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#### 1(a). Show that the fundamental r.m.s. value of per-phase output voltage of low- frequency for an m-pulse cycloconverter is given by Vor =Vph(m/pie)sin(x/m)

#### (b) Shown in figure below the resonant pulse commutation (or class B commu- tation) circuit has C-20 uF and L-5 uH. Initial voltage across capacitor is V₂-230 V. For a constant load current of 300 A, calculate- (i) conduction time for the auxiliary thyristor; (ii) voltage across the main thyristor when it gets commutated; (iii) the circuit turn-off time of the main thyristor.

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*ALL IN ONE BEU PYQ SOLUTION power electronics 2021 IS PROVIDED BELOW*

#### 2 (a) Explain the I-V characteristics of a thyristor. Label the various voltages, currents and the operation modes.

#### (b) A 3-phase full-converter delivers a ripple free load current of 10 A with a firing delay of 45°. The input voltage is 3-phase, 400 V, 50 Hz- (i) draw the waveform of source current and write the expression of Fourier series; (ii) calculate input power factor and THD.

#### 3(a) A single-phase full-bridge inverter is connected to a d.c. source Vs. Resolve the output voltage wave shape into Fourier series.

#### (b) Describe the operation of a single- phase two-pulse mid-point converter with relevant voltage and current waveforms.

#### 4(a) For type-A chopper connected to RLE load, write the basic voltage equations and derive the expression for the maximum and minimum value of load current in terms of source voltage V R, E, T, a and Ta

#### (b) Hence show that the expression for per unit ripple in the load current of a type-A chopper (Question No. 5(a) is given by

#### 5 (a) A 1-phase full-converter delivers ripple current to RL load with R= 150. The source voltage is 230 V, 50 Hz. For a firing angle of 30°, calculate- (i) rectification efficiency; (ii) voltage ripple factor (iii) THD.

#### (b) Describe the switching characteristics of power SCR.

#### 6(a) Explain the constructional details and working of n-channel power MOSFET. (B) Describe the switching characteristics of power MOSFETs.

#### 7. With the help of equivalent circuit, obtain the nature of waveform of phase voltage of a star-connected resistive load fed from a three-phase DC to AC bridge-inverter operating in 180° conduction mode.

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#### 8. Discuss the principle of operation of following choppers, with necessary circuit diagram and waveforms: (a) Step-down chopper (b) Step-up chopper Give comments on chopping frequency.

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