basic electrical technology (ele 101) · pdf filedepartment of electrical and electronics...

Department of Electrical and Electronics Engineering

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MANIPAL INSTITUTE OF TECHNOLOGY, MANIPAL (A Constituent Institute of Manipal University, Manipal)

FIRST SEMESTER B.E. DEGREE MAKEUP EXAMINATION

(REVISED CREDIT SYSTEM: 2007)

30 December 2008

BASIC ELECTRICAL TECHNOLOGY (ELE 101)

Time: 3 hours Max. Marks: 50

Note : Answer any FIVE full questions. Missing data, if any, may be suitably assumed.

1A. Using node voltage analysis find the voltage across 4 Ω resistor in the circuit shown in Fig. 1A. (04)

1B. Consider the circuit shown in Fig. 1B. The switch is closed at t=0. Determine the expression for the current and find the current at t = 5msec. (03)

1C. For the circuit shown, apply star-delta transformation to obtain the equivalent resistance across the input terminals. (03)

2A. A series RC circuit is energized by an alternating voltage v(t)=Vmax Sinωt volts. Derive an expression for the average power consumed by the circuit over a cycle. (03)

2B. A coil having a resistance of 15Ω and an inductance of 0.2H is connected in series with another coil having a resistance of 25Ω and an inductance of 0.04H to a 230V, 50Hz supply. Determine (i) current through the circuit (ii) the voltage across each coil (iii) active power dissipated in each coil (iii) the p.f of the circuit. (04)

2C. A mild steel ring has a mean circumference of 600 mm and a uniform cross sectional area of 250 mm2

. Calculate the mmf required by the coil to produce a flux of 450 μWb. in the air gap of 1.0 mm. Assume the relative permeability of the mild steel to be 220. Also find the current through the coil if the coil has 800 turns. (03)

3A. A load takes a current of 20 A at 0.707 pf lagging from a 240V, 50Hz supply. A capacitor is connected across the same supply so as to improve the power factor of the supply to 0.9 lagging. Find the value of the capacitance of the capacitor. (03)

3B. Three identical impedances of 5∠−30° Ω are connected in delta to a three phase three wire 400 Volts RYB system. Find the phase and line currents and the total power consumed by the load. (04)

3C. A coil has resistance of 400Ω and inductance of 318μH. Find the capacitance of a capacitor when connected in parallel with the coil makes the circuit to resonate at 1MHz. (03)

4A. A 50 KVA, 4400/220 V transformer has R1=3.45Ω, R2=0.009Ω, X1=5Ω, X2=0.014Ω. Calculate for the transformer

a) Equivalent impedance as referred to primary.

b) copper loss, at full load

c) secondary terminal voltage at full load 0.8 pf lag (04)

4B. A 1-phase transformer has 400 primary and 1000 secondary turns. The net cross sectional area of the core is 80 cm2. If the primary winding is connected to a 50Hz supply at 500V, calculate i) the peak value of the flux density in the core and ii) the voltage induced in the secondary winding. (03)

4C. Why starters are required for 3 phase induction motor? With a neat sketch, explain star delta starter for a 3 phase induction motor. (03)

Reg. No. :


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5A. Explain the principle of operation of 3 phase induction motor.

A 415 V, 50 Hz, 8 poles, 3 phase induction motor has a input of 50 kW, when running at 720 rpm. The motor has a stator loss of 750 W and friction and windage losses are 1500 W. Determine the (i) slip (ii) gross power in the rotor (iii) the rotor copper loss/phase (iv) efficiency (05)

5B. A 3 phase load takes a power of 15 KW at a power factor of 0.6 lagging. Determine the wattmeter readings, if the power is measured by 2 wattmeter method. (03)

5C. Explain why single phase induction motor is not self starting. (02)

6A. With the help of a neat sketch, explain the principle and working of a PMMC instrument? Why it is not suitable for AC measurements? (04)

6B. What is a steeper motor? How it is controlled? List some of its applications. (03)

6C. Two coils with self inductances of 500 mH and 400 mH respectively when connected in series aiding results in a total inductance of 1000 mH. Determine the mutual inductance between the two coils. What would be the total inductance if the coils are connected in series opposition? Also find the coefficient of coupling. (03)

12 V

6 Ω 3 Ω

4 Ω

6 Ω 2 A

2 Ω 3 A

Fig. 1A

100V

10Ω

0.1H

Fig. 1B

5 Ω

6 Ω6 Ω6 Ω

3 Ω

3 Ω6 Ω

A

BFig. 1C


Page 1 of 2


FIRST SEMESTER B.E. DEGREE END SEMESTER EXAMINATION


22 November 2008

BASIC ELECTRICAL TECHNOLOGY (ELE 101/102)



