Aligarh College of Engineering & Technology (College Code: 109) Affiliated to UPTU, Approved by AICTE Electrical Engg.

Transcription

1 Aligarh College of Engineering & Technology (College Code: 19) Electrical Engg. (EE-11/21) Unit-I DC Network Theory 1. Distinguish the following terms: (a) Active and passive elements (b) Linearity and no-linearity (c) Unilateral and Bilateral 2. Derive an expression for delta to-star transformation with their usual meanings. 3. Determine the current in 4Ω resistor using Thevenin s theorem in the circuit shown in Fig. 1. Draw its equivalent circuit also. (Ans: R Th =2Ω,I L =2A). 4. Find the value of R L in the circuit of fig.2 such that the maximum power transfer takes place. Also calculate maximum power transferred. (Ans: R Th =25Ω,P m =186.25W). 3Ω 1Ω 2Ω 1Ω 27V 3A 6Ω 4Ω 25V 1Ω 2A R L Fig. 1 Fig State and prove maximum power transfer theorem. 6. What do you mean by star-delta transformation? If three resistances each having the value of 3 Ω are connected in delta then what will be the value of each resistances in star? 7. Find the nodal voltages, V 1 & V 2 for the circuit shown in fig.3. (Ans: 9.21V, 11.21V). 8. Using Delta Star transformation determine the resistance terminals A-B and the total power drawn from the supply in the circuit shown in fig. 4. (Ans: Req=5.4Ω,P=19.84W). 5Ω V 1 4Ω 5V V 2 8Ω 8Ω A 2V 3Ω 2Ω 3V 3Ω 7Ω 5V 5V 1V 4Ω 1Ω B Fig-3 Fig-4 9. In the circuit shown in fig.5 find the current through 6Ω resistor using superposition theorem. (Ans: 9.91A). 3Ω 9Ω 2Ω 12Ω 12V 6Ω 6V 8Ω 9V 6V 2A Fig.5 Fig.6 1. In the circuit shown in fig.6 find the current through 2Ω resistor using superposition theorem. (Ans: 2.27A). 11. Using loop current method finds the current I 1 and I 2 shown in fig.7. (Ans: I 1 =3.163A, I 2 =1.55A). 12. In the circuit shown in fig.8, find the current through 6Ω resistor by using: (Ans: 6A). (a) By Superstition theorem, (b) By Thevenin s theorem, (c) By Mesh analysis method and, (d) By Nodal analysis method. 2Ω 3Ω 6Ω 6Ω 5Ω 6Ω I 1Ω 1V I 1 6Ω I 2 2V 24V 6Ω 12V Fig.9 (1) Compiled By: Er. Arif Hassan

2 Aligarh College of Engineering & Technology (College Code: 19) Electrical Engg. (EE-11/21) 6V 7Ω 8Ω Fig.7. Fig8. 2V 13. Find the magnitude and direction of the current in the 1Ω resistor by using Thevenin s theorem in the circuit shown in fig.9 (Ans: I L =.1479A). 14. State the Superposition theorem for dc circuit? Also verify by taking suitable circuit diagram. 15. Find the node voltages and the currents in various resistors using nodal analysis method in fig. 1. 5Ω I 1 a I 2 b I 6 c I 3 3Ω I 5 1Ω I 4 2A 15Ω 3Ω R 1A 2Ω 5Ω 4Ω 2A 8V Fig.1 { V a =12.5V, V b =5.93V, V c =3.796V} Ans: {R th =2.625Ω, V th = 11V, P m = W} Ans: { I 1 =1.658A, I 2 =2.3151A, I 3 =6.25A} {I 4 =.949A, I 5 =1.186A, I 6 =1.297A} 16. Find the value of R to have maximum power transfer in the circuit shown in fig. 11. Also obtain the amount of maximum power. Unit-II Steady-State Analysis of Single-Phase AC Circuits 1. Define following in terms of single-phase a.c. circuits: (a) Apparent Power (b) Active Power (c) Reactive Power (d) Power Factor 2. Derive an expression for resonant frequency in R-L-C parallel circuits? What do you understand by the term bandwidth of resonant circuit? 3. A voltage of v t = 1 sin ωt is applied to a series RLC circuit. At resonant frequency of the circuit the maximum voltage across capacitor is found to be 5 V. moreover, the bandwidth is known to be 4 rad/sec. and the impedance at resonance is 1 Ω. Find: (a) The resonant frequency (Ans: Hz). (b) Compute the upper and lower limits of the bandwidth. (Ans: f 1 = Hz, f 2 = Hz). (c) Determine the value of the L and C for the circuit. (Ans:.25 H,.5 µf). 