Power Electronics & Drives Test - 3 - PDF Flipbook
Power Electronics & Drives Test - 3
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Electronics&Drives
Test-03Solutions
POWER ELECTRONICS & DRIVES
1. A thyristorised three phase fully controlled converter feeds a dc
load that draws a constant current. Then the input ac line current
to the converter has
a) An r.m.s value equal to the dc load current
b) An average value equal to the dc load current
c) A peak value equal to the dc load current
d) A fundamental frequency component, whose r.m.s value is
equal to the dc load current
Answer: (c)
Solution:
Magnitude of ac input current is same as DC load current
2. In dc choppers, for chopping period T, the output voltage can be
controlled by FM by varying
a) T keeping Ton constant
b) Ton keeping T constant
c) Toff keeping T constant
d) none of these
Answer: (a)
1
3. Figure-I shows a 3-phase inverter fed by a constant voltage
source VDC and connected to a balanced resistive load at the
output. Each switching device may conductor 1200 or for 1800.
The wave form shown in the Figure-II is the
a) line voltage with 1200 firing.
b) load phase voltage with 1200 firing.
c) line voltage with 1800 firing.
d) load phase voltage with 1800 firing.
Answer: (b)
Solution:
1. For 1800 mode:
• Phase voltage remains 2Vdc for each 600.
3
• Line voltage remains Vdc for 1200.
2
2. For 1200 mode
• Phase voltage remains Vdc for 1200.
2
• Line voltage remains Vdc for 600.
2
4. Thermal resistance in SCRs has the units of 0C W and heat sinks
are made from
a) copper
b) steel
c) aluminium
d) none of these
Answer: (a)
5. A single-phase full-bridge, voltage source inverter, feeds a pure
inductive load L and the inverter input voltage is VS. The
current through L, with a period of T second, would be a
a) triangular wave with peak value VS/L.T/2
b) triangular wave with peak value VS/L.T/4
c) square wave with peak value VS/L.T/2
d) pulse wave with peak value VS/L.T/2
Answer: (b)
Solution:
= ∆ 0
∆
3
∆ 0 = 2 0
∆ =
2
= 2 0 ⇒ 0 = ∙
/2 4
6. A 3 phase to 3-phase cycloconverter requires
a) 18 SCRs for 3-phase device
b) 18 SCRs for 6-pulse device
c) 36 SCRs for 3-phase device
d) 36 SCRs for 6-pulse device
Answer: (d)
7. A single - phase half wave uncontrolled converter circuit is
shown in figure. A 2-winding transformer is used at the input for
isolation. Assuming the load current to be constant and Vin = Vm
sinωt, the current waveform through diode D2 will be
4
Answer: (d)
Solution:
Freewheeling diode current flows during negative half cycle
through D2. D2 will be off in first half cycle.
8. Which one of the following statements is correct? The peak
inverse voltage rating of a thyristor in a McMurray inverter
a) can be as small as the forward voltage drop of diode, i.e.,
equal to 1 to 1.5 V
b) should be the same as forward break over voltage.
c) should be much greater than the forward break over voltage.
d) is a very significant factor in the design of the inverter.
Answer: (a)
Solution:
Thyristor are reverse biased with forward voltage drop of diode
which is 1 - 2 V.
9. An ideal chopper operating at a frequency of 500 Hz, supplies a
load having resistance of 3 ohms and inductance of gm H from a
60 V battery. The mean value of the load voltage P or on/off
ratio of 4/1, (assuming that load is shunted by a perfect-
commutating diode and batter is loss-less) is
a) 240 V
b) 48 V
c) 15 V
d) 4 V
Answer: (b)
5
Solution:
V0 = αVin = 4 × 60
4+1
= 4 × 60 = 48V
5
10. Which of the following are capable of regenerative braking?
1. PWM voltage source inverter-fed induction motor drive.
2. Current source inverter-fed induction motor drive.
3. Cyclo-converter-fed synchronous motor drive.
Select the correct answer using the codes given below:
a) 1 and 2 only
b) 2 and 3 only
c) 1 and 3 only
d) 1, 2 and 3
Answer: (d)
Solution:
It can be done for all the above.
11. An inter-group reactor is used in a single-phase cycloconverter
circuit to
a) Reduce current-ripples
b) Reduce voltage-ripples
c) Limit circulating current
d) Limit di/dt in the semiconductor switch
Answer: (d)
6
12. For normal SCRs, turn-on time is
a) less than turn-off time tq
b) more than turn-off time tq
c) equal to turn-off time tq
d) half of turn-off time tq
Answer: (a)
13. The figure (a) shows an inverter circuit with a dc source
voltage Vs. The semiconductor switches are operated in such a
manner that the pole voltages V10 and V20 are shown in figure
(b). What is the rms value of the voltage V12?
