Power Electronics Question Bank

(1)

http://electrical-mentor.blogspot.in/

POWER ELECTRONICS

Question Bank

by

Shankar

Version: PEQBTNC06

Conventional, Objective and Interview questions in

Power Electronics for GATE |IES | All PSUs

(2)

(3)

Version Code: PEQBTNC06 Page 3333 of 120120120120

PREFACE

I would like to present this Question bank on Power electronics to my student community at free of cost.

I have prepared both conventional and objective questions in the subject of Power

Electronics from various sources and knowledge gained from my teaching experience over a span of 7 years. The content of this Question bank is mainly useful for GATE and

Engineering Service (ESE) aspirants to gain in depth analysis into the subject. As previous GATE and ESE papers are available in various modes, I have not repeated those questions here.

It is expected that the reader must have basic knowledge in the area of Power Electronics and its applications at under graduate level before solving this booklet.

This booklet contains the following sections:

Conventional Questions: By solving these questions, the reader can enhance his/her basic concepts in Power electronics and can establish the link between other branches of

electrical engineering. By solving these types of questions, I am sure your confidence levels in the subject will increase which is the key thing for success in any competitive exam and in career as well. I have provided answers for around 90% of questions and remaining 10% is left as open for the readers so that they can sharpen their knowledge. I will address these questions in the next release of this booklet based on response and will provide some more open questions in subsequent releases

Objective Questions: In the present trend, every exam is based on Objective questions. After solving the conventional questions, the reader can test his/her understanding in the concepts by taking 4 practice tests based on objective questions

Interview Questions: These questions are collected from various interviews like M.Tech admissions in IITs, OCES & DGFS interviews in BARC etc from student community itself. In fact, these questions are not my creation and collected from various students. If you attend any interview, you can also share your experience for the benefit of your next generation

And then I have given practical approach for compensator design for PE converter

After solving this booklet, I am expecting you can face any exam, or interview very confidently especially in the field of Power Electronics.

With initial thoughts in my mind, this booklet came out. I am planning to update this booklet based on feedback received and will revise in regular intervals and need basis Finally I would like to express my sincere thanks to Mr Saida (my student) for his valuable suggestions and efforts in the drafting corrections

If you have any suggestions for further development of this booklet, if you find any mistakes or corrections required, please feel free to write an email to

(4)

INDEX

Sl. No Description Page No 0 Preface 3 1 Conventional Questions 5

2 Objective Questions - Practice Test 1 35 3 Objective Questions - Practice Test 2 47 4 Objective Questions - Practice Test 3 60 5 Objective Questions - Practice Test 4 68

6 Interview Questions 76

7 Answers for Conventional Questions 89 8 Answers for Objective Questions 92

10 Compensator Design 93

11 Useful units for Electrical Engineering 114

(5)

Power Electronics

Conventional Questions

Q1.

In a power electronics laboratory, an experiment is conducted to find circuit component value and its circuit diagram is shown in Fig. The voltage and current waveforms for periodic time of 20 ms are captured from oscilloscope are also shown below. Find out what could be the circuit element and its value

Fig for Q1 Q2.

In a power electronics laboratory, an experiment is conducted to find circuit component value and its circuit diagram is shown in Fig. The voltage and current waveforms for periodic time of 10 ms are captured from oscilloscope are also shown below. Find out what could be the circuit element and its value

Fig for Q2 Q3.

In a power electronics laboratory, the impedance Z(s) diagram (bode plot) for a pure inductor is captured using network analyzer as shown in Fig

ii vi 10V 1A t −10V 2 Ts Vi(t) i1(t) Ii(t) v(t) vi Ii 10A 1V t 2 Ts

(6)

Fig for Q3 (a) If ω0 = 100 rad/s, find the value of inductance

(b) If ω1 = 50 rad/s and ω2 = 200 rad/s then find the value of Z (s) in dB and Ω at ω =

1000 rad/s Q4.

In a power electronics laboratory, the impedance Z (s) diagram (bode plot) for a pure capacitor is captured using network analyzer as shown in Fig

Fig for Q4

(a) If C = 10µF then find the values of ω0, ω1 and ω2 in rad/s (These frequencies are

in decade fashion)

(b) Find the frequency in rad/s when Z (s) = 2 Ω Q5.

The current through and the voltage across a power semi conductor switch is shown in Fig. 6 dB 0 dB −6 dB ω1 ω0 ω2 log₁₀(ω) Z(s) dBΩ 20 dB 0 dB −20 dB ω1 _ω 0 ω2 log10(ω) Z(s) dBΩ

(7)

Evaluate,

(a) The average current and the RMS current rating of the device. (b) The conduction loss in the device

Q6.

The approximate wave shape of a capacitor current in a commutation circuit is shown in Fig. The capacitor has an equivalent series resistance (ESR) of 20 mΩ.

Fig for Q6 Evaluate the power dissipation in the capacitor Q7.

In an inverter, the current through the active device is measured and found to be as shown in Fig. The switching frequency may be considered very high compared to the fundamental frequency of the output current.

Fig for Q7 20A

(8)

Determine,

(a) The average and RMS current rating of the switch.

(b) If the power device is a power transistor with a Vce drop of 1.2 V, evaluate the

conduction loss Q8.

The SCR is used in an application carrying half sinusoidal current of period 1 ms and a peak of 100 A as shown in Fig. The SCR may be modeled during conduction to have a constant voltage drop of 1.1 V and a dynamic resistance of 8 mΩ. Calculate the average conduction loss in the device for this application

Fig for Q8 Q9.

The periodic current through a power-switching device in a switching converter application is shown in Fig.

Fig for Q9 (a) Evaluate the average current through the device. (b) Evaluate the RMS current through the device.

(c) A BJT with a device drop of 1.2 V and a MOSFET with an of 150 mΩ are considered for this application. Evaluate the conduction loss in the device in either case.

Q10.

A power diode (ideal in blocking and switching) shown in Fig, is capable of dissipating 75 W. For square wave operation, it is rated for peak current of 100 A and 135 A at duty ratios 0.5 and 0.33 respectively.

(9)

(a) Evaluate the ON state model of the diode (This procedure is known as piece-wise modeling of semiconductor device).

(b) The above diode while dissipating 40W at an ambient temperature of 350_{C, is}

running with a case temperature of 750_{C and 125}0_{C respectively. Evaluate the}

thermal resistances of the device Q11.

The diode (20ETS08) is a 20 A, 800 V rectifier diode. It has a voltage drop of 0.8 V at 2 A and 1.2 V at 30 A.

(a) Find a piece-wise linear model for this diode consisting of a cut-in voltage and dynamic resistance.

(b) With this piece-wise model evaluate its conduction loss for a 30 A peak half sine wave of current.

Q12.

A power-switching device is rated for 600 V and 30 A. The device has an on state voltage drop of 1.5 V to 2.4 V for conduction current in the range of 15 to 30 A. The device has a leakage current of 5 mA while blocking 600 V.

Evaluate

(a) The maximum conduction loss, (b) The maximum blocking loss, and

(c) The ratio of the conduction and blocking loss with maximum possible power that may be controlled by this switch and make your comment on the result.

Q13.

A composite switch used in a power converter is shown in Fig. The periodic current through the switch is also shown.

(10)

Fig for Q13 Evaluate,

(a) The average current and RMS current through the composite switch. (b) The power loss in the MOSFET and the diode of the composite switch. Q14.

A power MOSFET has an Rds(on) of 50 mΩ. The device carries a current as shown in

Fig. Consider the switching process to be ideal and evaluate the conduction loss in the device. (Explore if you can simplify the evaluation of RMS value by applying superposition).