1A. In the circuit shown in Fig. 1A, Find the resistance between A and D using star-delta

reduction technique. (04)1B. In the circuit shown in the Fig. 1B, determine the current through 4 Ω resistance and

the power supplied by the battery. Use mesh analysis. (03)1C. Three Magnetically coupled coils are connected in series as shown in Fig. 1C. The

coefficient of coupling are K12 = 0.5, K23 = 0.7 and K13 = 0.6. Find the equivalent inductance and the current drawn from the supply. (03)

2A. For the circuit shown in Fig. 2A, the switch is in position 1 for a long time and it is changed to position 2 at t=0. Determine the capacitor voltage and current through 200 kΩ resistor at t=10 msec. (03)

2B. Two identical coils of 1200 turns each are placed side by side such that 60% of the flux produced by one coil links the other. A current of 10A in the first coil sets up a flux of 0.12mwb. If the current in the first coil changes from +10A to -10A in 20msec find (i) the self inductances of the coils (ii) the emf’s induced in both the coils. (03)

2C. A soft iron ring 25cm mean diameter and circular cross-section 5cm diameter is wound with a coil carrying a current of 5A to produce a flux of 2.5mwb in the air-gap which is 0.25cm in length. Assuming μr = 900, find the number of turns in the coil. (04)

3A. Discuss the drawbacks of low power factor. A single phase motor takes 10A at a power factor of 0.866 lagging when connected to a 230V, 50Hz, AC supply. Calculate the value of capacitor to be connected across the motor to improve the overall power factor to .95 lag. (03)

3B. A sinusoidal voltage v(t)= 200sinωt volts is applied to a network of three parallel branches. The currents in the two branches are i1= 14.14sin(ωt-370)A and

i2=28.28sin(ωt-14.30)A. The source current is 63.8 sin(ωt+12.80)A. Determine a. The current in the third branch b. Network elements in each branch.

Assume ω = 314 rad/sec. (04)3C. A resistor and coil connected in series across a 240V single phase ac supply takes a

current of 3A, lagging by 37° behind the supply voltage. If the voltage across the coil is 171V, find the resistance of the resistor and resistance and reactance of the coil. (03)

4A. Determine the line currents in a star connected load supplied from a symmetrical 3 phase, 3 wire, 400 V system using mesh analysis. The branch impedances of the load are Ω°∠= 3010AZ , Ω°∠= 4510BZ , Ω°∠= 6010CZ . Assume the phase sequence as ABC. Verify the answer by neutral displacement method. (05)

Reg. No. :


Page 2 of 2

Fig. Q 2A

1 2

t = 0

10 V

100 kΩ

200 kΩ0.1 μF

8 V 4 Ω

+_

1 Ω

2 Ω 3 Ix

3 A

Ix

Fig. Q 1B

4B. In the circuit shown in Fig. 4B, RL = 400 Ω, RC = 700Ω; C = 36uF and L = 0.2H. Find the resonant frequency. Derive the formula used, if any. (03)

4C. Explain the principle and working of a single phase auto transformer. (02)5A. The primary and secondary windings of a 30kVA, 6000V/230V transformer have

resistances of 10Ω and 0.016Ω respectively. The total reactance of the transformer referred to the primary side is 23Ω. Find the secondary terminal voltage when the transformer is supplying rated current at power factors of i) 0.8 lag, ii) 0.8 lead iii) upf. (04)

5B. Show that only two watt meters are sufficient to measure total power in 3-phase delta connected load supplied from balanced 3-phase supply. Draw the relevant phasor diagram. (03)

5C. Compare squirrel cage and slip ring induction motors. What are the applications of 3 phase induction motor? (03)

6A. A 3 phase, 20 Hp, 500V, 50Hz, 6 pole, 950 rpm induction motor is running on full load at a power factor of .86 lag. The mechanical losses are 700 watts. Calculate for this load a) slip b) rotor copper loss c) input if the stator losses total 1500 W d) the line current e) Efficiency (04)

6B. With the help of a neat circuit diagram explain the working principle of a capacitor start single phase induction motor. (03)

6C. With the help of a neat sketch, explain the principle and working of attraction type Moving Iron instrument? (03)

