POWER ELECTRONICS

Choose the correct answer of the following (any seven) :

Show that the fundamental r.m.s. value of per-phase output voltage of low-frequency for an m$-pulse cycloconverter is given by $V_{or} = V_{ph}(m/\pi)\sin(\pi/m)

Shown in figure below the resonant pulse commutation (or class B commutation) circuit has $C = 20\mu\text{F}$ and $L = 5\mu\text{H}$. Initial voltage across capacitor is $V_s = 230\text{ V}$. 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.

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

A 3-phase full-converter delivers a ripple free load current of 10 A with a firing delay of $45^\circ$. 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.

A single-phase full-bridge inverter is connected to a d.c. source $V_s$. Resolve the output voltage wave shape into Fourier series.

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

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 $V_s, R, E, T, \alpha \text{ and } T_a$.

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 $[(1 - e^{-\alpha T/T_a})(1 - e^{-(1-\alpha)T/T_a})] / (1 - e^{-T/T_a})$. Where $T = \text{Chopping period, } \alpha = \text{duty cycle, } T_a = L/R$.

A 1-phase full-converter delivers ripple current to RL load with $R = 15\Omega$. The source voltage is 230 V, 50 Hz. For a firing angle of $30^\circ$, calculate- (i) rectification efficiency; (ii) voltage ripple factor; (iii) THD.

Describe the switching characteristics of power SCR.

Explain the constructional details and working of n-channel power MOSFET.

Describe the switching characteristics of power MOSFETs.

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 $180^\circ$ conduction mode.

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.

Choose the correct alternative from any seven of the following :

Derive the relation for average output voltage, RMS output voltage and voltage ripple factor of 1-phase full-wave uncontrolled bridge rectifier.

A 1-phase full-wave bridge rectifier circuit fed from a $220\text{ V, 50 Hz}$ supply. It consists of four diodes, a load resistance $20\Omega$ and a very large inductance so that the load current is constant. Determine the (i) average output voltage, (ii) average output current, (iii) average value of diode current, (iv) RMS value of diode current, (v) RMS value of input current, (vi) d.c. output power and (vii) input power factor.

Compare at least 8 performance parameters of 1-phase and 3-phase uncontrolled rectifiers.

A 1-phase $230\text{ V, 1 kW}$ heater is connected across 1-phase $230\text{ V, 50 Hz}$ supply through a diode. Calculate the power delivered to the heater element. Find the (i) peak, average and RMS values of diode current and (ii) input power factor.

A d.c.-d.c. converter is connected to a $150\text{ V d.c.}$ source with an inductive load $R = 10\Omega$ and $L = 10\text{ mH}$. A freewheeling diode is also connected across load. Assume the load current varies from 10 A to 15 A. Find the time ratio of d.c.-d.c. converter.

Explain the operation of class A chopper with neat waveforms.

A single-phase half bridge inverter connected to $230\text{ V d.c.}$ source feeds a resistive load of $10\Omega$. Determine the following : (i) Fundamental RMS output voltage and total output power (ii) Average and peak current through each thyristor (iii) Input power factor and distortion factor (iv) Total harmonic distortion and harmonic factor for lowest order harmonic

Explain a.c. voltage control and series inverter control in single-phase inverters.

Write the applications of current source inverters. What is the condition to get sinusoidal output wave of the current source inverter?

A single-phase bridge inverter fed from $230\text{ V d.c.}$ is connected to load $R = 10\Omega$ and $L = 0.03\text{ H}$. Determine the power delivered to load in case the inverter is operating at 50 Hz with- (i) square-wave output; (ii) quasi-square-wave output with an on period of 0.5 of a cycle; (iii) two symmetrically spaced pulses per half cycle with an on period of 0.5 of a cycle.

A single-phase voltage controller has input voltage of 230 V, 50 Hz and a load of $R = 15\Omega$. For 6 cycles on and 4 cycles off, determine the- (i) RMS output voltage; (ii) input power factor; (iii) average and RMS thyristor currents.

Draw the phasor diagram of a highly inductive series R-L-C circuit when excited with a sinusoidal voltage source.

