West Bengal Institute of Technology (WBIT) 2008-6th Sem B.Tech Electronics and Communications Engineering Electronics & Comm ( - ) Power Electronics - Question Paper
CS/B.TBCHgKCE)/SKM-6/EC'803/06 3
ENGINEERING & MANAGEMENT EXAMH|AnONS, JUNE - 2008
Time : 3 Hours ] I Full Marks : 70
GROUP-A (Multiple Choice Type Questions)
1. Choose the correct alternatives for any ten of the following : 10 x 1 * 10
i) A single phase full converter supplying a very high inductive load can operate In
a) 4 quadrants b) 3 quadrants
c) 2 quadrants d) 1 quadrant, of V-J plane.
ii) A current controlled device is
a) MOSFET b) BJT
c) Capacitor d) Inductor.
itQ Hie main reason for connecting a pulse transformer at the output stage of an SCR firing circuit is to
a) amplify power of the triggering pulse
b) provide electric isolation
c) reduce the turn-on time of the SCR
d) avoid spurious triggering of SCR due to noise.
iv) In a 3-phase full converter, the output voltage pulsates at a frequency equal to supply frequency, / b) 2f
c) 3/ d) 6/.
4
I
v) Each diode of a 3-phase half-wave diode rectifier conducts for
b) 120
a) 60
dj 90
c) 180
' vl) The transformer used In a switched mode power supply is
b) Iron core
a) Air core
d) All of these;
c) Ferrite core
vU) If the gate current of an SCR Is increased, the forward breakdown voltage VreR win
b) decrease
a) increase
d) infinite.
c) not be effected
viii) In a single phase full converter, if a & P are firing & extinction angles respectively, then the load current as
a) discontinuous if ( P - a) < k
b) discontinuous if (P-a) >n
c) discontinuous If ( P - a) = n
d) continuous if (P - a) < n. r
ix) Which of the following devices exhibits second breakdown phenomenon ?
a) SCR b) Power MOSFET
c) Power BJT d) GTO. I
x) SCR can be triggered by '
a) positive feedback action
b) negative feedback action
c) both positive & negative feedback action
d) none of these. _
y
CS/B.TXCHt*C*)/8M-e/EC-603/06 5
id) Resonant converter controls the output power by
a) varying the switching frequency around resonating frequency
b) varying the on-time of the switch
c) controlling the power loss in the switch
d) none of these.
xil) Reverse recovery current in a diode depends upon
a) forward field current
b) storage current
c) temperature . d) PIV. .
f
GROUP - B ( Short Answer Type Questions)
Answer any three of the following. 3x5= 15
2. Derive an equation for anode current of an SCR considering two-transistor analogy of SCR.
3. In a single phase semi-converter, connected to 100 V, 50 Hz supply, find the average & r.m.s values of the output voltage If the firing angle is 60*.
4. What is a current source inverter ? Mention its merits & demerits compared to voltage source Inverter.
5. Explain briefly why maximum triggering angle available from a resistance triggering circuit is 90'. ,
6. Explain various triggering methods of an SCR.
' . ' . ' ' . . N
1 c
CflkT*CH(BCB)/8Elf-6/EC-603/08 6 ,
, GROUP-C (Long Answer Type Questions )
Answer any three questions. 3 x 15 = 45
7. a) Why Is a three phase bridge controlled rectifier called a six pulse converter ?
Explain briefly with circuit diagram find output voltage waveform.
b) A three phase six pulse converter is operated from a 3-phase star connected 400 V, 50 Hz supply and with R load ( R = 10 ohm).
It is required to obtain an average output voltage equal to 50% of the maximum possible output voltage of the rectifier.
Find out at this condition
i) the firing angle
10 the average output voltage
: * .
HI) the average current of each thyristor
iv) PIV requirement of each thyristor.
c) Explain how the above mentioned converter can act as rectifier and inverter.
6 + 6 + 3
, . . - - ' ' " ' '4 . . ' ' - . ' ' "
8. a) Explain with a neat circuit diagram the operation of a boost converter.
b) A boost converter has a supply voltage of 250 volts, while the output voltage is 500 V. If the period of Converter is 100 usee, determine the conduction of the switch. .
. . 4 .
If the period is reduced to one third for constant frequency operation, find the output voltage.
c) With a neat circuit diagram explain the operation of CuK converter.
d) State the advantages of CuK converter over Buck-Boost Converter.
/
C8/B.TECH(ECE)/8E14-6/EO-603/08 7
9. a] How is the working of a full bridge single phase Inverter different from that of
half bridge circuit. Explain with the help of diagrams.
b) Draw and explain the operation of a fly-back converter. How does it differ from forward converter ?
c) Explain how zero voltage switching can be achieved in a series resonant converter. 5 + 6 + 4
10. a] Explain the operation of IGBT. State the advantages of IGBT.
b) How are di/dt & du/dt protections achieved In SCR ?
c) Define turn-on & turn-off time of an SCR with the help of dynamic characteristics. 6 + 5 + 4
11. Write notes on any three of the following : 3x5
a) Electronic ballast
b) Induction heating
c) UPS
d) Active front end converter.
e) Need for power electronics converter. .
END
VI-369433 (3-AJ
|
Attachment: |
| Earning: Approval pending. |
