# Multiple choice question for engineering

## Set 1

1. Identify the below given circuit.

a) Three-phase bridge regulator

b) Three-phase bridge type semi-converter circuit

c) Three-phase bridge thyristor inverter

d) Three-phase bridge IGBT inverter

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2. A three-phase bridge inverter requires minimum of _____________ switching devices.

a) 3

b) 4

c) 6

d) 8

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3. The below given inverter circuit is a __ step inverter.

a) 3

b) 2

c) 6

d) none of the mentioned

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4. In the three-phase bridge inverter, each step consists of

a) 30°

b) 60°

c) 90°

d) will depend on the value of the firing angle

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5. In inverters, to make the supply voltage constant

a) an inductor is placed in series with the load

b) capacitor is connected in parallel to the load side

c) capacitor is connected in parallel to the supply side

d) none of the mentioned

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6. In the 180° mode VSI, ___________ devices conduct at a time.

a) 5

b) 2

c) 3

d) 4

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7. In the figure given below, for 180° mode of operation if T1 is fired at 0°. Then SCRs T3 and T5 should be fired at _________ and _________ respectively.

a) 180°, 360°

b) 90°, 180°

c) 120°, 240°

d) none of the mentioned

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8. For a three phase bridge inverter in the 180° mode, ___________ devices are conducting from 120° to 180°.

a) T1, T6, T5

b) T2, T6, T5

c) T1, T6, T5

d) T1, T2, T3

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Group I | T1 | T1 | T1 | T4 | T4 | T4 | T1 | T4 |

Group II | T6 | T6 | T3 | T3 | T3 | T6 | T6 | T6 |

Group III | T5 | T2 | T2 | T2 | T5 | T5 | T5 | T2 |

Step No | I | II | III | IV | V | VI | I | II |

Each step consists of 60°. 120° to 180° will be step III.

9. _________ SCRs conduct from 300° to 360°.

a) T1, T2, T3

b) T4, T5, T6

c) T4, T3, T2

d) T1, T6, T5

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Group I | T1 | T1 | T1 | T4 | T4 | T4 | T1 | T4 |

Group II | T6 | T6 | T3 | T3 | T3 | T6 | T6 | T6 |

Group III | T5 | T2 | T2 | T2 | T5 | T5 | T5 | T2 |

Step No | I | II | III | IV | V | VI | I | II |

Each step consists of 60°. 300° to 360° will be step VI.

10. The diodes D4 and D1 will conduct from

a) they will never conduct

b) 300° to 360°

c) 120° to 180°

d) insufficient information

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## Set 2

1. The conducting SCRs from 180 to 240 degrees would be

a) 6, 1, 2

b) 2, 3, 4

c) 3, 4, 5

d) 5, 6, 1

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2. What is the peak value of phase voltage in case of 3-phase VSI with 180° mode. The supply side consists of a constant dc voltage source of Vs.

a) Vs

b) 3Vs/2

c) 2Vs/3

d) 3Vs

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3. What is the R phase voltage when T1, T6 and T5 are conducting? Consider a star connected R load.

a) Vs

b) 2Vs/3

c) Vs/3

d) –Vs/3

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_{RN}= V

_{BN}= Vs/3. V

_{YN}= -2Vs/3.

4. What are the phase voltages when T6, T1 and T2 are conducting? Consider a star connected R load.

a) V_{YN} = V_{BN} = Vs/3, V_{RN} = 0

b) V_{YN} = V_{RN} = – Vs/3, V_{BN} = 2Vs/3

c) V_{RN} = V_{BN} = – Vs/3, V_{YN} = 2Vs/3

d) V_{YN} = V_{BN} = – Vs/3, V_{RN} = 2Vs/3

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_{YN}= V

_{BN}= -Vs/3. V

_{RN}= 2Vs/3. It should be noted that at any time, the summation of these phase voltages must be zero.

5. What are the phase voltages when T1, T2 and T3 are conducting? Consider a star connected R load.

a) V_{YN} = V_{BN} = V_{RN} = 0

b) V_{YN} = V_{RN} = Vs/3, V_{BN} = – 2Vs/3

c) V_{RN} = V_{BN} = – Vs/3, V_{YN} = 2Vs/3

d) V_{YN} = V_{BN} = Vs/3, V_{RN} = – 2Vs/3

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_{RN}= V

_{YN}= Vs/3. V

_{BN}= -2Vs/3. It should be noted that at any time, the summation of these phase voltages must be zero.