A single-phase to single-phase mid-point cycloconverter is delivering power to a resistive load $R = 20\Omega$. The supply transformer has turns ratio of 1:1:1. The frequency ratio is $f_o / f_s = 1/5$. The firing angle delays for all the four SCRs are the same. Sketch the variations of the following waveforms for $\alpha = 0^\circ$ and $\alpha = 30^\circ$ : (i) Supply voltage (ii) Output current (iii) Supply current

Explain the operating principle of 1-phase to 1-phase step-up cycloconverter with the help of mid-point and bridge-type configurations. Illustrate your answer with appropriate circuit and waveforms. The conduction of various thyristors must also be indicated on the waveforms.

Discuss two-transistor analogy of a thyristor. Derive an expression for the anode current and discuss therefrom the turn on mechanisms of a thyristor.

Explain the UJT as relaxation oscillator.

It is preferable to use a train of pulse of high frequency for gate triggering of SCR in order to reduce

Reverse recovery current in a diode depends upon

In a 3-phase full-wave a.c. to d.c. converter, the ratio of output-ripple frequency to the supply-voltage frequency is

A fully controlled natural commutated 3-phase bridge rectifier is operating with a firing angle $ \alpha = 30^{\circ} $. The peak to peak voltage ripple expressed as a ratio of the peak output d.c. voltage to the output of the converter bridge is

Natural commutation of a thyristor takes place when

A four-quadrant chopper cannot be operated as

A boost regulator has an input voltage of 5 V and the average output voltage of 15 V. The duty cycle is

PWM switching is preferred in voltage source inverter for the purpose of

Compared to a single-phase half-bridge inverter, the output power of a single-phase full-bridge inverter is higher by a factor of

How many switches are used to construct a 3-phase to 3-phase cycloconverter?

A single-phase bridge-type cycloconverter feeds a load R. For an output frequency equal to one-third of the input frequency, sketch output voltage waveform for a firing angle of about $ 30^{\circ} $. Also derive an expression for the r.m.s. output voltage.

Explain the processes of $ dv/dt $ triggering and temperature triggering.

A 3-phase bridge inverter delivers power to a resistive load from a 450 V d.c. source. For a star-connected load $ 10~\Omega $ per phase, determine the following for $ 120^{\circ} $ mode of operation:
(i) r.m.s. value of output phase and line voltages
(ii) r.m.s. value of transistor current
(iii) r.m.s. value of load current
(iv) Load power

Describe the structural features of power diodes. How do these differ from signal diodes?

A single-phase full-converter delivers a constant load current $ I_0 $. Express its source current in Fourier series.

Derive the expressions for the following performance parameters with the help of waveforms and Fourier series expression in Question No. 4(a): Input displacement factor, Current distortion factor, Power factor, Total harmonic distortion, Voltage ripple factor, Active and Reactive power inputs.

A battery is charged through a resistor R. If $ v_s = 220\sqrt{2} \sin \omega t $, $ E = 100 \text{ V} $ (d.c.) and $ R = 20~\Omega $, calculate the battery charging current and power supplied to the battery.

Write down any four applications of cycloconverters.

Discuss with relevant waveforms, class-B (resonant-pulse commutation) type of commutation employed for thyristor circuits.

What are forward blocking losses and gate power loss in a thyristor?

For step-down chopper circuit, source voltage $ V_s = 220 \text{ V} $, chopping period $ T = 2000~\mu s $, on period $ = 600~\mu s $, load circuit parameters $ R = 1~\Omega $, $ L = 5 \text{ mH} $ and $ E = 24 \text{ V} $.
(i) Find whether load current is continuous or not.
(ii) Calculate the value of average output current.
(iii) Compute the maximum and minimum values of steady state output currents.

Draw the circuit diagram of step-down chopper, also draw the waveforms of source voltage $ V_s $ and output voltage $ V_0 $ for duty ratio $ \alpha = 0.4 $ and time period $ T = 10~\mu s $.

Describe unipolar-voltage switching in a single-phase full-bridge inverter by suitable waveforms for carrier wave, reference wave and output voltage. Show the turn-on and turn-off of various switches in waveforms.

Justify why it is called unipolar-voltage switching inverter or three-level PWM inverter.

Explain the operation of boost converter with voltage and current waveforms across the boost inductor. Assume continuous conduction.

Derive its output voltage equation in terms of duty cycle and input voltage.