RL

XL XC

RC

Fig. Q 4B

2Ω 2Ω

2Ω

2Ω

2Ω 2Ω

4Ω

4Ω 4Ω

Fig. Q 1A

A

D

230 V, 50 Hz

2 H 3 H 4 H

Fig. Q 1C


Page 1 of 2


FIRST SEMESTER B.E. DEGREE MAKEUP EXAMINATIONS


04 January 2010 BASIC ELECTRICAL TECHNOLOGY (ELE 101)



1A. For the circuit shown in Fig. Q1A, apply star‐delta transformation to obtain the equivalent resistance across the input terminals A and B. (05)

1B. For the network shown in Fig. 1B, determine the voltages VA and VB using Node voltage analysis. Also find the power supplied by each voltage source. (05)

2A. A coil of resistance 30 Ω and inductance 0.6H is connected to a 240 V DC supply through a switch which is closed at time t = 0. Assuming zero initial energy, determine

i) The time constant τ of the circuit, ii) Current through the circuit & voltage across the inductor at t = 2τ, iii) Sketch the current and voltage variations as a function of time. (04)

2B. A circular ring has a mean circuference of 56 cm including an air gap of 2 mm with a uniform cross sectional area of 16 cm2. A coil of 400 turns is wound on the ring. For a leakage factor of 1.3 and relative permeability of 4000 for the material, Find the exciting current required to cause a flux density of 0.8Wb/m2 in the air‐gap. (03)

2C. Three magnetically coupled inductive coils having the following data are connected in series as shown in Fig. 2C.

L1 = 12 H; L2 = 14 H; L3 = 14 H; k12 = 0.33; k23 = 0.37; k31 = 0.65; Find the equivalent inductance of the circuit. (03)

3A. Explain how the sign of mutual inductane is determined based on the dot rule. (02)

3B. Two impedances (14+j5) Ω and (18+j10) Ω are in parallel across a single phase. 200V, 50Hz supply. Determine i) the admittance of each branch and of the entire circuit ii) total current and complex power iii) the capacitance to be connected in parallel with the above circuit to improve the power factor to unity. (05)

3C. For a sinusoidal alternating wave form, derive expressions for the average and RMS values in terms of its peak value and hence find the value of form factor. (03)

4A. A star connected load is supplied from a symmetrical 3 phase, 3 wire 400 V system. The branch impedances of the load are ZA = °∠3010 Ω, ZB= °∠4510 Ω,ZC= °∠6010 Ω. Determine the neutral shift voltage and the three line currents. Assume the phase sequence as ABC and consider VAN as reference. (05)

4B. The circuit shown in Fig. 4B is connected to a varible frequency voltage source. There is no mutual effect on inductances. Find

(i) Net resistance, inductance and capacitance (ii) Resonant frequency (iii) Half power frequencies (iv) Quality factor (05)

Reg. No. :


Page 2 of 2

5 Ω 10 Ω1 μF 2 μF4 mH

2 mH

Fig. Q 4B

A

B

6 Ω

6 Ω

6 Ω

6 Ω

5 Ω

18 Ω

12 Ω

FIG. Q 1A

100 V 60 V20 Ω 20 Ω 10 Ω

25Ω

10Ω

VA VB

Fig. Q 1B

Leq

L1 L3L2

Fig. Q 2C

5A. With relevant phasor diagram prove that in a 3 phase induction motor the rotating magnetic field produced is of constant magnitude but rotates at synchronous speed.

(05)

5B. The primary winding of a transformer has 300 turns. When connected across a 220 V, 50 Hz supply, it draws a current of 5A at a power factor of 0.25 on no load. Determine the following: (i) The maximum value of the core flux. (ii) Magnetizing reactance (iii) Equivalent Core loss resistance. (03)

5C. When balanced 3 phase power is measured by 2 wattmeter method, the two wattmeter readings are W1 = 10,000 W and W2 = − 2000 W. The line voltage is 400 V. Determine the total Power, Power factor and line current. (02)

6A. Explain Double field Revolving Theory as aplied to a single phase induction motor. (03)

6B. Give a brief analogy between Electrical and Magnetic circuits. (03)

6C. With a neat diagram, explain the construction and working of a single phase energy meter. (04)


Page 1 of 2


FIRST SEMESTER B.E. DEGREE END SEMESTER EXAMINATIONS

(S & T SECTIONS) (REVISED CREDIT SYSTEM: 2007)

07 December 2009 BASIC ELECTRICAL TECHNOLOGY (ELE 101)



1A. With neat connection diagrams explain the difference between ideal and practical voltage source. (02)