6. What is the maximum line voltage value in case of a three-phase VSI in 180° mode?

a) 2Vs

b) Vs

c) 3Vs

d) 2Vs/3

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7. Find V_{YB}, when T6, T1 and T2 are conducting. Consider a star connected R load.

a) Vs

b) Vs/3

c) 0

d) 2Vs/3

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_{YN}= V

_{BN}= -Vs/3. V

_{YB}= V

_{YN}– V

_{BN}= (-Vs/3) – (-Vs/3) = -Vs/3 + Vs/3 = 0.

8. Find V_{RY}, when T6, T1 and T2 are conducting. Consider a star connected R load.

a) Vs

b) Vs/3

c) 0

d) 2Vs/3

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_{YN}= V

_{BN}= -Vs/3. V

_{RN}= 2Vs/3 V

_{RY}= V

_{RN}– V

_{YN}= (2Vs/3) – (-Vs/3) = 3Vs/3 = Vs.

9. What is the Y phase voltage, when T2, T3 and T4 are conducting?

a) 0

b) Vs/3

c) 2Vs/3

d) -Vs/3

### View Answer

_{RN}= V

_{BN}= -Vs/3. V

_{YN}= 2Vs/3.

10. What is the voltage between the B phase and the neutral, when T2, T3 and T4 are conducting?

a) -2Vs/3

b) Vs/3

c) 2Vs/3

d) -Vs/3

### View Answer

_{RN}= V

_{BN}= -Vs/3. V

_{YN}= 2Vs/3.

## Set 3

1. The 120° mode of operation of a three phase bridge inverter requires ___________ number of steps.

a) 2

b) 4

c) 6

d) 8

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2. In case of the 120° mode of operation, __________ devices conduct at a time.

a) 2

b) 3

c) 4

d) none of the mentioned

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3. Safe commutation can be achieved in case of the ____________ operating mode.

a) 180°

b) 120°

c) 360°

d) none of the mentioned

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4. What is the R phase voltage when only T1 and T2 are conducting from 60° to 120°. Consider star connected R load.

a) Vs

b) Vs/2

c) –Vs/2

d) 0

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_{RN}= Vs/2 V

_{YN}= -Vs/2.

5. If T1 is gated at 0 °, T3 and T5 will start conducting at _______ and _________ respectively.

a) 180°, 270°

b) 120°, 240°

c) 180°, 300°

d) 240°, 360°

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Devices | T1 | T1 | 0 | T4 | T4 | 0 | T1 | T1 | 0 |

T6 | 0 | T3 | T3 | 0 | T6 | T6 | 0 | T3 | |

0 | T2 | T2 | 0 | T5 | T5 | 0 | T2 | T2 |

6. What is the B phase voltage when only T1 and T2 are conducting from 60° to 120°. Consider star connected R load.

a) Vs

b) Vs/2

c) –Vs/2

d) 0

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Devices | T1 | T1 | 0 | T4 | T4 | 0 | T1 | T1 | 0 |

T6 | 0 | T3 | T3 | 0 | T6 | T6 | 0 | T3 | |

0 | T2 | T2 | 0 | T5 | T5 | 0 | T2 | T2 |

V_{RN} = Vs/2
V_{BN} = -Vs/2.

7. What is the R phase voltage and Y phase voltage when only T3 and T4 are conducting? Consider a star connected balanced R load.

a) Vs, -Vs

b) Vs/2, -Vs/2

c) –Vs/2, Vs/2

d) –Vs, Vs

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Devices | T1 | T1 | 0 | T4 | T4 | 0 | T1 | T1 | 0 |

T6 | 0 | T3 | T3 | 0 | T6 | T6 | 0 | T3 | |

0 | T2 | T2 | 0 | T5 | T5 | 0 | T2 | T2 |

V_{YN} = Vs/2
V_{RN} = -Vs/2.

8. If the SCR T1 is gated at 0°, then for 120° mode of operation, from ωt = 240° to 300° __________ devices would conduct.

a) T3, T4

b) T4, T5

c) T1, T6

d) T5, T6

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Devices | T1 | T1 | 0 | T4 | T4 | 0 | T1 | T1 | 0 |

T6 | 0 | T3 | T3 | 0 | T6 | T6 | 0 | T3 | |

0 | T2 | T2 | 0 | T5 | T5 | 0 | T2 | T2 |

ωt = 240° to 300°, would form the 5th step.

