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Code No: R5100206
I B.Tech (R05) Supplementary Examinations, June 2009 ELECTRICAL CIRCUITS (Common to Electrical & Electronics Engineering, Electronics & Control Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Explain the concept of self and mutual inductance. (b) Distinguish between static induced emf and dynamically induced e.m.f. (c) Two inductors have self inductance of 0.1mH and 0.4mH and a mutual inductance of 0.15mH. What is the value of coefficient of coupling between them? If a current of I(t) = 3 sint + 1.5 sin 2t is passed through the first inductor, what is the expression for the voltage induced in the second coil? [4+4+8] 2. (a) Distinguish between statically induced e.m.f and dynamically induced e.m.f? (b) Derive the expressions for self inductance and mutual inductance interms of number of turns, flux and current in a coupled circuit. (c) A coil of 150 turns is linked with a flux of 0.01 wb when carrying a current of 10A, calculate the inductance of the coil. If this current is uniformly reversed in 0.01 sec, calculate the induced e.m.f. [6+6+4] 3. (a) Bring out the differences between series and parallel resonance? (b) A series RLC circuit consists of resistance R = 20Ω, inductance, L=0.01H and capacitance, C = 0.04 µF. Calculate the frequency at resonance. If a 10 Volts of frequency equal to the frequency of resonance is applied to this circuit, calculate the values of VC and VL across C and L respectively. Find the frequencies at which these voltages VC and VL are maximum? [6+10] 4. (a) Two wattmeters are used to measure power in a 3-phase three wire load. Determine the total power, power factor and reactive power, if the two wattmeters read i. 1000w each, both positive ii. 1000w each, but of opposite sign. (b) What is phase sequence? Explain its significance? (c) What are the advantages of a poly phase system over a single phase system. [8+4+4] 5. (a) What is phase sequence? Explain its significance. (b) A star connected three phase load has a resistance of 8 ohms and a capacitive reactance of 10 ohms in each phase. It is fed from a 400v, 3-φ balanced supply. i. Find the line current, total volt-amperes, active and reactive power ii. Draw phasor diagram showing phase voltages, line voltages and currents. [6+10] 6. (a) Derive the expressions between phase and line voltages, and phase and line currents for balanced 3-φ delta connected loads. (b) A 3-phase, 3 wire, 240V system supplies three delta connected loads. ZAB = 25 6 900 ; ZBC = 15 6 300 ; and ZCA = 20 6 00 . The phase sequence is ACB. Find the line currents and total power. [8+8] 7. (a) Calculate the time taken by a capacitor of 1µF and in series with a 1 MΩ resistance to be changed up to 80% of the final value. (b) In a series RL circuit, the application of a dc voltage results in a current of 0.741 times the final steady state value of current after 1 sec. However, after the current has reached its final value, the source is short circuited. What would be the value of current after one second? [8+8] 8. (a) A resistance R and a 3.5 µF capacitor is connected is series across a 230V dc supply. A voltmeter is connected across the capacitor. Calculate R, so that the voltmeter reads 165V at 5.65 sec. after closing the switch. (b) In a series RLC circuit R = 7Ω, L = 1H and C = 1F a dc voltage of 50V is applied at t = 0. Obtain i (t) and Imax . [8+8] ?????