1B. Using mesh current analysis, determine the voltage across 4Ω resistance, current through 2Ω resistance and power dissipated in 5Ω resistance in the circuit shown in Fig. Q1B. (05)

1C. Find resistance between terminals AB of the circuit shown in Fig. Q1C using star‐ delta transformation. (03)

2A. A coil of resistance 8Ω and inductance 19.1 mH is connected in parallel with another impedance of resistance 6Ω in series with capacitance of 398µF .What 50Hz voltage is to be applied across this parallel combination in order that a current of 10A may flow in the capacitor. Calculate the current in the coil and p.f. of whole circuit. Draw the complete phasor diagram. (04)

2B. Obtain the equations for the capacitor current for the circuit given in Fig. Q2B for the time interval 0 ≤ t ≤ ∞. The switch is closed to position A at time t = 0 and it is changed to position B at time t = 0.1 second. Plot the current variation with respect to time. (03)

2C. A circuit‐I has a resistance of 2Ω and a capacitance of 20µF. A second ciruit‐II consists of resistance 3Ω and a variable inductance. A voltage source of 230 V, 800 Hz is available. Determine the value of inductance for resonance if circuit‐I and circuit‐II are connected to supply

(i) in series and (ii) in parallel. (03)

3A. An iron ring has 32cm outer diameter and 30cm inner diameter and is of circular section. At one point on the ring a saw cut is made normal to its cross section equivalent to an air gap of 1mm. It is wound uniformly with 600 turns of wire carrying a current of 2.5A. The iron path takes 40% of the total magnet motive force. Estimate (i) Reluctance of air (ii) Magnetic flux (iii) Reluctance of iron (iv) flux density. Neglect magnetic leakage. (04)

3B. Two identical 1000 turn 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 0.05Wb. Calculate

(i) the self inductance of each coil and the mutual inductance of the arrangement. (ii) Emf induced in Y, if the current in X changes from 6A to ‐6A in 0.01 sec. (iii) Energy stored in coil X at the instant when the current through it is 5A (04)

3C. Define the terms: (i) RMS current (ii) Magnetomotive force (02)

4A. With the aid of the torque‐slip characteristics of a 3 phase Induction motor, explain how the rotor resistance can alter the performance of Induction motor. (03)

4B. What is the need of a starter for 3 phase induction motor, even though it is self starting? (02)

4C. A symmetrical 440V, 3 phase system supplies a star connected load with the following branch impedances ZR = 10Ω, ZY = j10Ω and ZB = ‐ j15Ω. Calculate the line currents drawn from the supply using mesh current analysis Assuming the phase sequence as RYB and VRY as reference vector. (05)

Reg. No. :


Page 2 of 2

A

B

1 Ω 2 Ω

8 Ω4 Ω

4 Ω

2 Ω

Fig. Q 1C

Fig. Q 2B

A

B

2 Ω

0.1 F10 V 8 V

+ -

+-

3 Ω

24 V

8 V

4 Ω 1 Ω

2 Ω5 Ω

Ib

16 Ib

2 Va

Va+ -

Fig. 1B

+_

+ _

5A. For a 3 phase star connected balanced lagging load, draw the connection diagram to measure the power by two wattmeter method. Derive the expressions for the two wattmeter readings with the aid of the phasor diagram and hence obtain an expression for the load power factor in terms of wattmeter readings. (05)

5B. A 3 phase induction motor has a star connected rotor. The rotor emf between the slip rings at standstill is 50V. The rotor resistance and standstill rotor reactance are 0.5Ω and 3Ω respectively. Determine (a) per phase rotor current at starting if a star connected rheostat of 6Ω per phase is connected across the slip rings (b) full load rotor current and rotor power factor for a full load slip of 4 % with slip rings short circuited. (03)

5C. The total power absorbed by a certain 3 phase balanced load is 40KW, the reactive power being 91.65KVAR. Determine the readings of the two watt meters if two wattmeter method is used to measure total power. (02)

6A. An approximate equivalent circuit as referred to the low tension side of a 250/2500V, 50Hz, single phase transformer has equivalent resistance and reactance values of 0.2Ω and 0.7Ω respectively. The shunt branch resistance and reactance values are 500Ω and 250Ω respectively. The load impedance connected to the high tension side is 450∠300Ω. For the primary applied voltage of 250V, Determine

(a) secondary terminal voltage (b) primary current (05)

6B. With the help of neat diagram and phasor diagram, explain the working of a capacitor start single phase induction motor. (03)

