason:] Vo (rms) = √0.4 x (230 – 2) = 144.2 V P = 144.22/10 = 2079.3 W = 2.08 kW.

4. For the step-down chopper shown in the figure below, dc source voltage = 230 V. Find the power input to the chopper If load of R = 10 Ω is connected. Duty cycle = 40 %.
power-electronics-questions-answers-quiz-q1-q2-q3-q4
a) 2300 W
b) 2097 W
c) 2560 W
d) 5290 W

View Answer

Answer: b [Reason:] Power input to the chopper is Vs . Io Vs = 230 V Vo = (Vs – 2) x Duty cycle = 91.2 V Io = Vo/R = 91.2/10 Pi = Vs.Io = 2097.6 W.

5. The below given chopper circuit is that of a
power-electronics-questions-answers-quiz-q5
a) step-up chopper
b) step-down chopper
c) step-up/step-down chopper
d) none of the mentioned

View Answer

Answer: c [Reason:] It is a step-up/step-down chopper. By controlling the duty cycle it can be made to behave like a step-up or a step down chopper.

6. The expression for a step-up/step-down chopper with α as the duty cycle and Vs as the dc input voltage is
a) Vs/1 – α
b) α x Vs
c) Vs (α/1-α)
d) Vs (α/1+α)

View Answer

Answer: c [Reason:] A step-up, step-down chopper can behave as a step up chopper for α > 0.5 and step –down chopper for α < 0.5.

7. For a step-up/step-down chopper, if the duty cycle > 0.5 then
a) Vo = Vs
b) Vo < Vs
c) Vo > Vs
d) None of the mentioned

View Answer

Answer: c [Reason:] Vo = Vs (α/1-α) For α is greater than 0.5, the chopper behaves as a step-up chopper. Hence, Vo > Vs.

8. A step down chopper has Vs = 230 V and R = 10 Ω. For a duty cycle of 0.4, the power taken by the chopper is 2097 Watts. Find the chopper efficiency. Take the voltage drop across the chopper switch as 2 V.
a) 98 %
b) 89.96 %
c) 99.14 %
d) 96.54 %

View Answer

Answer: c [Reason:] Vo (rms) = √0.4 x (230 – 2) = 144.2 V Po = 144.22/10 = 2079.3 Watts Pi = 2097 Watts η = Po/Pi = 99.14 %.

9. A step down chopper has input dc voltage of 220 V and R = 10 Ω in series with L = 65 mH. If the load current varies linearly between 11 A and 17 A, then find the duty cycle α.
a) 1
b) 0.4
c) 0.6
d) 0.7

View Answer

Answer: d [Reason:] Average load current = 11+17 / 2 = 14A Average load voltage = IxR = 14×10 = 140V But, Vo = α x Vs Therefore, α x 220 = 140, α = 0.7.

10. For a step-up/step-down chopper, if α (duty cycle) = 0.5 then
a) Vo = Vs
b) Vo < Vs
c) Vo > Vs
d) none of the mentioned

View Answer

Answer: a [Reason:] Vo = Vs (α/1-α) For α = 0.5 Vo = Vs x (0.5/0.5).

Set 2

1. The thyristor turn-off requires that the anode current
a) falls below the holding current
b) falls below the latching current
c) rises above the holding current
d) rises above the latching current

View Answer

Answer: a [Reason:] For effective turn-off of the SCR the anode current must fall below the holding current value.

2. In case of class A type commutation or load commutation with low value of R load the
a) L is connected across R
b) L-C is connected across R
c) L is connected in series with R
d) L-C is connected in series with R

View Answer

Answer: d [Reason:] In case of Class A commutation the requirement is that the circuit should be an under-damped RLC circuit.

3. The class A commutation or load commutation is possible in case of
a) dc circuits only
b) ac circuits only
c) both DC and AC circuits
d) none of the above mentioned

View Answer

Answer: a [Reason:] 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.

4. 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.
a) 560 A
b) 460 A
c) 360 A
d) 260 A

View Answer

Answer: b [Reason:] Ip = Vs x √C/L.

5. 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.
a) 0.23 μs
b) 6.57 μ
c) 31.41 μs
d) 56 μs

View Answer

Answer: c [Reason:] Ip = Vs x √C/L ω = 1/√LC t = π/ω.

6. An SCR is connected in series with L = 5 mH and C = 20 μF. Find the resonant frequency of the circuit.
a) 2569 rad/s
b) 3162 rad/s
c) 2400 rad/s
d) 7889 rad/s

View Answer

Answer: b [Reason:] ω = 1/√LC.