It is preferable to use a train of pulse of high frequency for gate triggering of SCR in order to reduce

Reverse recovery current in a diode depends upon

In a 3-phase full-wave a.c. to d.c. converter, the ratio of output-ripple frequency to the supply-voltage frequency is

A fully controlled natural commutated 3-phase bridge rectifier is operating with a firing angle $ \alpha = 30^{\circ} $. The peak to peak voltage ripple expressed as a ratio of the peak output d.c. voltage to the output of the converter bridge is

Natural commutation of a thyristor takes place when

A four-quadrant chopper cannot be operated as

A boost regulator has an input voltage of 5 V and the average output voltage of 15 V. The duty cycle is

PWM switching is preferred in voltage source inverter for the purpose of

Compared to a single-phase half-bridge inverter, the output power of a single-phase full-bridge inverter is higher by a factor of

How many switches are used to construct a 3-phase to 3-phase cycloconverter?

A single-phase bridge-type cycloconverter feeds a load R. For an output frequency equal to one-third of the input frequency, sketch output voltage waveform for a firing angle of about $ 30^{\circ} $. Also derive an expression for the r.m.s. output voltage.

Explain the processes of $ dv/dt $ triggering and temperature triggering.

A 3-phase bridge inverter delivers power to a resistive load from a 450 V d.c. source. For a star-connected load $ 10~\Omega $ per phase, determine the following for $ 120^{\circ} $ mode of operation:
(i) r.m.s. value of output phase and line voltages
(ii) r.m.s. value of transistor current
(iii) r.m.s. value of load current
(iv) Load power

Describe the structural features of power diodes. How do these differ from signal diodes?

A single-phase full-converter delivers a constant load current $ I_0 $. Express its source current in Fourier series.

Derive the expressions for the following performance parameters with the help of waveforms and Fourier series expression in Question No. 4(a): Input displacement factor, Current distortion factor, Power factor, Total harmonic distortion, Voltage ripple factor, Active and Reactive power inputs.

A battery is charged through a resistor R. If $ v_s = 220\sqrt{2} \sin \omega t $, $ E = 100 \text{ V} $ (d.c.) and $ R = 20~\Omega $, calculate the battery charging current and power supplied to the battery.

Write down any four applications of cycloconverters.

Discuss with relevant waveforms, class-B (resonant-pulse commutation) type of commutation employed for thyristor circuits.

What are forward blocking losses and gate power loss in a thyristor?

For step-down chopper circuit, source voltage $ V_s = 220 \text{ V} $, chopping period $ T = 2000~\mu s $, on period $ = 600~\mu s $, load circuit parameters $ R = 1~\Omega $, $ L = 5 \text{ mH} $ and $ E = 24 \text{ V} $.
(i) Find whether load current is continuous or not.
(ii) Calculate the value of average output current.
(iii) Compute the maximum and minimum values of steady state output currents.

Draw the circuit diagram of step-down chopper, also draw the waveforms of source voltage $ V_s $ and output voltage $ V_0 $ for duty ratio $ \alpha = 0.4 $ and time period $ T = 10~\mu s $.

Describe unipolar-voltage switching in a single-phase full-bridge inverter by suitable waveforms for carrier wave, reference wave and output voltage. Show the turn-on and turn-off of various switches in waveforms.

Justify why it is called unipolar-voltage switching inverter or three-level PWM inverter.

Explain the operation of boost converter with voltage and current waveforms across the boost inductor. Assume continuous conduction.

Derive its output voltage equation in terms of duty cycle and input voltage.

Draw the symbols of BJT, MOSFET and SITH.

The reverse recovery time of a diode is $ t_{rr} = 3~\mu s $ and the rate of fall of the diode current is $ di/dt = 30 \text{ A}/\mu s $. Determine the stored charge $ Q_{RR} $ in reverse recovery zone.

What is the softness factor of diodes?

Explain forward-biased safe operating area (FBSOA) of BJT.

Define distortion factor (DF).

A step-up d.c.-d.c. converter has input voltage $ V_s = 10 \text{ V} $, inductance $ L = 1 \text{ mH} $, duty ratio $ d = 0.5 $ and time period $ T = 1 \text{ ms} $. Find the peak-to-peak ripple current in inductor.

The H-bridge inverter has an R-L-C load with $ R = 10~\Omega $, $ L = 31.5 \text{ mH} $ and $ C = 112~\mu F $. The inverter frequency is $ f_0 = 60 \text{ Hz} $ and d.c. input voltage is $ V_s = 220 \text{ V} $. Find the output fundamental r.m.s. voltage.

Find the line-to-line r.m.s. voltage of three-phase inverter operating in $ 180^{\circ} $ conduction mode if d.c. input voltage $ V_s = 220 \text{ V} $.