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Code No: R5100206
I B.Tech (R05) Supplementary Examinations, June 2009 ELECTRICAL CIRCUITS (Common to Electrical & Electronics Engineering, Electronics & Control Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Explain i. Self inductance ii. Mutual Inductance. (b) Two identical 1000 turns coils X and Y lie in parallel planes such that 60% of the magnetic flux produced by one coil links the other. A current of 5A in X produces in it a flux of 0.05mwb. If the current in X changes from +6A to −6A in 0.01sec what will be the magnitude of the e.m.f induced in Y? Also, Calculate the self inductance of each coil and the mutual inductance? (c) Define leakage factor and its effect in a magnetic circuit. [6+6+4] 2. (a) Explain i. Statically induced e.m.f and ii. Dynamically induced e.m.f (b) The combined inductance of two coils connected in series is 0.6H or 0.1H, depending upon the relative directions of the currents in the coils. If one of the coils when isolated has a self inductance of 0.2H, Calculate i. Mutual inductance, and ii. The Coefficient of coupling. (c) Explain the terms i. MMF ii. Reluctance. [6+6+4] 3. (a) Explain the concept of i. Susceptance and ii. Admittance (b) An inductive coil takes 10A and dissipates 1000 watts when connected to a supply of 250v, 25Hz. Calculate. i. the impedance ii. the effective resistance iii. reactance iv. the inductance v. power factor. Also, Draw the vector diagram. [6+10] 4. (a) Explain the phenomenon of “ Acceptor resonance” in electrical circuits. (b) Proceeding analytically, sketch the resonance curves for a series resonant circuit with variable frequency and constant R, L and C. (c) A series circuit comprising R, L and C is supplied at 220v, 50Hz. At resonance, the voltage across the capacitor is 550v. The current at resonance is 1A. Determine the circuit parameters R, L and C. [5+5+6] 5. (a) A balanced 3-ph star connected load of 150 Kw takes a leading current of 100A with a line voltage of 1100 V, 50Hz. Find the circuit constants of the load per phase? (b) Three equal star connected inductors takes 8Kw at P.f of 0.8 when connected to 460V, 3-φ, 3 wire supply. Find the line currents, if one conductor is short circuited. [6+10] 6. (a) Two resistors each of 100Ω are connected in series. The phases a and c of a three phase 400V supply are connected to the two ends and phase b is connected to the junction of the two resistors. Find the line currents. (b) Derive the expressions for wattmeter readings in two wattmeter method with balanced star connected load. How do you calculate the power factor of the balanced load from wattmeter readings? [8+8] 7. (a) Draw the model graphs for the following (clearly indicate axis) i. Current decay transient in RC discharging circuit ii. VR and Vc in RC discharging circuit iii. Charging current profile in RL circuit (b) A series R.C circuit has R =20Ω and c = 100µF. A voltage ϑ = 200 sin 314t is applied at t =2.15m sec. Obtain an expression for i. Also, find the value of current after time 1m. sec from the switching instant. [6+10] 8. (a) A direct voltage of 200V is suddenly applied to series RL circuit having R = 20Ω and inductance 0.2H. Determine the voltage drop across the inductor at the instance of switching on and at 0.03 sec later. (b) A constant voltage of 100V is applied at t = 0 to a series RC circuit having R = 5MΩ and C = 20 µ F. Assuming no initial charge to the capacitor, find expression for i, voltage across R and C. [7+9] ?????
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Code No: R5100206
I B.Tech (R05) Supplementary Examinations, June 2009 ELECTRICAL CIRCUITS (Common to Electrical & Electronics Engineering, Electronics & Control Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Explain the terms magnetic field strength, magnetic circuit, magnetic flux and magnetic flux density. (b) What is coefficient of coupling? What is the range of variation of this quantity? In which type of circuits is it minimum and in which type of circuits is it maximum? (c) The combined inductance of two coils connected in series are 0.6H and 0.1H in series aiding and series opposing connections. If the self inductance of each coil is 0.2H, find the coefficient of coupling? [4+8+4] 2. (a) Explain the Faraday’s Law of electromagnetic induction? (b) A cast steel ring has a circular cross section 3cm in diameter and a mean circumference of 80cm. The ring is uniformly wound with 600 turns. i. Estimate the current required to produce a flux of 0.5 mcob in the ring. ii. If a saw cut 2mm wide is made in the ring, find approximately the flux produced by the current found in (i). iii. Find the current value which will give the same flux as in (i). Assume the gap density to be the same as in the iron and neglect fringing. [6+10] 3. (a) An inductor L and a resistor R are connected in series. A capacitor C is shunted across L and R. At what frequency will the total current in the circuit be independent of the value of R? What is the value of the current when the applied voltage is V? [12+4] (b) Define complex power? Explain it? 4. (a) The power taken by a 440V, 50Hz, 3-phase induction motor on full load is measured by two wattmeters, which indicate 250Ω and 1000Ω respectively. Calculate i. the input ii. the power factor iii. the current and iv. the motor output if the efficiency is 80%. (b) Derive the expressions for wattmeter readings for a balanced 3-phase load by two wattmeter power measurement. Explain the effect of power factor of load on the readings of two wattmeters. [8+8] 5. (a) Two wattmeters are used to measure the power in a 3-phase balanced system. What is the power factor of the load, when i. both the meters read equal ii. both the meters are read equal but one is ‘-ve’ iii. one reads twice the other? (b) The power factor of a three-phase, 440V motor load is 0.8. During power measurement by the two wattmeter method, the input is 25KW. Calculate the reading on each wattmeter. [9+7] 6. (a) Calculate the time taken by a capacitor of 1µF and in series with a 1 MΩ resistance to be changed up to 80% of the final value. (b) In a series RL circuit, the application of a dc voltage results in a current of 0.741 times the final steady state value of current after 1 sec. However, after the current has reached its final value, the source is short circuited. What would be the value of current after one second? [8+8] 7. (a) A direct voltage of 200V is suddenly applied to series RL circuit having R = 20Ω and inductance 0.2H. Determine the voltage drop across the inductor at the instance of switching on and at 0.03 sec later. (b) A constant voltage of 100V is applied at t = 0 to a series RC circuit having R = 5MΩ and C = 20 µ F. Assuming no initial charge to the capacitor, find expression for i, voltage across R and C. [7+9] 8. (a) Draw the current, impedance and admittance loci for an RC Series circuit having constant resistance but variable capacitance. (b) A circuit consists of a 15 ohm resistance in one branch, a 7.5 ohm inductive reactance in the second and a capacitive reactance of 12 ohm in the third branch. It is fed from a 230V, 50Hz supply. Find i. Current in each branch ii. Power factor iii. Parameters of equivalent circuits. iv. Also, draw the phasor diagram of the circuit. [6+10] ?????
