# Questions & Answers of Power Electronics

#### Topics of Power Electronics 56 Question(s) | Weightage 10 (Marks)

A single phase fully controlled rectifier is supplying a load with an anti-parallel diode as shown in the figure. All switches and diodes are ideal. Which one of the following is true for instantaneous load voltage and current?

Four power semiconductor devices are shown in the figure along with their relevant terminals. The device(s) that can carry dc current continuously in the direction shown when gated appropriately is (are)

The waveform of the current drawn by a semi-converter from a sinusoidal AC voltage source is shown in the figure. If $I_0=20\;A$, the rms value of fundamental component of the current is ___________A (up to 2 decimal places).

A phase controlled single phase rectifier, supplied by an AC source, feeds power to an R-L-E load as shown in the figure. The rectifier output voltage has an average value given by $V_\circ=\frac{V_m}{2\mathrm\pi}(3+\cos\;\alpha)$, where $V_m=80\mathrm\pi$ volts and $\mathrm\alpha$ is the firing angle. If the power delivered to the lossless battery is 1600 W, $\mathrm\alpha$ in degree is________ (up to 2 decimal places).

The figure shows two buck converters connected in parallel. The common input dc voltage for the converters has a value of 100 V. The converters have inductors of identical value. The load resistance is 1 $\Omega$. The capacitor voltage has negligible ripple. Both converters operate in the continuous conduction mode. The switching frequency is 1 kHz, and the switch control signals are as shown. The circuit operates in the steady state. Assuming that the converters share the load equally, the average value of ${\mathrm i}_{\mathrm S1}$, the current of switch S1 (in Ampere), is _____ (up to 2 decimal places).

A dc to dc converter shown in the figure is charging a battery bank, B2 whose voltage is constant at 150 V. B1 is another battery bank whose voltage is constant at 50 V. The value of the inductor, L is 5 mH and the ideal switch, S is operated with a switching frequency of 5 kHz with a duty ratio of 0.4. Once the circuit has attained steady state and assuming the diode D to be ideal, the power transferred from B1 to B2 (in Watt) is ___________ (up to 2 decimal places).

For the power semiconductor device IGBT, MOSFET, Diode and Thyristor, which one of the following statements is TRUE?

In the converter circuit shown below, the switches are controlled such that the load voltage $v_0(t)$ is a $400\;Hz$ square wave.

The RMS value of the fundamental component of $v_\circ(t)$ in volts is __________.

A 3-phase voltage source inverter is supplied from a 600 V DC source as shown in the figure below. For a star connected resistive load of 220 $\Omega$ per phase, the load power for 120° device conduction in kW, is____________.

The input voltage VDC of the buck-boost converter shown below varies from 32 V to 72 V. Assume that all components are ideal, inductor current is continuous, and output voltage is ripple free. The range of duty ratio D of the converter for which the magnitude of the study-state output voltage remains constant at 48 V is

The figure below shows an uncontrolled diode bridge rectifier supplied from a 220 V, 50 Hz, 1-phase ac source. The load draws a constant current I0 = 14 A. The conduction angle of the diode D1 in degrees (rounded off to two decimal places) is _____________ .

A phase-controlled, single-phase, full-bridge converter is supplying a highly inductive DC load. The converter is fed from a 230 V, 50 Hz, AC source. The fundamental frequency in Hz of the voltage ripple on the DC side is

In the circuit shown, the diodes are ideal, the inductance is small, and I0 ≠ 0. Which one of the following statements is true?

A three-phase voltage source inverter with ideal device operating in 180° conduction mode is feeding a balanced star-connected resistive load. The DC voltage input is $V_{dc}$. The peak of the fundamental component of the phase voltage is

The figure below shows the circuit diagram of a controlled rectifier supplied from a 230 V, 50 Hz, 1-phase voltage source and a 10:1 ideal transformer. Assume that all devices are ideal. The firing angles of the tyristors T1 and T2 are 90° and 270°, respectively.

The RMS value of the current through diode D3 in amperes is__________.

The figure below shows a half-bridge voltage source inverter supplying an RL-load with $R=40\Omega\;and\;L=\left(\frac{0.3}{\mathrm\pi}\right)H$. the desire fundamental frequency of the load voltage is 50 Hz. The switch control signals of the converter are generated using sinusoidal pulse width modulation with modulation index, M = 0.6 . At  50 Hz, the RL-load draws an active power of 1.44 kW. The value of DC source voltage VDC in volts is

In the circuit shown all elements are ideal and the switch S is operated at 10 kHz and 60% duty ratio. The capacitor is large enough so that the ripple across it is negligible and at study state acquires a voltage as shown. The peak current in amperes drawn from the 50 V DC source is ______. (Give the answer up to one decimal place.)

A buck converter, as shown in Figure (a) below, is working in steady state. The output voltage and the inductor current can be assumed to be ripple free. Figure (b) shows the inductor voltage vL during a complete switching interval. Assuming all devices are ideal, the duty cycle of the buck converter is ________.