6C. A moving coil instrument has a resistance of 10Ω and gives full scale deflection when carrying a current of 50mA. Calculate the shunt resistance required to extend the range of the meter to measure upto 100A. (02)


Page 1 of 2


FIRST SEMESTER B.E. DEGREE END SEMESTER EXAMINATION


16 November 2009 BASIC ELECTRICAL TECHNOLOGY (ELE 101)



1A. Define and explain the significance of the term “temperature coefficient of resistance”. The field winding of a motor has a resistance of 500 Ω at 150C. By how much will the resistance increase if the motor attains a temperature of 450C, when it is running? The winding is made of copper and its temperature coefficient of resistance at 00C is 0.00428/0C. (03)

1B. Determine the resistance between the points A and B using star‐delta transformation in Fig Q1B (04)

1C. In the circuit shown in Fig. Q1C, find the current through all resistors by mesh current analysis. (03)

2A. Two similar inductive coils with negligible resistance are wound on the same core. When excited by a 200V, 50Hz source, the first coil takes 5.2A and the emf induced in the second coil is 50V. Find the self inductance of the two coils, mutual inductance between the two coils and the coupling coefficient. (04)

2B. For the network shown in Fig Q2B the switch is in position 1 at t = 0 and is moved to position 2 at t = 10 ms. Determine iL(t) for 0 ≤ t ≤ ∞ and sketch the current variation with respect to time. (03)

2C. A mild steel ring having a cross sectional area of 400 mm2 and a mean circumference of 400 mm has a coil of 200 turns wound uniformly around it. Relative permeability of mild steel is 300. It is required to produce a flux of 800x10‐6 wb in the ring. Determine a) Reluctance of the ring b) Excitation current required. (03)

3A. Analytically derive the phasor relationship between voltage and current in a pure inductor. Hence show that average power consumed by the inductor is zero. Draw the waveform showing voltage, current and power. (03)

3B. A single phase circuit comprises of two lagging loads, which are connected in series and dissipate 800W and 1000W respectively. Two voltmeters connected across each load read 100V and 200V respectively. If the current flowing through the circuit is 10A, determine (i) Applied voltage (ii) power factor of the combination. (04)

3C. An inductive load takes 60A of a single phase a. c. supply of 230 volt, 50 Hz at a p. f. of 0. 6 lagging. Calculate the value of capacitance to be connected across the load to raise the power facter of the combined circuit to 0.9 lagging. Also calculate the active, reactive and apparent powers after power factor improvement. (03)

4A. A symmetrical 3 phase, RYB sequence, 3 wire, 400V, 50 Hz supply feeds an unbalanced star connected load. The branch impedances of the load are ZR= (8.66+j5) Ω, ZY = (7.07‐j7.07) Ω and ZB=(10+j17.32) Ω. Calculate the three line currents by converting star connected load to equivalent delta connected load. Hence determine the voltages across the original star connected impedances. Take VRY as reference. (06)

4B. With a neat sketch, explain the operation of a Star‐Delta starter used with 3 phase induction motor. (04)

Reg. No. :


Page 2 of 2

A

B

6 Ω

3 Ω 5 Ω

4 Ω

5 Ω 4 Ω2 Ω

Fig. Q 1B

Vx

2Vx

+ -

5 V 6 V3 Ω

2 Ω

4 Ω

3 A

Fig. Q 1C

t = 10 ms

1 2

10 Ω

15 Ω

0.1 H

10 V

Fig. Q 2B

5A. A 6 pole 50 Hz 3‐phase induction motor running on full load develops a useful torque of 160 Nm and the rotor emf has a frequency of 120 cycles per minute. Calculate the following:

i) Gross power developed if the torque loss in windage and friction is 12Nm. ii) The copper loss in rotor windings iii) The input to the motor, if the stator loss is 800W and iv) Full load efficiency

(05)

5B. A 30KVA, 2400/120 V, 50Hz transformer has high voltage winding resistance of 4.5 Ω and leakage reactance of 10 Ω. The corresponding values on low voltage winding are 0.01 Ω and 0.025 Ω. The iron losses are 1.5 KW. Calculate

(a) equivalent impedance referred to high voltage side, (b) total losses at (i) full load (ii) 60% of full load. (03)

5C. Explain the purpose of capacitor and centrifugal switch in a single phase induction motor. (02)

6A. The two wattmeter method is used to measure total active power in a 3 phase balanced circuit. Indicate the power factors for the following types of readings: (i) W1 and W2 are positive and equal, (ii) W1 is positive and W2 is zero (iii) W1 and W2 are equal and opposite and (iv) W1 and W2 are positive, but not equal (02)