7. The type of commutation when the load is commutated by transferring its load current to another incoming thyristor is
a) class A or load commutation
b) class B or resonant commutation
c) class C or complementary commutation
d) class D or impulse commutation

View Answer

Answer: c [Reason:] In the Class C type commutation also called as complementary commutation the load is commutated by transferring the current th another device.

8. The type of commutation in which the pulse to turn off the SCR is obtained by separate voltage source is
a) class B commutation
b) class C commutation
c) class D commutation
d) class E commutation

View Answer

Answer: d [Reason:] In class E commutation, another voltage source is used. It is also called as external pulse commutation.

9. The natural reversal of ac supply voltage commutates the SCR in case of
a) forced commutation
b) only line commutation
c) only natural commutation
d) both line & natural commutation

View Answer

Answer: d [Reason:] Both line and natural commuataion are used in converters.

10. ___________ commutation technique is commonly employed in series inverters.
a) line
b) load
c) forced
d) external-pulse

View Answer

Answer: b [Reason:] 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.

Set 3

1. Sequence control of ac voltage controllers is employed for the improvement of _________
a) output frequency
b) input frequency
c) commutation
d) system power factor

View Answer

Answer: d [Reason:] It is used to improve the power factor at both the input and output side.

2. A two stage sequence control is
a) two SCRs in anti parallel
b) two voltage controllers in parallel
c) two voltage controllers in series
d) a voltage controller having two voltage level

View Answer

Answer: b [Reason:] Sequence control of ac voltage controller means the use of two or more stages of voltage controllers in parallel for the regulation of output voltage.

3. From the below given statements regarding sequence control of ac voltage, which of them are true?
i) It improves system power factor
ii) It reduces the harmonic content at the output
iii) Wider control of output voltage is possible
a) only (i)
b) only (i) and (ii)
c) only (i) and (iii)
d) all of them are true

View Answer

Answer: d [Reason:] All of the above are the advantages of using two or more stages of voltage controllers in parallel. Harmonic content is reduced at two common parameters get cancelled out.

4. The below given circuit is that of a
power-electronics-questions-answers-sequence-controller-q4
a) four stage sequence controller
b) two stage sequence controller
c) full wave ac voltage controller
d) none of the mentioned

View Answer

Answer: b [Reason:] Sequence control of ac voltage controller means the use of two or more stages of voltage controllers in parallel for the regulation of output voltage.

5. If T1, T2 are kept off and T3, T4 are having a firing angle of 180° each, then the output voltage is ____ if the turns ratio is 1:1
power-electronics-questions-answers-sequence-controller-q4
a) Vs
b) √2Vs
c) 0
d) 2Vs

View Answer

Answer: c [Reason:] If T3 and T4 are fired at an angle of 180°, they are equivalent to an off switch, as they are fired and naturally commutated at the same time, hence they are never on. Hence, as all the four SCRs are always off, the output voltage is zero.

6. For obtaining the output voltage control from V to 2V, the firing angle must be
power-electronics-questions-answers-sequence-controller-q4
a) always 0 for T3, T4
b) always 180 for T3, T4
c) 0 to 180 from T3, T4
d) none of the mentioned

View Answer

Answer: a [Reason:] For obtaining the voltage control from V to 2V, the lower controller i.e. T3, T4 pair must always be on hence the firing angle from them must always be 0° ideally.

7. For obtaining the output voltage control from V to 2V, the firing angle must be
power-electronics-questions-answers-sequence-controller-q4
a) always 0 for T1, T2
b) always 180 for T1, T2
c) 0 to 180 from T1, T2
d) none of the mentioned

View Answer

Answer: c [Reason:] For obtaining the voltage control from V to 2V, the upper controller must be controlled by varying its firing angle from 0 to 180° whereas the T3, T4 pair must always be on hence the firing angle from them must always be 0° ideally.

8. A single-phase two stage sequence controller is designed to work on 230 V supply, and upper and lower current ratings must be 20 A and 21 A respectively. Find the transformer rating.
a) 230 VA
b) 4600 VA
c) 9430 VA
d) 9200 VA

View Answer

Answer: c [Reason:] Transformer rating is Vs(I1 + I2).

9. In a N-stage sequence controller, each secondary is rated for __________
a) n x Vs
b) Vs
c) Vs/n
d) Vs x (n-1)

View Answer

Answer: c [Reason:] In a N-stage sequence controller, n voltage controllers are used each having to anti parallel SCR pairs and each secondary is rated for Vs/n.