4. A resistance and an inductance are connected in series across a voltage: v = 283 sin 314t. The current expression is found to be 4 sin 314 t. Find the value of resistance, inductance and power factor. 4 (Ans: R=5.28 Ω, L=.1593 H). 5. A two element series circuit is connected across an a.c. voltage source v = 3 cos (314t + 2 ) volts. The current drawn is 15 cos (314t - 1 ) amperes. Determine the circuit impedance magnitude and phase angle. What is the average power drawn? (Ans: 2 3, W). 6. A series R-L-C circuit R = 1 Ω, and L =.1 H and C = 8 µf. determine: (a) Resonant frequency (b) Q-factor of the circuit at resonance (c) The half power frequencies (Ans: Hz, and f 1 = Hz, f 2 = Hz). 7. The following fig.1 shows a series-parallel circuit. Find: 4Ω +j3ω (i) Admittance of each parallel branch 1.6Ω +j7.2ω (ii) Total circuit impedance 6Ω -j8ω (iii) Total power supplied by the source (Ans: Ω, Ʊ, Ʊ). ac 6Ω 6V Fig.11 (2) Compiled By: Er. Arif Hassan

3 Aligarh College of Engineering & Technology (College Code: 19) Electrical Engg. (EE-11/21) Fig.1 1 V, 5 Hz 8. A 2 Ω resistor is connected in series with an inductor and a capacitor, across a variable frequency 25 Volts Supply. When the frequency is 4 Hz, the current is at its maximum value of.5 A. and the potential difference across capacitor is 15 Volts, calculate the resistance and inductance. (Ans: r = 3Ω, L=.1193H). 9. An ac. Voltage e(t) = sin 12 t is applied to a series R-C circuit. The current through the circuit is obtained as i(t) sin 12 t cos (12 t + 3 ). Determine the value of the resistance and capacitance. (Ans: R = 1 Ω, C = µf). 1. A constant voltage at a frequency of 1MHz is applied to an inductor in series with a variable capacitor. When the capacitor is set 5 pf, the current has its maximum value while it is reduced to one-half when the capacitance is 6pF. Find: (a) The inductance, (b) The resistance, (c) The Quality-factor of the inductor. (Ans: L= 5.66µF, R=3.62Ω, Q.F.= 1.4). 11. An inductive coil of resistance 1 Ω and inductance.1 H is connected in parallel with a 15 µf capacitor to a variable frequency, 2 V supply. Find the resonant frequency at which the tot al current taken from the supply is in phase with the supply voltage. Also find the value of this current. Draw the Phasor-diagram. (Ans: f r = Hz, I = 3 A) 12. A series circuit has resistance of 15 Ω and inductive reactance of 1 Ω. Calculate the value of capacitor which is connected across this series combination so that system has unity power factor. The frequency of A.C. supply is 5 Hz. (Ans: C = µf, L= mh) 13. What are the features of resonance in parallel circuits of fig.2? Calculate the value of C which in results for the shown in figure when frequency is 1 Hz and find Q r for each branch. 4 Ω 5 Ω 25Ω J8Ω C A.5H 5µF Fig.2 (Ans: C = µf, Q r = 2 & Q r = 1.) 14. For the circuit as shown in fig.3. Determine: (a) Resonant frequency, (b) Total impedance of the circuit at resonance, Fig.3 (c) Bandwidth, (d) Quality factor. (Ans: f r = 1.34 Hz, Z D = 4 Ω, BW = Hz, Q.f. = 12.65) 15. Derive the expression for resonant frequency of a parallel resonance. An inductive circuit of resistance 2 and inductance.14h is connected to a 25V, 5Hz supply. What capacitance placed in parallel will produce resonance. 16. A 46 mh inductive coil has a resistance of 1 Ω. How much current will it draw, if connected across 1 V, 5 Hz source? Also determine the value of the capacitance that must be connected across the coil to make the power factor of the circuit be unity. (Ans: µf). 17. Determine the following in the circuit shown in fig.4. (a) The current phasors I, I 1, I 2. (b) Active power dissipated in the three resistive branches I 1 3Ω j4ω (c) Power factor of the circuit 8Ω j6ω I 2 Fig.4 I 5Ω -j12ω ac (3) Compiled By: Er. Arif Hassan