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Code No: R5100206
I B.Tech (R05) Supplementary Examinations, June 2009 ELECTRICAL CIRCUITS (Common to Electrical & Electronics Engineering, Electronics & Control Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Write down the expressions used in conversion of Y to ∆ and ∆ to Y transformations. (b) What is the difference between an ideal source and a practical source? Draw the relevant characteristics of the above sources. (c) Write short notes on source transformation. [6+4+6] 2. (a) Explain i. Self inductance ii. Mutual Inductance. (b) Two identical 1000 turns coils X and Y lie in parallel planes such that 60% of the magnetic flux produced by one coil links the other. A current of 5A in X produces in it a flux of 0.05mwb. If the current in X changes from +6A to −6A in 0.01sec what will be the magnitude of the e.m.f induced in Y? Also, Calculate the self inductance of each coil and the mutual inductance? [6+6+4] (c) Define leakage factor and its effect in a magnetic circuit. 3. (a) What is magnetic coupling? What is its effect? How can you arrange two coils so that they donot have magnetic coupling? (b) Two coils having 30 and 600 turns are wound side by side on a closed iron circuit of 100 cm2 cross section and mean length 150cm. Calculate i. The self inductance of the two coils and mutual inductance if relative permeability of iron is 2000. Assume no magnetic leakage. ii. Calculate from 0 to 10A steadily in 0.01sec. (c) Define reluctance? Give its units. 6+8+2] 4. (a) The admittance of a circuit is (0.05-j0.08) mhos. Find the value of resistance and inductive reactance of the circuit if they are connected. i. in parallel and ii. in series (b) What is an impedance triangle? Draw impedance triangles for. i. Series RL circuit and ii. Series RLC circuit. (c) An RLC Series circuit has R = 4 ohms: XL = 19.5 ohms and Xc = 12 ohms. Calculate the power factor of the circuit. [6+6+4] 5. (a) Define power factor? What is its importance in a.c circuits? (b) The impedances of a parallel circuit are Z1 =(6 + j8)Ω and Z2 = (8 - j6) Ω. If the applied voltage is 120v, find. i. current and p.f of each branch ii. overall current and p.f of the combination iii. power consumed by each impedance. Draw a phasor diagram. [4+12] 6. (a) Derive the expressions between phase and line voltages, and phase and line currents for balanced 3-f star connected loads. (b) A 3-phase 4 wire, 400V system feeds three loads (10-j8)Ω, (12+j0)Ω and (8+j10) Ω connected in star. Find the line currents, neutral current and total active power. [8+8] 7. (a) Show that two wattmeters are sufficient to measure power in a 3-phase circuit. Derive the expressions for wattmeter readings for a balanced 3-phase load. (b) Three similar resistors are connected in star across 450 V, 3-phase lines. The line current is 10 Amps. If the same resistors are connected in delta across the same supply, calculate i. Phase current ii. Line current iii. The power consumed. [8+8] 8. (a) Draw the model graphs for the following (clearly indicate axis) i. Current decay transient in RC discharging circuit ii. VR and Vc in RC discharging circuit iii. Charging current profile in RL circuit (b) A series R.C circuit has R =20Ω and c = 100µF. A voltage ϑ = 200 sin 314t is applied at t =2.15m sec. Obtain an expression for i. Also, find the value of current after time 1m. sec from the switching instant. [6+10] ?????