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Multiple choice question for engineering

Set 1

1. A three-phase, three-pulse, M-3 type controlled converter uses ____________ number of SCRs.
a) 1
b) 2
c) 3
d) 4

View Answer

Answer: c [Reason:] It uses three SCRs with a three-phase transformer. M-3 type 3-pulse converters are not practically used.

2. A three-phase, three-pulse, M-3 type controlled converter has firing angle for one of the SCRs set as 15°. This SCR would start conducting at
a) 0°
b) 15°
c) 30°
d) 45°

View Answer

Answer: d [Reason:] In a three phase controller, the actually conduction starts at 30° + α. Hence, ωt = 30+15 = 45°.

3. In a three-phase, three-pulse, M-3 type controlled converter T1 starts to conduct at 30 + n°. At what angles do T2 and T3 start to conduct? Assume that the conduction sequence is T1-T2-T3.
a) 2n° and 3n°
b) 150 + n° and 270 + n°
c) n° each
d) 30 + n° and 60 + n°

View Answer

Answer: b [Reason:] In three-phase three pulse converter the conduction can start only after 30°. As each SCR conducts from 120°, T2 would conduct on 30+120+n° = 150+n° and so on.

4. A three-phase three pulse type controlled converter is constructed using 3 SCR devices. The circuit is supplying an R load with α < 30°. As such, each SCR device would conduct for
a) 60° each cycle
b) 120° each cycle
c) 180° each cycle
d) 360° each cycle

View Answer

Answer: b [Reason:] Each conduct for 120° per cycle is the firing angle is less than 30°. 120 x 3 = 360°.

5. Find the expression for average output voltage at R for the below given configuration. Take firing angle as α = 15°, transformer ratio as 1:1:1 and Vmp as the maximum value of phase voltage at the supply.
power-electronics-questions-answers-three-phase-converters-1-q5
a) (3Vmp/2π) x cosα
b) (3√3Vmp/2π) x cosα
c) (3√3Vmp/2π) x sinα
d) (3Vmp/2π) x sinα

View Answer

Answer: b [Reason:] The circuit is that of a three-pulse M-3 connection. The firing angle is less than 30°. Therefore, each device conducts for an angle of 120°. Vo = 3 x [ 1/2π ∫ Vmp sin⁡ωt d(ωt) ] Where, the integration runs from α+π/6 to α+5π/6. Vo = (3√3/2π) x Vmp x cosα.

6. In the circuit shown below, SCR T1 conducts first. If T1 is fired at an angle of α > 30°, then T1 would conduct from
power-electronics-questions-answers-three-phase-converters-1-q5
a) α to 180°
b) 30 + α to 180°
c) 30 + α to 150°
d) 30 + α to 120°

View Answer

Answer: b [Reason:] When firing angle is more than 30°, T1 would conduct from 30 + α to 180°. Irrespective of the firing angle, T1 will be turned on at 180° because it conducts first which means it is connected to the R phase and the phase sequence is R-Y-B. As R starts at 0° its value is 0 at 180° which reverse biases the SCR T1.

7. Find the expression for average output voltage for the given circuit if firing angle is greater than 30°. Take Vmp = secondary side maximum value of phase voltage.
power-electronics-questions-answers-three-phase-converters-1-q5
a) (3√3Vmp/2π) x cosα
b) (3√3Vmp/2π) x (1+cosα)
c) (3√3Vmp/2π) x [1+cos(30+α)].
d) (3√3Vmp/2π) x [3+cos(30+α)].

View Answer

Answer: c [Reason:] Vo = 3 x [ 1/2π ∫ Vmp sin⁡ωt d(ωt) ] Where, the integration runs from α+π/6 to π. Because conduction takes place from 30 + α to 180° for T1 and than the waveform is symmetrical for all other SCRs. Vo = (3√3/2π) x Vmp x [1+cos(30+α)].

8. A three-phase three-pulse converter would operate as a line commutated inverter when
a) 30° < α < 60°
b) 90° < α <180°
c) 90° > α
d) it can never operate as a line commutated inverter

View Answer

Answer: b [Reason:] The output voltage is proportional to cosine of the firing angle α. If α goes above 90° then the output voltage is negative, meaning that it is operating as an inverter.

