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Electrical Machines I Question Paper - Jun 17 - Electrical And Electronics (Semester 3) - Jawaharlal Nehru Technological University (JNTUH)
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written 6.9 years ago by gravatar for teamques10 teamques10 ★ 70k

Electrical Machines I - Jun 17

Electrical And Electronics (Semester 3)

Total marks: 80
Total time: 3 Hours INSTRUCTIONS
(1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Draw neat diagrams wherever necessary.

PART_A (25 MARKS)

1.a. Give an expression for force in single excited machine.
(2 marks) 00

1.b. Difference between electrical and magnetic circuit.
(3 marks) 00

1.c. Give the function of yoke and pole core in a D.C. machine.
(2 marks) 00

1.d. Differentiate between Lap and Wave winding in a D.C. machine.
(3 marks) 00

1.e. Define critical speed in D.C. Generator. Gives its significance.
(2marks) 00

1.f. Define critical resistance in D.C. Generator. Give its significance.
(3 marks) 00

1.g. Give the merits and demerits of word-leonard speed control method.
(2 marks) 00

1.h. Give the importance of OLR and NVC in 3-point starter.
(3marks) 00

1.i. Give the condition for maximum efficiency in a D.C. motor.
(2 marks) 00

1.j. Enumerate the various losses in D.C. Machine.
(3 marks) 00

PART_B (50 MARKS)

2.a. A still ring has a mean diameter of 20cm, a cross section of 25cm³ and radial air get of 0.8mm cut across it. When excited by a current of 1A through a coil of 1000 turns wound on the ring core, it produces an air gap flux of 1mWb. Negligible leakage and fringing calculate (i) related permeability of Steel and (ii)total reluctance of the magnetic circuit.
(5 marks) 00

2.b. State and explain the two kinds of power losses that occur when a magnetic material undergoes cyclic magnetization.
(5 marks) 00

OR

3.a. A straight conductor of 1.5m length carries a current of 40A. It is lying at right angle to a uniform magnetic flux density of 0.8T. find (i) the force is developed on the conductor (ii) the power required to drive the conductor at a uniform speed of 25m/second and (iii) the emf included in the conductor.
(5 marks) 00

3.b. Drive an expression for torque produced by doubly excited system.
(5 marks) 00

4.a Drive the e.m.f. equation of D.C. generator.
(5 marks) 00

4.b. What is armature reaction? Explain with neat diagram.
(5 marks) 00

OR

5.a. A separately excited DC generator running at 1000 rpm supplied 110A at 220V to a resistive load if the load resistance remain constant, what will be the load current if the speed is reduced by 800 rpm? Armature resistance is 0.02$\Omega$/ field current is unaltered assume a voltage drop of 1V per brush. Ignore the effect of armature reaction.

(5 marks) 00

5.b. Explain different methods to improve communication in d.c. machine.
(5 marks) 00

6.a. Explain the procedure to obtain magnetization, external and internal characteristics of a series generator.
(5 marks) 00

6.b. Explain the procedure to connect two series generators to share load in proportion.
(5 marks) 00

OR

7.a. Classify DC machines according to forms of excitation and draw its circuit diagram.
(5 marks) 00

7.b. The relation between excitation current and emf generated by DC shunt wound generator running on open circuit at 850 rpm in as following: Excitation (Amperes): 2 3 4 5 6 EMF (Volts): 68 87 100 109 112 The shunt field resistance is 22.2$\Omega$/. Find the voltage at the terminals of the machine when it run at 850 rpm self excited.
(5 marks) 00

8.a. Explain the constructional details of a d.c. motor with a neat sketch.
(5 marks) 00

8.b. A DC motor takes an armature current of 110A at 480V. The armatur circuit resistance is 0.2 $\Omega$/. The machine has 6 pole and the armature is lap connected with 864 conductors. The flux per pole is 0.05Wb.Calculate (i) speed (ii) Torque development in the armature.
(5 marks) 00

OR

9.a. Explain the working of a 4-point starter.
(5 marks) 00

9.b. Discuss different methods of speed control of a DC motor.
(5 marks) 00

10.a. Explain the swinburne's test with the help of a neat diagram to find out the efficiency of a DC Machine. what are the main advantage and disadvantage of this test?
(5 marks) 00

10.b. A 230V, 10 HP motor takes a full load line current of 40A. The armature and field resistance are 0.2$\Omega$/ and 230$\Omega$/ respectively. The total brush contact drop is 2V, and the core and the friction losses are 380W. Calculate the efficiency of the motor. Assume that the stray load loss is 1% of the rated output.
(5 marks) 00

OR

11.a. A field's test on two mechanically coupled DC series motor (with their field winding counted in series) give the following test data: motor :Armature current 50A Armature voltage = 500V; field winding voltage drop 38V. Generator: Armature current = 38A Armature voltage = 400V; Field winding voltage drop = 36V. Resistance of each armature is 0.2$\Omega$/. Calculate the efficiency of each machine at this load.
(5 marks) 00

11.b. With a neat circuit diagram explain the Hopkinson's test.
(5 marks) 00

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