Explain dynamic current sharing in parallel operation of thyristors.

The full converter is connected to a 120 V, 60 Hz supply. The load current $ I_a = 8 \text{ A} $ is continuous and its ripple content is negligible. If the delay angle is $ \alpha = \pi/3 $, calculate power factor.

The three-phase bidirectional delta-connected controller has a resistive load of $ R = 10~\Omega $. The line-to-line voltage is $ V_s = 208 \text{ V} $ (r.m.s.), 60 Hz and the delay angle is $ \alpha = 2\pi/3 $. Determine the r.m.s. output voltage $ V_0 $, the input PF and the r.m.s. current of a thyristor $ I_R $.

Explain three-phase full-wave delta-connected controller with various waveforms.

A single-phase half-wave converter is operated from a 120 V, 60 Hz supply and the resistive load is $ R = 10~\Omega $. If the average output voltage is 50% of the maximum possible output voltage, calculate the delay angle, the r.m.s. and average output currents.

Explain three-phase full converter with R-L load with various waveforms.

Explain two-transistor model of a thyristor.

A single-phase half-bridge inverter has resistive load of $ R = 5~\Omega $ and the d.c. input voltage is $ V_s = 100 \text{ V} $. Determine the r.m.s. output voltage at the fundamental frequency $ V_{01} $ and the output power $ P_0 $.

A single-phase full-bridge inverter is operated at 1 kHz and uses a uniform PWM with four pulses per half-cycle for voltage control. Plot the fundamental component, distortion factor and THD against the modulation index M.

The bridge inverter has an R-L-C load with $ R = 10~\Omega $, $ L = 31.5 \text{ mH} $ and $ C = 112~\mu F $. The inverter frequency is $ f_o = 60 \text{ Hz} $ and d.c. input voltage is $ V_s = 220 \text{ V} $. Calculate the r.m.s. load current at the fundamental frequency $ I_{o1} $ and the THD of the load current.

The buck converter has d.c. input voltage $ V_s = 110 \text{ V} $, average load voltage $ V_a = 80 \text{ V} $ and average load current $ I_a = 15 \text{ A} $. The chopping frequency is $ f = 10 \text{ kHz} $. The peak-to-peak ripples are 5% for load voltage, 2.5% for load current and 10% for filter $ L_e $ current. Determine the values of $ L_e $, $ L $, $ C_e $.

Write on the steady state analysis of buck converter.

Explain briefly about Cuk-converter.

The buck-boost regulator has an input voltage of $ V_s = 12 \text{ V} $. The duty cycle $ k = 0.25 $ and the switching frequency is 25 kHz. The inductance $ L = 150~\mu\text{H} $ and filter capacitance $ C = 220~\mu\text{F} $. The average load current $ I_a = 1.25 \text{ A} $. Determine the peak-to-peak output voltage ripple $ \Delta V_c $ and the peak-to-peak ripple current of inductor $ \Delta I_L $.

Draw the symbols of BJT, MOSFET and SITH.

The reverse recovery time of a diode is $ t_{rr} = 3~\mu s $ and the rate of fall of the diode current is $ di/dt = 30 \text{ A}/\mu s $. Determine the stored charge $ Q_{RR} $ in reverse recovery zone.

What is the softness factor of diodes?

Explain forward-biased safe operating area (FBSOA) of BJT.

Define distortion factor (DF).

A step-up d.c.-d.c. converter has input voltage $ V_s = 10 \text{ V} $, inductance $ L = 1 \text{ mH} $, duty ratio $ d = 0.5 $ and time period $ T = 1 \text{ ms} $. Find the peak-to-peak ripple current in inductor.

The H-bridge inverter has an R-L-C load with $ R = 10~\Omega $, $ L = 31.5 \text{ mH} $ and $ C = 112~\mu F $. The inverter frequency is $ f_0 = 60 \text{ Hz} $ and d.c. input voltage is $ V_s = 220 \text{ V} $. Find the output fundamental r.m.s. voltage.

Find the line-to-line r.m.s. voltage of three-phase inverter operating in $ 180^{\circ} $ conduction mode if d.c. input voltage $ V_s = 220 \text{ V} $.

Explain dynamic current sharing in parallel operation of thyristors.