4 Aligarh College of Engineering & Technology (College Code: 19) Electrical Engg. (EE-11/21) 2 V, 5 Hz (Ans: , , , W, 641 W, 158W, W, PF=.8228 lag). 18. Draw a Phasor diagram showing the following voltages: v 1 = 1 sin 5t, v 2 = 2 sin (5t + ), v3 = - 5 cos 5t and v 4 = 15 sin (5t - ).Find the 3 R.M.S. value of resultant and write the equation, v r = V r m sin (5t ± ɸ). (Ans: ). 19. A resistance of 12 Ω and a capacitive reactance of 25 Ω are connected in series across a AC voltage source. If a current of.9 A is flowing in the circuit find, (i) power factor (ii) supply voltage (iii) voltages across resistance and capacitance (iv) Active power and reactive power. 2. Find the r.m.s. value, average value and form factor of the voltage waveform shown in fig.5. Voltage 1V Fig π 3 2 2π (Ans: V rms = 7.716V, V av = V, K f = 1.11) 21. Calculate the average value, effective value, form factor and peak factor of the output voltage waveform of a half wave rectifier as shown in fig.6. Current 1V Three-Phase AC Circuits 2 π 2π 3π Unit-III Fig.6 (Ans: V rms = 5 V, V av = V, K f = 1.57) 1. What is necessity and advantages of three-phase system? Derive V L = 3 V P for star connected system. 2. Define phase-sequence, balanced load in a 3-phase system? 3. Explain power factor measurement by means of two wattmeters readings in a three-phase ac circuit? 4. Prove that the power consumed in delta connected system is 3-times the power consumed in star connected system if they are connected to a equal load of same nature. Three-Phase AC Circuits Numerical Problems 5. Three identical resistors of 2 Ω each are connected in star to a 415 V, 5 Hz, three-phase supply, calculate: (a) The total power consumed, (Ans: W). (b) The total power consumed, if they are connected in delta, (Ans: W). (c) The total power consumed, if one of the resistor is opened. (Ans: W, W). 6. Three similar resistors are connected in star draw a line current of 5 A from a 4 V, 5 Hz, 3 -phase supply. To what value should the line voltage be changed to obtain the same line-current when the resistor connected in delta system? (Ans: V). (4) Compiled By: Er. Arif Hassan

5 Aligarh College of Engineering & Technology (College Code: 19) Electrical Engg. (EE-11/21) 7. A 3-wire, 3-phase supply feeds a load consisting of three equal resistors, by how much is the load is reduced if one of the resistor is removed? (a) When the load is star-connected, (Ans: 5%). (b) When the load is delta-connected. (Ans: 33.33%). 8. Three identical impedances, each consisting of R and L in series are connected in star and are supplied from a 4 V, 5 Hz, 3-wire balanced supply system. The power input to the load is measured by twowattmeter method and the two-wattmeters read 3 kw and 1 kw. Determine the value of R and L connected in each. (Ans: Ω, 63.8mH). 9. A balanced delta-connected load of impedance (16 + j12) Ω per phase is connected to a three-phase 4 V supply. Find the phase-current, line-current, power factor, active power, reactive VA and total VA. (Ans: 2 A, A,.8 lag, 192 W, 144 VA, 24 VAR). 1. A balanced 3-phase star connected load 12 kw taking a leading current of 85 amperes when connected across a 11 V, 5 Hz supply. Find the values and nature of the load components. Also calculate the power factor of the load. (Ans: Ω, x1 4 F,.749 Leading). 