9. A three-phase M-3 converter is operated from a 3-phase, 230 V, 50 Hz supply with load resistance R = 10 Ω. Find the value of firing angle if an average output voltage of 50% of the maximum possible output voltage is required.
Hint: α > 30°.
a) 92.7°
b) 67.7°
c) 45°
d) 75.7°

View Answer

Answer: b [Reason:] We need , Vo = 0.5 Vom. α>30° hence we use the equation Vo = (3√3/2π) x Vmp x [1+cos(30+α)] √3Vmp = Vml = √2×230 Therefore, Vo = (3/2π) x √3 Vml x [1+cos(30+α)] = 0.5Vom (1/√3) x [1+cos(30+α)] = Vo x 2π/3Vml = Vo/Vom = 1/2 α = 67.7°.

10. A three-phase half-wave controlled converter is fed from a 3-phase, 400 V source and is connected to a load which takes a constant current of 36 A. Find,the value of average output voltage and average current rating of SCR for a firing angle of 30°.
a) 234 V, 36 A
b) 234 V, 12 A
c) 135 V, 36 A
d) 135 V, 12 A

View Answer

Answer: b [Reason:] Vo = (3√3/2π) x Vmp x cosα = 233.874 V. Ia = Io/3 = 12 A.

Set 2

1. A three phase full converter will require __________ number of SCRs.
a) 3
b) 6
c) 9
d) 2

View Answer

Answer: b [Reason:] Three legs having two SCRs each, six in total.

2. A three phase six pulse full converter works as a ac to dc converter for firing angles in the range
a) α > 90
b) 90 < α < 180
c) 0 < α < 90
d) 0 < α < 360

View Answer

Answer: c [Reason:] When α is less than 90°, the SCRs conduct for 120° and the current and voltage are positive on an average hence, the power flows from AC source to DC load.

3. For the below given circuit, α = 60°. T2 will start conduction at ωt = __________ Assume the inductor L value to be negligible.
power-electronics-questions-answers-three-phase-converters-2-q3
a) 60°
b) 120°
c) 90°
d) 150°

View Answer

Answer: d Expansion: Assuming the phase sequence is R-Y-B. T1 would start conducting at 30+60 = 90°, T2 at 90+210/2 = 150°. This is because after T1, T3 would conduct from the upper group, as T2 belongs to the lower group it will start to conduct exactly between T1 and T3 i.e. between 90 and 210(90+120) which is 150°.

4. For a three phase full controlled converter, with 3 thyristors in the upper or positive group and 3 thyristors in the lower or negative group, at any given time
a) two thyristors are conducting from each group
b) one thyristor is conducting from each group
c) one thyristor is conducting from either of the groups
d) all 6 thyristors are conducting at a time

View Answer

Answer: b [Reason:] Let’s say T1, T3 and T5 belong to the positive group and T2, T4 and T6 to the negative group. At any given time one SCR from each group conducts. e.g. T1 and T6 or T1 and T2.

5. In case of a three phase full controlled converter with 6 SCRs, commutation occurs every
a) 120°
b) 60°
c) 180°
d) 30°

View Answer

Answer: b [Reason:] Every SCR conducts for 120°. This means that the SCRs from the positive group are fired 120° among themselves, same is true for SCRs from negative group. For example, if T1 starts conducting at 90° it will conduct till 90+120 = 210°. But while T1 is conducting, half of the time i.e. from 90 to 150, T6 is conducting and another half of the time T2 is conducting. Hence, commutation (change in the SCR which is conducting) takes place every 60 degrees irrespective of the firing angle. Construct the firing sequence table for better understanding.

6. For the below given circuit, the conduction sequence for the negative group of SCRs is
power-electronics-questions-answers-three-phase-converters-2-q3
a) T4-T6-T2
b) T1-T2-T3
c) T2-T6-1
d) T2-T4-T6

View Answer

Answer: d [Reason:] The negative group of SCRs has T2, T4 and T6. The conduct as T2-T4-T6, as T2 is connected to the B phase, T4 to the R phase and like-wise.

7. For a three-phase full controlled converter with R load, the average value of output voltage is zero for
a) α = 0°
b) α = 90°
c) α = 180°
d) It can never be zero

View Answer

Answer: b [Reason:] For α = 90 degrees, the voltage waveform is equally symmetrical about the ωt axis, hence the average value is zero. This can also be found by using the formula for average output voltage, Vo = (3Vml/π) cos α, For α = 90°, cosα = 0, Vo = zero.

8. A three-phase full converter charges a battery from a three-phase supply of 230 V. The battery emf is 200 V and the internal resistance of the battery is 0.5 Ω. Find the value of the continuous current which is flowing through the battery if its terminal voltage is 210 V
a) 10 A
b) 20 A
c) 0.5 A
d) 25 A

View Answer

Answer: b [Reason:] Vo = 210 V Vo = E + Io x R 210 = 200 + 0.5 x Io Io (Current through the battery) = 20 A.

