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Multiple choice question for engineering

Set 1

1. In Azeotropic distillation __________ remains low.
a) Heat
b) Volatility
c) Relative volatility
d) None of the mentioned

View Answer

Answer: c [Reason:] Same vapour pressure of the mixture has nearby volatility.

2. Find the distillation preferred for the binary mixture having component A and B with vapour pressure 360mmHg and 355mmHg.
a) Multi-component distillation
b) Reactive distillation
c) Azeotropic distillation
d) All the mentioned

View Answer

Answer: c [Reason:] Relative volatility = Vapour pressure of A/ Vapour pressure of B = 360/355 = 1.014 ( Azeotropic composition).

3. The term entrainer used in
a) Multi-component distillation
b) Reactive distillation
c) Azeotropic distillation
d) None of the mentioned

View Answer

Answer: c [Reason:] Entrainer increase the relative volatility.

4. By adding Entrainer the __________ azeotropes forms.
a) Low boiling
b) High boiling
c) No
d) None of the mentioned

View Answer

Answer: a [Reason:] Since when we add entrainer the relative volatility increases by formation of low boiling azeotropes.

5. The use of solvent for increasing the relative volatility is for
a) Multi-component distillation
b) Reactive distillation
c) Azeotropic distillation
d) Extractive distillation

View Answer

Answer: d [Reason:] Generally, solvent is used for extraction. As it help to alter the relative volatility it is used in extractive distillation.

6. In the distillate use of heat is to separate the distillate from the entrainer.
a) True
b) False

View Answer

Answer: a [Reason:] See, in Azeotropic distillation the overhead products will be composed of entrainer added. In order to pure product heat is given to the distillate product.

7. Find the Azeotropic mixture.
a) Air-water
b) Acetic acid- water
c) Acetic acid- alcohol
d) Air-alcohol

View Answer

Answer: b [Reason:] Acetic acid- water mixture has the nearby same boiling point.

8. If the feed mixture has the same vapour and liquid compositions at equilibrium then the distillation is normal.
a) True
b) False

View Answer

Answer: b [Reason:] If the liquid and the vapour compositions are same the relative volatility becomes 1 and there must be an azeotropes formation.

9. Will it possible to separate the mixture having relative volatility value 2 with azeotropic distillation.
a) True
b) False

View Answer

Answer: b [Reason:] Because if the relative volatility is 2 there will be a large difference in vapour pressure so there won’t be any Azeotropic formation.

10. By adding entrainer or solvent one of the component’s _________ gets reduced.
a) Boiling point
b) Bubble point
c) Dew point
d) All of the mentioned

View Answer

Answer: a [Reason:] By adding entrainer or solvent the relative volatility increased due to decrease in the boiling point of one component.

Set 2

1. A diode rectifier with a C filter will have average output voltage _______ than that obtained by a rectifier without the C filter.
a) lower than
b) higher than
c) same as
d) none of the mentioned

View Answer

Answer: b [Reason:] C filter improves the efficiency and eliminates ripples by keeping the output voltage constant.

2. An inductor filter connected in series with a resistive load provides a
a) smoothing of the output voltage waveform
b) smoothing of the input voltage waveform
c) smoothing of the output current waveform
d) smoothing of the input current waveform

View Answer

Answer: c [Reason:] Filter is always connected to the load side (output). Inductor has the property of keeping the current smooth and constant.

3. In the below given configuration, L is connected as a filter across the R load.
power-electronics-multiple-choice-questions-answers-q3
The average ammeter current is
a) 2Vm/R
b) 2Vm/πR
c) Vm/πR
d) Vm/R

View Answer

Answer: b [Reason:] I = Vo/R Vo = 2Vm/π.

4. The current ripple factor (CRF) is the ratio of
a) Average value/RMS value
b) RMS value/Average value
c) Average value/Maximum value
d) Maximum value/RMS value

View Answer

Answer: b [Reason:] CRF = Ir/Io.

5. In case of an L filter, the ripple current increases with
a) increase in Load
b) decrease in Load
c) increase in the value of L
d) ripple current never increases

View Answer

Answer: b [Reason:] When the load is reduced the resistance R is increased & ripple current increases.