6B. A series resonant circuit with a resistance is 4 Ω resonates at frequency of 2.9 KHz. The bandwidth is 3x103 rad/sec. Calculate (i) the inductance and capacitance of the circuit, (ii) lower half power frequency and (iii) Quality factor. (04)

6C. With the help of a neat sketch, explain the principle of a Permanent Magnet Moving Coil instrument. Why it is not suitable for AC measurements? (04)


Page 1 of 2


SECOND SEMESTER B.E. DEGREE MAKEUP EXAMINATION (REVISED CREDIT SYSTEM: 2007)

17 July 2009



Note: Answer any FIVE full questions.

Missing data, if any, may be suitably assumed.

1A. For the network shown in Fig. Q1 A, determine the equivalent resistance between terminals A and B by star-delta conversion.

(04)

1B. A coil having a resistance of 20 Ω and an inductance of 0.8 H is connected to a 400 V DC supply through a switch. If the switch is closed at time t = 0, find

(i) Time constant of the circuit.

(ii) Time taken for the current to reach half of its final value.

(iii) Energy stored in the inductance at that instant as in case (ii) above

(03)

1C. In the circuit shown in Fig. Q1 C, determine the voltages at nodes A and B using Node voltage analysis. (03)

2A. Determine the currents in 5 Ω and 8 Ω resistances in the network shown in Fig. Q2 A by mesh analysis. (04)

2B. A cicular iron ring having a mean circumference of 120 cm and a cross sectional area of 3.14 cm2 has a saw cut equivalent to an air gap of 1.5 mm across its cross section. It is uniformly wound with 2000 turns of copper wire. Calculate the exciting current required to produce a useful flux of 1.2 mWb in the air gap. Assume the relative permeability of iron as 1200 and leakage factor of 1.14. (06)

3A. For a pure capacitance circuit connected to a single phase supply, sketch the wave forms of voltage, current and active power. Hence analytically prove that the average power consumed by pure capacitor is zero. (03)

3B. A resistance of 35 Ω is connectrd in series with an inductive coil having an internal resistance ‘r’ and inductance ‘L’. When connected across a 230 V, 50 Hz single phase supply, the voltage across the coil is 100 V and the current drawn is 4 A. Find the unknowns ‘r’ and ‘L’. (04)

3C. Two magnetically coupled coils have a coefficient of coupling of 0.832 with self inductances of 65 mH and 5 mH respectively Determine

(i) Mutual inductance between them (ii) Equivalent inductance when the two coils are connected in (a) series addtion and (b) series opposition (03)

4A. Three inductive coils, each with an internal resistance of 15Ω and an inductance of 31.83 mH are connected in delta to a 3-phase, 3 wire, 400V, 50Hz supply. Two wattmeters are connected in any two lines to measure the total power. Calculate

(i) Magnitude of the line current, total power absorbed and readings of Wattmeters

(ii) Same parameters if the connection is changed to star.

(06)

4B. A circuit having a resistance of 5Ω, an inductance of 0.4H and a variable capacitance in series is connected across a 110V, 50Hz supply. Calculate

(i) The value of capacitance for the circuit to be resonant at the supply frequency

(ii) Voltage across the inductance

(iii) Upper and lower half power frequency in Hz (04)

Reg. No. :


Page 2 of 2

Fig. Q2 A

2

15

V

6 A

10 A

8 5

12

V

6

6

6

3

3

3

1

1 1

A

B

Fig. Q1 A

Fig. Q1 C

18 V

6 20

8

10

20 V 4

A B

5A. With the aid of phasor diagram, show that a constant magnitude rotating magnetic field at synchronous speed is produced when a three phase balanced winding is supplied from a three phase voltage.

(04)

5B. The approximate equivalent circuit for a 200/400V, step up transformer has the following parameters referred to the low-voltage side:

(i) equivalent winding resistance and reactance = 0.15 Ω and 0.37 Ω respectively,

(ii) core loss component = 600 Ω,

(iii) Magnetizing reactance = 300 Ω.

When the transformer supplying a primary current of 20A, 200 V at a pf of 0.8 lag,

calculate

(a) The secondary load current (b) Total losses (c) Secondary terminal voltage and

(d) efficiency (06)

6A. Explain the Double Field Revolving theory applied to single phase Induction Motor and hence show that the starting torque is zero. (04)

6B. A 3 phase slip ring Induction motor has 80 volts between slip rings at stand still conditions. The rotor resistance and stand still reactance are 1.5 Ω and 5 Ω per phase respectively. Determine the rotor current and power factor

(i) at starting with slip rings connected to an external star connected impedance of (5+j2) Ω per phase.

