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Multiple choice question for engineering

Set 1

1. The below given circuit has Vs = 230V and R = 20 Ω. Find the value of the average output voltage at the R load for a firing angle of 45°.
power-electronics-questions-answers-ac-voltage-controllers-3-q1
a) 224 V
b) -15.17 V
c) 15.17 V
d) –224 V

View Answer

Answer: b [Reason:] Vo = [(√2 x 230) x (cos45 – 1)]/2π = -15.17 V. Negative value is due to the fact that the average value in the positive half cycle is less than that in the negative half cycle.

2. The below given circuit has Vs = 230V and R = 20 Ω. Find the value of the average output load current at the R load for a firing angle of 45°.
power-electronics-questions-answers-ac-voltage-controllers-3-q1
a) – 0.7585
b) 0.7585
c) -0.6396
d) -0.5

View Answer

Answer: a [Reason:] Vo = [(√2 x 230) x (cos45 – 1)]/2π = -15.17 V. Io = (Vo)/R = -0.7585 Negative value is due to the fact that the average value in the positive half cycle is less than that in the negative half cycle.

3. A single phase voltage controller has input of 230 V and a load of 15 Ω resistive. For 6 cycles on and 4 cycles off, determine the rms output voltage.
a) 189 V
b) 260 V
c) 156 V
d) 178 V

View Answer

Answer: d [Reason:] Vrms = Vo x √k k = (6/6+4) = 6/10 = 0.6 Vrms = √0.6 x 230 = 178.157 V.

4. A single phase voltage controller has input of 230 V and a load of 15 Ω resistive. For 6 cycles on and 4 cycles off, determine the input pf.
a) 0.6
b) 0.7746
c) 0.855
d) 0.236

View Answer

Answer: b [Reason:] k = (6/6+4) = 6/10 = 0.6 input pf = √0.6 = 0.7746.

5. A single phase voltage controller has input of 230 V and a load of 15 Ω resistive. For 6 cycles on and 4 cycles off, determine the power delivered to the load.
a) 2.1 W
b) 2.1 kW
c) 516 W
d) 5.16 kW

View Answer

Answer: b [Reason:] Vrms = Vo x √k k = (6/6+4) = 6/10 = 0.6 Vrms = √0.6 x 230 = 178.157 V P = (Vrms)2/R = 2116 W = 2.116 kW.

6. A single phase voltage controller has input of 230 V and a load of 15 Ω resistive. For 6 cycles on and 4 cycles off, determine the average value of SCR current.
a) 21.68 A
b) 200 mA
c) 4.14 A
d) 2.07 A

View Answer

Answer: c [Reason:] Peak current Im = (230 x √2)/15 = 21.681 A k = (6/6+4) = 6/10 = 0.6 Avg current = (k Im)/π = 4.14 A.

7. Pulse gating is suitable for
a) R loads only
b) R and RL loads
c) RL loads only
d) all types of loads

View Answer

Answer: a [Reason:] In RL loads with pulse gating, the incoming SCR may be fired during the interval when it is reversed biased by the outgoing SCR, thus it won’t get turn on even after the outgoing SCR is not reveres basing the incoming SCR is the pulse width is over before forward biasing the SCR.

8. In continues gating
a) overlap angle is very high
b) SCR is heated up
c) size of the pulse transformer is small
d) commutation cannot be achieved effectively

View Answer

Answer: b [Reason:] As the gating is applied for a longer duration, the device is heated up.

9. High frequency gating uses a
a) train of pulses
b) continuous gating block
c) carrier signal
d) none of the above

View Answer

Answer: a [Reason:] In high frequency gating a train of pulses are used to overcome the thermal problems due to continuous gating.

10. A single-phase voltage controller, using one SCR in anti parallel with a diode, feeds a load R and Vs = 230 V. For a firing angel of 90° for the SCR, the PMMC voltage connected across R would read
a) 0
b) 51.8 V
c) –51.8 V
d) –36.82 V

View Answer

Answer: c [Reason:] As firing angle is 90, there is ideally be no conduction in the positive half. Hence, the average value will be zero. Vo = (√2 Vs)/2π x (cos90 – 1) = – 51.8 V.

Set 2

1. In a 3 phase M-6 controlled converter for continuous conduction mode, each SCR conducts for __________ per cycle.
a) 2π/6 radians
b) 30 degrees
c) 3π/2 radians
d) 120 degrees

View Answer

Answer: a [Reason:] Each SCR conducts for 60° or 2π/6 radians. 6 x 60 = 360°.