Fig for Q14 Q15.

A power-switching device is ideal in conduction and blocking (0 V during conduction and 0 A in blocking). It is used in a circuit with switching voltages and currents as shown. The switching waveforms under resistive loading and inductive loading are shown in Fig. The switching times tr and tf are 100 ns and 200 ns respectively.

Evaluate,

(a) The switch-on and switch-off energy loss (in joule) for resistive loading (b) The switch-on and switch-off energy loss (in joule) for inductive loading

(c) The resistive and inductive switching losses in watt for a switching frequency of 100 kHz.

(11)

Q16.

The current through and the voltage across a switching device is given in Fig. Evaluate the approximate switch-off and switch-on energy loss in the device.

Fig for Q16 Q17.

A disc type Thyristor is shown with its cooling arrangement in Fig. The device is operating with a steady power dissipation of 200 W.

(12)

Various thermal resistances are defined as below:

= 0.3 C/W; = 0.3 C/W; = 0.05 C/W; = 0.05 C/W;

= 0.5 C/W; = 0.4 C/W;

Evaluate the steady state temperature rise of the junction Q18.

A composite switch (Q1 and Q2 in parallel) carrying a load current of 10 A is shown

in Fig. The switches may be considered ideal in switching. The on-state resistances of the devices Q1 and Q2 are respectively 0.8 Ω and 0.2 Ω. The devices are mounted

on a common heat sink held at a temperature of 800_C.

Fig for Q18 Evaluate,

(a) RMS values of I1 and I2

(b) The average power dissipation (P1 and P2) in Q1 and Q2.

(c) The junction temperatures of Q1 and Q2 (Note: RJC1 and RJC2 is thermal

resistances from Junction to case of Q1 and Q2).

Q19.

(13)

http://electrical-mentor.blogspot.in/ Copyright © Reserved 2014 The voltage across a capacitor used for a power electronic application is shown in Fig. The capacitance value is 2.5 µF. The capacitor has an equivalent series resistance (ESR) of 10 mΩ. The dielectric of the capacitor has a thermal resistance of 0.2 0_{C/W to the ambient.}

(a) Sketch the current waveform through the capacitor for one cycle (b) Evaluate the losses in the capacitor

(c) Evaluate the temperature rise in the dielectric of the capacitor Q20.

A power electronic capacitor is specified to have the following values. Capacitance = 10 µF; ESR = 30 mΩ; ESL = 75 nH; Sketch the impedance of the capacitor as a function of frequency in the dBΩ vs log ω. Determine the range of frequency for which the capacitor may be idealized to be a pure capacitance of 10 µF

Q21.

The current through a diode is shown in Fig. Consider the following data for waveform analysis.

t1 = 100 µs, t2 = 350 µs, t3 = 500 µs, f = 250 Hz, fs = 5 kHz, Im = 450 A and Ia = 150 A

Determine, (a) Average diode current and (b) RMS diode current

Fig for Q21 *Q22.

Visit a manufacturer's website, identify a controlled power switching device (BJT, or MOSFET, or IGBT etc) of rating > 10A and > 600V. Download the datasheet and fill in the following.

(a) Manufacturer (b) Device and Type No (c) On-state voltage (V) (d) ON-state current (A) (e) Transient switching times (s) (f) Maximum junction temperature (K) (g) Recommended drive conditions (?) (h) Conduction loss at rated current (W) (i) Blocking loss at rated voltage (W) (j) Switching energy loss (J).

t1 t₂ t3 T t i2 i1=Imsinωst Im Ia T= f 1

(14)

Q23.

The magnetic circuit of a coupled inductor is shown in Fig. The magnetic material of the core may be assumed to be ideal.

N1= 100 T; N2 = 200 T; Ag1= Ag2 = 40mm2; Ag = 80mm2; lg1 = 1mm; lg2 = 2mm;

lg = 1.5mm

Fig for Q23 Evaluate the inductances L1; L2; L12; L21

*Q24.

The following figures (a, b, and c) show three magnetic circuits with an exciting winding on each having 100 turns. The core in (c) is obtained by assembling together one each of cores shown in (a) and (b). The magnetic material for the core may be considered to have very large permeability with saturation flux density of 0.2 T.

(a) Evaluate the expression for flux linkages (N

ϕ

) for cores (a) and (b) as a function of the exciting current ia and ib.

(b) Plot the characteristics N

ϕ

vs i for the cores (a) and (b). (c) From the above plot N

ϕ

vs i for the composite core (c). (d) Comment on the inductance of the circuit (c).

(15)

Fig for Q25

(a) For the waveforms shown in Fig, calculate their average value, RMS values of the fundamental and the harmonic frequency components

(b) For the waveforms shown in Fig, consider A = 100 and µ = 600_{where applicable.}

Calculate their total RMS values

(c) For the waveforms shown from a to d in Fig shown, calculate the ratio of (i) the fundamental frequency component to the total RMS value and (ii) the distortion component to the total RMS value

(d) For the waveforms shown from e to f in Fig shown, calculate the ratio of the average value to the total RMS value (form factor)

0 ₀ A _π A 2π 0 0 2 µ 2 µ 2 µ 2 µ 0 0 0 2 µ 2 µ 2 µ 2 µ A A 0 0 t ωt ωt A A A A DT T (a) (b) (c) (d) (e) (f)

(16)

(e) For some common rectifiers, the line currents may be like the waveforms shown in a to b of Fig 2 with µ = 600_{. The need for power per phase is the same in the two}

cases i.e, the RMS value of the fundamental component the line currents are 100 A in both cases.

(i) Calculate the amplitude and the RMS value for waveform a in Fig shown (ii) Calculate the amplitude and the RMS value for waveform b in Fig shown (iii) Comment on the above answers

Q26.

An inductive load connected to a 120 V, 60 Hz ac source draws 1 kW at a power factor of 0.8. Calculate the capacitance required in parallel with the load in order to bring the combined pf to be 0.95 lag

Q27.

A 110 V/220 V, 60 Hz single phase 1 kVA transformer has a leakage reactance of 4 %. Calculate its total leakage inductance referred to (a) the 110 V side and (b) 220 V side

*Q28.

An input voltage of a repetitive waveform is filtered and the applied across the load resistance as shown in Fig. Consider the system to be in steady state. It is given that L = 5 µH and Pload = 250 W

Fig for Q28 (a) Calculate the average output voltage V0

(b)_{Assume that C ∞ so that}

v

o (t) = V0. Calculate Iload and the RMS value of the

capacitor current

i

c

(c) In part (b), plot

v

oand

i

L

*Q29.

The voltage

v

across load and current

i

into the positive polarity are as follows (

ω

1

≠

ω

3) = + √2 !"# + √2"$%# + √2& !"#&'V iload R (load) v0 ic iL vi + − + − + (Fig 3) vi 15V 0 6 µs t 4 µs

(17)

Calculate the following:

(a) The average power P supplied to the load (b) The RMS value of $and

(c) The power factor at which the load is operating Q30.

A single phase half wave diode rectifier is designed to supply dc output voltage of 200 V and load resistance of 10 Ω. Calculate the average and RMS current ratings of diode, PIV of diode and transformer for this circuit arrangement

Q31.

(a) In the circuit shown in Fig, The PMMC ammeter reads 10 A. Find the inductance value. Also find volt meters reading if they are PMMC type

Fig for Q31

(b) If all the meters in part (a) are replaced with MI type instruments, then find the meter readings

Q32.