(ii) at 5 % slip when the slip rings short circuited (04)

6C. Explain how the current range of Measuring Instruments can be extended. (02)


Page 1 of 2

15 5

10

4 i1

5 A 4 A

i1


SECOND SEMESTER B.E. DEGREE END SEMESTER EXAMINATION (REVISED CREDIT SYSTEM: 2007)

22 May 2009



Note: Answer any FIVE full questions.

Missing data, if any, may be suitably assumed.

1A.

Derive expressions for the equivalent star connected resistances for a given delta

connected resistance network.

(03)

1B. Using Node voltage method, determine the current I1 in the circuit shown below:

(04)

1C. The series RL circuit with a resistance of 50 Ω and inductance of 10 H has a constant

voltage of 100 V applied at time t = 0 by closing of a switch.

(i) From fundamentals, derive the equations for the circuit current, voltages across

resistance and inductance,

(ii) Current at time t = 0.5 sec,

(iii)The time at which the voltages across resistance and inductance are equal. (03)

2A. State and explain the Faraday’s Laws of Electromagnetic Induction. (02)

2B. When two coils are connected in series, their effective inductance is found to be 10 H.

When the connections of one coil are reversed, the effective inductance is 6 H. Given

the co-efficient of coupling as 0.6, calculate the self inductance of each coil and the

mutual inductance. (03)

2C. A steel ring of 25 cm mean diameter and of circular cross section with 3 cm in diameter

has 1mm air gap in it. It is wound uniformly with 700 turns of wire carrying a current

of 2 A. Given that there is no magnetic leakage with negligible fringing effect and iron

path takes 35% of total mmf, calculate

(i) the magnetomotive force

(ii) flux density

(iii) reluctance of steel ring

(iv) relative permeability of steel (05)

Reg. No. :


Page 2 of 2

3A. For a periodic alternating quantity, define the term RMS value and derive expression

for the RMS value of a sinusoidal alternating current. (02)

3B. Three impedances Z1=345 Ω, Z2=14.1460 Ω and Z3=5-90 Ω are connected in

series across an altrnating voltage source. The voltage across Z1 is 27-10 V. Find

(i) the voltages across other two impedances and

(ii) applied voltage V. (03)

3C. An unbalanced isolated star connected load with ZR = (4+j8) Ω, ZY = (3+j4) Ω and ZB =

(5+j8) Ω is supplied from a 3 phase, 400V, RYB System. Taking VRY as reference and

using Mesh analysis, calculate

(i) Line currents (ii) Neutral Displacement voltage and (iii) The total active power. (05)

4A. A series circuit consisting of resistance R and capacitance C is connected in parallel

with an inductance L. This combination is connected across a variable frequency

voltage source. Derive an expression for frequency in Hz at which the circuit resonates.

(04)

4B. Certain load takes 5 kW at 0.707 power factor lagging from 230 V, 50 Hz single phase

supply. An additional load of 2 kW at upf is to be connected in parallel with the

existing load. Calculate the combined power factor. Also determine the value of

capacitance required to improve the overall power factor to 0.9 lagging. (03)

4C. Two wattmeters are connected to measure power in a 3-phase circuit. The reading of

the one of the meter is 5kW when the load p.f. is unity. In each phase, a pure inductance

is coneected in parallel with existing load to change the power factor to 0.4 lagging,

Calculate the new readings of the two wattmeters. (03)

5A. The high voltage and low voltage windings of a 2500/250 V single phase transformer

has resistances of 4 Ω and 0.035 Ω and reactances of 16 Ω and 0.15 Ω respectively.

The low voltage winding is connected to a load of (4+j3) Ω. Neglect no load current.