10. A single-phase sinusoidal voltage controller has
a) one primary and n secondary windings
b) one primary and (n-1) secondary windings
c) n primary and n secondary windings
d) (n-1) primary and n secondary windings

View Answer

Answer: b [Reason:] A sinusoidal voltage controller is used to obtain continuous voltage control over wide range with low harmonic content.

Set 4

1. In the below shown semi-converter circuit, T1 and T2 are fired at an angle α, then from ωt = α to π
power-electronics-questions-answers-single-phase-semi-converters-1-q1
a) T1 and T2 conduct
b) T1 and D1 conduct
c) D1 and D2 conduct
d) FD conducts

View Answer

Answer: b [Reason:] At ωt = α T1 is gated and current starts to flow from T1-R-L-E-D1-Vs-T1.

2. In the below shown semi-converter circuit T1 & T2 are fired at an angle α, then from ωt = π to α+π
power-electronics-questions-answers-single-phase-semi-converters-1-q1
a) T1 is conducting
b) T2 is conducting
c) D1 is conducting
d) FD conducting

View Answer

Answer: d [Reason:] None of the SCRs are gated, FD is forward biased and starts to conduct charging the L.

3. In the below shown semi-converter circuit T1 & T2 are fired at an angle α, the output voltage is zero when
power-electronics-questions-answers-single-phase-semi-converters-1-q1
a) π<ωt<α
b) 0<ωt<α+π
c) π<ωt<π+α
d) π<ωt<2π

View Answer

Answer: c [Reason:] FD is conducting as none of the SCRs are gated which S.C’s the load circuit hence zero voltage.

4. In a semi-converter with RLE load during the freewheeling period, the energy is
a) fed back to the source
b) fed to the inductor(L) and absorbed by E
c) absorbed by the L & E and dissipated at R
d) fed to the L & E and dissipated at R

View Answer

Answer: d [Reason:] The energy is fed back fed to the L & E and dissipated at R.

5. A semi-converter circuit gives the following voltage waveform. Find the expression for the average output voltage with Vs = Vm sinωt and firing angle α
power-electronics-questions-answers-single-phase-semi-converters-1-q5
a) (Vm/π) cosα
b) (Vm/π) (1+cosα)
c) (2Vm/π) cosα
d) (2Vm/π) (1+cosα)

View Answer

Answer: b [Reason:] Vo = 1/π ∫ Vm sinωt d(ωt), Where the integration runs from α to π.

6. For the below given circuit configuration, the SCR is fired at angle α. For continuous current mode from ωt = π to 2π
power-electronics-questions-answers-single-phase-semi-converters-1-q6
a) FD conducts
b) SCR and D1 conduct
c) D3 and D2 conducts
d) None of the mentioned

View Answer

Answer: c [Reason:] From ωt = π to 2π, the negative cycle is active and the SCR is reversed biased. D3 and D2 are ative and are supplying power to the load.

7. For the below given circuit configuration, the SCR is fired at angle α. For continuous current mode, find the expression for the average output voltage
power-electronics-questions-answers-single-phase-semi-converters-1-q6
a) (Vm/2π) (1+ cosα)
b) (Vm/2π) (2+ cosα)
c) (Vm/2π) (3+ cosα)
d) (3Vm/π)

View Answer

Answer: c [Reason:] The output voltage will have two parts V1 = [ ∫ Vm sinωt d(ωt) ] where the integration would run from α to π V2 = [ ∫ Vm sinωt d(ωt) ] where the integration would run from π to 2π Vo = 1/2π x V1+V2.

8. A semi-converter circuit gives the following voltage waveform on R load. Find the expression for the average output current with Vs = Vm sinωt and firing angle α
power-electronics-questions-answers-single-phase-semi-converters-1-q5
a) (Vm/Rπ) cosα
b) (Vm/Rπ) (1+cosα)
c) (2Vm/Rπ) cosα
d) (2Vm/Rπ) (1+cosα)

View Answer

Answer: b [Reason:] Vo = 1/π ∫ Vm sinωt d(ωt) Where the integration runs from α to π Io = Vo/R.

9. A semi-converter with RLE load and a freewheeling diode has discontinuous load current with firing angle α and extinction angle β. The freewheeling period is
a) π>ωt<β
b) π>ωt<β
c) π<ωt>β
d) π>ωt>β

View Answer

Answer: a [Reason:] The freewheeling period is from π to the extinction angle.

10. A semi-converter with RLE load and a freewheeling diode has discontinuous load current with firing angle α and extinction angle β. If β < π and Vm sinβ < E then
a) the converter operates as a line commutated inverter
b) the conduction period is absent
c) the freewheeling period is absent
d) none of the mentioned

View Answer

Answer: c [Reason:] As the extinction angle is less than 90 degrees, the freewheeling period is absent.

