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Multiple choice question for engineering

Set 1

1. In case of TRC (Time Ratio Control), _________ is varied
a) duty cycle
b) firing angle
c) supply frequency
d) supply voltage magnitude

View Answer

Answer: a [Reason:] In case of TRC, α (Duty cycle) is varied to change the average value of output voltage.

2. In constant frequency TRC or pulse width modulation scheme, ________ is varied.
a) Vs
b) Ton
c) T
d) f

View Answer

Answer: b [Reason:] In pulse width modulation scheme also known as constant frequency time ratio control, the on-time Ton is varied keeping total time T constant. Toff also varies automatically with Ton to keep Ton constant.

3. In case of variable frequency system __________ is varied
a) T
b) Ton
c) Toff
d) supply frequency

View Answer

Answer: a [Reason:] In frequency modulation scheme, the frequency (f) hence T (1/f) is varied by either keeping Ton constant or keeping Toff constant.

4. In pulse width modulation scheme, _________ is kept constant.
a) Vs
b) Ton
c) T
d) Toff

View Answer

Answer: b [Reason:] In pulse width modulation scheme also known as constant frequency time ratio control, the on-time Ton is varied keeping total time T constant. Toff also varies automatically with Ton to keep Ton constant.

5. In case of a constant frequency system, Ton = (1/4)T. If the chopping frequency 2 kHz, find the value of Toff.
a) (1/8) ms
b) (3/8) ms
c) (1/8) μs
d) (3/8) μs

View Answer

Answer: b [Reason:] Ton = (1/4) T T = 1/2kHz = 0.5 ms Ton = T/4 Therefore, Toff = T – Ton = T – T/4 = 3T/4 = 3/8 msec.

6. In case of frequency modulation system, ________ is kept constant.
a) T
b) Ton
c) Toff
d) Either Ton or Toff

View Answer

Answer: d [Reason:] In frequency modulation scheme, the frequency (f) hence T (1/f) is varied by either keeping Ton constant or keeping Toff constant.

7. The control strategy in which on and off time is guided by the pervious set of values of a certain parameter is called as
a) time ratio control
b) pulse width modulation
c) current limit control
d) constant frequency scheme

View Answer

Answer: c [Reason:] In current ratio control, the on and off time is guided by the pervious set of value of load current.

8. In the current limit control method, the chopper is switched off when
a) load current reaches the lower limit
b) load current reaches the upper limit
c) load current falls to zero
d) none of the mentioned

View Answer

Answer: b [Reason:] In CLC, when the current reaches the upper limit the chopper is switched off and current starts to decay. When current reaches a predefined lower limit, the chopper is again switched on and current starts to rise, the process is thus again repeated.

9. Which of the following mentioned control strategy/strategies would require a feedback loop?
a) pwm
b) constant frequency system
c) current limit control
d) none of the mentioned

View Answer

Answer: c [Reason:] CLC would require a feedback loop as it is required to measure the values of current constantly before switching on or off the chopper.

10. In the current limit control method, when the load current reaches a predefined lower value, then
a) the chopper is switched off
b) the chopper is switched on
c) the source voltage is removed
d) load voltage goes to zero

View Answer

Answer: b [Reason:] In CLC, when the current reaches the upper limit the chopper is switched off and current starts to decay. When current reaches a predefined lower limit, the chopper is again switched on and current starts to rise, the process is thus again repeated.

Set 2

1. In the ___________ type of chopper, two stage conversions takes place.
a) AC-DC
b) AC link
c) DC link
d) None of the mentioned

View Answer

Answer: In AC link chopper, DC is converter to AC than stepped up/down than again AC to DC conversation takes place.

2. Choppers converter
a) AC to DC
b) DC to AC
c) DC to DC
d) AC to AC

View Answer

Answer: c [Reason:] Choppers are used to step up or step down DC voltage/current levels. Hence, they are DC to DC converters.

3. A chopper may be thought as a
a) Inverter with DC input
b) DC equivalent of an AC transformer
c) Diode rectifier
d) DC equivalent of an induction motor

View Answer

Answer: b [Reason:] It is a DC equivalent of an AC transformer because it behaves in the similar manner i.e. converting fixed DC to variable DC.

4. Which device can be used in a chopper circuit?
a) BJT
b) MOSFET
c) GTO
d) All of the mentioned

View Answer

Answer: d [Reason:] All of the devices which can be used as a switch can be used in a chopper.

