final exam paper for electrical systems

7/28/2019 Final exam paper for electrical systems

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Sri Lanka Institute of Information Technology

B.Sc. Eng. Honours Degree inEngineering

Final ExaminationYear 1, Semester 1 (2013)

EC102 - Electrical Systems

Duration: 2 Hours

July 2013

Instructions to Candidates:

This paper contains 4 questions on 5 pages (including the cover page).

Answer all questions

All questions carry equal marks. The marks assigned for each part of a question isindicated in brackets.

This examination accounts for 70% of the module assessment.

You are permitted to use CALCULATORS


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Question 1: (25 marks)

Application of the superposition theorem. The circuit given in Figure Q1a has a

current I flowing through the 8 resistor.

3 6

4

85A

12V

I

Figure Q1a

(a) Briefly explain the concept behind how a super mesh is formed using a circuit

diagram to illustrate. [2 marks]

(b) Identify and mark the super-mesh(s) in Figure Q1a. [2 marks]

(c) Write down the super mesh equation(s) for the circuit in Figure Q1a. Note that

the mesh currents and the resulting voltage directions you have assumed for the

analysis should be clearly shown. It is not necessary to write any of the other

circuit equations. [4 marks]

(d) Use superposition analysis method to compute the current I through the 8 resistor. Clearly indicate the mesh currents and voltages directions. [15 marks]

(e) If the 8 resistor was replaced by a load resistor RL, what should be its value

for maximum power to be transfered to the load. [2 marks]

Page 2 of 5


3/5


AC Theory. The circuit shown in Figure Q2a has E= 1000 V and I = 200200 mA.

300

100

600

200

100

E I

Figure Q2a

(a) Use the nodal analysis technique to determine the current and voltage across

the 200 impedance. Note: you may simplify the circuit to reduce the number

of loops if necessary. The directions of the currents and voltages of the network

under analysis should be clearly shown. [12 marks]

(b) Using the above network equations, show how the Cramers rule can be applied

to find the voltage across the 100 resistance. [4 marks](c) Sketch the phasor diagrams of the current and voltage across the 200 impedance

and the 100 resistance respectively. [3 marks]

(d) Calculate the active power, reactive power and the total power for the 200

impedance. [3 marks]

(e) Calculate the active power, reactive power and the total power for the 100

resistance. [3 marks]

Page 3 of 5


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Polyphase circuits. The three phase circuit shown in Figure Q3a shows a -connected

source delivering power to a balanced -connected load. The load impedance Z0 =

2 j6 . For a phase sequence of ACB, the line voltages of the source are EAB =

220

00

V, EBC = 220

1200

V, and ECA = 220

1200

V

3, 3 Wire

-ConnectedSource

Phase sequence: ACB

A

Z0

Z0

Z0

C

B

Figure Q3a

(a) Calculate the phase currents at the load. [6 marks]

(b) Estimate the line currents of the network. [3 marks]

(c) Sketch the phasor diagram for the currents of the three phase -connected

source. [6 marks]

(d) Calculate the average power to each phase and the total load. [4 marks]

(e) Calculate the reactive power to each phase and the total load. [4 marks]

(f) Find the power factor of the load. [2 marks]

Page 4 of 5


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Power supply design. A DC power supply with a capacitor filter is shown in Figure Q

4b. D1 to D4 are diodes having a forward drop of 0.7 V. The expected DC output

at the load RL is 12 V with a maximum allowable ripple of 0.2 V. The input to the

power supply Vin is the 230 V 50 Hz AC line voltage.

Vin

+

n1 n2

D1

D2

D4

D3

Rs

C RL

Figure Q4b

(a) Explain what is eddy current loss in a transformer, and explain how it will

influence the design of a DC power supply. [5 marks]

(b) If the secondary voltage of the transformer is Vs = vm sint, show that the ripple

voltage Vr at the load RL is given by [10 marks]

Vr =10vm 14

CRLmV

(c) Compute a suitable value for the capacitor C if the load RL can vary from 10K

to 100K. Give reasons for this selection. [5 marks]

(d) What is the most suitable transformer turns ratio n1/n2 for the designed power

supply. [5 marks]

End of the Question Paper

Formula Sheet

Finding the determinant of a 2

2 matrix

a11 a12a21 a22

= a11a22 a21a12.

Finding the determinant of a 3 3 matrix

a11 a12 a13a21 a22 a23a31 a32 a33

= a11

a22 a23a32 a33

a12

a21 a23a31 a33

+ a13

a21 a22a31 a32

Page 5 of 5

final exam paper for electrical systems

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