6. In case of a C filter, if R (load resistance) is increased
a) ripple factor is reduced
b) ripple factor is increased
c) ripple factor is not affected
d) increases noise in the circuit

View Answer

Answer: a [Reason:] The time constant RC is increased therefore the ripple factor is reduced.

7. When a certain type of filter is connected across the R load, of a full wave bridge diode biased rectifier the following output current waveform is obtained.
power-electronics-multiple-choice-questions-answers-q7
The filter connected is most likely a
a) L filter
b) C filter
c) LC filter
d) None of the above mentioned

View Answer

Answer: a [Reason:] The above waveform is that of the output current. The peak per half cycle is on the right, hence it is more likely to be an L filter.

8. C filters are suitable for ___________ load resistances and L filters are suitable for _____________ load resistances.
a) low, low
b) high, high
c) high, low
d) low, high

View Answer

Answer: c [Reason:] In case of L filter, if R is lowered, the time constant (L/R) increases, therefore ripple factor reduces. In case of a C filter, the time constant RC is increased, therefore, ripple factor is reduced.

9. An LC filter will have ripple factor value___________ (For the same value of L & C)
a) lower than that obtained by L filter but higher than that obtained by C filter
b) lower than that obtained by C filter but higher than that obtained by L filter
c) lower than that obtained by either L or C filter
d) higher than that obtained by either L or C filter

View Answer

Answer: c [Reason:] Simply, an LC filter combines both the advantages of L and C filters.

10. In a single-phase full wave rectifier ___________ order harmonics are the most dominant
a) first
b) second
c) third
d) fourth

View Answer

Answer: b [Reason:] The output voltage is given by, Vo = (2Vm/π) – (4Vm/3π)cos 2ωt – (4Vm/15π)cos 4ωt . . . . Hence the other values after the second factor (4Vm/3π) become very very small hence they can be eliminated.

Set 3

1. In practice the output from the diode rectifier has
a) AC component only
b) DC component only
c) AC + DC component
d) None of the mentioned

View Answer

Answer: c [Reason:] The output contents along with the DC components the AC harmonics which does no useful work & reduces the efficiency.

2. Choose the correct statement
a) The AC component in the output of rectifier does the useful work
b) The AC component in the output of rectifier increases the efficiency of the system
c) The AC component in the output of rectifier causes ohmic losses
d) The AC component in the output of rectifier does not affect the operation

View Answer

Answer: c [Reason:] A rectifier is used to convert AC to DC. Lower the AC (Non-DC) components in the output lower are the ohmic losses.

3. An L filter is connected ________
a) in series
b) in parallel
c) in both series and parallel
d) none of the mentioned

View Answer

Answer: a [Reason:] Inductor (L) has a very important property that the current through it cannot change rapidly. We can make use of this property by connecting it in series.

4. In case of an L filter connected with a rectifier in series with the load, it offers ________ impedance to ac whereas _______ resistance to dc respectively.
a) high, high
b) high, low
c) low, high
d) low, low

View Answer

Answer: b [Reason:] It offers high impedance to AC such as the AC ripples do not pass through the load.

5. In case of a C filter, the AC is not allowed to pass to the load by
a) offering it high impedance
b) offering it low impedance
c) short circuiting the AC component
d) open circuiting the AC component

View Answer

Answer: c [Reason:] AC ripples are not allowed to pass, by S.C the AC ripples as the C is always connected in parallel with the load.

6. A capacitor filter or C filter can be used in a rectifier by connecting it
a) in parallel with the load
b) in series with the load
c) in parallel with the supply
d) in series with the supply

View Answer

Answer: a [Reason:] AC ripples are not allowed to pass, by S.C the AC ripples as the C is always connected in parallel with the load.

7. Examine the below shown circuit.
power-electronics-questions-answers-filters-1-q7
While D7 & D10 are reversed biased the
a) capacitor is charging through Vs
b) capacitor is discharging through Vs
c) capacitor is charging through R
d) capacitor is discharging through R

View Answer

Answer: d [Reason:] When the two are reversed biased and the other two SCR’s are not yet gated, the stored energy in the capacitor is supply the load R.

