WBUT Question Papers Power Electronics B Tech 6th Sem June 2008

WBUT Question Papers Power Electronics B Tech 6th Sem June 2008

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
  2. 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.

HQ The 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 tum-on time of the SCR

d)              avoid spurious triggering of SCR due to noise.

tvj In a 3-phase full converter, the output voltage pulsates at a frequency equal to supply frequency

b) 2f

c)       3/   d) 6/.


v) Each diode of a 3-phase half-wave diode rectifier conducts for

b)                   120

d)                 90

‘ vl) The transformer used In a switched mode power supply is

b)                 Iron core

d)                 All of these;

vii) If the gate current of an SCR Is increased, the forward breakdown voltage VreR Will •

b)                 decrease

c)               not be effected

vili) In a single phase full converter, if a & P are firing & extinction angles respectively, then the load current as

a)              discontinuous if ( P – a I < n

b)              discontinuous if (P-a) >n

c)               discontinuous if ( P – a) = n

d)             continuous if (P – a) < n.

r tx) 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.

xi)             Resonant converter controls the output power by

a) varying the switching frequency around resonating frequency

b)              varying the on-tlme of the switch

c)               controlling the power loss in the switch

d)              none of these.

xii)           Reverse recovery current in a diode depends upon

a)              forward field current

b)              storage current

c)               temperature

d)              PIV.

GROUP -B ( Short Answer Type Questions)

Answer any three of the following.    3×5= 15

  1. Derive an equation for anode current of an SCR considering two-transistor analogy of SCR. •
  2. 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*.
  3. What is a current source inverter ? Mention its merits & demerits compared to voltage source inverter.
  4. Explain briefly why maximum triggering angle available from a resistance triggering circuit is 90*.    ,
  5. Explain various triggering methods of an SCR.

v GROUP -C (Long Answer Type Questions)

Answer any three questions.       3 x 15 = 45

  1. a) Why is a three phase bridge controlled rectifier called a six pulse converter ?
  2. 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 suppfy 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

1) the firing angle

10 the average output voltage

iU) 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.

  1. 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 iiSec, determine the conduction of the switch.

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.

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

  1. a) Explain the operation of IGBT. State the advantages of IGBT.

b)              How are 61/dt & du/dt protections achieved in SCR ?

c)               Define tum-on & turn-off time of an SCR with the help of dynamic characteristics.  6 + 5 + 4

  1. Write notes on any three of the following :                                                                           3×5

a)              Electronic ballast

b)              Induction heating

c)               UPS

d)              Active front end converter.

e)               Need for power electronics converter.


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