JNTU Kakinada B-Tech 2-1 R1621022 ELECTRICAL MACHINES – I R16 May 2018 Question Paper

Code No: R1621022
II B. Tech I Semester Supplementary Examinations, May – 2018
ELECTRICAL MACHINES ? I (Electrical and Electronics Engineering)Time: 3 hours Max. Marks: 70
Note: 1. Question Paper consists of two parts (Part-A and Part-B) 2. Answer ALL the question in Part-A
3. Answer any FOUR Questions from Part-B
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PART ?A
1. a)Explain the significance of interpoles in DC machines (2M)
b)Write the applications of series, shunt and compound DC motors (3M)
c)What is Hopkinson?s test and give its merits over other tests (3M)
d)Why is the core of a transformer laminated? (2M)
e)Explain why parallel operation of transformer is necessary? (2M)
f)How to eliminate third harmonic voltages in polyphase transformers (2M)
PART -B
2. a) Why most practical energy conversion devices use magnetic field as the coupling
medium between electrical and mechanical systems? (7M)
b) State the electromagnetic phenomena useful for the electromagnetic energy
conversion in rotating electric machines (7M)
3. a) Explain why the emf generated in the armature of a DC Motor is called ?back
emf? (7M)
b) The armature of a 4 pole lap wound dc machine has core length = 45 cm,
diameter = 50 cm, total conductors = 500, speed =1200 rpm and current = 25 A.
For an average flux density of 0.6 T, find the electromagnetic (or gross
mechanical) power developed and the internal torque. (7M)
4. a) Explain with a neat connection diagram , the working of a Three point starter
used for a DC Shunt motor (7M)
b) A 230 V dc Shunt motor takes 36 A at full load. Find the back emf on full load if
the resistances of motor armature and shunt field windings are 0.26 ohms and 130
ohms respectively (7M)
5. a)Explain the operation of transformer on no load with a neat vector diagram (7M)
b) A 220 V , 2.8 KVA single phase transformer has an iron loss of 120 W at
45 Hz and 70 W at 35 Hz. Find the hysteresis and eddy current losses at 50 Hz. (7M)
6. a)What is sumpner?s test and explain its principle with a neat circuit diagram (7M)
b) A single phase, 25 KVA, 2000/200V transformer has iron loss is 350W and full
load copper loss is 400W. Calculate the efficiency at unity power factor on full
load and half load? (7M)
1 of 2
SET – 1
R16

Code No: R1621022
II B. Tech I Semester Supplementary Examinations, May – 2018
ELECTRICAL MACHINES ? I (Electrical and Electronics Engineering)Time: 3 hours Max. Marks: 70
Note: 1. Question Paper consists of two parts (Part-A and Part-B) 2. Answer ALL the question in Part-A
3. Answer any FOUR Questions from Part-B
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PART ?A
1. a)Explain the significance of interpoles in DC machines (2M)
b)Write the applications of series, shunt and compound DC motors (3M)
c)What is Hopkinson?s test and give its merits over other tests (3M)
d)Why is the core of a transformer laminated? (2M)
e)Explain why parallel operation of transformer is necessary? (2M)
f)How to eliminate third harmonic voltages in polyphase transformers (2M)
PART -B
2. a) Why most practical energy conversion devices use magnetic field as the coupling
medium between electrical and mechanical systems? (7M)
b) State the electromagnetic phenomena useful for the electromagnetic energy
conversion in rotating electric machines (7M)
3. a) Explain why the emf generated in the armature of a DC Motor is called ?back
emf? (7M)
b) The armature of a 4 pole lap wound dc machine has core length = 45 cm,
diameter = 50 cm, total conductors = 500, speed =1200 rpm and current = 25 A.
For an average flux density of 0.6 T, find the electromagnetic (or gross
mechanical) power developed and the internal torque. (7M)
4. a) Explain with a neat connection diagram , the working of a Three point starter
used for a DC Shunt motor (7M)
b) A 230 V dc Shunt motor takes 36 A at full load. Find the back emf on full load if
the resistances of motor armature and shunt field windings are 0.26 ohms and 130
ohms respectively (7M)
5. a)Explain the operation of transformer on no load with a neat vector diagram (7M)
b) A 220 V , 2.8 KVA single phase transformer has an iron loss of 120 W at
45 Hz and 70 W at 35 Hz. Find the hysteresis and eddy current losses at 50 Hz. (7M)
6. a)What is sumpner?s test and explain its principle with a neat circuit diagram (7M)
b) A single phase, 25 KVA, 2000/200V transformer has iron loss is 350W and full
load copper loss is 400W. Calculate the efficiency at unity power factor on full
load and half load? (7M)
1 of 2
SET – 1
R16

Code No: R1621022

7. a)Explain open delta (or V ? V ) connection with neat diagrams (7M)
b) In a Sumpner?s test on two identical single ? phase transformers rated 750 kVA,
11/0.5 kV, 50 Hz the wattmeter reading on h.v side is 8000 W and on the l.v side
is 16000W. Find the efficiency of each transformer on half full load and 0.78
power factor. What will be its maximum efficiency (7M)

2 of 2
SET – 1
R16