8. When a certain type of filter is connected across the R load of a full wave bridge type diode biased rectifier, the following output voltage waveform is obtained
power-electronics-questions-answers-filters-1-q8
The filter connected is most likely to be a/an
a) L filter
b) C filter
c) LC filter
d) None of the above mentioned

View Answer

Answer: b [Reason:] It is to be noted that the waveform is that of a voltage parameter. The voltage goes up for a very small time and than goes down (discharges) for longer time. For every half wave the peak of the waveform is to the left, if it were to be on the right it would be more likely to be a L filter.

9. For the below given circuit,
power-electronics-questions-answers-filters-1-q7
When the output current rises, the capacitor current
a) is increasing
b) is decreasing
c) is constant
d) is zero

View Answer

Answer: b [Reason:] When the load current is increasing, the capacitor is charging through the source voltage & its current keeps on falling & voltage rises.

10. A single-phase diode B-2 rectifier is fed from a 250 V, 50 Hz source & connected to a load of R = 400 Ω.
Design a capacitor filter such that the ripple factor of the output voltage is less than 5 %. Find the value of the capacitance of the C filter.
a) 156.4 μF
b) 500 μF
c) 189 μF
d) 246 μF

View Answer

Answer: c [Reason:] Use, C = K * [1 + 1/√2RF] Where, K = 1/4fR.

Set 4

1. Name the circuit shown in the figure below.
advanced-power-electronics-questions-answers-q1-q4-q8-q10
a) Single-phase circulating current type dual converter
b) Single-phase non-circulating current type dual converter
c) Three-phase circulating current type dual converter
d) None of the mentioned

View Answer

Answer: d [Reason:] The converters are connected in parallel not in anti-parallel.

2. Dual converters handle ________ during no load.
a) very high temperature
b) no current
c) only circulating current
d) load current

View Answer

Answer: c [Reason:] During no load period, both the converters handle only the circulating current as the load current is zero.

3. In a three phase dual converter, converter 1 is operating with α = 95° and converter 2 is operating with α2 = 85°. Choose the correct statement.
a) Converter 1 is operating as a rectifier and converter 2 as an inverter
b) Both the converters are operating as a rectifier
c) Converter 1 is operating as a inverter and converter 2 as an rectifier
d) Both the converters are operating as an inverter

View Answer

Answer: c [Reason:] As α1 > 90, converter 1 is operating as a inverter and with α2<90, converter 2 is working as an inverter.

4. The circuit shown below is that of a
advanced-power-electronics-questions-answers-q1-q4-q8-q10
a) single-phase circulating current type dual converter
b) three-phase non-circulating current type dual converter
c) three-phase circulating current type dual converter
d) none of the mentioned

View Answer

Answer: c [Reason:] It is a 3-phase as it uses 6 SCRs on either sides, it is circulating current type as there is a reactor (L) placed on either sides of the load to compensate for the circulating current.

5. In circulating current type of dual converters, the nature of voltage across the reactor is
a) triangular
b) pulsating
c) constant
d) alternating

View Answer

Answer: d [Reason:] The current is alternating in nature.

6. When a line commutated converter operates in the inverter mode then
a) it draws real and reactive power from the supply
b) it delivers both real and reactive power to the supply
c) it delivers real power to the supply
d) it delivers reactive power to the supply

View Answer

Answer: c [Reason:] A converter is given an AC supply, when it is in the inverter mode, it is sending power (only real power) to the AC supply and absorbing power (only reactive) power from the supply.

7. The reactor is required in a circulating current type dual converter to
a) improve the power factor
b) smooth-en the waveform of circulating current
c) limit the circulating current
d) increase the circulating current

View Answer

Answer: c [Reason:] Reactor (inductance) is introduced in the circulating current mode between both the converters. The reactor limits the current to a reasonable value.