Set 5

1. A single-phase symmetrical semi-converter employs
a) one SCR and one diode in each leg
b) two SCRs and two diodes in each leg
c) two SCRs in each leg
d) two diodes in each leg

View Answer

Answer: a [Reason:] A symmetrical semi-converter will have one SCR and one diode in each leg. Two legs connected in parallel with each other having a symmetrical configuration.

2. A single-phase asymmetrical semi-converter employs
a) one SCR and one diode in each leg
b) two SCRs in one leg and two diodes in the other
c) two SCRs in both the legs
d) two diodes in both the legs

View Answer

Answer: b [Reason:] An asymmetrical semi-converter will two SCRs in one leg and two diodes in the other.

3. In the below given circuit, from ωt = π to α+π and continuous load current configuration
power-electronics-questions-answers-single-phase-semi-converters-2-q3
a) T1 and T2 conduct
b) T2 and D1 conduct
c) T1 and D2 conduct
d) T1 and D1 conduct

View Answer

Answer: c [Reason:] At ωt = π, D1 is reversed biased but T1 is still ON due to the nature of the load. Hence, the load current or inductor current flows from T1 and D2 and short circuiting the load terminals. Hence, due to the diode D2 freewheeling action takes place without even having a FD diode.

4. Find the circuit turn-off time (Tc) for the below given semi-converter configuration. Take α = 30° and Vs = 230√2 sin100t
power-electronics-questions-answers-single-phase-semi-converters-2-q3
a) 20π radians
b) 5π/6 radians
c) π/120 radians
d) 3/π radians

View Answer

Answer: c [Reason:] For the given circuit, ωt = π-α. tc = π – α/ω α = π/6 and ω = 100 … (given) tc = (π – π/6)/100.

5. For the circuit shown below, T1 and T2 (from top T1 and T2) are both fired at an angle α. Then from ωt = π to ωt = π+α
power-electronics-questions-answers-single-phase-semi-converters-2-q5
a) None of the devices conduct
b) T1 and one diode will conduct
c) T2 and one diode will conduct
d) Both the diodes will conduct

View Answer

Answer: d [Reason:] From ωt = π to ωt = π+α, None of the SCRs are fired and both the diodes are forward biased due to the nature of the load. As such, they start to conduct and freewheel all the inductor energy.

6. In any AC-DC circuit, the freewheeling action
a) improves the power handling capabilities
b) increases the THD
c) improves CDF
d) all of the mentioned

View Answer

Answer: c [Reason:] Freewheeling action reduces THD (total harmonic distortion), Improves CDF (current distortion factor and has no effect over the power handling capabilities as such.

7. A single-phase semi-converter is connected to a 230 V source and is feeding a load R = 10 Ω in series with a large inductance that makes the load current ripple free. Find the average output current for α = 45°.
a) 14 A
b) 17 A
c) 10 A
d) 0 A

View Answer

Answer: b [Reason:] Vo =( Vm/π) (1 + cosα) Io = Vo/R.

8. A single-phase semi-converter is connected to a 230 V source and is feeding a load R = 10 Ω in series with a large inductance that makes the load current ripple free. For α = 45°, find the rectification efficiency. The RMS value of output voltage is 219.3 V
a) 96.54 %
b) 75.25 %
c) 89.45 %
d) 80.58 %

View Answer

Answer: d [Reason:] Vo =( Vm/π) (1 + cosα) = 176.72V Io = Vo/R Irms = Io = Vo/R = 17.67 A Vrms = 219.3 V (given) η = (Vo x Io) / (Vrms X Irms) = 0.8058 = 80.58 %.

9. A single-phase semi-converter circuit is supplying power to a motor load. The average value of load voltage is 176.72 V and the rms value is 219.3 V. Find the VRF (voltage ripple factor).
a) 0
b) 0.735
c) 0.569
d) 2.48

View Answer

Answer: b [Reason:] First find the form factor (FF), FF = Vrms/Vo = 1.241 Now, VRF = √(FF2-1) = 0.735.

10. For the same triggering angle and ratings
a) a semi-converter operates at lower output voltage than a full converter
b) a semi-converter operates at higher output voltage than a full converter
c) a semi-converter has lower values of input p.f as compared to a full converter
d) a semi-converter has more THD as compared to a full converter

View Answer

Answer: b [Reason:] A semi-converter gives more output voltage than an equivalent full converter circuit. It also has less THD and high p.f (power factor).