The full converter is connected to a 120 V, 60 Hz supply. The load current $ I_a = 8 \text{ A} $ is continuous and its ripple content is negligible. If the delay angle is $ \alpha = \pi/3 $, calculate power factor.

The three-phase bidirectional delta-connected controller has a resistive load of $ R = 10~\Omega $. The line-to-line voltage is $ V_s = 208 \text{ V} $ (r.m.s.), 60 Hz and the delay angle is $ \alpha = 2\pi/3 $. Determine the r.m.s. output voltage $ V_0 $, the input PF and the r.m.s. current of a thyristor $ I_R $.

Explain three-phase full-wave delta-connected controller with various waveforms.

A single-phase half-wave converter is operated from a 120 V, 60 Hz supply and the resistive load is $ R = 10~\Omega $. If the average output voltage is 50% of the maximum possible output voltage, calculate the delay angle, the r.m.s. and average output currents.

Explain three-phase full converter with R-L load with various waveforms.

Explain two-transistor model of a thyristor.

A single-phase half-bridge inverter has resistive load of $ R = 5~\Omega $ and the d.c. input voltage is $ V_s = 100 \text{ V} $. Determine the r.m.s. output voltage at the fundamental frequency $ V_{01} $ and the output power $ P_0 $.

A single-phase full-bridge inverter is operated at 1 kHz and uses a uniform PWM with four pulses per half-cycle for voltage control. Plot the fundamental component, distortion factor and THD against the modulation index M.

The bridge inverter has an R-L-C load with $ R = 10~\Omega $, $ L = 31.5 \text{ mH} $ and $ C = 112~\mu F $. The inverter frequency is $ f_o = 60 \text{ Hz} $ and d.c. input voltage is $ V_s = 220 \text{ V} $. Calculate the r.m.s. load current at the fundamental frequency $ I_{o1} $ and the THD of the load current.

The buck converter has d.c. input voltage $ V_s = 110 \text{ V} $, average load voltage $ V_a = 80 \text{ V} $ and average load current $ I_a = 15 \text{ A} $. The chopping frequency is $ f = 10 \text{ kHz} $. The peak-to-peak ripples are 5% for load voltage, 2.5% for load current and 10% for filter $ L_e $ current. Determine the values of $ L_e $, $ L $, $ C_e $.

Write on the steady state analysis of buck converter.

Explain briefly about Cuk-converter.

The buck-boost regulator has an input voltage of $ V_s = 12 \text{ V} $. The duty cycle $ k = 0.25 $ and the switching frequency is 25 kHz. The inductance $ L = 150~\mu\text{H} $ and filter capacitance $ C = 220~\mu\text{F} $. The average load current $ I_a = 1.25 \text{ A} $. Determine the peak-to-peak output voltage ripple $ \Delta V_c $ and the peak-to-peak ripple current of inductor $ \Delta I_L $.

Choose the correct option of the following (any seven) :

Explain the fully-controlled rectifier with R-L load with neat conduction waveforms.

Draw the V-I characteristics of SCR and explain various modes of operation.

Discuss about power MOSFET and its characteristics.

If the half-wave controlled rectifier has a purely resistive load of $R$ and the delay angle is $60^\circ$, determine the-

Draw and explain the equivalent circuit and V-I characteristics of the UJT in detail.

Compare the R firing circuit and R-C firing circuit.

For a single-phase quasi square wave inverter having resistive load, derive an expression for (i) r.m.s. output voltage, (ii) r.m.s. output current and (iii) load power.

A 3-phase bridge inverter is operated from 200 V DC supply in $180^\circ$ mode. Determine (i) r.m.s. output voltage, (ii) r.m.s. value of fundamental component of line voltage.

A DC chopper circuit connected to a 100 V DC source supplies an inductive load having 40 mH in series with a resistance of $5\Omega$. A freewheeling diode is placed across the load. The load current varies between the limits of 10 A and 12 A. Determine the time ratio of the chopper.

Describe the principle of operation of step-down chopper or buck converter.

Explain the time ratio control and current limit control of DC choppers.

Explain the bidirectional AC voltage controller with R-load.

The number of p-n junctions in a thyristor is

When a thyristor gets turned on, the gate drive

Once SCR starts conducting a forward current, its gate loses control over

In a single-phase full converter, for continuous conduction, each pair of SCRs conduct for

Commutation overlap in the phase-controlled a.c. to d.c. converters is due to

A converter which can operate in both 3-pulse and 6-pulse modes is a

In a 3-phase semi-converter, the three SCRs are triggered at an interval of

In single-pulse modulation of PWM inverters, third harmonic can be eliminated if pulse width is equal to

In d.c. choppers, the waveforms for input and output voltages are

In a 1-phase to 1-phase step-down cycloconverter

Explain the two-transistor model of a thyristor.