a) Vs
π
b) Vs ∙ �2∅π
7
c) Vs ∙ �π∅
d) Vs∙∅
π√2
Answer: (c)
Solution:
Output,
V12 = V10 – V20
Rms value = �21 �∫ +∅ 02 + ∫ +∅ 02 � = 0� ∅
14. A chopper operating at a fixed frequency is feeding an R-L
load. As the duty ratio of the chopper is increased from 25 % to
75 %, the ripple in the load current
a) Remains constant
b) Decreases, reaches a minimum at 50 % duty ratio and then
increases
8
c) Increases, reaches a maximum at 50% duty ratio and then
decreases
d) Keeps on increasing as the duty ratio is increased
Answer: (c)
Solution:
15. The three-phase ac to dc converter which requires neutral point
connection is
a) 3-phase semi-converter
b) 3-phase full converter
c) 3-phase half-wave converter
d) 3-phase full converter with diodes
Answer: (c)
16. Match List-I (Power Converters) with List-II (Industrial
Drive) and select the correct answer:
List-I List-II
A. Dual converter 1. Cement Kiln drive
B. DC chopper 2. Induction heating
C. Cyclo converter 3. Rolling mill drive
D. Inverter 4. Electrical vehicle
5. Sugar mills
9
Codes:
ABC D
a) 2 1 5 3
b) 3 4 1 2
c) 2 4 1 3
d) 3 1 5 2
Answer: (b)
Solution:
DC chopper → Electrical vehicle
CSI or VSI Inverter → Induction heating
Cyclo converter → Cement Kiln drive
Dual converter → Rolling mill drive
17. Secondary breakdown occurs in
a) MOSFET but not in BJT
b) both MOSFET and BJT
c) BJT but not in MOSFET
d) none of these
Answer: (c)
18. Resonant converters are basically used to
a) generate large peaky voltages.
b) reduce the switching losses.
c) eliminate harmonics.
d) convert a square wave into a sine wave.
Answer: (b)
10
Solution:
The converter in which switching is carried out at zero voltage
or zero current to reduce switching losses are called resonant
converters.
19. A forward voltage can be applied to an SCR after its
a) anode current reduces to zero
b) gate recovery time
c) reverse recovery time
d) anode voltage reduces to zero
Answer: (b)
20. Match the switch arrangements on the top row to be steady -
state V - I characteristics on the lower row. The steady state
operating points are shown by large black dots.
11
a) A – i, B – ii, C – iii, D – iv
b) A – ii, B – iv, C – i, D – iii
c) A – iv, B – iii, C – i, D – ii
d) A – iv, B – iii, C – ii, D – i
Answer: (c)
21. Holding current in a p-n-p-n diode is the
a) maximum operating current
b) normal operating current
c) current corresponding to break over voltage
d) minimum current to keep the device ON
Answer: (d)
22. A dc to dc transistor chopper supplied from a fixed voltage dc
source feeds a fixed resistive - inductive load and a free -
wheeling diode. The chopper operates at 1 kHz and 50% duty
cycle. Without changing the value of the average dc current
through the load, if it is desired to reduce the ripple content of
load current, the control action needed will be
a) Increase the chopper frequency keeping its duty cycle
constant
b) Increase the chopper frequency and duty cycle in equal ratio
12
c) Decrease only the chopper frequency
d) Decrease only the duty cycle
Answer: (a)
Solution:
The average d.c. current through the load 0 = 0 and V0 = αVs
Ripple =
4
If duty cycle ‘α’ changes average value of output current and
voltage changes. So ‘α’ should be maintained constant. Current
ripple can be decreased by increasing chopping frequency.
23. Diac is a
a) 2 terminal bidirectional switch
b) 2 terminal unilateral switch
c) 3 terminal bidirectional switch
d) 3 terminal unilateral switch
Answer: (a)
24. The peak current through the resistor in the circuit of Figure
assuming the diode to be ideal is
a) 12 mA
b) 4 mA
c) 16 mA
13
d) 8 mA
Answer: (d)
25. Match List-I (Power device) with List-II (Property) and
select the correct answer:
List-I
A. Thyristor
B. MOSFET
C. IGBT
D. BJT
List-II
1. Secondary breakdown
2. Large on-state drop
3. Small on-state drop
4. Slow device
Codes:
ABC D
a) 4 3 2 1
b) 1 2 3 4
c) 4 2 3 1
d) 1 3 2 4
Answer: (c)
Solution:
Thyristor → Slow
MOSFET → Large on-state drop
14
BJT → Secondary breakdown A drop in collector emitter
voltage at large collector current.
IGBT → Advantage of both MOSFET and BJT low on-state
loss and high input impedance.
26. In SCR, the turn-ON time
a) is independent of ambient temperature T
b) increases with increase of T
c) varies 1/T
d) varies is 1/T2
Answer: (b)
27. A p-n-p-n diode is a
a) negative resistance device
b) voltage controllable device
c) controlled rectifier
d) current controlled negative resistance device
Answer: (d)
28. When compared with BJTs, MOSFETs have lesser turn-off
time, enabling them to operate at high operating frequencies.
What is the reason that can be attributed to that property?
a) High input impedance of the MOSFETs.
b) Positive temperature coefficient of the MOSFETs.
c) The absence of minority storage charge in the MOSFETs.
d) Smaller leakage current of MOSFETs.
Answer: (c)
15
Solution:
Conduction in MOSFET is due to majority carriers. Time delay
caused by removal or recombination of minority carriers are
eliminated.
29. In a p-n-p-n diode, break over condition is marked by
a) a sudden decrease in current
b) a sudden increase in current
c) diode getting burnt off
d) a sudden glow taking place
Answer: (b)
30. Natural commutation of a thyristor takes place when
a) anode current becomes zero
b) gate current becomes zero
c) voltage across the device becomes zero
d) voltage across the device becomes negative
Answer: (d)
16
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