Question 1

Plot I-V characteristics of SCR.

Accepted Answer

The SCR I-V characteristic has a forward blocking region, a breakover point, a forward conducting region with low voltage drop, and a reverse blocking region.

Question 2

Name the methods to turn on the thyristor.

Accepted Answer

A thyristor can be turned on by: forward voltage breakover, gate triggering (most common), thermal triggering, light triggering, and dv/dt triggering.

Question 3

Define the latching current in SCR.

Accepted Answer

Latching current IL is the minimum anode current required to keep the SCR in the ON state immediately after the gate pulse is removed during turn-on.

Question 4

What is a Snubber circuit?

Accepted Answer

A snubber circuit is an RC or RCD network connected across (or in series with) a power device to limit dv/dt or di/dt during switching, protecting the device from stress and reducing EMI.

Question 5

Define delay angle and extinction angle

Accepted Answer

The delay angle α is the angle from the natural commutation point at which the SCR is triggered; the extinction angle β is where the SCR current falls to zero in discontinuous conduction mode.

Question 6

Define Transformer Utilization Factor (TUF), Form Factor and Ripple Factor fo…

Accepted Answer

TUF is the ratio of DC power delivered to the VA rating of the transformer; Form Factor is Vrms/Vavg; Ripple Factor measures the AC content relative to DC output.

Question 7

Write the expression of the output voltage of a single-phase full-wave fully …

Accepted Answer

The average output voltage of a single-phase fully controlled bridge converter is Vdc = (2Vm/π)cosα, where Vm is the peak supply voltage and α is the firing angle.

Question 8

Define the duty cycle in a chopper.

Accepted Answer

The duty cycle D of a chopper is the ratio of the ON time to the total switching period; it directly sets the average output voltage as a fraction of the input.

Question 9

Define the modulation index.

Accepted Answer

The modulation index (amplitude modulation ratio ma) in a PWM inverter is the ratio of the reference signal amplitude to the carrier signal amplitude; it controls the output voltage magnitude.

Question 10

List a few applications of inverters in domestic and industrial appliances.

Accepted Answer

Inverters are used in UPS systems, variable frequency motor drives, solar grid-tie systems, induction heating, HVDC transmission, electric vehicle drives, and household appliances like air conditioners and refrigerators.

Question 11

Discuss the construction, characteristics, working, and applications of TRIAC.

Accepted Answer

TRIAC is a bidirectional thyristor that can conduct in both directions and is triggered in all four gate-current quadrants, making it ideal for AC power control.

Question 12

Discuss the R and RC triggering method for a thyristor.

Accepted Answer

R-triggering uses a resistor to limit gate current and provides crude firing angle control (α: 0-90°); RC-triggering adds a capacitor for phase shift, extending control to nearly 180°.

Question 13

Discuss the symmetrical and asymmetrical AC-DC converters.

Accepted Answer

Symmetrical converters use fully controlled (all SCR) bridges, while asymmetrical converters use half-controlled (mixed SCR and diode) bridges; they differ in output voltage range and freewheeling behaviour.

Question 14

(a) Discuss the working principle of a single-phase full-wave fully controlle…

Accepted Answer

A single-phase fully controlled converter with RL load conducts continuously when α < φ and discontinuously when α > φ; the waveforms differ significantly in load current shape and source current polarity.

Question 15

A single-phase full bridge converter is connected to 'R' load. The source vol…

Accepted Answer

For a single-phase full-bridge converter with R load at 230 V, 50 Hz, average load current 10 A, R=20 Ω, the firing angle α ≈ 48.8°.

Question 16

Derive the mathematical relationship between the duty ratio (D) and the avera…

Accepted Answer

In a boost converter, volt-second balance on the inductor gives Vo = Vin/(1-D), showing output voltage is always greater than input and increases as duty ratio D approaches 1.

Question 17

What is pulse modulation? List the various PWM techniques. How do these diffe…

Accepted Answer

Pulse Width Modulation (PWM) controls the output voltage of an inverter by varying the width of voltage pulses; common techniques are single-pulse, multi-pulse, sinusoidal (SPWM), space vector (SVPWM), and hysteresis PWM.

Question 18

A single-phase full wave fully controlled converter with RLE load of $R=0	ex…

Accepted Answer

For a fully controlled bridge with R=0, L=2 mH, Vs=230 V, 50 Hz: for E=+120 V, α≈31.1° and for E=-120 V, α≈148.9°; the power factor is computed from real power and apparent power.

Question 19

Discuss the three-phase 6-pulse AC-DC converter with RL load. Sketch the inpu…

Accepted Answer

A three-phase 6-pulse fully controlled bridge converter produces an average output of (3√6/π)Vph·cosα; for α=45° partial inversion begins; for α=90° average output is zero.

Question 20

Explain the construction, operation, and output waveforms of both boost and b…

Accepted Answer

Buck converter steps voltage down (Vo = D·Vin); boost converter steps it up (Vo = Vin/(1-D)). Both use an inductor to store and transfer energy controlled by a switching transistor.

Question 21

A buck converter as shown in Figure (a) is working in steady state. The outpu…

Accepted Answer

For a buck converter with a given inductor voltage waveform VL, the duty cycle D is determined from the volt-second balance condition: the positive and negative volt-second areas must be equal in steady state.

Question 22

Discuss the principle of working of a three-phase bridge inverter with an app…

Accepted Answer

A three-phase bridge inverter has six SCRs firing every 60°; with 180° conduction and star-connected resistive load, the phase voltages are six-stepped waveforms and line voltages are 12-stepped waveforms.

Power Electronics