(a) In the circuit shown in Fig, Ideal PMMC instruments are placed. Find voltmeter readings Fig for Q32 V1 A

∼

L V2 220V 50Hz V1 A

∼

1µF V2 220V 50Hz C

(18)

(b) In case voltmeter 2 in part (a) is replaced by MI type, then find its reading Q33.

A battery is to be charged by a single phase half wave diode rectifier. The supply voltage is 30 V, 50 Hz and the battery emf is constant at 6 V. Determine,

(a) The resistance to be inserted in series with the battery to limit the charging current to 4 A. Take a voltage drop of 1 V across the diode when it is ON

(b) PIV of diode

(c) In case battery capacity is 100 W._{h, find the charging time in hours}

Q34.

Find the time required to deliver a charge of 200 A._{h through a single phase half}

wave diode rectifier with an output current of 100 A (RMS) and with sinusoidal input voltage. Assume diode conduction over a half cycle.

*Q35.

(a) A dc battery is to be charged through a resistor R from a single phase half wave uncontrolled rectifier. For an ac source voltage of 230 V 50 Hz, find the value of average charging current and supply power factor for R = 8 Ω and E = 150 V

(b) In case, if diode is replaced by SCR and fired continuously through a constant dc signal, the repeat part (a)

(c) In case, SCR in part (b) is triggered after 1 ms from its forward bias point. Then repeat part (b)

(d) Comment on all the calculations Q36.

Fig for Q36

(19)

(c) What will be the required V

RRM rating if the capacitor is removed? Assume a

resistive load.

(d) The source of the single phase rectifier circuit has an internal resistance of 2 Ω. Find out the required Non repetitive peak surge current rating of the diode. Also find the i2t rating of the protective fuse to be connected in series with the diode. *Q37.

A single phase midpoint converter is shown in Fig, where we assume the transformer is to be ideal and the dc side load to be represented by a current stiff load. Calculate the VA rating of the transformer as a ratio of the average power supplied to the load.

Fig for Q37 Q38.

A single bridge consists of one SCR and three diodes operating with a firing angle of 450_{. Find the average load current and power delivered to the load in case the load}

consists of R = 8.356 Ω, L = 8 mH and E = 100 V. Assume the load current is constant in the entire working range

Q39.

A single phase full converter feeds power to RLE load with R = 10 Ω, L = 6 mH and E = 60 V. The ac source voltage is 220 V, 50 Hz. In case one of the four SCRs gets open circuited due to fault, find the average value of load current by assuming the load current as continuous and firing angle is 450_.

Q40.

A three phase half wave phase controlled rectifier delivers power to a resistive load of 10 Ω. Input to the rectifier is 400 V, 50 Hz three phase ac supply. Find power delivered to the load for a firing angle of (a) 150_{and (b) 60}0

∼

Vp=Vmsinωt n : 1 : 1 D1 Id D2

(20)

Q41.

A three phase half wave phase controlled rectifier is operated from a 3-ph 230 V, 50 Hz supply with load resistance of 10 Ω. An average output voltage of 50 % of the maximum possible output voltage is required. Determine, (a) Firing angle of the converter (b) Average and RMS values of load current

Q42.

A three phase half wave phase controlled rectifier is fed from a 3-ph, 400 V 50 Hz source and is connected to load taking a constant current of 30 A. SCRs are having a voltage drop of 1.9 V during their conduction. Calculate,

(a) Average value of load voltage for a firing angle of 300_{and 60}0

(b) Average and RMS current ratings of SCRs as well as PIV of SCRs (c) Power loss in each SCR

(d) In case, if freewheeling diode (FD) is connected across load, find the average value of output voltage, average and RMS value of FD current for firing angles of 300_{and 60}0

Q43.

A three phase half wave phase controlled rectifier is operating from a 3-ph, 400 V 50 Hz and delivers power to the armature of a dc motor with negligible resistance and large inductor in the dc bus. The source transformer has DY-11 connection with unity phase turns ratio. Back emf of the motor is 300 V. Find the firing angle of the rectifier

Q44.

A three phase fully controlled rectifier is delivers a ripple free load current of 10 A with a firing angle of 300_{. The average output voltage is 400 V. Find active and}

reactive power input to the bridge and input power factor of the converter Q45.

A battery consists of R = 5 Ω and E = 150 V is charging through a three phase half wave phase controlled rectifier. Input voltage to the converter is 230 V (RMS) from any line to neutral and firing angle for SCRs is 300_{. Find average current flowing}

through the battery Q46.

A three phase full converter is fed from 230 V, 50 Hz supply having source inductance of 4 mH per phase. The load current is 10 A and ripple free

(a) Calculate the voltage drop in dc output voltage due to source inductance (b) If dc output voltage is 210 V, find firing angle and overlap period

(c) In case, the bridge is made to operate as a line commutated inverter with dc voltage of 210 V, find firing angle for the same load current

(21)

A three phase half wave diode rectifier delivers power to an inductive load which takes ripple free current of 100 A. The source voltage to the bridge is 3-ph 440 V, 50 Hz. Determine,

(a) The average and RMS current ratings of diode (b) PIV of diode

(c) RMS value of source current *Q48.

A battery with a nominal voltage of 200 V and internal resistance of 10 mΩ has to be charged at constant current of 20 A from a 3-phase 220 V, 50 Hz AC power supply. Which of the following converter circuit will give better performance in terms of Distortion factor in source current, fundamental power factor, and total input power factor? (i) 3-ϕ Full converter (ii) 3-ϕ Semi converter

Q49.

(a) For the same average DC output voltage of 100 V, calculate the PIV of SCR for the following configurations (Consider α = 00₎

(i) 1-ϕ full wave center tap converter (midpoint converter) (ii) 1-ϕ full converter

(iii) 1-ϕ semi converter (iv) 3-ϕ half wave converter (v) 3-ϕ full converter

(vi) 3-ϕ semi converter

(b) From the above calculations, which configuration is having maximum and minimum PIV rating for SCR

Q50.

(22)

Two six pulse converters, used for a bipolar HVDC transmission system (shown in figure) are rated at 1000 MW, ±200 kV. Evaluate, the RMS current and peak reverse voltage ratings for each of the SCRs

Q51.

A buck converter has an input voltage of 16 V. The required average output voltage is 8 V and peak to peak ripple in output voltage is 10 mV. The switching frequency of the converter is 25 kHz. If the peak to peak ripple in inductor current is limited to 0.7 A. Determine, (a) Duty cycle ratio (b) Filter inductance (c) Filter capacitance Q52.

The input voltage to a boost converter is 8 V. The required average output voltage is 16 V and the average output load current is 0.5 A. The switching frequency of the converter is 30 kHz. If L = 160 µH and C = 380 µF, calculate, (a) Duty cycle ratio (b) The peak to peak ripple in inductor current (c) The peak current of the switch (d) The ripple voltage in capacitor

Q53.

The input voltage to a buck- boost converter is 10 V. The switch is operating with a duty ratio of 0.3 and the switching frequency is 25 kHz. The filter inductance is 150 µH and filter capacitance is 220 µF. The average load current is 1.2 A. Determine, (a) The average output voltage (b) The peak to peak ripple in output voltage (c) The peak to peak ripple in inductor current (d) The peak and average current of the switch

*Q54.

