Solved model Question Paper

SOLVED MODEL QUESTION PAPER

( [As Per New Syllabus]

 Electrical Machines - I

 Semester - III (EEE)

Time: Three Hours]             [Maximum Marks: 100

Answer ALL Questions

PART A (10 x 2 = 20 Marks)

Q. 1 What is leakage flux? How it is incorporated in machine models? (Refer Two Marks Q. 24 of Chapter - 1)

Q. 2 Write an expression for the stored energy in the magnetic field. (Refer Two Marks Q. 17 of Chapter - 2)

Q. 3 State the e.m.f. equation of a d.c. machine. (Refer Two Marks Q. 13 of Chapter 3)

Q. 4 What are the conditions for parallel operation of D.C. generators? (Refer Two Marks Q. 34 of Chapter - 3)

Q. 5 How does 4-point starter differ from 3-point starter? (Refer Two Marks Q. 30 of Chapter - 4)

Q. 6 Why d.c. series motor is not suitable for belt driven loads? (Refer Two Marks Q. 33 of Chapter - 4)

Q. 7 The e.m.f. per turn for a single phase 2200/220 V, 50 Hz transformer is 11 V. Calculate the number of primary and secondary turns. (Refer Two Marks Q. 28 of Chapter - 6)

Q. 8 What is the application of equivalent circuit of a single phase transformer? (Refer Two Marks Q. 12 of Chapter - 6)

Q. 9 What are the advantages of autotransformer over two winding transformer? (Refer Two Marks Q. 3 of Chapter - 7)

Q. 10 State the advantages of three phase transformers. (Refer Two Marks Q. 6 of Chapter - 7)

PART B (5 × 13 = 65 Marks)

Q. 11 a) i) Derive the expression for the magnitude of the dynamically induced e.m.f. (Refer section 1.22) [8]

ii) The field winding of dc electromagnets is wound with 800 turns and has a resistance of 40 Ω  when exciting voltage is 230 volt, magnetic flux around the coil is 0.004 Wb. Calculate self inductance and energy stored in magnetic field. (Refer example 1.26.3)  [5]

OR

b) i) The magnetic flux density on the surface of an iron face is 1.8 T which is a typical saturation level value for ferromagnetic material. Find the force density on the iron face. (Refer example 2.6.1) [7]

 ii) Derive expression for co-energy in a multiply excited magnetic field system. (Refer section 2.7)[6]

Q. 12 a)  i) Briefly explain the methods to overcome the adverse effects of the armature reaction. (Refer section 3.11) [13]

OR

b) i) A 4-pole, lap connected D.C. machine has 540 armature conductors. If the flux per pole is 0.03 Wb and runs at 1500 r.p.m., determine the e.m.f. generated. If this machine is driven as a shunt generator with the same field flux and speed, calculate the terminal voltage when it supplies a load resistance of 40 Ω. Given armature resistance as 2 and shunt field circuit resistance as 450 Ω. Also find the load current. (Refer example 3.19.2) [7]

ii) Draw and explain load characteristics of d.c. series generator. (Refer section 3.26)[6]

Q. 13 a) i) Derive from the first principle an expression for the torque developed in a d.c. motor. (Refer section 4.6) [7]

ii) A 230 V d.c. shunt motor, takes an armature current of 3.33 A at rated voltage and at no load speed of 1000 r.p.m. The resistances of the armature circuit and field circuit are 0.3 Ω and 160 Ω respectively. The line current at full load and rated voltage is 40 A. Calculate, at full load, the speed and the developed torque in case the armature reaction weakens the no load flux by 4%. (Refer example 4.11.1)[6]

OR

b)  i) With neat sketch, explain the function of 3 point starter. (Refer section 4.17)

[7]

ii) A 500 V d.c. shunt motor running at 700 r.p.m. takes an armature current of 50 A and effective armature resistance is 0.4 Ω.  What resistance must be placed in series with the armature to reduced the speed to 600 r.p.m., the torque remaining constant. (Refer example 4.20.3) [6]

Q. 14 a) i) Explain in detail step by step the procedure to draw the equivalent circuit of transformer. (Refer section 6.13)[7]

ii) A 100 kVA, 6.6 kV/415 V single-phase transformer has an effective impedance of (3+j8)Ω referred to HV side. Estimate the full-load voltage regulation at 0.8 pf lagging and 0.8 pf leading. (Refer example 6.14.3) [6]

OR

b) i) Explain the back to back method or Sumpner's test of testing of two identical single phase transformers. (Refer section 6.19)[7]

ii) A 200 kVA, single phase transformer has an efficiency of 98 % at full load 0.8 p.f. lag. If the maximum efficiency occurs at three quarters full load, calculate the iron loss and full load copper loss. (Refer example 6.16.4) [6]

Q. 15 a) With proper connection and phasor diagrams describe the different ways of connecting three phase transformers. (Refer section 7.4) [13]

OR

b) i) Prove that the amount of copper saved in autotransformer is (1- K) times that of ordinary transformer (Refer section 7.1)[5]

ii) With the help of phasor diagram explain how 2 phase supply can be obtained from 3 phase supply using Scott connection. (Refer section 7.6) [8]

PART C (1 × 15 = 15 Marks)

Q. 16 a) The no load test of a 44.76 kW, 220 V d.c. shunt motor gave the following results: Input  current = 13.25 A, Field current = 2.55 A, Resistance of armature at 75 °C = 0.032 Ω Brush drop = 2 V Estimate the full load current and efficiency. (Refer example 5.5.1)[15]

OR

b) Draw a phasor diagram to represent conditions in a single phase transformer supplying load at: biort or 1) Unity p.f. 2) Lagging p.f. and 3) Leading p.f. (Refer section 6.12) [15]