A steady dc current of 100 A is flowing through a power module (S, D) as shown in Figure (a). The V-I characteristics of the IGBT (S) and the diode (D) are shown in Figures (b) and (c), respectively. The conduction power loss in the power module (S, D), in watts, is ________.

A single-phase thyristor-bridge rectifier is fed from a 230 V, 50 Hz, single-phase AC mains. If it is delivering a constant DC current of 10 A, at firing angle of 30o, then value of the power factor at AC mains is

The switches T1 and T2 in Figure (a) are switched in a complementary fashion with sinusoidal pulse width modulation technique. The modulating voltage ${v}_{m}\left(t\right)$=0.8 sin $\left(200\mathrm{\pi t}\right)$ V and the triangular carrier voltage $\left({v}_{c}\right)$ are as shown in Figure (b). The carrier frequency is 5 kHz. The peak value of the 100 Hz component of the load current (iL), in ampere, is ________ .

The voltage $\left({v}_{s}\right)$ across and the current $\left({i}_{s}\right)$ through a semiconductor switch during a turn-ON transition are shown in figure. The energy dissipated during the turn-ON transition, in mJ, is _______.

A single-phase full-bridge voltage source inverter (VSI) is fed from a 300 V battery. A pulse of 120o duration is used to trigger the appropriate devices in each half-cycle. The rms value of the fundamental component of the output voltage, in volts, is

A three-phase diode bridge rectifier is feeding a constant DC current of 100 A to a highly inductive load. If three-phase, 415 V, 50 Hz AC source is supplying to this bridge rectifier then the rms value of the current in each diode, in ampere, is _____________.

A buck-boost DC-DC converter, shown in the figure below, is used to convert 24 V battery voltage to 36 V DC voltage to feed a load of 72 W. It is operated at 20 kHz with an inductor of 2 mH and output capacitor of 1000 μF. All devices are considered to be ideal. The peak voltage across the solid-state switch (S), in volt, is ____________.

A full-bridge converter supplying an RLE load is shown in figure. The firing angle of the bridge converter is 120º. The supply voltage   . The inductor L is large enough to make the output current IL a smooth dc current. Switches are lossless. The real power fed back to the source, in kW, is __________.

A three-phase Voltage Source Inverter (VSI) as shown in the figure is feeding a delta connected resistive load of 30 Ω/phase. If it is fed from a 600 V battery, with 180o conduction of solid-state devices, the power consumed by the load, in kW, is __________.

A DC-DC boost converter, as shown in the figure below, is used to boost 360V to 400 V, at a power of 4 kW. All devices are ideal. Considering continuous inductor current, the rms current in the solid state switch (S), in ampere, is _________.

A single-phase bi-directional voltage source converter (VSC) is shown in the figure below. All devices are ideal. It is used to charge a battery at 400 V with power of 5 kW from a source Vs = 220 V (rms), 50 Hz sinusoidal AC mains at unity p.f. If its AC side interfacing inductor is 5 mH and the switches are operated at 20 kHz, then the phase shift (δ) between AC mains voltage (Vs) and fundamental AC rms VSC voltage (VC1), in degree, is _________.

In the following chopper, the duty ratio of switch S is 0.4. If the inductor and capacitor are sufficiently large to ensure continuous inductor current and ripple free capacitor voltage, the charging current (in Ampere) of the 5 V battery, under steady-state, is ________.

The circuit shown is meant to supply a resistive load ${R}_{L}$ from two separate DC voltage sources. The switches S1 and S2 are controlled so that only one of them is ON at any instant. S1 is turned on for 0.2 ms and S2 is turned on for 0.3 ms in a 0.5 ms switching cycle time period. Assuming continuous conduction of the inductor current and negligible ripple on the capacitor voltage, the output voltage ${V}_{0}$(in Volt) across ${R}_{L}$ is ________.

A self commutating switch SW, operated at duty cycle is used to control the load voltage as shown in the figure.

Under steady state operating conditions, the average voltage across the inductor and the capacitor respectively, are

The single-phase full-bridge voltage source inverter (VSI), shown in figure, has an output frequency of 50 Hz. It uses unipolar pulse width modulation with switching frequency of 50 kHz and modulation index of 0.7. For = 100V DC, = 9.55 mH, = 63.66 µF, and = 5 $\mathrm{\Omega }$, the amplitude of the fundamental component in the output voltage (in volt) under steady-state is_____________.

In the following circuit, the input voltage ${\mathrm{V}}_{\mathrm{in}}$ is 100$\mathrm{sin}\left(100\pi t\right)$. For 100$\pi RC$=50, the average voltage across R(in Volts) under steady-state is nearest to

A buck converter feeding a variable resistive load is shown in the figure. The switching frequency of the switch S is 100 kHz and the duty ratio is 0.6. The output voltage ${V}_{\mathit{0}}$ is 36 V. Assume that all the components are ideal, and that the output voltage is ripple-free. The value of R (in Ohm) that will make the inductor current (iL) just continuous is _________.

