Power Electronics (BEG335EC )

Size: px

Start display at page:

Download "Power Electronics (BEG335EC )"

Monica Charles
5 years ago
Views:

1 1 Power Electronics (BEG335EC )

2 2 PURWANCHAL UNIVERSITY V SEMESTER FINAL EXAMINATION The figures in margin indicate full marks. Attempt any FIVE questions. Q. [1] [a] A single phase full converter is fed form the 230V, 50HZ ac source. If R 1 = 10Ω and large inductance is connected to output side of the converter to give the constant load current. if firing angle α = 30. Determine: [i] Average output voltage. [ii] Average output current [iii] Average and r.m.s current of thyristor. [iv] Input power factor. [b] If there is source inductance of 1.5 mh in above problem determine: [i] Average output voltage. [ii] Angle of overlap. [c] Discuss the turn on process of the thyristor with the help of two transistors model. [4] Q. [2] [a] What do you mean by the forced commutation. Explain the operation of the resonant pulse communication circuit with neat diagram and waveform if necessary. [b] Explain the switching characteristics of the power diode. [5] [c] What are the advantages of GTO over thyristor? Explain the turn on and turn off mechanism of the GTO. [5] Q. [3] [a] Why (dv/dt ) and (di/dt) protection is needed? A thyristor permits (dv/dt) max = 100 v /µs and (di/dt) max = 30 A/ µs peak current I p =125Amp. Design the snubber Resistance, capacitance and inductance assume damping coefficient Q = [5] [b] Explain the physics of UJT. What is intrinsic stand off ratio? [5] [c] A single phase AC voltage controller feed power to resistance load of 3Ω from 230V and 50HZ, determine: [i] Maximum value of Average and r.m.s current of thyristor for any firing angle α. [ii] Maximum circuit turns off time for any firing angle α [iii] Ratio of the third harmonics and fundamental Voltage for α = π 3 Q. [4] [a] Discuss the speed control of separately excited DC motor. Why field control and armature control are used for constant power and constant torque drive. [4] [b] Develop the Separately excited DC motors block diagram for open loop condition. Find the transfer function when there is: [i] Change in speed command only. [ii] Change in load torque only. [c] Draw the circuit diagram for step down chopper and waveforms for Vo and Io assuming R-L load and continuous mode. Also derive the expression for I max and I min. Q. [5] [a] What is Power factor conditioning? Explain with circuit diagram and necessary wave forms. [b] A 40 Kw, 240 v 1150 rpm separately excited dc motor is used in speed control system. The field circuit is held constant at a value for which K Φ =1.9 V-sec/rad, Armature Resistance Ra = 0.089Ω and viscous coefficient B = 0.275NM-sec/rad. The tachogain delivers 10v at 1000 rpm. The amplification with controller is 200. Determine: [10] [i] The value of the reference voltage required to run the motor at rated speed at no-load. [ii] If the reference voltage is unchanged determine speed at which motor runs with rated load. Q. [a] Explain the Power transistor's static characteristics. Explain the quasi saturation and hard saturation region. [5]

3 3 [b] Explain the Dynamic characteristics of the Power MOSFET. [c] Explain the base drive control circuit of Power transistor. [5]

