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Electrical Circuits and Machines (143102/183102/233102)

P. Pages : 4

Time : Three Hours Max. Marks : 80

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Instructions to Candidates : 1. Do not write anything on question paper except Seat No. 2. Answer sheet should be written with black ink only. Graph or

diagram should be drawn with the same pen being used for writing paper or black HB pencil.

3. Students should note, no supplement will be provided. 4. Attempt any two sub-questions from each unit. 5. Assume suitable data wherever necessary and state the

assumption made 6. Diagrams / sketches should be given wherever necessary. 7. Use of Logarithmic table, drawing instruments & non-

programmable Calculators is permitted. 8. Figures to the right indicate full marks.

UNIT - I

1. a) i) Using Thevenins Theorem Calculate current flowing through the Load across terminals A & B & also Calculate power delivered to the load.

_ j10 4 A

10

_ j10

B

100 0

10

j10

4

*1739* Seat Number

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ii) Find the Nortons equivalent Network of terminals A and B of the circuit as shown below.

3 _ j4

2+j5

5

12 0

B

A

4

b) i) Derive the relationship between line voltage and phase voltage for star connected 3 phase Balanced load.

4

ii) A star Connected balanced load Take 7 Ampere of 0.8 Lagging power factor from 400 V power supply. Calculate loading of wattmeter W 1. If current coil is connected in R line and Voltage coil is connected in between R and Y line. For W 2 if current coil is in R line and Voltage coil is connected in between B & Y line. Phase sequence is R-Y-B and also draw net phasor diagram.

4

c) To derive and expression for total power Consumed by three phase balanced Delta connected inductive load using two wattmeter. Draw net circuit & phasor diagram.

8

UNIT - II

2. a) Explain why starter is necessary for DC motor? Explain with neat diagram the working of 4 -point starter?

8

b) i) Derive the speed equation of DC motor.

4

ii) Draw and explain following characteristics of DC series motor. a) Ta-Ia b) N-Ia c) N-Ta

4

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c) i) A shunt generator delivers 200 A of a terminal voltage of

220 volt the armature and shunt field resistance are 0.03

and 55 . The iron and friction losses is 800 W. Find i) EMF generated. ii) Copper Losses. iii) B.H.P of the prime more. iv) Electrical, Mechanical and Commertial efficiency.

4

ii) The armature winding of a 4 - pole, 250 V DC shunt motor is lap connected these are 120 slots each slots containing 8 conductors. The flux per pole is 20 mwb and current taken by motor is 25 A. The resistance of armature and field circuit are

0.1 and 125 respectively. If the rotational Losses amount to 810W find. i) Gross Torque. ii) Useful torque. iii) Efficiency.

4

UNIT - III

3. a) Draw open circuit and short circuit Test of single phase transformer and explain the steps to find out efficiency and voltage regulation from observation.

8

b) i) Explain the working principal of transformer.

4

ii) Draw the phasor Diagram for transformer on load of Lagging and Leading power factor with winding resistance?

4

c) Write a short notes on. i) Potential transformer. ii) Current transformer.

8

UNIT - IV

4. a) Derive and expression of emf equation of an alternator and calculation of Kc and Kd.

8

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b) i) Explain the principle of operation of an alternator.

4

ii) What are the different methods of starting synchronous motor and explain any one in detail?

4

c) Explain with vector diagram for synchronous motor with different excitations for various power factors.

8

UNIT - V

5. a) Explain working principle of 3 Induction motor and concept of rotating Magnetic field.

8

b) i) Draw and explain the torque-slip characteristics of 3 Induction motor?

4

ii) Write a short note on Star Delta starter.

4

c) i) With the help of neat sketch explain the working and applications of stepper motor.

4

ii) With the help of neat sketch explain the working and applications of Servo motor.

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4

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