Code: A-26                                                                              Subject: POWER ELECTRONICS

Time: 3 Hours                                                                                                     Max. Marks: 100

NOTE: There are 11 Questions in all.

· Question 1 is compulsory and carries 16 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

· Answer any THREE Questions each from Part I and Part II. Each of these questions carries 14 marks.

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

Q.1       Choose the correct or best alternative in the following:                                           (2x8)

a. The MOSFET is a __________ controlled device, also in power applications the drain-source voltage of the device must be ___________ to reduce the conduction losses. 

(A)  current, large                                (B)  voltage, small

(C)   current, very small                        (D)  voltage, very large

             b.   The maximum output voltage obtainable from a 3-phase fully controlled converter which is fed by a 3-phase source having an rms value of ‘V’ volts as the phase to neutral voltage is

(A) (B)

(C) (D)

             c.   The transformer in Fig.1 delivers a DC

                   load current of 20 A at the load and

                   has a DC load voltage of 115V. 

                   If it is supplied from a single phase

                   transformer of ratio 5 : 1 the primary

                   voltage and current are respectively

                   ________ and ____________.

(A) 445.8 V, 5.25A                          

(B) 638.5V, 4.44A

(C) 714.0 V, 4.96 A                         

(D) 512.5 V, 6.23 A

             d.   For the inverting mode of a converter the firing angle is ________ and ______ has to be in series with the load.

(A) less than , a large capacitor in series with a battery

(B) more than , a battery

(C) less than , a small capacitor 

(D) more than , a battery

             e.   In the two quadrant chopper shown in Fig.2 the devices_______ are connected to the source for one part of the period  and the devices _________ for the other part.

(A) , (B)  ,

(C)  (D)  none of these.

             f.    For the single phase chopper of Fig.3, the control of load voltage is possible for a firing angle that approximately obeys the inequality

(A)

(B)

(C)

(D)

             g.   As compared to simultaneous control of a cycloconverter, nonsimultaneous control has the drawback of _________. 

(A) large ratings of the thyristor           (B)  more loss

(C)  high circuit cost                             (D)  large voltage rating for thyristors         

             h.   If the turn off time of a thyristor is 2.5 m sec, then the maximum permissible frequency of the single-phase a.c.-voltage source whose voltage is to be rectified is __________ Hz.

(A) 400                                              (B) 125

(C) 200                                               (D) 250

PART I

Answer any THREE Questions. Each question carries 14 marks.

Q.2     a.   Define the terms ‘recombination time’ and ‘turn off time’ of a thyristor.  Explain the process of turn-off of a thyristor.                                              (6)

             b.   A resonant pulse commutation circuit has a capacitor with  and inductance L = 5.  The initial capacitor voltage V_c (0) = 220 V. Determine the circuit turn-off time  if the load current  is 60 A.                                                           (8)

Q.3     a.   With the help of waveforms discuss the effect of source inductance on the average output voltage of a three phase full bridge converter.                  (6)

             b.   A three phase full bridge converter is supplied from a 300V, 50 Hz a.c. supply.  If the average load current  is 180A and the commutator inductance is 0.12 mH, determine the overlap angle when  is .                                                   (8)

Q.4     a.   Fig.4 shows the circuit of a two-quadrant chopper.  Describe its operation.         (4)

b.   Give the circuit of a buck-boost regulator and derive the expressions for

(i) peak-to-peak ripple current and

(ii) peak-to-peak ripple voltage.

                         With source voltage  as 15, duty cycle k of 0.3, switching frequency f of 30 KHz and L of 120 determine the peak-to-peak ripple current.                                                 (10)

Q.5     a.   With the help of the circuit diagram of a three-pulse single phase cyclo converter discuss how the positive and negative groups are controlled to provide maximum possible output voltage. (6)

b.

For the complementary commutation circuit of Fig.5 derive expressions for the capacitor current, capacitor voltage and compute the turn-off time for R₁= and .  Also draw the relevant waveforms to explain the operation.                                                                                         (8)

  Q.6     a. For the circuit of Fig.6 derive the expressions for the output voltages for the following two conditions:

(i)   the input voltage is connected to the load for n cycles and disconnected for m cycles.

                   (ii)  a bi-directional control is implemented with a firing angle of .

                   Derive the expressions for the output voltages in both the cases.                          (6)

             b.   With the help of a block diagram describe the features of a switch mode regulator.  Explain the part played by the control voltage and the saw tooth waveform in this regulator.                                    (8)

PART II

Answer any THREE Questions. Each question carries 14 marks.

Q.7     a.   Describe the features of a PWM in which a sinusoidal reference signal is compared with a triangular wave.  With the help of waveforms explain its merits with respect to multiple PWM.       (8)

b.   Describe the phase-displacement technique that is applied for the voltage control of a single-phase inverter.                                                            (6)

Q.8     a.   Give the circuit and waveforms of a three-phase inverter in which 180-degree conduction type of control is applied.                                               (6)

             b.   The buck regulator in Fig.7 has an input voltage  as 16V.  The required average output voltage  is 7V.  The peak-to-peak output ripple voltage is 22mV and the switching frequency is 28 KHz.  If the peak-to-peak ripple current of the inductor is limited to 0.75A, determine the sizes of the filter inductor L and the filter capacitor C.                                 (8)

Q.9     a.   Explain the principle of a Current Source Inverter (CSI); also describe the operation of a single phase CSI.                                                            (8)

b.   In a typical UJT triggering circuit R is given as a 50 K pol and as 18.2V, given .  Check the correctness of R and ; and in the latter case compute the percentage error with respect to the correct value.          (6)

Q.10           a.                                                        Give the circuit of a dual converter which is constituted of two three phase bridge converters and explain how it is made free of circulating current.  Discuss any other uses of the dual converter.                                     (8)

             b.   Write briefly as to how constant Torque and constant power operation can be obtained from a separately excited DC motor using a solid-state converter, for a wide range of speed.                         (6)

Q.11                                                                      Write notes on any TWO of the following:

(i)  and  protection of a thyristor.

(ii) IGBT vs MOSFET.

(iii) PUT.                                                                                    (2x7)