4 PURWANCHAL UNIVERSITY V SEMESTER BACK-PAPER EXAMINATION The figures in margin indicate full marks. Attempt any FIVE questions. Q. [1] [a] Two diodes are to be connected for (i) Voltage sharing (ii) Current sharing. Discuss all the issues to implement this strategy. [b] Explain the significance of Power semiconductor devices in modern technology. [4] [c] Design a full bridge rectifier with output 3 volt dc from a source of 220 volt AC. Calculate the transformer turn ratio. Q. [2] [a] Explain the two-transistor model of a Thyristor. [5] [b] Explain the dynamic characteristic of GTO and compare it with a SCR. [5] [c] Draw and Explain force commutation technique with proper diagram. Q. [3] [a] Draw the line current, output voltage, input current, input voltage across a pair of Thyristor in a single- phase semiconverter for continuous conduction mode. Assume the firing angle be 45 and find out the equation for the output voltage. [b] Explain the operation of three phase half wave converter circuit. [4] [c] Explain the output characteristics of a BJT and show how it can be used as an amplifier and as a switch. Q. [4] [a] Explain the operating principle of a N-channel MOSFET and discuss its drain and transfer characteristics. [4] [b] Describe how a buck-boost regulator works. If a constant output dc voltage of 12 volt has to be obtained from a variable dc input voltage, which is obtained from a solar 4 panel, find out the range of duty ratio that has to be varied. Given, Input voltage = 10,11, 12, 13, 14, volts. [c] Explain the operating principle of a SMPS. [4] Q. [5] [a] Obtained the close loop transfer function of a separately excited DC motor considering mechanical as well as electrical time constant. [b] Draw the following wave-form for a discontinuous conduction mode of a DC series motor supplied form a single phase full converter: (i) Input Voltage (ii) Input current (iii) output current (iv) output voltage (v) The voltage across a pair of thyristor. [5] [c] Deduce the equation for the output torque in a DC series motor supplied from a single phase semi converter. [5] Q. [a] Explain with proper diagram the operation of snubber circuit to protect from a high current and voltage transient in a Semi conductor switch. [b] Explain the Turn-on and off phenomenon in a semi conductor switch and suggest its significance in power conversion application. [c] What is power factor conditioning? Explain some method to improve it using Semi-conductor switch. [4]

5 PURWANCHAL UNIVERSITY V SEMESTER FINAL EXAMINATION All questions carry equal marks. The marks allotted for each sub-question is specified along its side. Attempt any FIVE questions. Q. [1] [a] Discuss briefly about the history of power electronics [2] [b] The reverse recovery time of a diode is 5 µ sec and the rate of fall of diode current is 80 A/ µ sec. If the softness factor is 0.5, determine the storage charge and peak reverse current. [4] [c] A 3-phase star rectifier has a purely resistive load with RΩ Determine: [10] [i] The form factor [ii] Efficiency [iii] The ripple factor. [ iv] The transformer utilization factor. [v] The PIV of each diode. Q. [2] [a] Discuss the principle of power factor conditioning with necessary diagram. [b] A 15 hp 220v rpm separately excited DC motor control a load requiring a torque of 45 N-M at a speed of 1200 rpm. The field circuit resistance is 147Ω, the armature circuit resistance is 0.25Ω and the voltage constant of the motor is V/A-rad/sec. The field voltage is 220v. The viscous friction and no load losses are negligible. The armature current may be assumed continuous and ripple free. Determine: [i] The back emf. [ii] The acquired armature voltage. 5 [iii] Rated armature current of the motor. Q. [3] [a] Discuss the working principle of step down operation of DC chopper. [b] Explain output characteristics of a npn transistor in CE configuration and hence prove that the total power loss in the two junction is P T = V BE I B +V CE I C, where the symbols have their usual meanings. Q. [4] [a] What are the types of forced commutation? Discuss in detail any one of them. [b] A 3-phase full converter is operated from a 3-phase 230v, 60Hz supply. The load is highly inductive and the average load current is 150A with negligible ripple content. If the delay angle is π/3, determine the rating of thyristors. Q. [5] [a] What are the different types of thyristors? Discuss the working principle and advantage of GTO. [b] What are the differences between polarized and non polarized snubber? [4] [c] The capacitance of reverse biased junction J2 in a thyristor is 20pF and can be assumed to be independent of the off state voltage. The limiting value of the charging current to turn on the thyristor is 16mA. Determine the critical value of dv/dt.