5. A chopper is a
a) Time ratio controller
b) AC to DC converter
c) DC transformer
d) High speed semiconductor switch

View Answer

Answer: d [Reason:] It is a high speed on/off semiconductor switch. Note that it behaves like a DC transformer, does not mean it is a DC transformer. There is no DC transformer.

6. What is the duty cycle of a chopper ?
a) Ton/Toff
b) Ton/T
c) T/Ton
d) Toff x Ton

View Answer

Answer: b [Reason:] It is the time during which the chopper is on (Ton) relative to the whole period (T = Ton+Toff).

7. The load voltage of a chopper can be controlled by varying the
a) duty cycle
b) firing angle
c) reactor position
d) extinction angle

View Answer

Answer: a [Reason:] The output voltage can be changed by changing the duty cycle (Ton/T).

8. The values of duty cycle (α) lies between
a) 0<α<1
b) 0>α>-1
c) 0<=α<=1
d) 1<α<100

View Answer

Answer: c [Reason:] The duty cycle is between 0 and 1. It can be 0 if the chopper switch is never on and it can be 1 when the chopper switch is always on.

9. If T is the time period for a chopper circuit and α is its duty cycle, then the chopping frequency is
a) Ton/α
b) Toff/α
c) α/Toff
d) α/Ton

View Answer

Answer: d [Reason:] α = Ton/T T = Ton/ α f = 1/T = α/Ton.

10. Find the output voltage expression for a step down chopper with Vs as the input voltage and α as the duty cycle.
a) Vo = Vs/α
b) Vo = Vs x α
c) Vo = Vs2
d) Vo = 2Vs/απ

View Answer

Answer: b [Reason:] The chopper output voltage is Duty cycle x the input voltage (ideal condition).

Set 3

1. The below given figure is that of a _________ (IGBT is used as a chopper switch)
power-electronics-questions-answers-choppers-2-q1-q2-q3
a) step-up/step-down chopper
b) step-down chopper
c) step-up chopper
d) none of the mentioned

View Answer

Answer: b [Reason:] When the switch is closed current flows through the load and also charges the inductor L, when the switch is off, the current freewheels through L-R-FD-L and voltage is zero because the FD short circuits the load.

2. In the below given circuit, when switch (SW) is on
power-electronics-questions-answers-choppers-2-q1-q2-q3
a) voltage is non-zero and current is rising
b) voltage is non-zero and current is decaying
c) voltage is zero and current is rising
d) voltage is zero and current is decaying

View Answer

Answer: a [Reason:] When the switch is closed current flows through the load and also charges the inductor L, hence voltage is Vo and current is rising.

3. In the below given circuit, when switch (SW) is off
power-electronics-questions-answers-choppers-2-q1-q2-q3
a) voltage is non-zero and current is rising
b) voltage is non-zero and current is decaying
c) voltage is zero and current is rising
d) voltage is zero and current is decaying

View Answer

Answer: d [Reason:] When the switch is off, the current freewheels through L-R-FD-L and voltage is zero because the FD short circuits the load.

4. In a step down chopper, if Vs = 100 V and the chopper is operated at a duty cycle of 75 %. Find the output voltage.
a) 100 V
b) 75 V
c) 25 V
d) none of the mentioned

View Answer

Answer: b [Reason:] Vo = Duty cycle x Vs = 0.75 x 100 = 75 V.

5. If Vo is the output voltage and Vs is the input DC voltage, then for the below given circuit.
power-electronics-questions-answers-choppers-2-q5-q6-q7-q8
a) Vo = Vs
b) Vo < Vs
c) Vo > Vs
d) Insufficient information

View Answer

Answer: c [Reason:] The given circuit is that of a step-up chopper. For a step up chopper the output voltage is always (even if duty cycle is very very small) greater than the input voltage.

6. In the below given circuit, when switch (SW) is on
power-electronics-questions-answers-choppers-2-q5-q6-q7-q8
a) voltage is non-zero and current is rising
b) voltage is zero and current is decaying
c) voltage and current both are non-zero
d) voltage and current both are zero

View Answer

Answer: c [Reason:] When the switch is on, the current flows from Vs-L-SW-Vs and charges the inductor (L), hence voltage and current both are zero.

7. In the below given circuit, when switch (SW) is first switched closed and then opened the
power-electronics-questions-answers-choppers-2-q5-q6-q7-q8
a) voltage is non-zero and current is rising
b) voltage is zero and current is decaying
c) voltage is non-zero and current is decaying
d) voltage is zero and current is rising

View Answer

Answer: c [Reason:] When the switch is off, the inductor is has stored energy from the earlier chopper on operation. The current starts to flow from Vs-L-D-Load-Vs, hence current is decaying as the inductor is discharging and the voltage is Vs + Vl (inductor voltage) (apply KVL).