Sketch switching (or dynamic) characteristics of a thyristor during its turn-on and turn-off processes. Show the variation of voltage across the thyristor and current through it during these two dynamic processes.

Describe a 1-phase full-wave diode-bridge rectifier with a circuit diagram and relevant waveforms for load R. Hence, derive expressions for average and r.m.s. values of output voltage and obtain therefrom voltage ripple factor (VRF), form factor (FF) and rectifier efficiency.

Enumerate the various techniques by which thyristors can be triggered into conduction.

Explain the working principle of snubber circuit used for protection of thyristor.

Discuss the principle of working of a three-phase inverter with an appropriate circuit diagram. Draw phase and line voltage waveforms on the assumption that each thyristor conducts for $ 120^{\circ} $ and the resistive load is star-connected.

A three-phase bridge inverter delivers power to a resistive load from a 450 V d.c. source. For a star-connected load of $ 10~\Omega $ per phase with $ 120^{\circ} $ mode of operation, calculate the following: (i) r.m.s. value of load current (ii) r.m.s. value of thyristor current (iii) Load power.

What is meant by step-up chopper? Explain its operation. Sketch the input voltage, input current, output voltage and output current waveforms. State the various assumptions made.

A d.c. chopper is fed from 100 V d.c. Its load voltage consists of rectangular pulses of duration 1 msec in an overall cycle time of 3 msec. Find the average output voltage and ripple factor for this chopper.

What is an a.c. voltage controller? List some of its industrial applications. Enumerate its merits and demerits.

A 3-phase half-wave controlled converter is fed from 3-phase, 400 V, 50 Hz source and is connected to load taking a constant current of 36 A. Thyristors have a voltage drop of 1.4 V. Find out the average value of load voltage for firing angle of $ 30^{\circ} $ and $ 60^{\circ} $.

Describe the modified McMurray-Bedford half-bridge single-phase inverter with relevant voltage and current waveforms. The working of this inverter may be explained in certain well-defined modes. Enumerate the simplifying assumptions made.

A single-phase bridge-type cycloconverter has input voltage of 230 V, 50 Hz and load of $ R=10~\Omega $. Output frequency is one-third of input frequency. For a firing angle delay of $ 30^{\circ} $, calculate (a) r.m.s. value of output voltage, (b) r.m.s. current of each converter, (c) r.m.s. current of each thyristor and (d) input power factor.

The number of p-n junctions in a thyristor is

When a thyristor gets turned on, the gate drive

Once SCR starts conducting a forward current, its gate loses control over

In a single-phase full converter, for continuous conduction, each pair of SCRs conduct for

Commutation overlap in the phase-controlled a.c. to d.c. converters is due to

A converter which can operate in both 3-pulse and 6-pulse modes is a

In a 3-phase semi-converter, the three SCRs are triggered at an interval of

In single-pulse modulation of PWM inverters, third harmonic can be eliminated if pulse width is equal to

In d.c. choppers, the waveforms for input and output voltages are

In a 1-phase to 1-phase step-down cycloconverter

Explain the two-transistor model of a thyristor.

Sketch switching (or dynamic) characteristics of a thyristor during its turn-on and turn-off processes. Show the variation of voltage across the thyristor and current through it during these two dynamic processes.

Describe a 1-phase full-wave diode-bridge rectifier with a circuit diagram and relevant waveforms for load R. Hence, derive expressions for average and r.m.s. values of output voltage and obtain therefrom voltage ripple factor (VRF), form factor (FF) and rectifier efficiency.

Enumerate the various techniques by which thyristors can be triggered into conduction.

Explain the working principle of snubber circuit used for protection of thyristor.

Discuss the principle of working of a three-phase inverter with an appropriate circuit diagram. Draw phase and line voltage waveforms on the assumption that each thyristor conducts for $ 120^{\circ} $ and the resistive load is star-connected.

A three-phase bridge inverter delivers power to a resistive load from a 450 V d.c. source. For a star-connected load of $ 10~\Omega $ per phase with $ 120^{\circ} $ mode of operation, calculate the following: (i) r.m.s. value of load current (ii) r.m.s. value of thyristor current (iii) Load power.

