Cork Institute of Technology
Bachelor of Engineering in Building Services Engineering – Award
(NFQ – Level 7)
Autumn 2007
Building Services (Electrical) Paper 2
(Time: 3 Hours)
Instructions
Answer FIVE questions.
All questions carry equal marks.
ETC.IRE Booklets required
Examiners: Mr. J. C. Duggan
Dr. M. Smyth
Mr. D. Leonard
Q1. (a) State the advantages of a three-phase a.c. system (5 Marks)
(b) Using phasor diagrams, or otherwise, explain the relationship between line and phase
voltages obtained from a star connected three phase source. (5 Marks)
(c) A delta connected motor is supplied from a star connected transformer. The line voltage
is 400 volts. The motor delivers an output of 30kW while operating at a power factor of
0.85 and an efficiency of 90%.
Determine the motor current and the corresponding transformer phase current.
(10 Marks)
Q2. A three-phase 415V 50Hz system has the following single phase and three phase loads
connected.
Single-phase:
R (L1) phase – 12 kW at p.f. 0.95 lagging
S (L2) phase – 8 kW at p.f. 0.85 lagging
T (L3) phase – 10 kW at p.f. 0.8 lagging
Three-phase:
A balanced three phase load of 15 kW at p.f. 0.9 lagging
Determine (i) if the system is star or delta connected. (5 Marks)
(ii) the current in each phase (5 Marks)
(iii) the neutral current (5 Marks)
(iv) the total installed power (5 Marks)