A switched mode power converter is shown in Fig. The switches S are ON during DTs and the switches S´ are ON during (1-D)Ts

Fig for Q54

(a) Evaluate the steady state performance of the circuit. Assume the switches, inductors and capacitors are to be ideal

(b) Indicate how the voltage conversion ratio will be modified if the inductor has a resistance of RL S _L _S′ V0+∆V0 I0 R C S S′ Vg + Ig _I_L_+∆I_L

(23)

Consider the circuit given in Fig. Carry out the steady state analysis for the same and evaluate the following

Fig for Q55

(a) Output voltage (b) Average input current (c) Output power (d) Efficiency (e) Power dissipation in the MOSFET and the diode

*Q56.

Figure P13 shows a boost converter cascaded by a buck converter. The switches S and S−_{are ON during DT}_S_{and (1-D)T}_S_{respectively.}

Fig for Q56

(a) Evaluate the steady state currents in L1 and L2 in terms of I0 and D.

(b) Evaluate the steady state voltages across C1 and C2 in terms of Vg and D

(c) Evaluate the current ripples in L1 and L2

(d) Evaluate the voltage ripple in C1 and C2

Q57.

A DC-DC converter circuit is shown in Fig. It consists of on active switch (S1) and

three passive switches D1, D2 and D3. It has four energy storage elements - two

inductors (L1, L2) and two capacitors (C1, C2). Consider that the currents through

the inductor and voltage across the capacitors are all continuous. The switch S1 is on

during Ton and off during Toff . The duty ratio of S1 may be designated as D. The

(24)

Fig for Q57

(a) Indicate the duty ratios of the three diodes D1, D2 and D3.

(b) Evaluate the steady-state inductor currents (I1, I2) and the steady state

capacitor voltages (VC1, VC2).

(c) Evaluate the voltage conversion ratio Vo/Vg.

(d) Sketch the steady-state waveforms of (I1, I2; VC1, and VC2).

(e) Evaluate the ripple currents ∆I1 and ∆I2 in terms of Vg, D, L1, L2 and R.

(f) Evaluate the ripple voltages ∆VC1 and ∆VC2 in terms of Vg, D, L1, L2, C1, C2, and

R.

(g) Calculate L1, L2, C1 and C2 by considering the circuit data as Vg = 100 V, D = 0.6,

R = 12 Ω and Ts = 20 µs.

Assume ripple in capacitor voltage is 1% of its average value and ripple in inductor current is 10 % of its average value

Q58.

In the buck converter shown the diode D has a lead inductance of 0.2µH and a reverse recovery change of 10 µC at iD =10A.

Fig for Q58 Find peak current through active switch.

Q59.

The following Figure shows a PI controller and its asymptotic magnitude bode plot. Select R1, R2, and C. make any suitable assumptions if necessary

(25)

*Q60.

A fly back converter operating at a duty ratio of 0.3 is shown in the following Fig. The transistor ON state drop is 1 V. The diode ON state drop is 0.7 V. The

resistance of the inductor windings is 0.5 and 0.25 for the primary and secondary respectively.

Fig for Q60

Evaluate the voltage conversion ratio and efficiency of the converter Q61

In a flyback converter, the required output voltage is 100 V for a nominal input voltage of 12 V. If the switch is operating at D = 0.5

(a) Find the turns ratio of flyback transformer. Assume voltage drop across switch is 0.8 V and diode is 0.8 V

(b) Find minimum and maximum values of D, if input voltage varies from 10 to 14 V, by maintaining V0 be constant. Assume the switching frequency of 2 kHz

(c) Find the value of Ls on secondary winding so that secondary current is just

continuous at the minimum value of D calculated in part (b). Consider load resistance of 100 Ω

(26)

Q62.

A fly-back converter is to be designed to operate in just-continuous conduction mode when the input dc is at its minimum expected voltage of 200 volt and when the load draws maximum power. The load voltage is regulated at 16 volts. What should be the primary to secondary turns ratio (N1/N2) of the transformer if the switch duty

ratio is limited to 80 %. Neglect ON-state voltage drop across switch and diodes Q63.

The average output voltage flyback converter is 24 V at a resistive load of 0.8 Ω. The duty cycle ratio is 0.5 and switching frequency is 1 kHz. The ON state voltage drops of BJT and Diode are VT = 1.2 V and VD = 0.7 V. The turns ratio of transformer is =>

=?= 0.25. Find the efficiency of the converter

Q64.

Find maximum voltage stress of the switch in the primary winding and diode in the tertiary winding if the forward converter-transformer has 10 primary turns and 15 tertiary turns and the maximum input dc voltage is 300 V

Q65.

If the turns ratio of the primary and tertiary windings of the forward transformer are in the ratio of 1:2, what is the maximum duty ratio at which the converter can be operated? Corresponding to this duty ratio, what should be the minimum ratio of secondary to primary turns if the input dc supply is 400 V and the required output voltage is 15 V. Neglect switch and diode conduction voltage drops.

Q66.

(27)

http://electrical-mentor.blogspot.in/ Copyright © Reserved 2014 A forward converter is operating at the boundary of continuous and discontinuous conduction. The switch is operating at 100 kHz. Assume µ = ∞ for the core so that energy recovery winding is ignored. A load of 10 A at 20 V is being supplied.

(a) Find the inductance value

(b) Find peak to peak ripple in output voltage as % of average output voltage (∆AB AB)

*Q67.

A forward converter operating at a duty ratio of 0.3 is shown in the following Fig. The transistor while ON drops a voltage of 1.0 V, and the diode while ON drops a voltage of 0.7 V.

Fig for Q67

Evaluate the output voltage and efficiency of the converter. *Q68.

A forward converter operating at a duty ratio of 0.4 is shown in the following Fig. Assume the components to be ideal.

(28)

Sketch the following waveforms under steady state.

(a) Inductor current. (b) Secondary current. (c) Primary current. (d) Output voltage.

Q69.

A single phase full bridge VSI is fed from 230 V dc. In the output voltage waveform, only fundamental component of voltage is considered.

(a) Determine the RMS current ratings of switches and diode of the bridge for the following types of loads:

(i) R = 2 Ω (ii) ωL = 2 Ω

(b) Find also the repetitive peak voltage that may appear across switches in part (a) Q70.

A single phase full bridge VSI delivers power to RLC load with R = 3 Ω and XL = 12

Ω. The bridge operates with periodicity of 0.2 ms. Calculate the value of C so that load commutation is achieved for the SCRs. Turn off time for thyristors is 12 µs and consider factor of safety 2. Assume that load current contains only fundamental component.

Q71.

A single phase full bridge VSI delivers power at 50 Hz to RLC load with R = 5 Ω, L = 0.3 H and C = 50 µF. The dc input voltage is 220 V.

Evaluate,

(a) Expression for load current up to 5th_harmonic

(b) Power delivered to load and fundamental power (c) The RMS and peak currents of each switch

(d) Conduction time of switches and diodes by considering only fundamental components

Q72.

A single phase full bridge VSI delivers power to a load of R = 12 Ω and L = 0.04 H from a 400 V DC source. If the inverter operates at a frequency of 50 Hz, determine the power delivered to the load for

(a) Square wave operation

(b) Quasi square wave operation with an on period of 0.6 of a cycle

(c) Two symmetrically spaced pulses per half cycle with an on period of 0.6 of a cycle

Q73.

A single phase current source inverter (CSI) with ideal switches has the following data:

Source current = 30 A, frequency = 500 Hz, and pure capacitive load = 20 µF For this inverter, Evaluate:

(29)

(b) The maximum value of reverse voltage that appears across switches Q74.