6 6 PURWANCHAL UNIVERSITY V SEMESTER FINAL EXAMINATION V in D R L I L All questions carry equal marks. The marks allotted for each sub-question is specified along its side. Attempt EIGHTquestions. Q. [1] Explain the reverse recovery characteristics of Diode. [10] Q. [2] Describe the basic behavior of the SCR with the help of I-V characteristics. Explain with the help of its two transistor model that it is a latching device. Describe any one technique for turning an SCR off. [3+5+2] Q. [3] [a] Given the relaxation oscillator in Fig. 3(a). [ ] [i] Determine R B1 and R B2 at Ig = 0 A. [ii] Calculate Vp, the voltage necessary to turn on the UJT. [iii] Determine whether R1 is within the permissible range of values to ensure firing of the UJT. [iv] Determine the frequency of oscillation if Ra1 =100Ω during the discharge. +12 v R BB = 5KΩ R η= =50kΩ V v = 1v, I v = 10mA c=0.1pf I p = 10 µ A Fig. 3[a] R 2 =0.1Ω + V R2 - Fig. 3[b] Q. [4] [a] Explain about the three phase bridge rectifier with RL load. [b] What are the different methods of communication schemes? Describe any one of the them with necessary circuit diagram and wave form. [4] Q. [5] [a] Show that the r.m.s output voltage of the step down dc chopper depends upon the duty cycle of the chopper. [4] [b] A step up chopper has input voltage of 220v and output voltage of 600v. If the non-conducting time of thyristor chopper is 100 microseconds, compute: [i] Pulse width of output voltage. [ii] In case pulse width is halved for constant frequency operation, find the new output voltage. [3+3] Q. [a]. dc supply voltage is Vcc = 200v and input voltage circuit is VB = 10v. If VCE(sat) = 1v and Vsc(sat) = 1.5 v. Find: [a] the value of R B that results in saturation with an ODF of 5. [b] β forced [c] Power loss (p t ) in the transistor. R P [b] In the figure given below Fig. 3[b] V in is 120 V r.m.s ac voltage and R L = 40Ω. Assume that the drop across the SCR is 1V when it is conducting, what should be the firing angle if it is desired to deliver an average current of 1A to the load? [3] V B R B I B Fig. R c I c I E - + Vc E V cc

7 7 Q. [7] Explain about a single phase full converter with RL load. Derive its average output voltage and output current. Find critical value of α at discontinuous case (α c ). [10] Q. Explain the operation of electronic chopper drives for the speed control of a series dc motor. What is the role of an thyristor in these chopper drives? [10] Q. [9] Write short notes on: (any Two) [2 5 = 10] [a] Pulse transformer [b] Subber circuit. [c] Switched mode dc power supply. [d] Switch mode ac power supply.

8 PURWANCHAL UNIVERSITY V SEMESTER FINAL EXAMINATION All questions carry equal marks. The marks allotted for each sub-question is specified along its side. Attempt EIGHTquestions. Q. [1] [a] What is Power electronics? Mention its application in modern electronics. [5] [b] The reverse recovery time of a diode is 3µs and rate of fall of a diode current is 30A/µs. [i] The storage chare, Q RR [ii] The peak reverse current, I RR [5] Q. [2] Describe thyristor characteristics with necessary figures. Explain its tow transistor model. [10] Q. [3] [a] Ten thyristor are used in a string to withstand a dc voltage of Vs = 15Kv. The maximum leakage current and recovery charge differences of thyristors are 10mA and 150 µc, respectively. Each thyristor has voltage sharing resistance of R = 56KΩ and capacitance of C 1 = 0.5 µf. Determine: (i) The maximum steady state roltage sharing V DS(max), (ii) The steady stage voltage derating factor, (iii) the maximum transient voltage sharing VD r(max), and (iv) the transient voltage derating factor. [5] [b] What do you mean by the forced commutation? Explain the operation of self commutation. [5] Q. [4] Draw the circuit diagram, quadrant and waveforms of single phase dual converters. Explain its operation. [10] Q. [5] Explain the constructional details an steady-state characteristics of power MOEFETs with relevant waveforms. [10] Q. [a] Explain the principle of step down operation using chopper. [5] 8 [b] A relaxation oscillator using an UJT, Fig. 6(b), is to be designed for triggering an SCR. The UJT has the following data: η = 0.75, Ip = 0.6mA, Vp = 18.0 V, V v = 1.0 v, I v = 2.5 ma, R BB = 5 KΩ, Normal leakage current with emitter open = 4.2 ma. The firing frequency is 2KHZ. For C = 0.04 uf, compute the values of R, R1 and R2. [5] Fig. 6[b] Q. [7] [a] What is the principle of power factor corditionin in diode rectifiers? [7] [b] Mention three differences between MOSFER and BJT. [3] Q. Explain the basic characteristics of DC motors. What are the parameters to be varied for speed control of dc series motors. [10] Q. [9] A 15-hp, 220v, 2000 rpm separately excited dc motor controls a load requiring a torque of T L = 45N m at a speed of 1200 rpm. The field circuit resistance is Ra = 0.25Ω, and the voltage constant of the motor is K v = V/A-rad/s. The field voltage is V f = 220 V. The viscous friction and no-load losses are negligible. [10] Determine: [a] The back emf Eg, [b] The required armature voltage Va, and [c] The rated armature current of the motor. Q. [10] Wirte short note on (any two) [2 5 =10] [a] Reverse recovery time and reverse recovery charge of a diode. [b] Switched mode dc power supply. [c] Heat Sinks [d] Diode with RL load. C R E B 1 B 2 R 2 V BB R 1 V o