Determine

(i) Currents in the two windings (ii) Secondary terminal voltage

(iii) % voltage regulation (iv) Power consumed by the load. (04)

5B. With a neat sketch explain the working of star-delta starter. (03)

5C. Why single phase Induction motor is not self starting? Explain any one method to make

it self starting? (03)

6A. With a neat sketch explain operation of attraction type Moving Iron type instrument. (04)

6B. Write a note on applications of Autotransformer. (02)

6C. A 3 phase, 4 pole, 50 Hz, 400 V Star connected Induction Motor runs at a slip of 5%

when drawing a line current of 15 A at 0.8 power factor lagging. The stator losses total

250 Watts and friction and windage losses 100 Watts. Determine

(i) Rotor speed (ii) Rotor copper losses

(iii) Gross torque (iv) Efficiency (04)

Department of Electrical and Electronics Engineering, MIT, Manipal

Reg. No.:

MANIPAL INSTITUTE OF TECHNOLOGY, MANIPAL (A Constituent Institute of MAHE, Deemed University)

I SEMESTER BE (Common to All Branches) DEGREE MAKE-UP EXAMINATION

(Revised credit system)

08 January 2007

ELE 101: BASIC ELECTRICAL TECHNOLOGY


Note: Answer any FIVE full questions. Q1A. Explain briefly the conventional and non conventional energy sources. (03) Q1B. Consider the circuit shown in Fig Q1B. The switch is closed after 5msec. Hence determine

the expression for the current for t>5msec. (03) Q1C. Find r1, r2, Vx using node analysis for the circuit shown in Fig Q1C (04)

Q2A. For the circuit shown in Fig Q 2A, Find the current that flows through the impedance z1 using the loop analysis method. (03)

Q2B. A mild steel ring has a mean circumference of 600 mm and a uniform cross sectional area of 250 mm2

. Calculate the mmf required to produce a flux of 450 µWb. An air gap, 1.0 mm in length is now cut in the ring. Determine the flux produced if the mmf remains constant. Assume the relative permeability of the mild steel remain constant at 220. (03)

Q2C. A coil of resistance R and inductance L is connected in series with a capacitor C across a variable frequency source. The voltage is maintained constant at 300 mV and the frequency is varied until a maximum current of 5 mA flows through the circuit at 6 kHz. If under these conditions, the Q factor of the circuit is 105, calculate: (i ) the maximum voltage that can appear across the capacitor (ii) the values of R,L,C. (04)

Q3A. Determine the line currents in a star connected load supplied from a symmetrical 3 phase,

400 V system. The branch impedances of the load are 10 30AZ = ∠ °Ω , . Assume the phase sequence as ABC.

Z 10 45= ∠ °ΩB

CZ 10 60 = ∠ °ΩIf the two wattmeters W1 and W2 are connected in such way that their current coils are in line A and line C respectively and their potential coils are connected to line B, find their readings. (06)

Q3B. A single phase motor takes 10 A at a power factor of 0.866 lagging when connected to a 230 V, 50 Hz supply. Draw the power triangle indicating reactive power, active power and apparent power. A capacitance bank is now connected in parallel with the motor to raise the power factor to 0.95, what is the value of the capacitance. (04)

Q4A. Why starters are required for 3 phase induction motor? With a neat sketch, explain star delta

starter for a 3 phase induction motor. (05) Q4B. Explain the principle of operation of 3 phase induction motor.

A 415 V, 50 Hz, 8 poles, 3 phase induction motor has a output of 50 kW , when running at 720 rpm. The motor has a stator loss of 750 W and friction and windage losses are 1500 W. Determine the

(i) slip (ii) gross power in the rotor (iii) the rotor copper loss/phase (iv) efficiency (05)

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Department of Electrical and Electronics Engineering, MIT, Manipal

5A. Two coils with a coefficient of coupling of 0.5 between them are connected in series so as to magnetize (i) in the same direction (ii) in the opposite direction. The corresponding values of equivalent inductance for (i) is 1.9 H (ii) 0.7 H. Find the self inductance of each coil, mutual inductance between the coils. (03)

5B. Explain the principle of operation of a stepper motor. (03) 5C. Derive an equation for the induced e.m.f. in the primary of the transformer.

The primary winding of a transformer has 300 turns. When connected across a 220 V, 50 cps supply. It draws a current of 5A at a power factor of 0.25 on no load. Determine the following (i) The maximum value of the core flux. (ii) Magnetizing reactance. (iii) Core loss component of resistance. (04)

6A. Explain with a neat sketch, explain the construction of single phase energy meter. (05) 6B. The applied voltage in a parallel RLC circuit is given by

)4

t5000sin(50v π+=

If the values of R, L, C be given as 20Ω, 1.6mH and 20 µF, find the expression for the current supplied by the source and draw the phasor diagram. (05)

100V

10Ω

10Ω

0.1H

Fig. Q1B

+-

-

+

Vx

10V3A

0.15Ω

1Ω

r2

10A

5A

r1

FigQ1C

°∠050

z1= (2+j5)Ω

+

10Ω

2Ω

4Ω

10Ω5A

Fig. Q2A

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