Electrical Machines-I | Nagpur University | Winter-17

B.E. (Electrical Engineering (Electronics & Power) /
Power Engineering) Fourth Semester (C.B.S.)

Electrical Machines – I

 NRJ/KW/17/4421/4450
Time : Three Hours
Max. Marks : 80

Notes : 1. All questions carry marks as indicated.
2. Solve Question 1 OR Questions No. 2.
3. Solve Question 3 OR Questions No. 4.
4. Solve Question 5 OR Questions No. 6.
5. Solve Question 7 OR Questions No. 8.
6. Solve Question 9 OR Questions No. 10.
7. Solve Question 11 OR Questions No. 12.
8. Due credit will be given to neatness and adequate dimensions.
9. Assume suitable data whenever necessary.
10. Illustrate your answers whenever necessary with the help of neat sketches.

1. a) Draw and explain phasor diagram of single phase transformer for leading P. F. [06 M]

b) Obtain the equivalent circuit of a 200 / 400 V, 50 Hz 1- phase transformer from the
following test data:
O. C. test – 200V, 0.7 A, 70W (on LV side)
S. C. test – 15V, 10 A, 85W – (on HV side)
Calculate the secondary voltage when delivering 5kW at 0.8 P.F. lagging, the primary voltage being 200V.  [07 M]

OR

2. a) Explain the short circuit test to calculate the equivalent circuit parameters of 3-phase 100 kVA D/Y, 11 kV/400V distribution transformer. Draw the circuit diagram and approximate ratings of the instruments used.  [07 M]

b) Explain the following terms along with the phasor diagram and connection diagram any two.[06 M]
i) Y_y0
ii) D_d6
iii) Y_d1

3. a) Explain with the help of connection & phasor diagram how SCOTT connections are used to obtain two phase supply from 3-phase supply mains. What are its application.  [07 M]

b) 3Ω 6.6 K.V. supply feeds 2 scott connected transformer to supply two furnaces at 100V on secondary side If the load on teaser secondary is 200 kW and on main transformer secondary is 100 kW both at UPF. Calculate the primary I/P currents. Also draw phasor diagram and connection diagram.
[07 M]

OR

4. a) Enumerate various conditions for parallel operation of two 3 phase transformer. [03 M]

b) Write short note on method of cooling of transformer. [04 M]

c) Describe ‘Back to Back’ test for determining the regulation and efficiency of a pair of transformer.  [07 M]

5. a) Draw and explain in brief following characteristics curves for D. C. series and shunt motor.   [06 M]
i) Torque – Armature current characteristics.
ii) Speed – Armature current characteristics.
iii) Speed – Torque characteristics.


b) Explain methods to improve commutation in d. c. generators. [07 M]

OR

6. a) Explain the term ‘Armature reaction’ in d. c. machine.  [06 M]

b) A 250V D. C. shunt motor has an armature resistance of 0.6 Ω and field resistance of 250 Ω when driving at 650 r.p.m. at constant load torque and armature takes 20A what resistance must be inserted in the shunt field circuit to raise the speed from 650 rpm to 850 rpm? Assume linear magnetic circuit in the machine.  [07 M]

7. a) A 746 kW 3-phase 50Hz 16 pole I. M. has a rotor impedance of (0.02 + j 0.15) /ph at stand still. Full load torque is obtained at 360 rpm
find:  [06 M]
i) Ratio of maximum torque to full load torque.
ii) Speed at maximum torque
iii) Rotor resistance to be added to get the maximum starting torque.

b) Draw and briefly explain the torque slip curve for 3 Φ I.M. in three different regions and explain the meaning of negative slip. [07 M]

OR

8. a) Prove that for an Induction motor. \frac{Rotor \; Cu \; Losses}{Gross \; Rotor\; output}=\frac{S}{1-S}      [06 M]

b) Explain in short the No load and blocked rotor test on 3Φ I. M. to find the parameters of equivalent circuit with necessary equations.  [07 M]

9. a) Write various braking methods of three phase I. M. and explain plugging in detail. [07 M]

b) Explain crawling & cogging phenomenon in Induction motor.  [07 M]

OR

10. a) Name various methods of speed control of I. M. and explain speed control of I. M. by changing I/P frequency.  [07 M]

b) What is necessity of starter in case of 3  I. M.? Explain working of Autotransformer starting in detail. [07 M]

11. a) Explain briefly the double field revolving theory to understand the concept of a single phase Induction motor. Draw torque -slip characteristics also.  [07 M] 

b) Explain a shaded Pole single phase Induction motor. With its applications.  [06 M]

OR

12. a) Draw a neat sketch explaining working of capacitor start capacitor run 1Φ  Induction motor. Draw the torque -speed characteristics also.  [07 M]

b) Explain split-phase 1Φ Induction Motor.   [06 M]