A single phase capacitor commutated CSI connected to the load R has the following data:

R = 40 Ω, C = 50 µF, Source current = 40 A, frequency = 500 Hz. Evaluate,

(a) Express the load current as function of time and its value at t = 0 and t = T/2 (b) The circuit turn off time

Q75.

A 3-phase 1200_{mode inverter feeds a star connected load of R = 5 Ω. DC source}

voltage is 230 V and output frequency is 50 Hz.

(a) Express the line to line output voltage, line to neutral output voltage and line current in fourier series up to 11th_{harmonic components.}

(b) RMS values of line to line and line to neutral voltages

(c) RMS values of line to line and line to neutral voltages at fundamental frequency (d) THD for line current

(e) Load power and average value of source current (f) Average and RMS value of switch currents Q76.

SCR T in the figure below is initially OFF and is triggered with a single pulse of width 10 s. It is given that C = DEE

F G μH and K = D EE

F G μF. Assume latching and

holding currents are zero and initial conditions L and C are zero. (a) Evaluate the conduction time of SCR T

(b) Voltage across device and capacitor after SCR is turned OFF

Fig for Q76 Q77.

A circuit employing current commutation as shown below has C = 20 µF and L = 3 µH. Initially capacitor is charged towards the source voltage (=230 V dc).

(30)

Fig for Q77

Determine the conduction time for auxiliary SCR (TA) and circuit turn off time for

main SCR (TM) in case constant load current is (a) 300 A and (b) 60 A

Q78.

In the circuit shown in the Figure below, has commutating elements L = 20 µH and C = 40 µF are connected in series with the load resistance of R = 1 Ω.

Fig for Q78

Check whether self commutation or load commutation, would occur or not. Find also conduction time of SCR

Q79.

For the circuit shown in Fig, (dv/dt) rating of thysristor T is 400 V/µs. and its junction capacitance is 25 pF. Switch S is closed at t = 0.

(31)

(a) Calculate the value of Cs so that thyristor T is not turned on due to dv/dt

(b) In case maximum current through thyristor in above circuit is limited to 40 A, determine the value of Rs

Q80.

For illustrating complementary commutation, the following circuit is employed where Vdc = 200 V and R1 = 10 Ω.

Fig for Q80

(a) Find the value of capacitor so that T1 is commutated in 50 µs.

(b) It is required that SCR T2 is turned off naturally when current through it falls below holding current of 4 mA. Find the value of R2.

Q81.

In the complementary scheme of commutation, Source voltage is 200 V dc, R1 = 10 Ω

and R2 = 100 Ω. Evaluate,

(a) Peak value of current through SCRs T1 and T2

(b) Capacitance value C if each SCR has turn off time of 40 µs. Take a factor of safety 2. Justify if you make any assumptions

(32)

*Q82.

(a) A separately excited dc motor is represented in block diagram form as show in Fig. Fill all the blocks with usual and your convenient notation

Fig for Q82-a

(b) From the block diagram given in part (a), find G1 (s) and G2 (s) as per the

definitions given below M" = N#O"

P" Q_R_ST_UEV%W M" = N

#O"

XYZ"Q_A_[_UE

(c) Now, express G1 (s) in terms of machine mechanical and electrical time constants which are defined as below:

\O = _`]^_

R`a V%W \b =

C_^ ]^

Where, KT and KE are torque and electrical constant

(d) The dc motor under consideration is having the following data. T rated = 10 N-m ; Nrated = 3700 rpm KT = 0.5 N-m/A ; KE = 53V/1000 rpm;

Ra = 0.37 Ω, 6e = 4.05ms, 6m = 11.7 ms

If this motor is controlled from a power electronic converter, Evaluate the terminal voltage required (in steady state) if motor is required to deliver a torque of 5 Nm at a speed of 1500 rpm

(e) If G1(s) in the given statement is expressed as M" = /cd efD_ijghGekg

ijg

Then find the values of D and

ωn

by using data given in part (d). Now, plot the asymptotic magnitude and phase plot of G1 (s) by means of bode plot. Then find

out Phase margin and Gain margin with approximate hand calculations or using MATLAB

V

_t

(s)

_Q(s)

?

I

a

(s)

T

e

(s)

?

T

_WL

-E

_a

(s)

(33)

Fig for Q82-f lY" = _##"_∗_{" =} 1 1 + " D2o_# pG + " #p

Where D = 0.5 and ωn = 300 rad/s

(g) Draw the bode plot of closed loop transfer function l_q" = qf

q∗_f if the gain KP =

60 is used for the proportional position regulator.

(h) What is the bandwidth of the above closed loop systems (Hint: This is indication of speed of the response)

Q83.

A 200 V, 1450 rpm, 100 A separately excited dc motor has an armature resistance of 0.04 Ω. The machine is driven by a 3-ϕ half controlled rectifier operating from a 3-ϕ 220 V, 50 Hz supply. The motor operates at rated speed and rated load torque. Assuming continuous conduction to evaluate (a) Firing angle of the converter (b) RMS value of fundamental input current (c) Fundamental power factor (d) THD in source current

Q84.

A 3-ϕ full converter is feeding a 100 HP, 400 V, 1500 rpm separately excited dc motor having armature resistance of 0.1 Ω and filter choke which is connected in series with armature to maintain constant current of 175 A. The bridge is connected to a 3-ϕ, 400 V, 50 Hz supply. The source has an inductance of 0.5 mH and back emf constant of the machine is 0.25 V/rpm. Evaluate (a) Firing angle (b) Overlap angle

Q

*

(s)

Σ

_K

_p

F

w

(s)

ω

*

(s)

ω(s)

−

s

(34)

Q85.

A 200 V, 875 rpm, 150 A separately excited dc motor has an armature resistance of 0.06 Ω. It is fed from a single phase fully controlled rectifier with an ac source voltage of 220 V, 50 Hz. Assuming continuous conduction calculate,

(a) Firing angle for rated motor torque and at 750 rpm (b) Firing angle for rated motor torque and at (-500) rpm (c) Motor speed for α = 1600_{and rated torque}

Q86.

A 220 V, 1500 rpm, 50 A separately excited dc motor with armature resistance of 0.5 Ω is fed from a 3-ϕ full converter. Available ac source has a line voltage of 440 V, 50 Hz. A Y-∆ connected transformer is used to feed the full converter so that motor rated terminal voltage equals the rated voltage when converter firing angle is zero. (a) Calculate the turns ratio of transformer (between phase windings of primary and secondary)

(b) Determine the firing angle of the converter when (i) motor is running at 1200 rpm and rated torque (ii) motor is running at -800 rpm and twice the rated torque Q87.

A 230V, 960 rpm and 200 A separately excited dc motor with armature resistance of 0.02 Ω. The motor is fed from basic chopper circuit which provides both motoring and baking operation. The source has a voltage of 230 V. Assuming continuous conduction

(a) Calculate the duty ratio of the chopper for motoring operation at rated torque and 350 rpm

(b) Calculate the duty ratio of the chopper for braking operation at rated torque and 350 rpm

(c) If maximum duty ratio of chopper is limited to 0.95 and maximum permissible motor current is twice the rated. Calculate maximum permissible motor speed obtainable without field weakening and power fed to the source

(d)If motor field is also controlled in part (c), calculate filed current as a fraction of its rated value for a speed of 1200 rpm

Q88.

A four pole, 10 HP, 460 V Induction motor is supplying its rated power to a centrifugal pump type of load at a 60 Hz frequency. Its rated speed is 1746 rpm (a) Calculate its speed, slip frequency, and slip when it is supplied by a 230 V, 30 Hz source

(b) If the starting torque is required to be 150 % of the rated torque for a constant air gap flux, find the starting frequency that need to apply to the motor

(35)

Objective Questions

Practice Test - 1

Q1.

