POWER ELECTRONICS - B.Tech 6th Semester Exam., 2015

When an SCR is reverse biased

1. two junctions are reverse biased and one junction is forward biased
2. all the three junctions are reverse biased
3. one junction is reverse biased and two junctions are forward biased
4. Any of the above depending on the magnitude of reverse bias

An R-C snubber circuit is used to protect a thyristor against

1. false triggering
2. failure to turn on
3. switching transients
4. failure to commutate

A single-phase semiconverter is feeding a highly inductive load and has free-wheeling diode across the load. The waveshapes of output voltage and output current

1. are similar
2. are not similar
3. may be similar or dissimilar
4. are similar only if firing angle is zero

A single-phase half-wave rectifier is feeding a resistive load. Input voltage \( v = V_m \sin \omega t \). The output d.c. voltage is \( V_{dc} \) and output r.m.s. voltage is \( V_{r.m.s.} \). If firing angle is \( 180^{\circ} \), \( V_{dc} \) and \( V_{r.m.s.} \) respectively are

1. 0 and 0
2. \( \frac{V_m}{\pi} \) and \( \frac{V_m}{\sqrt{2}} \)
3. 0 and \( -V_m \)
4. \( -\frac{V_m}{\pi} \) and \( -\frac{V_m}{2} \)

In a step-down chopper using pulse-width modulation \( T_{on} = 3 \times 10^{-3} \text{ s} \) and \( T_{off} = 1 \times 10^{-3} \text{ s} \). The chopping frequency is

1. 333.33
2. 500
3. 250
4. 1000

In a three-phase full-wave regulator feeding a star-connected resistance load the input voltage is 400 V line to line. The firing angle is \( 160^{\circ} \). The line to line output voltage would be

1. 400 V
2. about 20 V
3. about 100 V
4. zero

Assertion (A): An inverter does not require forced commutation. Reason (R): A series inverter is a forced commutation inverter.

1. Both A and R are correct and R is correct explanation of A
2. Both A and R are correct but R is not correct explanation of A
3. A is correct but R is wrong
4. A is wrong but R is correct

A three-phase bridge inverter is fed by 400 V battery. The load is star-connected and has a resistance of \( 10~\Omega \) per phase. The peak value of load current is

1. 8 A
2. 20 A
3. 10 A
4. 5 A

The waveshape of output voltage of half-bridge inverter is

1. sinusoidal
2. square
3. triangular
4. Either (i) or (ii)

In dielectric heating, the thyristor circuits consist of

1. rectifier-chopper combination
2. a.c. regulator
3. controlled rectifier
4. rectifier-inverter combination

Discuss some of the advantages and disadvantages of power electronic converters.

Define latching and holding currents as applicable to an SCR. Show these on the V-I characteristics.

Why are \( dv/dt \) and \( di/dt \) protections in case of thyristor important?

An SCR operating from a peak supply voltage of 400 V has the following specifications: Repetitive peak current \( I_p = 200 \text{ A} \), \( (di/dt)_{max} = 50 \text{ A}/\mu\text{s} \), \( (dv/dt)_{max} = 200 \text{ V}/\mu\text{s} \). Choosing a factor of safety of 2 for \( I_p \), \( (di/dt)_{max} \) and \( (dv/dt)_{max} \), design a suitable snubber circuit. The minimum value of load resistance is 10 ohms.

Explain the principle of phase control with suitable diagram and waveform.

Discuss in detail three-phase full converter with R-L-E load with suitable diagram and waveform.

What is inverter? List a few industrial applications of inverter.

Discuss in detail three-phase 120 degree mode voltage source inverter.

Describe the principle of d.c. chopper operation.

Derive an expression for its average d.c. output voltage.

Describe integral cycle control type a.c. voltage controller.

Enumerate the merits and demerits of a.c. voltage controller.

Draw the basic integrated structure and the V-I characteristics of a TRIAC and briefly explain its principle of working.

Briefly explain the working of an oscillator employing a UJT. Derive the expressions for the frequency of triggering firing angle delay in terms of \( \eta \), and the charging resistance values.