# 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

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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°

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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°

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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

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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.

a) (3Vmp/2π) x cosα

b) (3√3Vmp/2π) x cosα

c) (3√3Vmp/2π) x sinα

d) (3Vmp/2π) x sinα

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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

a) α to 180°

b) 30 + α to 180°

c) 30 + α to 150°

d) 30 + α to 120°

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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.

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)].

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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

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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°

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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

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## Set 2

1. A three phase full converter will require __________ number of SCRs.

a) 3

b) 6

c) 9

d) 2

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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

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3. For the below given circuit, α = 60°. T2 will start conduction at ωt = __________ Assume the inductor L value to be negligible.

a) 60°

b) 120°

c) 90°

d) 150°

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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

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5. In case of a three phase full controlled converter with 6 SCRs, commutation occurs every

a) 120°

b) 60°

c) 180°

d) 30°

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6. For the below given circuit, the conduction sequence for the negative group of SCRs is

a) T4-T6-T2

b) T1-T2-T3

c) T2-T6-1

d) T2-T4-T6

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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

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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

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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°

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^{-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

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^{2}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°

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^{-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) Im^{2}/3

c) √2Im/√3

d) √2Im/3

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_{S}over π radians would be (I

_{S})

^{2}= (1/π) x (Im)

^{2}x (2π/3) = Im√2/√3.

3. Name the below given circuit.

a) Full controlled, bridge converter

b) Full controlled, semi converter

c) Bridge type semi-converter

d) Half controlled, full converter

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4. In the below given circuit, each SCR and diode conduct for

a) 60° and 120° respectively

b) 120° and 60° respectively

c) 120°

d) 60°

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5. In the below given circuit, __ and __ conduct along with T2.

a) T1, T3

b) D1, D2

c) D1, D3

d) T1, T2

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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?

a) D2 only

b) D3 only

c) D1 and D2

d) D2 and D3

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7. What is the value of voltage at the output terminal when the freewheeling diode (FD) is conducting?

a) Zero

b) Maximum

c) E

d) It could be anything depending on α

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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

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^{2}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 %

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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

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## 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

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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

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3. Below given circuit is a

a) current commutated chopper

b) voltage commutated chopper

c) load commutated chopper

d) none of the mentioned

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4. In the below shown voltage commutated chopper circuit, the chopper operation can only start when

a) t1 is gated

b) the capacitor is charged

c) the diode is forward biased

d) the capacitor is dis-charged

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5. In a voltage commutated chopper circuit, the capacitor C can be charged to Vs by

a) gating T1

b) gating TA

c) closing the switch

d) both gating TA and closing the switch

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6. In the below given circuit, when the main thyristor T1 is triggered after charging the capacitor, the commutation current flows through the path

a) Vs – T1 – Load – Vs

b) S – C – T1 – Load – S

c) C – T1 – L – D – C

d) C – T1 – TA – C

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7. In the below given circuit, when the main SCR T1 is commutated at t_{2}, the load voltage is

a) Vs

b) 0

c) 2Vs

d) Vs + Ldi/dt

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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

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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

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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

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## Set 5

1. For the below given voltage commutation circuit, find the peak value of voltage across the freewheeling diode FD.

a) Vs

b) 0

c) 2Vs

d) Vs + Ldi/dt

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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

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^{-6})/60 = 40 x 10

^{-6}Farads.

3. Identify the below given circuit.

a) current commutated chopper

b) voltage commutated chopper

c) load commutated chopper

d) load commutated inverter

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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

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^{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

a) Vs

b) 0

c) 2Vs

d) Vs + Ldi/dt

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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

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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

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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

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^{’}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

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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