9. Find the line voltage V_{YR} when only T3 and T4 are conducting? Consider a star connected balanced R load.

a) 2Vs/3

b) Vs/2

c) Vs

d) –Vs

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_{YN}= Vs/2 V

_{RN}= -Vs/2. V

_{YR}= V

_{YN}– V

_{RN}= Vs.

10. The peak value of the line voltage in case of 120° mode of operation of a three-phase bridge inverter is

a) Vs/2

b) 3Vs/2

c) Vs/√2

d) Vs

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## Set 4

1. AC voltage controllers convert

a) fixed ac to fixed dc

b) variable ac to variable dc

c) fixed ac to variable ac

d) variable ac to fixed ac

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2. In AC voltage controllers the

a) variable ac with fixed frequency is obtained

b) variable ac with variable frequency is obtained

c) variable dc with fixed frequency is obtained

d) variable dc with variable frequency is obtained

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3. Earlier then the semiconductor technology, ___________ devices were used for voltage control applications.

a) cycloconverters

b) vacuum tubes

c) tap changing transformer

d) induction machine

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4. The AC voltage controllers are used in __________ applications.

a) power generation

b) electric heating

c) conveyor belt motion

d) power transmission

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5. In the principle of phase control

a) the load is on for some cycles and off for some cycles

b) control is achieved by adjusting the firing angle of the devices

c) control is achieved by adjusting the number of on off cycles

d) control cannot be achieved

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6. A single-phase half wave voltage controller consists of

a) one SCR is parallel with one diode

b) one SCR is anti parallel with one diode

c) two SCRs in parallel

d) two SCRs in anti parallel

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7. In the below given voltage controller circuit

a) the positive half cycle at the load is same as the supply Vs

b) the negative half cycle at the load is same as the supply Vs

c) the positive and negative half cycles at the load are identical to the supply

d) none of the mentioned

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8. The below shown controller circuit is a

a) half wave controller

b) full wave controller

c) none of the mentioned

d) will depend upon the firing angle

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9. The below given controller circuit is a

a) half wave controller

b) full wave controller

c) none of the mentioned

d) will depend upon the firing angle

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10. In the below given voltage controller circuit

a) only the negative cycle can be controlled

b) only the positive cycle can be controlled

c) both the cycles can be controlled

d) none of the mentioned

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## Set 5

1. In the below shown circuit, the diode conducts for

a) 90°

b) > 90°

c) < 90°

d) 0°

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2. The SCR T1 is fired at an angle of α, and the supply Vs = Vm sinωt. Find the average value of the output voltage.

a) (Vm/2π) (cosα + )

b) (Vm/2π) (cosα)

c) (Vm/2π) (cosα – 1)

d) Vm

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3. The below given output voltage waveform can be obtained by a

a) half wave ac voltage controller

b) full wave ac voltage controller

c) half wave controller with firing angle = 0° for T1

d) full wave controller with firing angle = 0° for both T1 and T2

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4. In the below shown AC converter circuit with firing angle = α for both the devices, T2 will conduct from

a) α to π

b) π + α to 2π

c) π to 2π

d) α to 2π

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5. The below given output voltage waveform can be obtained by a

a) half wave ac voltage controller

b) full wave ac voltage controller

c) full wave inverter circuit

d) none of the mentioned

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6. In the integral cycle control method of ac voltage controller

a) the average power delivered to the load is controlled

b) the instantaneous power delivered to the load is controlled

c) the frequency of output voltage is controlled

d) none of the mentioned

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7. In the integral cycle control of ac voltage controller, is the load is on for n cycles and off for m cycles, then the periodicity is given by? Consider the output is sinusoidal.

a) m/2π(m+n)

b) n/2π(m+n)

c) m/π(m+n)

d) n/π(m+n)

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8. If k is the duty cycles of the controller, then the rms value of the output voltage in case of a integral cycle control circuit will be?

Consider the input to be sinusoidal with peak value Vm and rms value Vs.

a) Vs x k

b) Vs/k

c) Vs x √k

d) Vs

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^{2 }sin

^{2}ωt d(ωt) ]

^{ 1/2 }Where the integral runs from 0 to 2π. Vrms = (Vm/√2) k = Vs x √k.

9. Find the power delivered to the load in the integral cycle control method of ac voltage control, having a sine input of Vs, R load and duty cycle = k.

a) Vs^{2}/R

b) k.Vs^{2}/R

c) √k .Vs^{2}/R

d) 0

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^{2}/R.

10. In the integral cycle control method with duty cycle = k and maximum load current = Im. Find the value of average SCR current.

a) Im/k.π

b) Im

c) k.Im

d) k.Im/π