What is meant by step-up chopper? Explain its operation. Sketch the input voltage, input current, output voltage and output current waveforms. State the various assumptions made.

A d.c. chopper is fed from 100 V d.c. Its load voltage consists of rectangular pulses of duration 1 msec in an overall cycle time of 3 msec. Find the average output voltage and ripple factor for this chopper.

What is an a.c. voltage controller? List some of its industrial applications. Enumerate its merits and demerits.

A 3-phase half-wave controlled converter is fed from 3-phase, 400 V, 50 Hz source and is connected to load taking a constant current of 36 A. Thyristors have a voltage drop of 1.4 V. Find out the average value of load voltage for firing angle of $ 30^{\circ} $ and $ 60^{\circ} $.

Describe the modified McMurray-Bedford half-bridge single-phase inverter with relevant voltage and current waveforms. The working of this inverter may be explained in certain well-defined modes. Enumerate the simplifying assumptions made.

A single-phase bridge-type cycloconverter has input voltage of 230 V, 50 Hz and load of $ R=10~\Omega $. Output frequency is one-third of input frequency. For a firing angle delay of $ 30^{\circ} $, calculate (a) r.m.s. value of output voltage, (b) r.m.s. current of each converter, (c) r.m.s. current of each thyristor and (d) input power factor.

Choose the correct option (any seven) :

Describe the different modes of operation of a thyristor with the help of its static V-I characteristic.

With the help of a neat diagram, explain the two-transistor analogy of an SCR.

Explain the working of an oscillator employing an UJT. Derive expression for the frequency of triggering.

A relaxation oscillator using a UJT is to be designed for triggering an SCR. The UJT has the following data : $\eta = 0.72$, $I_P = 0.6\text{ mA}$, $V_P = 18\text{V}$, $V_V = 1\text{V}$, $I_V = 2.5\text{ mA}$, $R_{BB} = 5\text{ k}\Omega$ Normal leakage current with emitter open $= 4.2\text{ mA}$. The firing frequency is $2\text{ kHz}$. For $C = 0.04\mu\text{F}$, compute the values of $R, R_1$ and $R_2$.

A single-phase half-wave controlled converter is operated from a $120\text{V, 50 Hz}$ supply. Load resistance $R = 10\Omega$. If the average output voltage is 25% of the maximum possible average output voltage, determine- (i) firing angle; (ii) r.m.s. and average output currents; (iii) average and r.m.s. SCR currents.

Explain the operation of single-phase half-controlled bridge converter (symmetrical configuration) with resistive load along with suitable waveforms. Also derive the expressions for average load voltage, average load current and RMS load voltage.

A three-phase fully controlled converter is connected to a resistive load. Show that the average output voltage is given by : $E_{dc} = \frac{3\sqrt{3}}{2\pi} E_m \cos\alpha$, for $0 < \alpha < \frac{\pi}{6}$ and $E_{dc} = \frac{3}{2\pi} E_m [1 + \cos(\alpha + \frac{\pi}{6})]$, for $\frac{\pi}{6} < \alpha < \frac{5\pi}{6}$

With the help of neat circuit diagram and associated waveforms, explain the operation of single-phase half-bridge voltage-source inverter with resistive load.

The single-phase half-bridge inverter has a resistive load of $10\Omega$ and the center-tap DC input voltage is $96\text{V}$. Compute- (i) RMS value of the output voltage; (ii) fundamental component of the output voltage waveform; (iii) first five harmonics of the output voltage waveform; (iv) RMS power consumed by the load.

Draw the schematic of step-down and step-up choppers and derive an expression for output voltage in terms of duty cycle for a step-up and step-down choppers.

Draw the schematic of class E chopper and explain the working of the same.

Describe the basic principle of working of a single-phase to single-phase cycloconverter for both continuous and discontinuous conductions for a bridge-type cycloconverter. Mark the condition of various thyristors also.

For a single-phase full-wave a.c. voltage controller feeding a resistive load, draw the waveforms of source voltage, gating signals, output voltage, source and output currents and voltage across SCRs. Describe its working with reference to the waveforms drawn.

A single-phase full-wave a.c. voltage controller feeds power to a resistive load of $4\Omega$ from $230\text{V, 50 Hz}$ source. Determine the peak values of average and RMS thyristor currents for any firing angle $\alpha$.