11. In two-wattmeter method of power measurement in a 3-phase, the readings of the two-wattmeters are 1 W and 55 W. what is the power factor of the load? (Ans:.528 ). 12. For a certain load, one of the wattmeter reads 2 kw and the other 5 kw after the current coil connection has been reserved. Calculate the power, power factor. (Ans: 15W,.3273). 13. Each phase of a star-connected load consists of a resistance of 1 Ω in parallel with a capacitance of 31.8 µf. calculate: (a) Line current, (Ans: A). (b) Total power absorbed, (Ans: 1.73 kw). (c) Total kva, (Ans: 2.45 kva). (d) Power factor of the load. (Ans:.77 Leading). 14. A balanced star connected load of (8 + j6) Ω per phase is connected to a 4 V, 5 Hz supply. Determine: (a) Line-current, (Ans: 23.94A). (b) Total power consumed, (Ans: 128W). (c) Power factor, (Ans:.8 lagging) (d) Power factor angle, (Ans: ). (e) Apparent power, (Ans: 16VA). (f) Draw the phasor-diagram. 15. A 3-phase balanced load connected across a 3-phase AC supply draws a line current of 1 Amperes. Two-wattmeters are used to measure input power. The ratio of 2-wattmeter readings is 2 : 1. Find the readings of 2-wattmeters. (Ans: 4W, 2W). 16. Three similar coils each of 2 Ω and an inductance of.5 H are connected in star across a three -phase source of 4 V, 5 Hz. Calculate the line current and power absorbed of the circuit. [Ans: A, W]. 17. A three-phase star connected load of (8+j6) Ω per phase is connected to a 3-φ 23 V supply. Find the line current, power factor, power, reactive volt-amperes and total volt-amperes. [Ans: A,.8 (lag), kw, kvar, 5.29 kva ]. 18. A balanced star-connected 3-phase load has 8 Ω and an inductive reactance of 6 Ω per phase is connected to a 3-φ, 4 V supply. Find the line current, power factor, and active power, reactive power and three -phase voltamperes. [Ans: I P =I L =23.94A,.8 (lag),p=128w, Q=96VAR S=16VA]. 19. In the two-wattmeter method of power measurement, the readings of the two-wattmeters are 1 W and 55 W. what is the power factor of the load? [Ans:.893 (lag)]. 2. A three-phase star connected load when supplied from 44 V, 5 Hz source takes a line current of 12 A lagging w.r.t. Line voltage by 7. Calculate (i) Impedance parameters (ii) Power factor and its nature (iii) Draw phasor diagram indicating all voltages and currents. [Ans: Ω, 13.6 Ω,.766 (lagging)]. 21. The readings of two-wattmeters are +15 kw and -4 kw for a three-phase balanced load. If the supply voltage is balanced 44 V, find the true power drawn by the load, the power factor and line current. (5) Compiled By: Er. Arif Hassan

6 Aligarh College of Engineering & Technology (College Code: 19) Electrical Engg. (EE-11/21) [Ans: 11 kw,.317 (lag), A]. 22. A 3-phase motor draws a line current of 5 A from 22 V source while starting. The p.f. is.4. Find the readings two-wattmeters connected to measure power. [Ans: 8.85 kw, kw]. 23. Three equal impedances, each consisting of R and L in series are connected in star and are supplied from a 4 V, 5 Hz, 3-phase, 3-wire balanced supply system. The power input to the load is measurement by twowattmeter method and the two-wattmeters read 3 kw and 1 kw. Determine the values of R and L connected in each phase. [Ans: Ω, 63 mh A 3-phase balanced load connected across a 3-phase, 4 V AC supply draws a line current of 1 A. twowattmeters are used to measure input power. The ratio of two-wattmeter readings is 2 : 1. Find the readings of the two-wattmeters. [Ans: 4 W, 2 W]. 