xua.38 - Electrical Engineering

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HIGHER COLLEGE OF TECHNOLOGY

DEPARTMENT OF ENGINEERING SECTION: CAE EEE MIE

Final Assignment Semester: 2 A. Y: 2019 / 2020

Date of Assignment posting: 1st May 2020 Time: 12:00 PM 48 hours

Date of Uploading: 3rd May 2020 Time: 12:00 PM

Student Name

Student ID

Specialization Electrical Engineering

Level DSY

Course Name / Course Code Electrical Power Technology/EEPW2251

Section No. 1,2,3



Page 2 of 5

Question No.

Max. Marks

Obtained

Marks

Question No. Max. Marks Obtained

Marks

PART- 1 10

PART- 2 10

PART- 3 20

Sub-Total

Marks Sub-Total Marks

Grand Total

Marks ____ / 40

Course Lecturer: Ms. Sowmya Saraswthi P Second Marker:

Assignment instructions from Course Lecturer:

a) Type the theory and problems answers in word file

b) Type the theory answers in your own words.

c) Diagrams alone – draw neatly, scan and add in the word file.

Student’s Declaration: (to be filled by student)

Student Name: __________________ ID: _________ Signature: _________

(Digital Signature)

Page 3 of 5

PART – 1 (10 marks)

SHORT ANSWER QUESTIONS

Q 1. The efficiency of transformer is more, because ______________________.

a). It is a static device

b). It has two windings

c). It has only one core

d). It has more losses

[1]

Q 2.

The eddy current loss & hysteresis losses of a transformer at 50Hz are 50W & 40W

respectively. The core loss of the same transformer at 25Hz is ____

a). zero

b). 90W

c). 45W

d). 32.5W

[1]

Q 3. The speed of dc shunt motor is ___________________________.

a). Inversely proportional to field current.

b). Directly proportional to field current

c). Inversely proportional to field circuit resistance.

d). Directly proportional to armature resistance.

[1]

Q 4. Three identical capacitors with 10ohms capacitive reactance are connected in star

takes 1A current from supply. The active power taken by the load is __________.

a). Equal to zero

b). Equal to real power.

c). 20W

d). 20VAR

[1]

Q 5. Which of the following statement is true for a dc series motor?

a). Back e.m.f is greater than terminal voltage.

b). Voltage drop in the armature winding is same as field winding voltage drop.

c). Field winding is connected in parallel with armature.

d). None of the above

[1]

Page 4 of 5

Fill in the blanks (5 x 1 = 5marks)

Q 6. Three loads, each of resistance 30Ω are connected in star to a 415V, 3-phase

supply. The power factor of load is ________________.

[1]

Q 7. A 5kVA ideal step- up transformer with a turn’s ratio of 2:7 is fed from a 240V

supply. Its output current on no load is ______________.

[1]

Q 8. In _____________ wound generators, the generated emf is independent of number

of poles?

[1]

Q 9. The angle between voltages in R phase and B phase of a supply with RYB phase

sequence is ___________.

[1]

Q 10. The angle between primary voltage and core magnetic flux in a transformer under

noload condition is _______.

[1]

PART – B (10marks)

NOTE: USE DIAGRAM, GRAPHS IF ANY FOR ANY EXPLANATION and/or DERIVATION/S FOR

THE ANSWERS BELOW.

Q 11. Draw the phasor diagram of a step-up transformer connected to an incandescent lamp. [3]

Q 12. Why delta connected load dissipates more heat than when they are connected in star

across same supply?

[2]

Q 13. While conducting OC test on a single phase transformer, instead of connecting the ac

source to the primary of transformer, you are connecting the primary to a rectifier. What

will happen to the transformer?

[3]

Q 14. Explain how the power factor of a 3-phase delta connected load can be measured? Draw

the circuit diagram.

[2]

Page 5 of 5

PART – C (20 marks)

Numerical Questions

Q 15. A DC shunt generator is driven by a diesel motor. The shaft torque of the diesel motor is

35Nm, when the current through the armature of the generator is 20A. If the shunt field

regulator is adjusted so that the flux is increased by 15%, the torque decreases to 25Nm.

Determine the armature current at this new value of torque.

[3]

Q 16. A DC shunt generator generates 400V when running at 1800rpm.Determine how much

percent the flux has to increase per pole, to generate 500V when running at 20rps.

[2]

Q 17. A three phase star connected alternator delivers a line current of 65A to a balanced

delta connected load at a line voltage of 380V.calculate

(i) The phase voltage of alternator,

(ii) The alternator phase current

(iii) The load phase current

[2]

Q 18. A shunt motor having an armature circuit resistance of 1Ω and a field resistance of 125Ω,

connected to of 250V supply. Determine the efficiency of the motor when it takes 50A

from the supply, assuming constant losses amount to 500W.

[2]

Q 19. A delta-connected load consists of three identical coils each of resistance 3 Ω and

inductive reactance of 4 Ω.this load is connected to a star connected alternator through

wires. The resistance of each wire is 0.025 Ω.The power dissipated by each phase of the

load is 19.2kW. How much power will be dissipated in the wires? Also find the

alternator voltage.

[5]

Q 20. A 200kVA, 4000V/2000V, 50Hz single-phase transformer has 100 secondary turns. The

transformer primary winding has resistance and reactance of 1 Ω & 0.5 Ω respectively and

the transformer secondary windings has resistance and reactance of 0.25 Ω & 0.1225 Ω

respectively. Determine

(a) the copper losses in primary and secondary windings at full load,

(b) The maximum value of flux density in the core if the area of cross section is 300 cm2.

(c) The efficiency of the transformer at full load,( assume UPF), if the total core losses is

100W.

[6]

__________________ End of the Assignment __________________