West Bengal Institute of Technology (WBIT) 2009-6th Sem B.Tech Electronics and Communications Engineering Electronics & Comm ( - ) Power Electronics - Question Paper
C/B.T*CH(*CE-N)/8EM-/E0603/09 3
ENGINEERING* MANAGEMENT EXAMINATIONS, JUNE-2009
1
Time; 3 Hours] [ Full Marks : 70
GROUP - A (Multiple Choice Type Questions)
1 Choose the correct alternatives for any ten of the following : 10 x 1 = 10
9 The main reason for connecting a pulse transformer at the Output stage of an SCR firing circuit is to *
4 amplify power of the triggering pulse f
b) provide electric isolation
c) reduce the turn on time of the SCR
d) avoid spurious triggering of SCR due to noise.
ii) Each diode of a 3-phase half diode rectifier conducts for a) 60 b) 120
c) 180 d) 90.
Hi) In a controlled rectifier* a freewheeling diode is recessaiy if the load is
a) inductive b) resistive
c) capacitive d) any of these.
iv) It ts possible to connect two or more MOSFETs in parallel because ai the threshold value of the gate to source voltage is only 2-3 V
b) fast switching times are obtainable with it
c) the MOSFET has a verey small power loss under high frequency conditions
d) the MOSFET resistance has a positive temperature coefficient.
v) The turn off loss In a GTO is ........................ that of an SCR.
a) smaller than b) greater than
c) of the same order as d) double.
vi)
A three phase controlled rectifier feeds a purely resistive load. The data are Vs * 240 V ( RMS ) and R = 24 SI . If the firing angle, a is 90, then the average
current delivered to the load is
a)
c)
8*5 A 3 -38 A
b)
d)
9-65 A 6-75 A.
Vii) In a single phase voltage source bridge inverter, the shape of the load current . depends on the
a) source voltage
b) duration of conduction ofSCRs
c) load impedance '
d) duration of conduction of the feedback diodes.
viii) In a voltage source inverter, the purpose of the diodes across the SCRs is to
a) help the commutation of theSCRs
b) see that excessive current does not pass through the SCRs
c) protect SCRs from excessive Voltages
d) feed the energy back from the load to the source under negative power
conditions.
lx) The condition suitable for the inverting mode of operation of a single phase bridge rectifier is
a) a greater than 90 ,
b) an extra inductance in series with a d.c. motor load, with a > 120
c) a battery in series with a d.c. motor, with a in the range 90 < a < 120
d) 'a battery and an extra inductance in series with load, with a > 120.
x) RC snubber circuit Is used to protect $n SCR against
a) false triggering b) flure to turn on
c) switching transients d) failure to commute.
xi) A 1 - $ full bridge inverter can operate in load commutation mode in case load consistsof
a) RtC overdamped b) RLC underdamped
c) RLC critically damped d) all of these. |
xii) The inverter circuit used in a UPS, the value of percentage total harmonic distortion should be
a) <5% b) <10%
c) <15% d) <2%. | "
GROUP-B
(8hort Answer TypeQuestions)
Answer any three of the following questions. .3x5*15
2. Draw and explain dynamic switching characteristics of BJT.
3. SCR is self latching device.sJSxplain the statement with the help of two transistor analogy of SCR.
4. Explain with necessary Waveforms, the principle of operation of an RC triggering circuit.
5. What are the advantages of zero-current and zero voltage converters over conventional D.C. to D.C. converters. State the concept of quasi-resonant converter. 3 + 2
6. A single phase halfwave inverter feeds a resistive load R L = 10 ft . Source voltage Is
240 V D.C. Determine the r.m.s. voltage, output power and r.m.s. value of current.
' ' . . .
, - : ... . - CMtOUP-C
Answer any three of the following questions. 3 x 15 = 45
7. a) Why is a three phase bridge full wave controlled rectifier called a six-pulse converter ? Explain with circuit diagram and output waveforms.
b) A three-phase, six-pulse converter is operated from a 3-phase star connected 400 V. 50 Hz supply and with resistive load of 10 a. Load inductance is negligible. It is required to obtain an average output voltage equal to 50% of the maximum possible output voltage of the rectifier.
Find at this condition,
I) firing angle
" ' x v .. , ' .
II) the average output voltage m) the aveftge cuitent of each SCR
iv) HV requirement of each SCR .
v) rectification efficiency.
5+10
Explain with a neat circuit diagram, the operation of a Buck-brost converter.
a)
b)
8.
A step-down chopper feeds a D.C. motor load, The data pertaining to this chopper based drive are E * 210 V, Ra 7 ft, L ( including armature
inductance) = 12 mH. Chopper frequency * 1-5 kHz, duty cycle = 0-55 and
Eb = 55 V. Assuming continuous conduction. determine
* . . , -1) the average load current
10 current ripple
Hi) RMS value of current through chopper. 5+10
Describe with the help of necessaiy voltage waveforms and circuit diagram, the operation of a three phase voltage source inverter with 120 conduction mode delivering power to star connected pure resistive load.
9. a)
b)
Explain how the output Voltage of a single phase Inverter is controlled by sinusoidal PWM. 10 + 5
CS/B.TECH (EC8-N)/8EM-6/BC-603/09 7
10. a) With the help ofblock diagram, explain the principle of operation of UPS.
b) Write note on resonant converter. 7 + 8
11. a| With the help of suitable circuit diagrams, explain the principle of a flyback
converter.
b) Discuss any scheme of microprocessor based trigging angle control. 7 + 8
' 5 . '' : ' '"V END; ' : ' ' . '
I ' . .. . ' '
i - '
6724 (09/06)
|
Attachment: |
| Earning: Approval pending. |