For the switching converter shown in the following figure, assume steady-state operation. Also assume that the components are ideal, the inductor current is always positive and continuous and switching period is ${\mathrm{T}}_{\mathit{s}}$. If the voltage ${\mathrm{V}}_{\mathrm{L}}$ is as shown, the duty cycle of the switch S is _______.

In the given rectifier, the delay angle of the thyristor ${\mathrm{T}}_{1}$ measured from the positive going zero crossing of ${\mathrm{V}}_{\mathrm{s}}$ is 30°. If the input voltage ${\mathrm{V}}_{\mathrm{s}}$ is $100\;\sin\left(100\prod\mathrm t\right)\mathrm V$, the average voltage across R (in Volt) under steady-state is _______.

Figure shows four electronic switches (i), (ii), (iii) and (iv). Which of the switches can block voltages of either polarity (applied between terminals ‘a’ and ‘b’) when the active device is in the OFF state?

A single-phase SCR based ac regulator is feeding power to a load consisting of 5 Ω resistance and 16 mH inductance. The input supply is 230 V, 50 Hz ac. The maximum firing angle at which the voltage across the device becomes zero all throughout and the rms value of current through SCR, under this operating condition, are]

The SCR in the circuit shown has a latching current of 40 mA. A gate pulse of 50 μs is applied to the SCR. The maximum value of R in Ω to ensure successful firing of the SCR is _________.

The separately excited dc motor in the figure below has a rated armature current of 20 A and a rated armature voltage of 150 V. An ideal chopper switching at 5 kHz is used to control the armature voltage. If La= 0.1 mH, Ra= 1$\Omega$ , neglecting armature reaction, the duty ratio of the chopper to obtain 50% of the rated torque at the rated speed and the rated field current is

Thyristor T in the figure below is initially off and is triggered with a single pulse of width 10 μs. It is given that $L=\left(\frac{100}{\mathrm{\pi }}\right)\mu \mathrm{H}$ and $C=\left(\frac{100}{\mathrm{\pi }}\right)\mu \mathrm{F}$. Assuming latching and holding currents of the thyristor are both zero and the initial charge on C is zero, T conducts for

The typical ratio of latching current to holding current in a 20 A thyristor is

Circuit turn-off time of an SCR is defined as the time

A voltage commutated chopper circuit, operated at 500Hz, is shown below.

If the maximum value of load current is 10A, then the maximum current through the main (M) and auxiliary (A) thyristors will be

A solar energy installation utilize a three – phase bridge converter to feed energy into power system through a transformer of 400V/400 V, as shown below

The energy is collected in a bank of 400 V battery and is connected to converter through a large filter choke of resistance10Ω.

The maximum current through the battery will be

A solar energy installation utilize a three – phase bridge converter to feed energy into power system through a transformer of 400V/400 V, as shown below

The energy is collected in a bank of 400 V battery and is connected to converter through a large filter choke of resistance10Ω.

The kVA rating of the input transformer is

The power electronic converter shown in the figure has a single-pole double-throw switch. The pole P of the switch is connected alternately to throws A and B. The converter shown is a

The circuit shows an ideal diode connected to a pure inductor and is connected to a purely sinusoidal 50Hz voltage source. Under ideal conditions the current waveform through the inductor will look like

Match the switch arrangements on the top row to the steady-state V-I characteristics on the lower row. The steady state operating points are shown by large black dots.

A single phase fully controlled converter bridge is used for electrical braking of a separately excited dc motor. The dc motor load is represented by an equivalent circuit as shown in the figure.

Assume that the load inductance is sufficient to ensure continuous and ripple free load current. The firing angle of the bridge for a load current of I0 = 10 A will be

A three-phase, 440 V, 50 Hz ac mains fed thyristor bridge is feeding a 440 V dc, 15 kW, 1500 rpm separately excited dc motor with a ripple free continuos current in the dc link under all operating conditions, Neglecting the losses, the power factor of the ac mains at half the rated speed is

In the circuit of adjacent figure the diode connects the ac source to a pure inductance L.

The diode conducts for

The circuit in the figure is a current commutated dc-dc chopper where, ThM is the main SCR and ThAUX is the auxiliary SCR. The load current is constant at 10 A. ThM is ON. ThAUX is trigged at t = 0. ThM is turned OFF between

A 1:1 Pulse Transformer (PT) is used to trigger the SCR in the adjacent figure. The SCR is rated at 1.5 kV, 250 A with IL = 250 mA, IH = 150 mA, and IGmax = 150 mA, IGmin = 100 mA. The SCR is connected to an inductive load, where L = 150 mH in series with a small resistance and the supply voltage is 200 V dc. The forward drops of all transistors/diodes and gate-cathode junction during ON state are 1.0 V.

The resistance R should be