2. In a 3 phase, 12-pulse controlled converter for continuous conduction mode, each SCR conducts for __________ per cycle.
a) π/6 radians
b) 60 degrees
c) 3π/2 radians
d) 12 degrees

View Answer

Answer: a [Reason:] Each SCR conducts for 30° or π/6 radians. 12 x 30 = 360°.

3. A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. The input voltage is 3-phase, 400 V, 50 Hz. The source current is given by the following relation.
power-electronics-questions-answers-advance-three-phase-convters-q3
Find the fundamental component of the source current amplitude.
a) 11.03
b) 2.205
c) 11.03 sin 45
d) 46.98

View Answer

Answer: a [Reason:] Put n = 1 (fundamental component) and the rest of the given values in the above given equation.

4. A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. Find the DF (distortion factor).
a) 1.414
b) 0
c) 0.707
d) 0.569

View Answer

Answer: c [Reason:] DF = cosα = cos45 = 0.707 Note that ripple free current does not mean that DF = 0. It means the current is continues in magnitude and direction, current will always contain harmonics (distortions) whether it is rippled or ripple free.

5. What is the relationship between DF, CDF and PF?
a) PF = CDF = DF
b) PF = CDF/DF
c) PF = DF/CDF
d) PF = CDF x DF

View Answer

Answer: d [Reason:] PF (power factor) = (distortion factor) x (current distortion factor).

6. The commutation period when both incoming and outgoing SCRs are conducting due to source inductance is called as the
a) conduction delay
b) overlap period
c) one on one period
d) distorting angle

View Answer

Answer: b [Reason:] Due to source inductance, SCRs cannot start and stop conducting immediately, hence a time occurs when both incoming and outgoing SCRs are conducting together. This delay is called as commutation angle or overlap period (μ).

7. A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. The input voltage is 3-phase, 400 V, 50 Hz. The source current is given by the following relation.
power-electronics-questions-answers-advance-three-phase-convters-q3
Find the value of 2nd harmonic source current amplitutue.
a) 11.25 A
b) 0.256 A
c) 2.69 sin (ωt – 45) A
d) 0 A

View Answer

Answer: d [Reason:] 2nd harmonics are absent in a 3-phase full converter, it has only odd number of harmonics i.e. 3rd, 5th etc.

8. In a p-pulse converter, each SCR conducts for (per cycle)
a) p radians
b) p degrees
c) p/2π radians
d) 2π/p radians

View Answer

Answer: d [Reason:] In a p-pulse controlled converter, each device conducts for p radians. p-pulse converter would have p devices. Hence, 2π/p x p = 2π = 360°.

9. Which of the below mentioned converter can operate in both 3-pulse and 6-pulse modes?
a) 3-phase half wave controller
b) 3-phase full converter
c) 3-phase semi-converter
d) None of the mentioned

View Answer

Answer: c [Reason:] Semi-converters can operate in 6-pulse modes gating the SCRs at appropriate intervals.

10. A M-6 controlled converter or 6-pulse half-wave controlled converter is obtained by using a transformer having
a) a double delta connected secondary winding
b) a double star connected secondary winding
c) a double delta connected primary winding
d) 6-windings on both primary and secondary sides

View Answer

Answer: b [Reason:] M-6 requires a transformer having a delta connected primary and a double star connected secondary such that 6 SCRs are connected to it on the secondary side.

Set 3

1. SMPS is used for
a) obtaining controlled ac power supply
b) obtaining controlled dc power supply
c) storage of dc power
d) switch from one source to another

View Answer

Answer: b [Reason:] SMPS (Switching mode power supply) is used for obtaining controlled dc power supply.

2. SPMS are based on the ________ principle.
a) Phase control
b) Integral control
c) Chopper
d) MOSFET

View Answer

Answer: c [Reason:] SMPS (Switching mode power supply) are based on the chopper principle. The output dc voltage is controlled by varying the duty cycle of the chopper circuit.

3. Choose the incorrect statement.
a) SMPS is less sensitive to input voltage variations
b) SMPS is smaller as compared to rectifiers
c) SMPS has low input ripple
d) SMPS is a source of radio interference

View Answer

Answer: c [Reason:] SMPS has higher output ripple and its regulation is worse.

4. _________ is used for critical loads where temporary power failure can cause a great deal of inconvenience.
a) SMPS
b) UPS
c) MPS
d) RCCB

View Answer

Answer: b [Reason:] Uninterruptible Power Supply is used where loads where temporary power failure can cause a great deal of inconvenience.