8. What is the voltage across the load when the SW is first closed and then opened?
power-electronics-questions-answers-choppers-2-q5-q6-q7-q8
a) Vs
b) duty cycle x Vs
c) Vs + L (di/dt)
d) zero

View Answer

Answer: c [Reason:] When the switch is off, the inductor is has stored energy from the earlier chopper on operation. The current starts to flow from Vs-L-D-Load-Vs, hence current is decaying as the inductor is discharging and the load voltage is Vs + Vl (inductor voltage) Vl = L(di/dt) Vo = Vs + L(di/dt).

9. Find the expression for output voltage for a step-up chopper, assume linear variation of load current and α as the duty cycle.
a) Vs
b) Vs/α
c) Vs/(1-α)
d) Vs/√2

View Answer

Answer: c [Reason:] During Ton, the energy stored in the inductor is Win = Vs x Ton x (I1+I2)/2 During Toff, the energy released by the inductor is Wout = (Vo – Vs) x Toff x (I1+I2)/2 Considering ideal condition, Win = Wout Vo = Vs x (T/Toff) α = Ton/T Vo = T/T-Ton = Vs/1-α.

10. Find the output voltage for a step-up chopper when it is operated at a duty cycle of 50 % and Vs = 240 V.
a) 240 V
b) 480 V
c) 560 V
d) 120 V

View Answer

Answer: b [Reason:] Vo = Vs/1-α.

Set 4

1. In voltage source inverters (VSIs), the amplitude of the output voltage is
a) independent of the load
b) dependent on the load
c) dependent only on L loads
d) none of the mentioned

View Answer

Answer: a [Reason:] In VSIs the input voltage is maintained at a constant value and the amplitude of the output voltage does not depend on the load conditions. However, the waveform of the load current as well as its magnitude depends upon the nature of the load impedance.

2. In voltage source inverters (VSIs), the output currents _____________
a) amplitude depends upon the load impedance
b) waveform depends upon the load impedance
c) amplitude as well as the nature of the waveform depends on the load
d) both amplitude and waveform are independent of the load impedance

View Answer

Answer: c [Reason:] In VSIs the input voltage is maintained at a constant value and the amplitude of the output voltage does not depend on the load conditions. However, the waveform of the load current as well as its magnitude depends upon the nature of the load impedance.

3. In current source inverters (CSIs)
a) the amplitude of the output current is independent of the load
b) the amplitude of the output current dependents on the load
c) the amplitude of the output voltage is independent of the load
d) none of the mentioned

View Answer

Answer: a [Reason:] In CSIs, the amplitude of the output current is independent on the load impedance, as the input current (to the CSI) is kept constant.

4. In current source inverters (CSIs), the output voltage’s
a) amplitude depends upon the load impedance
b) waveform depends upon the load impedance
c) amplitude as well as the nature of the waveform depends on the load
d) both amplitude and waveform are independent of the load impedance

View Answer

Answer: c [Reason:] In CSIs, the amplitude of the output current is independent on the load impedance, as the input current (to the CSI) is kept constant. However, the magnitude of output voltage and its waveform depends upon the nature of the load impedance.

5. In current source inverters
a) L filter is used after the CSI (load side)
b) L filter is used before the CSI (input side)
c) C filter is used after the CSI (load side)
d) C filter is used before the CSI (input side)

View Answer

Answer: b [Reason:] In order that the current input to the CSI must remain ripple free an constant, L-filter is used before the CSI (in series with the energy source).

6. A CSI converters
a) the input dc current to an an current at output
b) the input ac current to dc current at output
c) the input dc current to amplified dc current at the output
d) the input ac current to amplified ac current at the output

View Answer

Answer: a [Reason:] CSI converts the input dc current to an ac current at its output terminals.

7. In the below shown, the ideal single-phase CSI circuit has constant input source current of I amps. The output current waveform varies from
power-electronics-questions-answers-current-source-inverters-q7
a) 2I to – 2I
b) I/2 to –I/2
c) I to –I
d) none of the mentioned

View Answer

Answer: c [Reason:] Considering the SCRs as ideal voltage sources, when T1, T2 are on the load current is = I. When T3, T4 conduct, the load current is negative with respect to the previous value and = -I.

