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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 1 Examinations 2010/
Electrical Circuit Analysis
Module Code: ELEC
School: Electrical and Electronic Engineering
Programme Title: Bachelor of Engineering in Electrical Engineering
Programme Code: EELEC_7-Y
External Examiner(s): Mr. Gerard Beecher Dr. Maeve Duffy Internal Examiner(s): Noel Mulcahy
Instructions: Answer all questions. Each full question is worth 20 marks.
Duration: 2 hours
Sitting: Winter 2010
Requirements for this examination: Use of a Scientific Calculator is permitted.
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt please contact an Invigilator.
Q1.
a. Determine the admittance (in complex form), of the following impedances. i. 15 <-70° Ω. ii. (12 + j6) Ω. (2 marks)
b. Using the following circuit determine i. The current flowing in branch 1 (I 1 ) ii. The current flowing in branch 2 (I 2 ) iii. The impedance of branch 1 (Z 1 ) iv. The impedance of branch 2 (Z 2 ) v. The total circuit admittance (YT) vi. The total circuit impedance (ZT) vii. The total circuit current (IT) viii. The total current phase angle ix. The total power consumed
200 F
50mH
15 200 (^0) V
50Hz
IT
I 1 I^2
20
(18 marks)
Figure Q
Q3.
a. A relay coil has an inductance of 75H and the current required to operate the relay is 12mA. When the coil is connected to a 40V d.c. power supply the steady state current is 200mA. The operating time between, switch-on and the start of the relay operation is 23.2ms. Calculate: i. the resistance of the coil ; (3 marks) ii. the time constant; (3 marks)
b. A 50Ω resistor is placed in series with the coil of part (a) i. Calculate the new time constant (2 Marks) ii. The operating time of the relay. (8 Marks)
c. The following filter was constructed in a Lab Experiment.
100Vrms @ Variable Frequency
2 F
V out
300
Figure Q
Determine the output voltage when a 100V sinusoidal supply of the following frequencies is applied. i. 1 Hz. ii. 1000 Hz. (4 marks)
Q4.
a. Explain why it is important to correct the power factor of an electrical installation to a reasonable value. (4 marks)
b. An engineering workshop is supplied at single-phase and has the listed loads connected. 5.2kW of heating at PF = 0.97 lag; 2.7kW of fluorescent lighting at PF = 0.67 lag; 3.7kW of motors on average PF = 0.67 lag; 3kVA of plant at PF = 0.73 lag. Calculate: i. the total load in kW, (5 marks) ii. the total kVAr supplied, (5 marks) iii. the total kVA supplied, (3 marks) iv. the overall PF of the installation. (3 marks)
Q5.
The following loads are connected to a 3-phase, 4-wire, 400/230V 50Hz supply: Between R line and Neutral: A 15 load at a power factor of 1 Between S line and Neutral: A 18.25 load at a power factor of 0.92 leading Between T line and Neutral: A 12.2 load at a power factor of 0.83 lagging
a. Calculate the phase current for each load. (3 marks) b. Calculate the phase angle for each load. (3 marks) c. Determine the current flowing in the Neutral conductor (10 marks) d. Suggest and sketch a suitable arrangement of Wattmeter’s to measure the power in the load above. (4 marks)
^
t iL I e [A]
^
t i (^) L I. e [A]
Delta – Star Transform
A B C
A B Z Z Z
Z Z Z
1 , A B C
B C Z Z Z
Z Z Z
2 , A B C
A C Z Z Z
Z Z Z
3
Star – Delta Transform
2
1 * 2 2 * 3 3 * 1 Z
Z Z Z Z Z Z Z (^) A , 3
1 * 2 2 * 3 3 * 1 Z
Z Z Z Z Z Z Z (^) B , 1
1 * 2 2 * 3 3 * 1 Z
Z Z Z Z Z Z ZC
Growth of Current in an Inductor in an RL Network
Decay of Current through an Inductor in an RL Network