25. A 3-wire, 3-phase supply feeds a load consisting of three equal resistors. By how much is the load is reduced if one of the resistors be removed? (i) When the load is star connected (ii) When the load is delta connected. [Ans: 5%, 33.33%]. 26. Three identical resistors of 2 Ω each are connected in star to a 415 V, 5 Hz, 3-φsupply. Calculate: (i) The total power consumed (ii) The total power consumed, if they are connected in delta (iii) The power consumed, if one of the resistors is opened. [Ans: W, W, W, W]. 27. A 22 V, 3-φ Ac voltage is applied to a balanced delta-connected load of impedance (15+j2) Ω. Find: (i) Phasor current in each line (ii) Power consumed per phase (iii) The phasor sum of the three line currents. [Ans: I R = , I Y = , I B = , (Zero)]. 28. A balanced 3-phase star-connected load of 12 kw takes a leading current of 85 amperes, when connected across a 3-phase 11 V, 5 Hz supply. Obtain the values of the resistance; impedance and capacitance of the load per phase also calculate the power factor of the load. *Ans: Ω, Ω, μf A 3-φ, 3-wire Y-connected system has 15 V between two-phases. Each phase has Z = 5 <-3 Ω. Find: (i) Current in each phase (ii) Total power consumed (iii) Draw phasor diagram. [Ans: A,.866 (lead), 3897 W]. 3. Three similar resistors are connected in star draw a line current of 5 A from 4 V, 3-phase mains. To what value should the line voltage be change to obtain the same line current with the resistors connected in delta? [Ans: V]. 31. Show that the power consumed in delta-connected system is three times the power consumed in starconnected system when connected across balanced 3-phase load of a 3-phase, 5 Hz, AC supply. 32. A delta-connected system load draws a current of 15 A at a lagging power factor of.85 from a 4 V, 3-phase, 5 Hz supply. Find the resistance and inductance of each phase. [Ans: Ω,.775 H The power in a 3-phase circuit is measured by two-wattmeters. If the input power is 15 kw and the power factor is.72 (lagging). Calculate: (i) What will be the readings of each wattmeter? (ii) For what power factor will one of the watt-meters read zero. [Ans: kw, 3.45 kw,.5]. 34. Three equal impedances, each (2+j3) Ω are connected in star across a 3-phase, 4 V, 5-Hz supply. Determine: (i) The phase and line currents,(ii) The power factor of the load, (iii) The readings of the wattmeters when the 2-wattmeter method is used to measure power input to the load. [Ans: I P =I L =6.46A,.5547 (lag), W 1 =2297W, W 2 =165W]. 35. Three identical resistors each of value 4 Ω are connected first in star and then in delta across a 2 V, 3-phase, 5 Hz supply. Calculate: (i) The power taken from the supply, (ii) If one of the resistors is taken out of the circuit in each case, what would be the new values of the power? [Ans: 1 W, 3 W, 5 W, 2 W]. 36. A balanced Y-connected load is connected from symmetrical 3-φ, 4 V, 5 Hz, supply system. The current in each phase is 3 A with lagging power factor at 3. Find: (i) Impedance in each phase (ii) Total power drew, (iii) Parameters of the load. [Ans: Z P =7.698 <3,P=18kW, R=6.67Ω, L=21.23mH]. 37. Prove that the power in a 3-phase balanced circuit can be deduced from the readings of two-wattmeters. Draw the circuit and phasor diagrams. Discuss the nature of the power factor, (i) When the two -wattmeter readings are equal and positive, (ii) When the two-wattmeter readings are equal but opposite in sign, (iii) When one of the wattmeters reads zero. 38. A balanced 3-phase star-connected load of 18 kw taking a leading current of 6 A when connected across a three-phase, 44 V, 5 Hz supply. Find the values and nature of the load components and also power factor of the load. [Ans: Z P = < , R=1.667Ω, C= μF+. (6) Compiled By: Er. Arif Hassan