9 PURWANCHAL UNIVERSITY V SEMESTER FINAL EXAMINATION All questions carry equal marks. The marks allotted for each sub-question is specified along its side. Attempt EIGHTquestions. Q. [1] [a] Why it is necessary to connect diode in series and parallel? Discuss the problems associated with the parallel and series operation of diodes and how these are overcome? [b] Two Diodes are connected in series to share a input DC Voltage of 10Kv. The resistances across the diodes are R 1 =R 2 = 10KΩ. The leakage currents are I s1 = 30mA and I s2 = 35 ma. Determine the voltage shared by each diode. Q. [2] [a] Describe with sketch, the effect of gate current on the forward break over voltage of an SCR. [4] [b] Describe the various mechanism by which thyristor can be triggered into conduction. [4] [c] Following are the specifications of a thyristor operating from a peak supply of 500V and repetitive peak current, Ip = 250A. (di/dt)max = 60A/µs. (dv/dt)max = 200V/µs. Design the Snubber circuit if the minimum load resistance is 20Ω. Take ζ = 0.65 Take a factor of safety 2 for the given data. Q. [3] [a] A single phase full wave bridge rectifier has purely resistive load of 5 ohm, the supply voltage is 220v and the supply frequency 50Hz. Determine (a) Efficiency, (b) the ripple factor, (c) the transformer utilization factor, (d) the peak inverse Voltage of Diode and (e) Crest Factor. 9 [b] Explain the need of communication in Thyristor circuits. What is the difference between Natural and forced commutation of thyristor? Describe with sketch and necessary waveform the any one method of forced commutation technique in Thyristor. Q. [4] [a] Describe the principle of step up chopper. Derive an expression for the average output voltage in terms of input dc Voltage and duty cycle for step up chopper. State the assumption made. [b] The step down chopper has resistive load of R = 20Ω and the input voltage is Vs = 220v. When the chopper remains on, its voltage drop is 1.5 V and the chopping frequency is f = 10 KHz. If the duty cycle is 80% Determine: [i] The average output Voltage [ii] RMS voltage [iii] Chopper efficiency [iv] Effective input resistance Ri of the chopper [v] On-time and off time of chooper. Q. [5] [a] Determine the open loop transfer function of separately excited DC-motor along with the corresponding circuit. How the single phase dual converter drive is used to control the speed of dc motor? [6+2] [b] A single phase semi converter circuit feeds power to a resistive load. Draw waveforms for the source voltage, load voltage, load current and voltage across the SCR for a given firing angle α. Hence obtain the expression for average and RMS load voltage and firing angle. Q. [a] What do you mean by SMPS? Explain the operation of on-line and off-line SMPS. Also describe about Power factor conditioning. [1+5+2] [b] Describe the electronics Crowbar protection scheme employed for the over current protection of power converter. [c] Write short note on(any TWO): [2 2 =4] [a] Optocupler [b] IGBT [c] Varistor

6. Explain control characteristics of GTO, MCT, SITH with the help of waveforms and circuit diagrams.

6. Explain control characteristics of GTO, MCT, SITH with the help of waveforms and circuit diagrams. POWER ELECTRONICS QUESTION BANK Unit 1: Introduction 1. Explain the control characteristics of SCR and GTO with circuit diagrams, and waveforms of control signal and output voltage. 2. Explain the different