LinkedIn Insight Electrical Machines-I | Nagpur University | Summer-2018 - Grad Plus

Electrical Machines-I | Nagpur University | Summer-2018

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

Electrical Machines – I

NJR/KS/18/4421/4450
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. Assume suitable data whenever necessary.
9. Illustrate your answers whenever necessary with the help of neat sketches.
10. Use of non programmable calculator is permitted.

1. a) A 40 – kVA distribution transformer has iron loss of 500 – W and full load copper loss of
500 – W. The transformer is supplying a lighting load (unity p.f.) The load cycle is as under:
Full load for 4 – hours;
Half load for 8 – hours;
No load for 12 – hours;
Calculate :
i) Efficiency of transformer at half full load. [02 M]
ii) ALL DAY EFFICIENCY of transformer. [05 M]

b) Draw a phasor diagram of single phase transformer supplying a ‘LEADING POWER FACTOR’ Load. [06 M]

OR

2. a) A 4 – kVA, 200 / 400 V, 50 Hz single phase transformer gave the following test results.
No load : (Low Voltage Data ) : 200 – V, 0.7 – A, 60 W
Short Circuit Test : (High Voltage Data) : 9V, 6 – A, 21.6 – W
Calculate :
i) the magnetising current and the component corresponding to iron loss at normal frequency and voltage. [03 M]
ii) the secondary terminal voltage and regulation of transformer at 0.8 LEADING POWER Factor.  [03 M]

b) Explain the following terms alongwith the PHASOR DIAGRAM and CONNECTION DIAGRAM.     [06 M]
i) Dd0 ii) Yd11

3. a) Explain the method of 3 phase to 2 phase conversion and prove that neutral divides the TEASER PRIMARY in the ratio 1 : 2.[07 M]

b) A SCOTT connected transformer set supplies two electric furnaces, A and B at 110 – V each from 3 – phase, 4400 – V system. If the furnace connected to teaser transformer secondary takes 270 – kW at upf and other connected to main transformer takes 450 – kW at upf. Find the LINE CURRENTS taken from a 3 – phase supply.   [07 M]

OR

4. Write short notes on :

a) ‘BACK TO BACK’ test for determining regulation & efficiency of a pair of similar transformers.    [06 M]

b) Conditions for parallel operation of two 3-phase transformers.  [04 M]

c) Necessity of cooling of transformer & ONAF method of cooling.  [04 M]

5. a) A 250 V shunt motor runs at 1000 rpm while taking a current of 25 A. The resistance of the armature is 0.2Ω & the resistance of the shunt field is 250Ω. Calculate the speed when loaded to take a current of 50 A. If the armature reaction weakens the field by 3%, the voltage drop per brush is 1 volt. Determine torque in both the cases.  [07 M]

b) Two different speed control methods of a d.c. series motor. Explain any two speed control methods of a d.c. series motor.  [06 M]

OR

6. a) Draw and explain magnetization curve for d.c. shunt generators at 2 – different speeds.   [05 M]

b) Explain the causes of failure of voltage build up of d.c. shunt generator.  [04 M]

c) Explain why the terminal voltage of d.c. shunt generator falls when it is loaded. [04 M]

7. a) A 3 – phase induction motor having 6 – pole, STAR connected stator winding runs on 240 -V, 50 Hz supply. The rotor resistance and standstill reactance are 0.12Ω and 0.85Ω per phase. The ratio of STATOR to ROTOR TURNS is 1.8 and full load slip is 4%.
Calculate :
i) the developed torque at full load.
ii) the maximum torque.
iii) the slip and speed at maximum torque.
[08 M]

b) Sketch the torque – slip curve of 3 – phase Induction motor and explain the performance for the following conditions :
i) Rotor resistance is doubled, keeping stator voltage and frequency unchanged.
ii) the rotor is made to run above the synchronous speed.  [06 M]

OR

8. a) The rotor of a 4 – pole, 50 Hz 3-phase SLIP RING Induction Motor has a resistance of 0.25Ω / phase and runs at 1440 rpm on full load. Calculate the external resistance per phase which must be added to lower the speed to 1200 rpm. The torque remaining constant in both the cases.  [05 M]

b) For a 3 – phase Induction Motor, show that the per phase input power to rotor can be divided in the ratio of :
1 : S : (1 – S) = Pg : Rotor Ohmic Loss : Pm.  [04 M]

c) Explain Blocked Rotor test for a 3 Phase Induction Motor. [05 M]

9. a) The full load power input to a 3-phase Induction Motor is 50 kW and the slip is 3%.  [06 M]
Neglecting Stator Losses, Calculate :
i) Full load rotor Cu-losses per phase.
ii) Total mechanical power developed.


b) Explain speed control of Induction motor by stator voltage control with necessary equations and characteristics. [07 M]

OR

10. Write short notes on :

a) Plugging of an Induction Motor. [07 M]

b) STAR – DELTA Starter of 3 – phase Induction motor.  [06 M]

11. a) Explain briefly ‘TWO FIELD REVOLVING THEORY’ to understand the concept of single phase Induction motor.  [07 M]

b) Explain “CAPACITOR START INDUCTION RUN” single phase Induction motor. [06 M]

OR

12. a) Explain split phase Induction motor. [07 M]

b) Justify : Single phase Inductor motor is not self starting.  [06 M]

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