Cork Institute of Technology
Bachelor of Engineering in Electrical Engineering – Award
(EELEC_7_Y3)
Autumn 2008
Electrical Engineering
(Time: 3 Hours)
Instructions
Answer any FIVE questions.
All questions carry equal marks.
In markings scripts, the examiner will take
account of clarity of expression and
communication skills.
Examiners: Mr. J. Buckley
Mr. M. Hennessy
Prof. E. McQuade
Q1. (a) “All electrical equipment using alternating current is designed to use a voltage with a
clean and regular sine wave. However, in present day networks, this type of curve is
extremely rare. Harmonic frequencies create distortions in the sine wave, causing
interference to equipment connected to the network”.
Discuss the above in the context of the common harmonies which stress networks.
(10 marks)
(b) An e.m.f in which the amplitude of the fundamental (frequency 50 Hz) is 500 volts, and
which contains a 21st harmonic having an amplitude of 1 per cent of the fundamental,
exists in a circuit in which R = 2 ohms, L = 0.115 henry, and C = 0.2 microfarad.
Calculate the amplitude of (i) the harmonic current and the harmonic p.d. across the
capacitor (ii) the fundamental current. (10 marks)
Q2. (a) Discuss the importance of power factor correction in a.c. systems. (10 marks)
(b) A 415V, 50HZ, three-phase motor takes a line current of 15A when operating at a
lagging power factor of 0.65. When a capacitor is connected across the motor terminals,
the line current is reduced to 11.5A. Calculate the kVAr rating and the capacitance per
phase of the capacitor bank for: (a) star connection and (b) delta connection. Find also
the new overall power factor. (10 marks)
