Code: AE-26                                                                           Subject: POWER ELECTRONICS

Time: 3 Hours                                                                                                     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. A voltage  is applied across a UJT.  The emitter voltage at peak point is ________.

(A) (B)

(C) (D) 

b. The stator voltage of an induction motor can be varied by

(A) varying the rotor current.              

(B) changing the supply frequency.

(C) pulse-width modulation inverters. 

(D) Rheostatic brake control.

             c.   If 4 Quadrant operation is required, we need ________.

(A) dual converter                              (B)   full converter

(C)  semi converter                              (D)   any of A, B, C

             d.   If a step down chopper is fed by a dc voltage V and  and  are on and off periods, then  is __________.

(A)  average output voltage                 (B)  rms output voltage

(C)  neither (A) nor (B) (D)  maximum output voltage         

             e.   In a single phase half wave ac regulator feeding a resistive load, the rms load current __________.

(A) is always positive                          (B)  is always negative

(C)  may be positive or negative           (D)  can be even zero

             f.    An SCR can be turned off by _________.

(A) reversing the anode voltage          

(B) reducing the anode current below the holding value

(C) both (A) and (B)

(D) none of A,B,C

g.   For an SCR, the  protection is achieved through the use of __________ 

(A) R-L in series with the SCR           (B)  R-C in series with the SCR

(C)  R-C across the SCR                    (D)  L in series with SCR

             h.   In a single phase full converter, for continuous conduction, each pair of SCRs conduct for ________. 

(A) (B)

(C) (D)

             i.    A single phase bridge inverter delivers its output to a series connected RLC load with , .  For this inverter, load commutation is possible in case the magnitude of  is __________. 

(A) (B)

(C) (D) zero

             j.    A phase controlled cycloconverter employs _________.

(A)  Line commutation                         (B)  Load commutation

(C)  Forced commutation                     (D) No commutation

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2     a.   Define turn-off time period of an SCR; Discuss turn-off characteristics of an SCR with suitable waveforms for line commutated type.                     (2+6)

             b.   A relaxation oscillator using an UJT is to be designed for triggering an SCR.  The UJT has , , , .  Normal leakage current with emitter open = 4 mA, firing frequency is 2 KHz. For , compute the values of .                           (8)

Q.3     a.   Draw the circuit diagram of a single-phase semi-converter with R-L load.  Discuss its operation with waveforms.                                          (3+6)

b.   A 3 phase half controlled bridge rectifier is feeding a R-L load.  If the input voltage is 400 Sin 314 t and SCR is fired at , find the average load voltage.  If any one supply line is disconnected, what is the average load voltage.                                                                                     (3+4)

  Q.4     a.   Explain the principle of operation of a step-up chopper with suitable circuit arrangement and current waveform.                                                                                                         (8)

b.   A dc chopper has a resistive load of  and input voltage .  When the chopper is ON, its voltage drop is 1.5 V and chopping frequency is 10 KHz.  If the duty cycle is 80%, find the average output voltage and the chopper on time.                                                                    (4+4)

Q.5     a.   With the aid of a circuit and waveforms, describe the working principle of resonant pulse commutation and obtain the expression for the circuit turn off time.                                                               (7+3)

     b.   Differentiate between natural and forced commutation with examples.             (3+3)

  Q.6     a. How is power transfer realised in AC voltage controllers by on-off and phase angle control?  Discuss with relevant circuits and waveforms.           (5+5)

             b.   A single phase voltage controller is used for controlling the power flow from a 220V, 50 Hz source into a load circuit consisting of , .  Calculate

(i) The control range of firing angle.

(ii) Maximum value of rms load current.                                       (3+3)

Q.7     a.   Explain with the help of a circuit diagram and waveforms, the operation of a single-phase cyclo-converter using a center-tap transformer.             (8)

b.   What is an inverter?  Discuss the inverting operation of a single phase bridge inverter with suitable diagrams.                                                           (1+7)

Q.8     a.   What is pulse width modulation control of inverters?  Discuss the sinusoidal pulse width modulation technique with gating signal waveforms.                                                                         (1+7)

             b.   Differentiate between voltage source and current source induction motor drives for speed control.                                                              (4+4)

Q.9           Write short  notes on any TWO of the following:-

(i) Chopper fed dc drive in power control.

(ii) MOS-controlled thyristor.

(iii) Single phase transformer tap changer.

(iv) Buck regulator.                                                                               (8  2)