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Cork Institute of Technology
Bachelor of Engineering in Electrical Engineering – Award
(National Diploma in Engineering in Electrical Engineering – Award)
(NFQ – Level 7)
Summer 2005
Electrical Engineering
(Time: 3 Hours)
Instructions Answer any FIVE questions. All questions carry equal marks.
Examiners: Mr. J. Buckley Mr. M. Ahern Mr. K. O Connell
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 most common harmonics which stress networks i.e. the third harmonic, the fifth harmonic and the seventh harmonic. (10 Marks) (b) an e.m.f. represented by:
e = 100Sin 314t + 20Sin (942t + 3
π (^) )
2 π (^) )
exists in a circuit in which R = 10 ohms, L = 0.02H. Determine the total power and the power factor. (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)
Q3.(a) Using differentiation determine the value of slip for maximum torque of a slip ring induction motor. T = Torque R 2 = Rotor Resistance X 2 = Rotor Reactance (10 Marks) (b) A 400V, 6-pole, 50 Hz, 3-phase induction motor has a rotor resistance of 0.15 ohm/phase and a rotor resistance of 0.15 ohm/phase and a rotor reactance at standstill of 0.7 ohms per phase. The rotor is star connected and at standstill the emf between slip rings is 238V. Find: (i) The gross torque at the full load slip of 5 per cent. (ii) The full load power output if friction and windage losses are 500 watts. (iii) the maximum torque and the speed at which it occurs. (10 Marks)
Q4.(a) Discuss armature and tachogenerator feedback in the context of speed control of direct current (d.c.) motors. (6 Marks) (b) A 90kW, 500V compound wound d.c. motor, running unloaded at normal speed and voltage has an armature current of 3.5 amps and a shunt field current of 6 amps. The resistances of the windings are armature 0.1Ω, interpoles 0.05Ω and series field 0.03Ω. The voltage drop across the brushes is 2V. Calculate the power output and efficiency. (12 Marks)
Q5.(a) Discuss the conditions necessary for satisfactory operation of transformers in parallel.
(6 Marks) (b) With the aid of a neat sketch, outline a method for connecting the secondary side of two 1000 kVA 10kV/400 V distribution transformers to two sets of 400 V bus-bars. Such that under normal working conditions, the transformers are not connected in parallel, but in the event of the failure of one transformer, the remaining transformer may be connected to the two sets of bus-bars. (6 Marks) (c) Two transformers A and B are connected in parallel and supply a load of 120 kW at 3, Volts, P.F. 0.8 lagging. The respective resistances and reactances referred to the secondary of each transformer are R (^) A = 3.25Ω , XA = 4.5Ω, R (^) B = 2.2Ω, XB = 6.7Ω. Find the output power and power factor of each transformer. (8 Marks)