Which of the following power semi conductor device is popularly using in wind and solar power converters?

(A) SCR (B) GTO (C) IGBT (D) BJT

Q2.

Which of the following MOSFET is most suitable as power electronic switch? (A) N – channel depletion MOSFET

(B) P – channel depletion MOSFET (C) N – channel enhancement MOSFET (D) P – channel enhancement MOSFET Q3.

Consider the following statements and choose the correct option. Statements about ideal switches:

1. In OFF state, current flowing through ideal switch is zero. 2. In ON state, voltage across ideal switch is zero.

3. The ideal switch need finite energy to switch ON/OFF or OFF/ON. 4. The switch can be turned ON and OFF instantaneously

(A) All statements are true (B) Only 1, 2 and 4 true (C) Only 1, 2 and 3 are true (D) None

04.

Match List – I (Transfer characteristics) and List – II (Devices) and select the correct option. List – I List – II P Q R S P Q R S (A) 1 3 2 4 (B) 2 1 4 3 (C) 4 1 3 2 (D) 1 2 3 4 Q5.

A BJT with a device drop of 1.2 V is carrying a current which is shown below. VGs ID (1) VGs ID (2) VGs ID (3) VGs ID (4) (P) _D G S (Q) G S D (R) G S D (S) G S i 10A 0 5 15 20 30 (µs)

(36)

Version Code: PEQBTNC06 Page 36363636 of 120120120120 ton υCE ic tof t ICs t T VCC (A) 5 W (B) 5.5 W (C) 6 W (D) 8 W Q6.

Which of the following device (s) will be considered as bipolar and unidirectional switch. 1. SCR

2. Symmetrical GTO 3. Asymmetrical GTO 4. BJT in series with diode

(A) Only 1, 2, 3 (B) 1, 3 and 4 (C) 1, 2, 3, 4 (D) 1, 2 and 4 Q7.

During forward conduction, a thyristor has static V – I characteristic as shown by a straight line in given figure. Find the average power loss in the thyristor in case thyristor is carrying a constant current of 80 A for one half cycle.

(A) 70.4 W (B) 11.6 W (C) 40 W (D) 50 W Q8.

The switching waveform for a power transistor are shown in below figure

In case, Ics=80A, Vcc=220V, ton=1.5 µs and toff=4 µs then power loss in the transistor during

turn ON is (A) 8.8 W (B) 23.46 W (C) 32.6 W (D) None Q9.

For a power diode, the reverse recovery time is 3.9 µs and the rate of diode current decay is 50 A/µs. For a softness factor of 0.3, Find the storage charge.

(A) 350 µC (B) 292.5 µC (C) 150 µC (D) 200 µC Q10.

Turn on time of an SCR in series with RL circuit can be reduced by (A) Increasing R (B) Decreasing R

(C) Increasing L (D) Decreasing L Ia

100A

(37)

For dynamic equalizing circuit used for series connected SCRs, the selection of C is based on

(A) Reverse recovery characteristics (B) Turn – on characteristics (C) Turn – off characteristics (D) Rise time characteristics Q12. The circuit symbol for GTO is

(A) P, Q and S are correct (B) Only P & Q are correct (C) Only Q & S are correct (D) All are symbols are GTO Q13.

Input power factor of any rectifier circuit can be defined as below: (A) rectifier the to input power Apparent rectifier the to input power Actual IPF=

(B) IPF = (distortion factor) × (displacement factor) (C)

(

)

2 THD 1 DPF IPF + =

(D) All the above Q14.

A single phase semi converter is operated from 230 V, 50 Hz AC supply and operated with a firing angle of

3 π

. The load on the converter is highly inductive with a resistance of 15.53 Ω and load current is ripple free. RMS value of the fundamental source current will be.

(A) 10A (B) 9A (C) 5A (D) 7.8A

Q15.

A single phase fully controlled rectifier is supplying power to a purely resistive load of 100 Ω and the bridge is triggered with α= 90°. The source voltage is 200 V, 50 Hz. The power delivered to the load is given by

(A) 1 kW (B) 2 kW (C) 3 kW (D) 4 kW

Q16.

The DC equivalent circuit of a single phase full converter is shown below. The net average output voltage is available across terminals X and Y. Find the source inductance by assuming input frequency of 50 Hz.

A G G (P) A K G (R) A K G (S) A K G (Q) I0 V0=200V x y V 220 cos V 2 _m = α π

(38)

(A) 1 H (B) 0.2 H (C) 0.01 H (D) none

Q17.

Match List – I (1-φ bridge configuration) with list – II (Average output voltage) and choose the correct option. (Assume I0 is constant)

List – I (1-φφφφ bridge) List – II [Average output voltage] (P) with 4 SCRs 1. π = m 0 V 2 V (Q) with 2 SCRs + 2 diodes 2. π = m 0 V 2 V cosα (R) with 1 SCR + 3 Diodes 3. [3 cos ] 2 0

α

π

+ =Vm V (S) with 4 diodes 4. ₀ [1 cos

α

]

π

+ =Vm V P Q R S P Q R S (A) 2 4 1 3 (B) 2 4 3 1 (C) 4 2 3 1 (D) 4 2 1 3 Q18.

Which of the following PE converter circuits can be used to operate the DC machine in regenerative braking mode? Select your choices from the given options.

1. 1-φ full converter 2. 1-φ semi converter 3. 3-φ full converter 4. 3-φ semi converter 5. Type E chopper 6. 1-φ full bridge VSI Options:

(A) 1, 3, 5 and 6 only (B) 1, 2, 3, 4 only (C) 1, 3 and 5 only (D) All can be used Q19.

In controlled rectifiers the nature of load current i.e. whether load current is continuous or discontinuous

(A) does not depend on type of load and firing angle delay (B) depends on type of load and firing angle delay

(C) depends only on the type of load (D) depends only on the firing angle delay Q20.

If α=90o_{in the following circuit, the RMS value of the output voltage is}

(39)

A 230 V, 50 Hz one-pulse SCR controlled converter is triggered at a firing angle of 40o_and

load current extinguishes at an angle of 210o_{. Find the circuit turn off time (in second) if}

load R=5 Ω and L=2 mH

(A) 120 (B) (1/120) (C) 60 (D) (1/60) Q22.

A four quadrant operation of the DC machine requires (A) Two full converters is series

(B) Two full converters connected back to back (C) Two full converters connected in parallel (D) Two semi converters connected back to back. Q23.

The output ripple frequency of 3-φ semi converter depends on (A) Input frequency (B) firing angle

(C) source inductance (D) both A&B Q24.

A 3-φ full converter feeds power to a resistive load of 10 Ω. For a firing angle of 45o_{, the load}

takes 1 kW. Find the magnitude of the line Voltage

(A) 100 2_V (B) 100 3_V (C) 200 V (D) 200 3_V Q25.

A 3-φ half wave controlled rectifier is delivering power to highly inductive load with continuous load current. Which of the following equation can be used to find average output voltage when firing angle is 450_?

(A)

[

+ α

]

π = 1 cos 2 V 3 3 V₀ mp (B) α π = cos 2 V 3 3 V₀ mp (C) _            π + α + π = 6 cos 1 2 V 3 V₀ mp (D) _            π + α + π = 6 cos 1 2 V 3 3 V₀ mp Q26.