9. A three-phase full converter charges a battery from a three-phase supply of 230 V. The battery emf is 200 V. Find the value of the firing angle if the battery terminal voltage is 210 V.
a) 36.54°
b) 56.7°
c) 89.3°
d) 47.45°

View Answer

Answer: d [Reason:] Vo = (3Vml/π) cos α α = cos-1(210π/3√2×230) = 47.453°.

10. A three-phase full converter charges a battery from a three-phase supply of 230 V. Find the value of the power delivered to the load if a continues current of 20A is flowing through the battery of emf 200 V and internal resistance of 0.5 Ω.
a) 0 W
b) 5600 W
c) 4200 W
d) 1040 W

View Answer

Answer: c [Reason:] Iavg = Irms = 20 A P = E x Iavg + Irms2 x R = 4200 W.

Set 3

1. A three-phase full converter supplied from a 230 V source is working as a line commutated inverter. The load consists of RLE type with R = 5 Ω, E = 200 V and L = 1 mH. A continues current of 10 A is flowing through the load, find the value of the firing angle delay.
a) 119°
b) 127°
c) 156°
d) 143°

View Answer

Answer: a [Reason:] Vo = 200 – 10×5 = 150V as the circuit is operating as an inverter Vo = -150V Now, Vo = (3Vml/π) cos α α = cos-1(-150π/3√2×230) = 118.88°.

2. A three-phase full converter is driving a DC motor. If a continues current of Im amperes is flowing through the motor load, then find the rms value of supply current drawn by the converter to drive the motor.
a) Im/√2
b) Im2/3
c) √2Im/√3
d) √2Im/3

View Answer

Answer: c [Reason:] The RMS value of the supply current IS over π radians would be (IS)2 = (1/π) x (Im)2 x (2π/3) = Im√2/√3.

3. Name the below given circuit.
power-electronics-questions-answers-three-phase-converters-3-q3
a) Full controlled, bridge converter
b) Full controlled, semi converter
c) Bridge type semi-converter
d) Half controlled, full converter

View Answer

Answer: c [Reason:] It uses 3 SCRs and 3 diodes, hence it is a semi-converter. Option (b) and (d) make no sense, because there can be no full controlled semi-converter.

4. In the below given circuit, each SCR and diode conduct for
power-electronics-questions-answers-three-phase-converters-3-q3
a) 60° and 120° respectively
b) 120° and 60° respectively
c) 120°
d) 60°

View Answer

Answer: c [Reason:] At any given time, one SCR and one diode is conducting, each conduct for 120° per cycle.

5. In the below given circuit, __ and __ conduct along with T2.
power-electronics-questions-answers-three-phase-converters-3-q3
a) T1, T3
b) D1, D2
c) D1, D3
d) T1, T2

View Answer

Answer: c [Reason:] When one SCR conducts, a diode conducts along with it at a time to provide the path of current flow. . For example, if T2 starts conducting at 90° it will conduct till 90+120 = 210°. But while T2 is conducting, half of the time i.e. from 90 to 150 D1 is conducting and another half of the time D3 is conducting. T2 and D2 cannot conduct together as it will cause a short circuit. Hence, T2-D1 conduct for 60° and then T2-D3 conduct for another 60°.

6. In a three-phase semi-converter, at a time one SCR and one diode conduct simultaneously. With SCR T1 conducting which diode(s) is most likely to conduct along with T1?
power-electronics-questions-answers-three-phase-converters-3-q6
a) D2 only
b) D3 only
c) D1 and D2
d) D2 and D3

View Answer

Answer: d [Reason:] T1 and D1 together will cause a S.C. D2 or D3 any of these two can conduct along with T1 depending on which phase voltage is currently active RB or RY.

7. What is the value of voltage at the output terminal when the freewheeling diode (FD) is conducting?
power-electronics-questions-answers-three-phase-converters-3-q6
a) Zero
b) Maximum
c) E
d) It could be anything depending on α

View Answer

Answer: a [Reason:] When FD is conducting it will short circuit the load terminal resulting in zero voltage. It won’t be E because the terminals are shorted. It can be E when none of the devices are conducting (This can happen only when α > 120°).

8. A 3-phase full converter feeds power to an R load of 10 Ω. For a firing angle delay of 30° the load takes 5 kW. An inductor of large value is also connected to the load to make the current ripple free. Find the value of per phase input voltage.
a) 133 V
b) 230/√3 V
c) 191/√3 V
d) 298/√3 V

View Answer

Answer: c [Reason:] Ior = Vo/R = (3Vml/Rπ) cos α P = 5 kW = Ior2 x R = [(3Vml/π) cos α]2 x 1/R] Therefore, Vs (line) = √50000 x (π/√2 x 3 x cos30) = 191.22 V Vs (phase) = 191/√3 V.