A single-phase one pulse diode rectifier is feeding an RL load with freewheeling diode across the load. For conduction angle $ \beta $, the main diode and freewheeling diode would, respectively, conduct for

When a thyristor is forward biased, the number of blocked p-n junctions is

During forward blocking state, a thyristor is associated with

When a UJT is used for triggering an SCR, the wave shape of the voltage obtained from UJT circuit is a

In a commutation circuit employed to turn off an SCR, satisfactory turn-off is obtained when

A step-up chopper has $ V_s $ as the source voltage and $ \alpha $ as the duty cycle. The output voltage for this chopper is given by

In an inverter with fundamental output frequency of 50 Hz, if third harmonic is eliminated, then the frequencies of other components in the output voltage wave, in Hz, would be

A cycloconverter is a frequency converter from
1. higher to lower frequency with one-stage conversion
2. higher to lower frequency with two-stage conversion
3. lower to higher frequency with one-stage conversion
4. AC at one frequency to DC and then DC to AC at different frequencies
From these, the correct statement(s) is/are

If, for a single-phase half-bridge inverter, the amplitude of output voltage is $ V_s $ and the output power is P, then their corresponding values for a single-phase full-bridge inverter are

In DC choppers, the waveforms for input and output voltages are respectively

Briefly discuss the V-I characteristic of SCR.

Explain the turn-on and turn-off characteristics of SCR with neat waveforms.

Explain the operation of three-phase half-wave controlled converter for a firing angle less than $ 30^{\circ} $ and feeding R load. Also derive the expression for its average output voltage.

The specification sheet for an SCR gives maximum rms on-state current as 35 A. If this SCR is used in a resistive circuit, compute average on-state current rating for half sine wave current for conduction angle of $ 180^{\circ} $.

Explain the effect of source inductance in the operation of single-phase fully controlled converter.

With neat circuit diagram and waveform, explain the working principle of single-phase AC voltage controller with R-L load.

Explain the principle of operation of three-phase inverter with $ 180^{\circ} $ conduction mode with necessary waveforms and circuit.

A step-up chopper has input voltage of 220 V and output voltage of 660 V. If the conducting time of thyristor-chopper is 100 $ \mu\text{s} $, compute the pulse width of output voltage. In case output-voltage pulse width is halved for constant frequency operation, find the average value of new output voltage.

Explain the two-transistor analogy of a thyristor.

Discuss the various mechanisms that can be used to trigger thyristors.

Discuss the operation of step-up chopper and prove that its output voltage is greater than input voltage.

Snubber circuit for an SCR should primarily consist of capacitor only. But, in actual practice, a resistor is used in series with capacitor. Discuss.

With neat circuit diagram and waveform, explain the operating principle of 1-$\phi$ to 1-$\phi$ step down mid-point type cycloconverter, with continuous load current. Assume the loads to be R and L in series.

A single-phase half-wave AC voltage controller feeds a load of $ R=20~\Omega $ with an input voltage of 230 V, 50 Hz. Firing angle of thyristor is $ 45^{\circ} $. Determine: (i) rms value of output voltage; (ii) average input current.

Briefly discuss the different types of PWM schemes available for voltage control in an inverter.

A single-phase one pulse diode rectifier is feeding an RL load with freewheeling diode across the load. For conduction angle $ \beta $, the main diode and freewheeling diode would, respectively, conduct for

When a thyristor is forward biased, the number of blocked p-n junctions is

During forward blocking state, a thyristor is associated with

When a UJT is used for triggering an SCR, the wave shape of the voltage obtained from UJT circuit is a

In a commutation circuit employed to turn off an SCR, satisfactory turn-off is obtained when

A step-up chopper has $ V_s $ as the source voltage and $ \alpha $ as the duty cycle. The output voltage for this chopper is given by

In an inverter with fundamental output frequency of 50 Hz, if third harmonic is eliminated, then the frequencies of other components in the output voltage wave, in Hz, would be

A cycloconverter is a frequency converter from
1. higher to lower frequency with one-stage conversion
2. higher to lower frequency with two-stage conversion
3. lower to higher frequency with one-stage conversion
4. AC at one frequency to DC and then DC to AC at different frequencies
From these, the correct statement(s) is/are

If, for a single-phase half-bridge inverter, the amplitude of output voltage is $ V_s $ and the output power is P, then their corresponding values for a single-phase full-bridge inverter are