A 3-φ full converter is supplying power to an inductive load and current of 31.42 A. The RMS value of fundamental input current will be

(A) 24.5 A (B) 31.42A (C) 25.65A (D) None

∼ D3 D2 D4 D1 T2 T1 230V 50Hz R υ0 + −

(40)

Q27.

In 12 pulse rectifier, the lowest harmonic in the input current will be (A) 3rd _{(B) 5}th _{(C) 7}th _{(D) 11}th

Q28.

Which of following rectifier can only be used as both 3-pulse and 6-pulse rectifier? (A) 1-φ full converter (B) 3-φ full converter

(C) 3-φ half wave converter (D) 3-φ half controlled converter Q29.

In step up chopper (boost converter), the average output voltage when duty cycle ratio=1.

(A) infinity (B) zero

(C) equal to source voltage

(D) less than infinity but greater than source voltage Q30.

In which of the following DC-DC converter, the nature of power conversion will be of current to voltage type

(A) Buck converter (B) Boost converter

(C) Buck-Boost converter (D) fly back converter (Isolated type) Q31.

A chopper circuit shown in the figure is operating at a switching frequency of 25 kHz with a duty cycle ratio of 0.25 The peak to peak ripple in the inductor current will be

(A) 1 A (B) 1.8 A (C) 0.5 A (D)0.8 A

Q32.

The fundamental difference between transformer and inductor used in power conversion circuits is

(A) both have same purpose

(B) Inductor is used to smoothen energy flow where as transformer will provide electrical isolation and voltage levels matching.

(C) Inductor will store energy whereas transformer will not store energy (D) both B & C are correct

Q33.

A type – A chopper is operating at 2 kHz from a 100 V dc source has a load time constant of 5 ms and load resistance of 10 Ω. Find the max value of inductor current for a mean load voltage of 50V.

(A) 5.25A (B) 5.025A

(C) 5.125A (D) 4.875A 12V sw 120µH + − V0

(41)

A diode is connected in series with LC circuit as shown in fig. Assume the capacitor is initially charged to a voltage of -50 V.

The voltage across capacitor at the time diode turns off is

(A) 230 V (B) -280 V (C) -220 V (D) 510 V

Q35.

Find the conduction time of SCR in the following circuit. Assume L and C are initially relaxed

(A) 0.314 ms (B) 0.314 µs (C) 3.14 ms (D) none Q36.

A complementary commutation scheme is shown in fig.

If Vdc = 200V, R1 = 10Ω and R2 = 100Ω determine peak value of current through T2.

(A) 24A (B) 42A (C) 21A (D) 12A

Q37.

In the circuit shown in figure, if Vdc = 200 V, C = 4 µF and L = 16 µH and R = 20 Ω. The

peak value of current through T1 and D can respectively be

Vdc

T1

R1

T₂ R2

(42)

100V

A B

(A) 110 A, 100 A (B) 110 A, 10 A (C) 10 A, 110 A (D) 100 A, 110 A Q38.

In 3-φ full bridge VSI, which of the following voltage will have 3rd_{harmonic in 180°}

operation

(A) line voltage (B) phase voltage (C) pole voltage (D) none Q39.

A single phase full bridge VSI is feeding power to a resistive load of 5 Ω. The fundamental output voltage is found to be 200 V (rms). Find the rms value of switch and diode currents at fundamental frequency (A) Isw,1 = 20 2Aand ID,1 = 10 2A (B) Isw,1 = 20 2Aand ID,1 = 0A (C) Isw,1 = 10 2Aand ID,1 = 0A (D) Isw,1 = 10 2Aand ID,1 = 5 2A Q40.

The source voltage of a 3 – φ full bridge VSI is 200 V. The rms value of phase voltage in 120° operation will be

(A) 40.82V (B) 20.41V (C) 81.64V (D) 141.42V Q41.

A single phase full bridge VSI is operating in 180° square operation. The phase angle between the pole voltages is 45°. The RMS value of the output voltage between two poles is

(A) 100 V (B) V

4

100×π (C) 200 V (D) 50 V

Q42.

A single phase full bridge VSI has a source voltage of 200 V. The load consists of RLC in series where R = 1Ω, ωL = 7Ω and = Ω

ωC 6 1

. Identify the fundamental component of load current from the following.

(A) 180 sin (ωt + 45°) (B) 180 sin(ωt - 45°) (C) 127.3 sin(ωt-45°) (D) 127.3 sin(ωt+45°)

(43)

The operating points of three power electronic switches on VI plane is shown below Consider the following statements regarding the switches P, Q, R

1. P is most suitable for VSI 2. P is most suitable for CSI 3. Q is the most suitable for VSI 4. Q is most suitable for CSI

5. P, Q and R can be used in either VSI or CSI

Now, select the correct option from the following (A) only 2 & 3 are correct

(B) only 1 & 4 are correct (C) 2, 3 and 5 are correct (D) All are correct

Q44.

In single pulse modulation used in PWM inverters, Vdc is the input dc voltage. For

eliminating third harmonic, the magnitudes of rms value of fundamental component of output voltage and pulse width are respectively

(A) 4Vdc ,60o π (B) o dc V _,₁₂₀ 6 π (C) 2 2 V_dc,60o π (D) o dc,120 V 4 π Q45.

A PWM inverter is capable of producing the following type of output voltage (A) variable in magnitude and frequency

(44)

Version Code: PEQBTNC06 Page 44444444 of 120120120120 ∼ Load (1) ∼ Load SCR (2) ∼ Load MOSFET/ BJT/ IGBT (4) ∼ Load TRIAC (3)

(B) variable voltage, fixed frequency (C) Fixed voltage, variable frequency (D) Fixed voltage, fixed frequency Q46.

Consider the following circuits

From the above circuits which one can be considered as AC voltage regulator (A) only 1 & 3 (B) only 1, 2 and 3 (C) only 1 (D) All circuits Q47.

An induction motor is required to run at a very low speed around 25 to 40 rpm from 50 Hz source. Which of the following circuit is most suitable for this application

(A) step up cycloconverter (B) inverter (C) step down cycloconverter (D) All the above Q48.

A load consisting of R = 10 Ω and ωL = 10 Ω is being fed from 230 V, 50 Hz source through a 1-φ AC voltage controller. For a firing angle delay of 45°, the rms value of load current will be (A) 23A (B) A 2 23 (C) A 2 23 > (D) A 2 23 < Q49.

A dc motor is driven from a 3-φ full converter draws a dc line current of 10A with negligible ripple. The rms value of thyristor current will be

(A) 10 A (B) 7.07 A (C) 5.77 A (D) 17.32 A

Q50.

Which of the following chopper circuit can be used to drive the dc motor in regenerative braking is

(45)

Q51.

A separately excited dc motor fed through a single phase semi converter runs at a speed of 1200 rpm. when ac supply voltage is 220 V, 50 Hz and the motor counter emf is 90 V. The firing angle is 90° and armature resistance is 1 Ω. Find the average armature current

(A) 7A (B) 8A (C) 9A (D) 10A

Q52.

In the speed control of dc motors in servo applications, where response time is very critical, which of the following PE converter circuit is most preferable

(A) DC-DC converter (switched mode) (B) phase controlled rectifiers (C) Dual converters (D) Any one is suitable

Q53.

A separately excited dc motor is driven from a 3-φ full converter. The armature current is ripple free. Find 3rd_{and 5}th_{harmonic components of line currents as a % of the fundamental}

component respectively

(A) 0% and 20% (B) 0% and -20% (C) 20% and 0% (D) 0% and -40%

Directions: the following items consist of two statements; one labeled as Assertion (A) and the other as Reason (R). You are to examine these two statements carefully and select the correct answers to these items using the codes given below.