7 Aligarh College of Engineering & Technology (College Code: 19) Electrical Engg. (EE-11/21) 39. In a 2-wattmeter method, the power measured was 3 kw at.7 p.f. lagging. Find the readings of each wattmeter. [Ans: kw, kw]. Measuring Instruments 4. Explain the working principle of moving iron type instruments? Why these instruments are suitable for the measurement of AC as well as DC quantities. 41. Explain the construction & working principle of attraction type moving iron instruments. Also enlist advantages and disadvantages. 42. Explain the principle of operation of a repulsion type moving iron instrument. 43. Describe the following in case of measuring instruments: (a) Deflecting torque (b) Controlling torque (c) Gravity control (d) Damping torque Unit-IV Single-Phase Transformer 1. Describe the working principle of single-phase transformer? 2. Derive an expression induced emf of a single-phase transformer? 3. Explain the following for the single-phase transformer: (a) Phasor diagram for inductive load in case of ideal transformer, (b) Develop Exact Equivalent circuit referred to primary with their referred values, (c) Voltage regulation, (d) Transformation ratio. 4. Draw and explain the Phasor diagram of a single-phase transformer on load with lagging power factor? 5. Define the efficiency of a single-phase transformer? Derive the condition for maximum efficiency? 6. Describe the power losses that take place in a transformer? On what factors these losses depend? 7. What is an autotransformer? Explain its merits and demerits? Also write applications. 8. Derive the expression for copper saving in case of step-up and step-down autotransformer? 9. A 44/11 V transformer takes a no load current of 4 A at.2 lagging power factor. Secondary winding supplies a load current 1 A at a power factor of.8 lagging. Find the primary input current. (Ans: I 1 = A). 1. A 25 kva 4/2 V, 5 Hz, single-phase transformer has R 1 =3.45 Ω, R 2 =.9 Ω, X 1 = 5.2 Ω and X 2 =.51 Ω. Calculate the equivalent resistance and reactance referred to (i) primary (ii) secondary. Also calculate the net power loss due to winding resistance. (Ans: R 1 =7.5 Ω, X 1 = 25.6 Ω, R 2 =.176 Ω, X 2 =.64 Ω and W). 11. A 2 kva, 33/24 V, 5 Hz single-phase transformer has 8 turns on secondary winding. Calculate: (a) Primary and secondary current on full load (b) Maximum value of flux (c) Number of primary winding turns (Ans: 6.6 A, and A, mwb N 1 = 11) 12. Efficiency of a 4 kva, single-phase transformer is % when delivering full load of.8 p.f. and it is % at half load at unity p.f. calculate: (i) Iron loss (ii) Full load copper loss (Ans: 112 W, 2973 W) 13. The iron loss and full load copper loss of a 1 kva, 66/4 V single-phase transformer are 6 W and 9 W. calculate the efficiency at full load and half load at.8 p.f. lagging. Also calculate the load at which maximum efficiency is obtained and its magnitude at same power factor. (Ans: ƞ HL =97.98%, ƞ FL =98.196%, & kva). 14. The primary and secondary winding of a 5 kva transformer have resistances of.42 Ω and.11 Ω respectively. The primary and secondary voltages are 66 V and 4 V respectively. The iron loss is (7) Compiled By: Er. Arif Hassan