9. A three-phase semi-converter circuit is given a supply of 400 V. It produces at the output terminals an average voltage of 381 V. Find the rectification efficiency of the converter circuit.
a) 99.65 %
b) 95.25 %
c) 91 %
d) 86.5 %

View Answer

Answer: b [Reason:] Rectification efficiency = Pdc/Pac Pdc = Vo x Io . . . (both average values of output current and voltage) Pac = Vrms x Irms . . . (both rms values of input current and voltage) For a semi-converter Irms = Io Therefore, Rectification efficiency = 381/400 = 95.25 %.

10. In a 3-phase semi-converter, for firing angle less than 60° the freewheeling diode conducts for
a) 30°
b) 60°
c) 120°
d) 0

View Answer

Answer: d [Reason:] In case of a semi-converter operating with α < 60°, FD does not comes into play, as the voltage never falls to zero and gives no chance for the inductor to discharge.

Set 4

1. The process of commutating a SCR by applying a reverse voltage to an SCR through a previously charged capacitor is called as
a) capacitor commutation
b) forced commutation
c) voltage commutation
d) current commutation

View Answer

Answer: c [Reason:] It is called as voltage commutation, which is a type of forced commutation.

2. In case of current commutation of SCR
a) a diode is connected in series with the main SCR
b) a diode is connected in parallel with the main SCR
c) a diode is connected in anti-parallel with the main SCR
d) none of the mentioned

View Answer

Answer: c [Reason:] The voltage drop across the anti parallel connected SCR reverse biases the main SCR.

3. Below given circuit is a
power-electronics-questions-answers-thyristor-chopper-circuits-1-q3
a) current commutated chopper
b) voltage commutated chopper
c) load commutated chopper
d) none of the mentioned

View Answer

Answer: b [Reason:] It is a voltage commutated chopper or impulse-commutated chopper. If the diode was directly in parallel with the main SCR it would be a current commutated chopper circuit.

4. In the below shown voltage commutated chopper circuit, the chopper operation can only start when
power-electronics-questions-answers-thyristor-chopper-circuits-1-q3
a) t1 is gated
b) the capacitor is charged
c) the diode is forward biased
d) the capacitor is dis-charged

View Answer

Answer: b [Reason:] The chopper can only start (effectively) when the C in the commutating circuit is charged first.

5. In a voltage commutated chopper circuit, the capacitor C can be charged to Vs by
power-electronics-questions-answers-thyristor-chopper-circuits-1-q3
a) gating T1
b) gating TA
c) closing the switch
d) both gating TA and closing the switch

View Answer

Answer: d [Reason:] When switch is closed, the C gets charged through Vs, C, Switch. When TA is triggered C gets charged through Vs, C, TA and the load.

6. In the below given circuit, when the main thyristor T1 is triggered after charging the capacitor, the commutation current flows through the path
power-electronics-questions-answers-thyristor-chopper-circuits-1-q3
a) Vs – T1 – Load – Vs
b) S – C – T1 – Load – S
c) C – T1 – L – D – C
d) C – T1 – TA – C

View Answer

Answer: c [Reason:] The commutation current is nothing but the capacitor current.

7. In the below given circuit, when the main SCR T1 is commutated at t2, the load voltage is
power-electronics-questions-answers-thyristor-chopper-circuits-1-q3
a) Vs
b) 0
c) 2Vs
d) Vs + Ldi/dt

View Answer

Answer: c [Reason:] The main SCR T1 is commutated by the triggering of the TA which then makes the capacitor revere bias the main SCR T1. The load current then flows through Vs-C-TA-Load. Hence, Vo = Vs + (capacitor voltage). As the capacitor is already charged to a voltage Vs. Vo = 2Vs.

8. In a voltage commutated SCR, the main SCR is commutated by
a) triggering the auxiliary SCR
b) sending a negative pulse to the main SCR
c) removing the supply Vs
d) none of the mentioned

View Answer

Answer: a [Reason:] The main SCR T1 is commutated by the triggering of the TA which then makes the capacitor revere bias the main SCR T1.

9. Which type of commutation circuit does not work on no load?
a) Voltage commutation
b) Current commutation
c) Both voltage and current commutation
d) None of the mentioned

View Answer

Answer: a [Reason:] This is because the capacitor won’t be able to charge on no load.