5. __________ is used in the rotating type UPS system to supply the mains.
a) DC motor
b) Self excited DC generator
c) Alternator
d) Battery bank

View Answer

Answer: c [Reason:] When the supply is gone, the diesel engine is started, which runs the alternator and the alternator supplies power to the mains. Non-rotating type UPS are not used anymore.

6. Static UPS requires __________
a) only rectifier
b) only inverter
c) both inverter and rectifier
d) none of the mentioned

View Answer

Answer: c [Reason:] Rectifier to converter the dc from the battery to ac. Inverter to charge the battery from mains.

7. No discontinuity is observed in case of
a) short break static UPS configuration
b) long break static UPS configuration
c) no break static UPS configuration
d) rotating type UPS configuration

View Answer

Answer: c [Reason:] No dip or discontinuity is observed in case of no break static UPS configuration, as the battery inverter set immediately takes over the mains.

8. Usually __________ batteries are used in the UPS systems.
a) NC
b) Li-On
c) Lead acid
d) All of the mentioned

View Answer

Answer: c [Reason:] Lead acid batteries are cheaper and have certain advantages over the other types. NC batteries would however be the best, but are three to four times more expensive than Lead Acid.

9. HVDC transmission has ___________ as compared to HVAC transmission.
a) smaller transformer size
b) smaller conductor size
c) higher corona loss
d) smaller power transfer capabilities

View Answer

Answer: b [Reason:] The conductor size is smaller as there is no sink effect, and the whole conductor is utilized for transmitting power.

10. The negative polarity is used in the monopolar link because it
a) uses less conductor size
b) is safer
c) produces less radio interference
d) has less resistance

View Answer

Answer: c [Reason:] The monopolar link uses just a single conductor, which is usually negative as it produces less radio interference and corona.

Set 4

1. HVDC transmission lines are __________ as compared to HVAC lines.
a) difficult to erect
b) more expensive for long distances
c) more expensive for short distances
d) less expensive for short distances

View Answer

Answer: c [Reason:] HVDC lines require additional cost of the converting and inverting equipments, hence they are more expensive for short distances as compared to HVAC lines.

2. In HVDC transmission lines
a) both the stations operate as an inverter
b) both the stations operate as a converter
c) one acts as a converter and other as an inverter
d) depends upon the type of the load

View Answer

Answer: c [Reason:] The supply side station is the converting station, power is transmitted in dc then again converted back to ac at the inverting or receiving station further which it is distributed in ac to houses.

3. Two six pulse converters used for bipolar HVDC transmission system, are rated at 1000 MW, +- 200 kV. What is the dc transmission voltage?
a) 200 kV
b) 400 kV
c) 500 kV
d) 100 kV

View Answer

Answer: b [Reason:] As the link is bipolar, the total voltage transmitted will 200 + 200 kV.

4. Two six pulse converters used for bipolar HVDC transmission system, are rated at 1000 MW, +- 200 kV. Find the dc current in the transmission line.
a) 500 A
b) 25 A
c) 2500 A
d) 5 A

View Answer

Answer: c [Reason:] Transmission voltage = 200 + 200 = 400 kV. I = P/V = 1000 mW/ 400 kV = 2500 A.

5. Two six pulse converters used for bipolar HVDC transmission system, are rated at 1000 MW, +- 200 kV. Find the rms current rating required for the SCRs.
a) 2500 A
b) 1350 A
c) 1445 A
d) none of the mentioned

View Answer

Answer: c [Reason:] Transmission voltage = 200 + 200 = 400 kV. I = P/V = 1000 mW/ 400 kV = 2500 A. As in 3-phase full converter each SCR conducts for 120° for a total period of 360°. Irms = 2500 x √(120/360) = 1443.4 A. Nearly about 1445 A as it is a standard current rating.

6. For high power applications _________ are used as static switches whereas for low power applications __________ are used.
a) Transistors, SCRs
b) SCRs, transistors
c) Diodes, transistors
d) SCRs, diodes

View Answer

Answer: b [Reason:] As SCR are of higher rating they are preferred in high power applications.

7. _________ can be used as a single phase static ac switch.
a) Diode
b) SCR
c) DIAC
d) TRAIC

View Answer

Answer: d [Reason:] SCR cannot be used, as it is unidirectional. Diode isn’t a switch nor is the DIAC.

