Code: A-26                                                                              Subject: POWER ELECTRONICS

Time: 3 Hours                                                                     Flowchart: Alternate Process: December 2005                                Max. Marks: 100

 

NOTE: There are 9 Questions in all.

·      Question 1 is compulsory and carries 20 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

·      Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.

·      Any required data not explicitly given, may be suitably assumed and stated.

 

 

Q.1       Choose the correct or best alternative in the following:                                         (2x10)

       

a.       The  device which has the highest reverse blocking voltage is 

 

                   (A)  MOSFET.                                   (B)  BJT.

(C)    SCR.                                           (D)  IGBT.

       

b.      The relation between  and , in a three phase dual converter is

 

(A)                               (B)   

(C)                            (D) 

            

             c.   A class D chopper operates in

                  

(A)    a single quadrant.                          (B)   two quadrants.

(C)  three quadrants.                            (D)  four quadrants.

 

             d.   A single phase Dual Converter uses

 

(A)    natural commutation.                  

(B)    forced commutation.

(C)  both natural and forced commutation.

(D)  none of this.                                

 

             e.   The output RMS voltage of a single phase AC voltage controller in on-off control mode with n cycles ON and m cycles OFF is

                  

(A)                             (B) 

(C)                           (D) 

 

             f.    The minimum number of thyristors required for constructing a single phase cycloconverter is

 

(A)     2                                                  (B)  4

(C)  8                                                  (D)  6

             g.   The modulation technique that is used for harmonic minimisation is

 

(A)     single pulse width modulation.      

(B)     multi pulse width modulation.

(C)     sinusoidal pulse width modulation.

(D)    none of the above.                       

 

             h.   In the case of rheostatic braking

 

(A)    power is fed back to source.         (B) power is wasted in the form of heat.

(C) power is stored in battery.             (D) none of the above.

 

             i.    The frequency of the average output D.C. voltage in the case of a three phase full bridge uncontrolled rectifier is

 

(A)   12 times supply frequency.            (B) 6 times supply frequency.

(C) 3 times supply frequency.               (D) 2 times supply frequency.

 

             j.    The triac is a _______________

 

(A)  bi-directional uncontrolled switch.

(B)  bi-directional controlled switch.

(C)  unidirectional uncontrolled switch.

(D) unidirectional controlled switch.

 

 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

 

  Q.2     a.   Draw the V-I characteristics of the SCR and explain the terms ‘latching current’ and ‘holding current’.                                                                   (10)

       

             b.   Compare the salient features of SCR and MOS-controlled thyristor.                   (6)

 

  Q.3     a.   Explain the inversion mode of operation with waveform for a single-phase fully controlled Bridge converter.                                                                                                                        (8)

 

             b.   Calculate the output dc voltages of a 3-phase fully controlled bridge ac-dc converters for constant load current with (i) firing angles of . Assume a thyristor drop of 1.5 V and an ac line  voltage of 220 V.                                                                                               (8)          

 

  Q.4     a.   Explain the operation of a resonant pulse chopper with waveforms.                     (8)

 

             b.   A buck chopper is shown in Fig.1. The input voltage is Vs=115V, the average load voltage is Va=62V, and the average load current is Ia=24A. The chopping frequency is f=25 KHz. The peak-to-peak ripples are 2.5% for load voltage, 6% for load current, and 12%for filter Le current. Determine the values of Le, L and Ce.                                                      (8)

 

 

 
 
 

 

 

 

 

 

 

 

 

 

 

 


  Q.5     a.   Explain line side commutation with a circuit diagram and derive the expression for circuit turn-off time.                                                                    (10)

       

             b.   Explain the process of ‘natural commutation’.                                                     (6)

 

  Q.6     a.   Explain the operation of a single phase A.C. voltage controller with an inductive load.                     (8)

 

             b.   Explain single phase tap changing transformer operation with waveforms.            (8)

 

  Q.7     a.   Explain the operation of a single-phase cyclo-converter with RL load.                 (8)   

 

             b.   Explain the technique for reduction of output harmonics in a               cyclo-converter.                    (8)

 

  Q.8     a.   Explain the multiple pulse-width modulation technique for a single-phase inverter, with waveforms.                                                             (10)

 

             b.   Compare multiple pulse-width modulation with sinusoidal pulse-width modulation.              (6)

 

  Q.9     a.   With a neat block diagram describe the speed control of a d.c. drive.                (10)

 

             b.   Draw the speed torque characteristics of A.C. induction motor for variable speed drive application.                                                                       (6)