8. In the below given figure the converter 1 (to the right side) has firing angle = α1, converter 2 has firing angle = α2. For the dual converter to operate in the second quadrant.
advanced-power-electronics-questions-answers-q1-q4-q8-q10
a) α1 > 90°
b) α2 > 90°
c) α1 < 90°
b) α2 < 90°

View Answer

Answer: b [Reason:] Converter 1 would control the first and the fourth quadrant and converter 2 would operate in the second and the third quadrant. In the second quadrant, voltage should be positive and current should be negative, hence inverter operation hence α2 > 90°.

9. If V1 and V2 are the converter output voltages then the reactor voltage is
a) V1 + V2
b) V1 – V2
c) V1 x V2
d) none of the mentioned

View Answer

Answer: b [Reason:] The reactor voltage Vr is the difference of the converter output voltages.

10. In the below given figure, the converter 1 (to the right side) has firing angle = α1, converter 2 has firing angle = α2. For the dual converter to operate in the fourth quadrant.
advanced-power-electronics-questions-answers-q1-q4-q8-q10
a) α1 > 90°
b) α2 > 90°
c) α1 < 90°
b) α2 < 90°

View Answer

Answer: a [Reason:] Converter 1 would control the first and the forth quadrant and converter 2 would operate in the second and the third quadrant. In the fourth quadrant, voltage should be negative and current should be positive, hence C1 as an inverter is operated with α1 > 90°.

Set 5

1. Which of the following is not a valid type of a BJT?
a) PNP
b) NPN
c) PPN
d) All of the mentioned

View Answer

Answer:c [Reason:] A BJT is a device with a layer of semiconductor sandwitched between 2 unlike types of semiconductors and hence, PPN is not a valid type of a BJT.

2. In a BJT, the outer layers are much more thick as compared to the middle layer.
a) True
b) False

View Answer

Answer:a [Reason:] In a BJT, the thickness of the middle layer or the base is thin as compared to the collector and emitter.

3. In a BJT, which of the following layers is heavily doped?
a) Collector
b) Emitter
c) Base
d) All of the mentioned.

View Answer

Answer:b [Reason:] In a BJT, emitter is heavily doped whereas base and collector are lightly doped.

4. Considering the resistances of emitter, collector and base to be Re, Rc and Rb respectively, which of the following is the correct statements?
a) Re > Rb > Rc
b) Rc > Rb > Re
c) Rb > Rc > Re
d) None of the mentioned

View Answer

Answer:c [Reason:] As the base is lightly doped, the number of free charge carriers are less and hence the resistance is high and as the emitter is the most highly doped, its resistance is low.

5. In a pnp-BJT, when the E-B junction is forward biased and no voltage is applied across C-B junction, what happens to the width of the depletion region in the E-B junction?
a) Increases
b) Decreases
c) Remains same
d) Can’t be determined

View Answer

Answer: b [Reason:] On application of a forward bias voltage across E-B junction, the width of the depletion region decreases.

6. Which of the following statements is true about proper functioning of a BJT?
a) One junction is forward biased and one is reverse biased.
b) Both junctions are forward biased
c) Both junctions are reverse biased
d) Can’t be determined

View Answer

Answer:a [Reason:] In a BJT, one junction is forward biased and the other is reverse biased depending upon the type of the transistor.

7. What is the typical order of magnitude of the base current for a BJT?
a) 10 -8
b) 10 -9
c) 10 -6
d) 10 -3

View Answer

Answer:c [Reason:] As the base current is quite lower as compared to the collector and emitter current, it is usually in the order of microamperes.

8. The collector current in a BJT is temparature-independent.
a) True
b) False

View Answer

Answer:b [Reason:] The collector current I C in a BJT is made up of two components – one due to majority carriers and the other due to minority carriers. The component of IC due to minority carriers i.e ICO is temparature sensitive.

9. Which of the following currents in a BJT is also called leakage current?
a) IC
b) IE
c) ICO
d) ICBO

View Answer

Answer:c [Reason:] Leakage current in a BJT is the name given to ICO, which is due to the flow of minority carriers.

10. Which of the following relations are correct?
a) IE + IB = IC
b) IC + IB = IE
c) IE + IC = IB
d) None of the mentioned

View Answer

Answer:b [Reason:] On applying KCL to the BJT, we get IC + IB = IE.