8 Aligarh College of Engineering & Technology (College Code: 19) Electrical Engg. (EE-11/21) 2.9 kw. Calculate the efficiency on (i) full load and (ii) half load assuming that the p.f. of the load to be.8 lagging. (Ans: ƞ FL =98.26%, ƞ HL =98.7%). 15. A single-phase 1 kva, 33/23 V, 5 Hz transformer has 89.5 % efficiency at.85 p.f. lagging both at full load and also half load. Determine the efficiency of the transformer at 75 % load and.9 p.f. leading. (Ans: 9.52%). 16. A 1 kva,, 4/2 single-phase, 5 Hz transformer has a maximum efficiency of 96 % at 75 % of full load at unity p.f.. Calculate full load the efficiency at full load.8 power factor lagging. (Ans: 94.8 %). 17. The following test results were obtained on a 22 kva, 22/22 V transformer: O.C. Test (L.V. side): 22 V, 1.1 A, 125 W S.C. Test (H.V. Side): 52.7 V, 3.4 A, 287 W Find parameters referred to primary, voltage regulation and the efficiency of the transformer. (Ans: %). 18. A 2/1 V, 5 Hz transformer has an impedance of (.3 + j.8) Ohm in the 2 V winding and impedance of (.1 + j.25) ohm in the 1 V winding. What are the currents on the high and low voltage sides, if a short circuit occurs on the 1 V side with 2 V applied to the HV side. (Ans: I 1 = 13.5 A, I 2 = 27 A). 19. A 4 kva transformer has a core loss of 4 watts and full load copper loss of 8 watts. If the power factor of the load is.9 lagging, calculate: (a) The full load efficiency and (b) Percentage of the load at which the maximum efficiency occurs. (Ans: %, 7.71 %) 2. In a 25 kva, 2/2 V transformer the iron and copper losses are 35 watts and 4 watts respectively. Calculate the efficiency at full load and.8 power factor lagging. Determine maximum efficiency and the corresponding load. (Ans: %, kva & 97.1 %). 21. A 6 kva, single-phase transformer has an efficiency of 92 % both at full load and half load at unity power factor. Determine its efficiency at 6 % of full load at.8 power factor lagging. (Ans: 9.59 %). 22. A 5 kva single-phase transformer draws a primary current of 25 A on full loads. The total resistance referred to primary side is.5 Ω. If the iron loss of the transformer is 1 W, calculate the efficiency on full load and half load at.8 p.f. lagging. (Ans: ƞ HL =99.11%, ƞ FL =98.97%). 23. The primary and secondary winding of a 3 kva, 6/23 V transformer has resistances of 5 and.8 Ω respectively. The total reactance of the transformer referred to the primary is 2 Ω. Calculate percentage regulation of the transformer when supplying full load current at a power factor of.8 lagging. (Ans: 1.72 %). Magnetic Circuits 24. Define leakage flux and magnetic fringing? How can you minimize these? 25. Explain magnetic and electric circuits? Give analogy between them? 26. A ring of ferromagnetic material has a circular cross-section. The inner diameter is 7.4 inch, the outer diameter is 9 inch and the thickness is.8 inch. There is a coil of 6 turns wound on the ring. When the coil carries a current of 2.5 A, the flux produced in the ring is 1.2 mwb. Find: (a) Magnetic field intensity (Ans: AT/m). (b) Reluctance (Ans: 1.25x1 6 AT/Wb). (c) Permeability (Ans: ). 27. A wrought iron bar 3 cm long and 2 cm in diagram is bent into a circular shape. it is than wounded with 5 turns of wire. Calculate the current required to produce a flux of.5 mwb. In magnetic circuit with an air gap of 1 mm, µ r (iron) = 4 assume constant. (Ans: A). Earthing 28. What is earthing and its purpose? Distinguish between system earthing and equipment earthing. (8) Compiled By: Er. Arif Hassan