(A) Both A and R are true and R is the correct explanation of A

(B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false

(D) A is false but R is true Q54.

Assertion (A): In SCR, latching current corresponds to turn ON process. Reason (R): In SCR, holding current corresponds to turn OFF process. Q55.

Assertion (A): Semi converters are not suitable for braking applications. Reason (R): V0 is always +ve for all values ‘α’ in semi converters.

Q56.

Assertion (A): In rectifiers, input power factor can be improved by using freewheeling diode across load.

Reason (R): With freewheeling, some power can fed back to the source. Q57.

Assertion (A): The output ripple frequency in 3-φ rectifiers is less than 1-φ rectifier.

Reason (R): Output voltage of 3-φ rectifier will have more number of pulses than 1-φ rectifier.

Q58.

Assertion (A): MOSFET is most suitable switch in DC – DC converters than SCR. Reason (R): Ripple content is less when tsw is high in DC-DC converters.

Q59.

(46)

Reason (R): Switches with anti parallel diodes should not be used in CSI. Q60.

Assertion (A): 1 - φ triac can be used in fan regulators.

(47)

Objective Questions

Practice Test - 2

Q1.

Consider the following statements

(1) The ON state voltage drop of GTO is higher than that of SCR. (2) GTO can be used for more switching frequency than that of SCR. (3) SCR can handle large currents than GTO.

(4) GTO can turn ON and OFF from same gate. Choose the appropriate options.

(a) Only 1 is true (b) 1,2 and 3 are true (c) 1,2 and 4 are true (d) all statements are true Q2.

In the following single-phase diode Rectifier circuit, the average and RMS current rating of the diode will be respectively

(a) IDav = 10.35 A and IDr = 12.68 A (b) IDav = 14.64 A and IDr = 10.35 A

(c) IDav = 7.32 A and IDr = 10.35 A (d) IDav = 10.35 A and IDr = 7.32 A

Q3.

The output voltage of a 3-phase voltage source inverter contains 5th_{and 7}th

harmonics. Assume the output is balanced.

If Va = V1m sin (ωt) +V5m sin (5ωt) +V7msin(ωt) then Vb can be expressed as

(a) Vb = V1msin       − 3 2π ωt +V5msin(5ωt)+V7msin(7ωt) (b) Vb = V1m sin       − 3 2π ωt _+V_5m_sin _      π − ω +       π + ω 3 2 t 7 sin V 3 2 t 5 ₇_m (c)       + +       − +       − = 3 2 7 sin 3 2 5 sin 3 2 sin 5 7 1 π ω π ω π ωt V t V t V V_b _m _m _m (d) None of these

(48)

Q4.

In a single phase semi converter, THD in source current is found to be 31%. Then firing angle could be

(a) 30° (b) 40° (c) 45° (d) 60°

Q5.

A three phase half wave phase controlled rectifier is operated from a 3-phase star connected 400 V, 50 Hz supply and the load resistance is R = 10 Ω If it is required to obtain an average output voltage of 50% of maximum possible output voltage then the converter circuit need to operate at

α

=

(a) 135° (b) 67.7° (c) 60° (d) 30°

06.

A boost converter is shown in the figure. The DC source voltage is 100 V and load resistance is 10 Ω. Assume that the inductor has an internal resistance of 0.5 Ω. The range of duty cycle which can give the stable operation of the converter circuit is

(a) 0< D< 0.78 (b) 0.78<D< 1 (c) 0.8<D<1 (d) 0<D<1 Q7.

A single phase full bridge VSI has a source voltage of 200 V DC. The load consists of RLC in series where R = 1Ω, = Ω ω Ω = ω 7 C 1 and 6

L . Identify the fundamental component of the load current from the following.

(a) 180 sin (ωt+45°) (b) 180 sin(ωt−45°) (c) 127.3 sin (ωt−45°) (d) 127.3 sin (ωt+45°) Q8.

A separately excited DC Motor is driven from a 3 phase full converter. The armature current is ripple free. Find 3rd_{and 5}th_{harmonic components of line}

currents as a percentage of the fundamental component respectively (a) 0% and 20% (b) 0% and -20%

(c) 20% and 0% (d) 0% and −40% Q9.

A buck converter circuit is shown in the figure

Vdc is input voltage and v0 is the output voltage. Consider iL and v0 as state

(49)

(a) Under damped (b) Critically damped (c) Over damped (d) Undamped

A single phase semi converter is operated from 230 V, 50 Hz AC supply and with a firing angle of (π/4). The load on the converter is highly inductive with a resistance of 17.765 ohm and load current is ripple free

Q10.

The RMS value of the freewheeling current is

(a) 10 A (b) 3.33A (c) 5 A (d) None

Q11.

RMS value of fundamental source current will be

(a) 10 A (b) 9 A (c) 5 A (d) 8.315 A

Q12.

A single phase full bridge inverter has RLC load of R = 4Ω, L = 35 mH and C = 155 µF. The DC input voltage is 230 V and the fundamental output frequency is 50 Hz. The conduction time of the diodes is (consider only fundamental components)

(a) 6.264 ms (b) 2.5 ms (c) 3.736 ms (d) None Q13.

(50)

(a) 90° (b) 135° (c) 45° (d) Load current does not have max Q14.

Regarding Buck-Boost converter, which of the statement is true 1. It will operate as Buck converter when 0<D<0.5

2. It will operate as Boost converter when 0.5<D<1.0 and stable throughout this range

3. It will operate as Boost converter when 0.5<D<Dmax and unstable if D>Dmax.

Select the options as below:

(a) only 1 & 3 are true (b) all are true (c) only 1&2 are true (d) None of these

Q15.

In the given circuit a thyristor converter is feeding a resistor R, The power consumed by R in the absence of SCR is P. In the presence of SCR, the power consumed by R at α = 90° would be reduced by a factor of

(a) 1/2 (b) 1/4 (c) 1/6 (d) 1/12 Q16.

A 3-φ full converter feeds power to a resistive load of 10 Ω. For a firing angle of 30° the load takes 5 kW. Find the magnitude of per phase input supply voltage

(51)

A 1−φ fully controlled rectifier is supplying power to a resistive load of 35 Ω as shown in figure. If the bridge is triggered at 90°, the average current drawn by the load is given by

(a) 0 A (b) 2A (c) 2 2A (d) A

2 1 Q18.

A 3−φ Fully controlled rectifier is operated from a 3−φ star connected 400 V, 50 Hz AC supply and the load resistance is R = 10 Ω. A large inductance is connected in series with the load to maintain ripple free load current. If it is required to obtain an average output voltage of 86.66 % of the maximum possible output voltage. Find the firing angle

(a) 30° (b) 60° (c) 45° (d) 90°

Q19.

A Power converter is shown in the figure has two power switching devices namely X and Y. The source voltage is 50 V. The inductor current is steady 5 A without any ripple.

On the V-I plane, identify the correct operating points of switches from the given options.

(52)

(d) None

Q20.

A single phase AC voltage Regulator has a load resistance of 10 Ω. Input voltage is 200 V, 50 Hz. If SCRs are triggered at π/2 with symmetrical triggering scheme, find the RMS value of SCR current.

(a) 0A (b) 20 A (c) 4.5 A (d) 10 A

Q21.

The following chopper circuit is operating at a switching frequency of 1 kHz with a duty cycle ratio of 50%. Assume a voltage drop of 2 V across the switch when it is ON. Find the converter circuit efficiency.