8. ___________ can be used as a dc static switch.
a) GTO
b) Transistor
c) Both GTO and transistor
d) TRIAC

View Answer

Answer: c [Reason:] Both GTO and transistor can be used as a dc static switch.

9. A single-phase ac switch is used in between a 230 V source and load of 2 kW and 0.8 lagging power factor. Determine the rms current rating required by the SCR. Use the factor of safety = 2.
a) 10.87 A
b) 87 A
c) 21.74 A
d) 32 A

View Answer

Answer: c [Reason:] I = (200√2)/(230 x 0.8) = 15.37 A. I(rms) = 15.37/1.414 = 10.87 Required rating = 10.87 x factor of safety = 15.37 x 2 = 21.74 A.

10. Solid State Relays (SSRs) have
a) moving parts
b) no moving parts
c) a coil
d) a contactor

View Answer

Answer: b [Reason:] SSRs have no moving parts, they simply consist of a LED and a transistor or photo diode.

Set 5

1. In the circuit shown below, find the expression of average output voltage if due to some faults the SCR T3 gets permanently open circuited.
power-electronics-questions-answers-campus-interviews-q1
a) 2Vm/π cosα
b) Vm/π cosα
c) Vm/√π
d) Zero

View Answer

Answer: b [Reason:] If one of the device gets O.C the circuit would behave just like a half-wave rectifier. Only Two SCRs T1 & T2 will operate, even T4 is useless in this case. Vo = 1/2π x [ ∫ Vm sinωt d(ωt) ] where the integration would run from α to π+α.

2. A single phase full converter, feeds power to a RLE load with R = 6 Ω and E = 60 V. The supply voltage is 230 V AC and the firing angle is 50°. Find the power delivered to the battery.
a) 542 W
b) 100 W
c) 960 W
d) 730 W

View Answer

Answer: d [Reason:] Vo = 2Vm/π cosα = 133.084 V I = Vo – E/R = 133.084-60 / 6 = 12.181 A Power delivered = E x I = 60 x 12.181 = 730.86 Watts.

3. In a single phase full converter with resistive load and firing angle α, the load current is
a) zero at α, π+α, …
b) remains zero for duration α
c) Vm/R sinα at α, π+α, …
d) remains zero for duration π-α

View Answer

Answer: c [Reason:] The current starts to flow from α, π+α, …

4. In a single phase semi converter with resistive load and a firing angle α, each SCR and freewheeling diode would conduct for
a) α, 0°
b) π-α, α
c) π+α, α
d) π-α, 0°

View Answer

Answer: d [Reason:] FD does not come into the picture for resistive loads.

5. A single phase full converter has average & peak voltage values of 133 V and 325 V respectively. Find the value of the firing angle.
a) 40°
b) 140°
c) 50°
d) 130°

View Answer

Answer: c [Reason:] Vm = 325V Vo = 2Vm/π cosα = 133 V.

6. A freewheeling diode placed across a RL load provides
a) fast turn-on time
b) slow turn-off time
c) poor utilization factor
d) better power factor

View Answer

Answer: d [Reason:] FD improves efficiency by providing freewheeling action which increase the power delivered to the load.

7. A single phase full converter has discontinues load current. The converter is supplying a dc shunt motor (R-L-E) load. When the current falls to zero the output voltage is
a) Zero
b) Vo
c) Vm
d) E

View Answer

Answer: d [Reason:] As the load is RLE output voltage will always exists even if the current goes to zero due to E.

8. A single phase full converter has discontinues load current. The converter is supplying a DC shunt motor (R-L-E) load. When the current falls to zero
power-electronics-questions-answers-campus-interviews-q8
a) none of the devices conduct
b) all of the devices conduct
c) only one device conducts
d) only one pair of devices conducts

View Answer

Answer: a [Reason:] The current falls to zero when the L has discharged completely and the devices are not yet fired.

9. For the circuit shown below to act as a line commutated inverter
power-electronics-questions-answers-campus-interviews-q8
a) 180°>α>90° & E is removed
b) 180°>α>90° & E is reversed
c) 180°>α<90° & E is removed
d) 180°>α<90° & E is reversed

View Answer

Answer: b [Reason:] For inverter operation the firing angle should be above 180 and E should be reversed.

10. If in the below given converter configuration, any 3 of the 4 SCRs are replaced by diodes
a) the average output voltage would increase
b) the average output voltage would decrease
c) the average output voltage would not change
d) the circuit would malfunction

View Answer

Answer: a [Reason:] If diodes are put in place of any 3 SCRs the conduction angle would increase and hence the voltage.