9 Aligarh College of Engineering & Technology (College Code: 19) Electrical Engg. (EE-11/21) 29. Explain the necessity and uses of earthing. 3. What types of earthing used in electrical system? 31. State the various safety precautions to be observed while using electric supply. 32. What is electric shock? Which factors affect the severity of the shock? 33. State the various safety rules to be observed while dealing with electricity. 34. What are the methods of earthing used in electrical system? Explain any one method. Unit-V DC Machines 1. Explain the working principle of DC motor. 2. Derive an expression for the torque developed in a DC motor? 3. Derive an expression for the generated e.m.f. in a DC generator? 4. Explain the Torque-Armature current characteristics of DC shunt and Dc series motors. 5. Explain the Speed- torque characteristics of DC shunt and DC series motors. 6. Explain the speed regulation of DC motors. 7. A 4-pole, 25 V, d.c. series motor has a wave connected armature with 2 conductors. The flux per pole is 25 mwb. When motor is drawing 6 amperes from the supply. Armature resistance is.15 Ω and series field winding resistance is.2 Ω. Calculate the speed under this condition. (Ans: 1374 r.p.m.). 8. A 25 V, d.c. shunt motor takes a line current of 2 A. resistances of field and shunt windings are 2 Ω and.3 Ω respectively. Find the armature current and back e.m.f. (Ans: A, V). 9. A d.c. shunt motor runs at a speed of 1 r.p.m. on no load taking a current of 6 A from the supply., when connected to 22 V d.c. source, its full load current is 5 A. calculate its speed on full load. Assume R a =.3 Ω and R sh = 11 Ω. (Ans: r.p.m.). 1. A d.c. series motor is running with a speed of 8 r.p.m. while taking a current of 2 A from the supply. If the load is changed such that the current drawn by the motor is increased to 5 A, calculate th e speed of the motor on new load. Armature and field windings resistances are.2 Ω and.3 Ω respectively. Assume the flux produced is proportional to the current and the supply voltage as 25 V. (Ans: 3 r.p.m.). Three-Phase Induction Motor 11. Explain the construction, working and applications of three-phase induction motor? 12. Draw and explain the Torque-Slip characteristics of three-phase induction motor? 13. Draw and explain the Speed-Torque characteristics of three-phase induction motor? 14. Explain what is the rotating magnetic field? And how it is made use of in electrical machinery? 15. Explain the term slip in induction motors? What is its value at starting and synchronous speed? 16. An 8-pole, 3-phase, induction motor is supplied from 5 Hz a.c. supply. On full load, the frequency of induced e.m.f. in rotor is 2 Hz. Find the full load percentage slip and corresponding speed. (Ans: 4%, 72 r.p.m.). 17. For a 4-pole, 3-phase, 5 Hz induction motor ratio of stator to rotor turns is 2. On a certain load its speed is observed to be 1455 r.p.m. when connected to 415 V supply, calculate: (a) Frequency of rotor e.m.f. in running condition, (b) Magnitude of induced e.m.f. in the rotor at standstill, (c) Magnitude of induced e.m.f. in the rotor at running condition. (Ans: 1.5 Hz, V and V). 18. A 3-phase, 4 V, 5 Hz, 4-pole induction motor has star connected stator winding. The rotor resistance and reactance are.1 Ω and 1. Ω respectively. The full load speed is 144 r.p.m. Calculate the torque developed on full load by the motor. (Ans: N-m). Single-Phase Induction Motor 19. Why single-phase induction motor is not self starting? What are the different methods of self starting? Explain one of them. 2. Explain the construction, working and applications of single-phase induction motor? (9) Compiled By: Er. Arif Hassan

10 Aligarh College of Engineering & Technology (College Code: 19) Electrical Engg. (EE-11/21) Three-Phase Synchronous Machines 21. Explain the principle of operation and applications of three-phase Synchronous motor? 22. Explain the working operation and applications of three-phase Synchronous alternator? (1) Compiled By: Er. Arif Hassan