10. What is the expression for the value of the capacitor in the voltage commutation circuit? Let tc be the circuit turn off time and Io be the constant load current.
a) Io x (dv/dt)
b) Vs/Io
c) (tc x Io)/Vs
d) none of the mentioned

View Answer

Answer: c [Reason:] Capacitor current = C dv/dt For constant load current Io, the above expression can be Io = C x (Vs/tc) or C = (tc x Io)/Vs.

Set 5

1. For the below given voltage commutation circuit, find the peak value of voltage across the freewheeling diode FD.
power-electronics-questions-answers-thyristor-chopper-circuits-2-q1
a) Vs
b) 0
c) 2Vs
d) Vs + Ldi/dt

View Answer

Answer: c [Reason:] The load current then flows through Vs-C-TA-Load. Hence, Vo = Vs + (capacitor voltage). As the capacitor is already charged to a voltage Vs. Vo = Voltage across FD = 2Vs.

2. A voltage commutated chopper feeds power to an electric car (battery-powered). The battery voltage is 60 V. The SCR turn-off time is 20 μs. Calculate the value of capacitor (C) required for the commutating circuit.
a) 400 μF
b) 40 nF
c) 40 μF
d) 0.4 mF

View Answer

Answer: c [Reason:] Capacitor current = C dv/dt For constant load current Io, the above expression can be Io = C x (Vs/tc) or C = (tc x Io)/Vs C = ( 40 x 60 x 10-6)/60 = 40 x 10-6 Farads.

3. Identify the below given circuit.
power-electronics-questions-answers-thyristor-chopper-circuits-2-q1
a) current commutated chopper
b) voltage commutated chopper
c) load commutated chopper
d) load commutated inverter

View Answer

Answer: a [Reason:] Diode in anti-parallel with the main SCR. Hence (a).

4. A voltage commutated chopper feeds power to an electric car (battery-powered). The battery voltage is 60 V. Starting current is 60 A and the SCR turn-off time is 20 μs. Calculate the value of inductance (L) required for the commutating circuit.
a) 400 μH
b) 40 nH
c) 40 μH
d) 0.4 mH

View Answer

Answer: c [Reason:] L = (Vs/Io)2 x C C = (tc x Io)/Vs C = ( 40 x 60 x 10-6)/60 = 40 x 10-6 F L = (60/60)2 x 40 x 10-6 = 40 uH.

5. In the below given circuit, at the instant T1 the main SCR is triggered, the voltage across the load terminal is
power-electronics-questions-answers-thyristor-chopper-circuits-2-q1
a) Vs
b) 0
c) 2Vs
d) Vs + Ldi/dt

View Answer

Answer: a [Reason:] When the main SCR is gated, the commutating circuit remains inactive. Hence the load is directly connected to Vs through the SCR. Considering SCR as an ideal switch Vo = Vs.

6. In current commutated copper circuit
a) the capacitor cannot charge on no load
b) the auxiliary thyristor is turned on automatically
c) the auxiliary thyristor is naturally commutated
d) the commutation is not reliable

View Answer

Answer: c [Reason:] It is naturally commutated as its commutating current passes through zero value in the ringing circuit formed by L and C.

7. A current commutated chopper is fed from a source of 230 V. Its commutating components are L = 20 μH and C = 50 μF. Find the value of peak commutating current.
a) 233 A
b) 59 A
c) 363 A
d) 198 A

View Answer

Answer: c [Reason:] I = V √(C/L) = 230 x √(50/20) = 363.66 A.

8. A voltage commutated chopper has the following parameters:
Vs = 220 V. Commutation circuit: L = 20 μH and C = 50 μF.
Find the effective on period for a constant load current of 80 A and Ton = 800 μsecs.
a) 1.075 μs
b) 10.75 ms
c) 1075 μs
d) 1.075 sec

View Answer

Answer: c [Reason:] The effective on period Ton is given by Ton + 2Vs.C/Io.

9. A voltage commutated chopper has the following parameters:
Vs = 220 V. Commutation circuit: L = 20 μH and C = 50 μF.
Find the turn-off time for the main SCR with a constant load current of 80 A and Ton = 800 μsecs.
a) 137.5 μs
b) 1075 μs
c) 49.67 μs
d) 200 μs

View Answer

Answer: a [Reason:] C = (tc x Io)/Vs tc = C.Vs/I = 137.5 μsecs.

10. The load commutated chopper circuit consists of
a) two thyristors and one commutating capacitor
b) four thyristors and one commutating capacitor
c) two thyristors and two commutating capacitors
d) four thyristors and two commutating capacitors

View Answer

Answer: b [Reason:] It consists of 4 SCRs and one capacitor. 4 SCRs forming 2 pairs of 2 SCRs conduct alternately.