8. T1, T2 are triggered at 0 and T3, T4 at T/2. If both the pairs of SCRs conduct for equal duration and the load consists of a capacitor. Then
power-electronics-questions-answers-current-source-inverters-q7
a) the load current waveform is a square wave
b) the load voltage waveform is a square wave
c) the load current is constant dc
d) the load voltage is constant dc

View Answer

Answer: a [Reason:] The load current waveform is a square wave with frequency = 1/T. As the load consists of a capacitor, dv/dt must be constant over every half cycle. This slope is positive from zero to T/2 and negative from T/2 to T.

9. In a 3-phase VSI operating in square-wave mode, the output line voltage is free from
a) 3rd harmonic
b) 7th harmonic
c) 11th harmonic
d) 13th harmonic

View Answer

Answer: a [Reason:] The line voltage is the difference of two phase voltages, the 3rd harmonic gets cancelled out, though it is present in the phase voltages.

10. Force-commutated CSIs need
a) capacitors for their commutation
b) inductors for their commutation
c) diodes for their commutation
d) none of the mentioned

View Answer

Answer: a [Reason:] All the CSIs need capacitors for their commutation if force commutation is required. Force commutation is essential for lagging power factors.

Set 5

1. A cycloconverter is a _________
a) one stage power converter
b) one stage voltage converter
c) one stage frequency converter
d) none of the mentioned

View Answer

Answer: c [Reason:] A cycloconverter converters input power at one frequency to output power at a different frequency with one-stage conversion.

2. Applications of cycloconverters include
a) speed control of ac drives
b) induction heating
c) static VAr compensation
d) all of the mentioned

View Answer

Answer: d [Reason:] Speed of induction machines can be controlled by controlling the input frequency. In induction heating, eddy current is proportional to the square of input frequency.

3. The single phase mid-point type cycloconverter uses __________ number of SCRs.
a) 4
b) 8
c) 6
d) none of the mentioned

View Answer

Answer: a [Reason:] 2 negative and 2 positive SCRs are employed in mid-point type cycloconverter.

4. The single phase bridge type cycloconverter uses __________ number of SCRs.
a) 4
b) 8
c) 6
d) none of the mentioned

View Answer

Answer: b [Reason:] 4 negative and 4 positive SCRs are employed in bridge type cycloconverter.

5. In the positive half cycle from ωt = 0 to π
power-electronics-questions-answers-cycloconverters-q5
a) P1 and P2 are forward biased
b) N1 and P2 are forward biased
c) P1 and N2 are forward biased
d) None of the mentioned

View Answer

Answer: c [Reason:] In the positive half cycle, upper terminal is positive of the upper secondary and the lower terminal is negative for the lower secondary. Hence, P1 and N2 are forward biased.

6. In the below given cycloconverter circuit, _________ and _________ conduct in one cycle together.
power-electronics-questions-answers-cycloconverters-q5
a) P1, P2 and N1, N2
b) P1, N2 and N1, P2
c) N1, P1 and N1, P2
d) None of the mentioned

View Answer

Answer: b [Reason:] In the positive half cycle, upper terminal is positive of the upper secondary and the lower terminal is negative for the lower secondary. Hence, P1 and N2 are forward biased. And likewise, N1 and P2 will conduct in the next half cycle.

7. In the positive half cycle, _________ SCRs are forward biased.
power-electronics-questions-answers-cycloconverters-q7
a) P1, P2, N1, N2
b) P1, P2, P3, P4
c) N1, N2, N3, N4
d) P3, P4, N1, N2

View Answer

Answer: a [Reason:] Pairs P1, P2 and N1, N2 are forward biased from ωt = 0 to π.

8. The principle of three phase cycloconverter is to
a) add and remove number of SCRs
b) vary progressively the firing angle of the devices
c) keep the firing angle as 0° for all the devices
d) none of the mentioned

View Answer

Answer: b [Reason:] The basic principle of three phase cycloconverter is to vary progressively the firing angle of the controlling devices.

9. In three phase cycloconverters, the reduction factor is given by
a) input frequency/output frequency
b) (input frequency/output frequency) -1
c) (input frequency/output frequency) -1/2
d) (input frequency/output frequency) 1/2

View Answer

Answer: b [Reason:] Reduction factor = output frequency/input frequency.

10. In a three phase half-wave cycloconverter ___________
a) both inverting and converting action takes place
b) only inversion action takes place
c) only converting action takes place
d) none of the mentioned

View Answer

Answer: a [Reason:] During the conversion process, the current flows in both the directions, hence both inverting and converting action takes place.

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