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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 1 Examinations 2011/
Module Title: Electrotechnology for Mech L
Module Code: ELEC
School: Mechanical & Process Engineering
Programme Title: Bachelor of Engineering (Hons) in Sustainable Energy – Year 3 Bachelor of Engineering in Mechanical Engineering – Year 3 Bachelor of Engineering in Manufacturing Engineering – Year 3
Programme Code: ESENT_8_Y EMECH_7_Y EMANF_7_Y
External Examiner(s): Dr Eamonn Ahearne, Mr. Paul Kenny, Mr. Conor Buckley, Mr. Mike McKeon Internal Examiner(s): Mr Sean McShera
Instructions: Attempt Four Questions
Duration: 2 Hours
Sitting: Winter 2011
Requirements for this examination:
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) Explain why it is important for Electrical Design Engineers to limit the voltage drop
on an installation. (4 marks)
b) A three phase 25kW induction motor is being installed for the yaw control of a wind turbine. Determine the cable size required supplying the electrical power to the motor, using the tables provided and assuming a multi-core XLPE cable is used. Additional application information:
- The motor is located at the turbine hub (90m high) and is supplied from a substation located 15m from the turbine
- The motor efficiency is 0.92 (92%)
- The motor power factor is 0.84 lagging
- The system voltage is 400V, 3-phase 50Hz
- 25kW refers to the shaft power of the motor.
- Overload protection of the motor is set to 1.5 times the full load current.
- Installation Method E from the ETCI Regulation Tables. (13 marks)
c) Explain why a Star-Delta motor starter would be chosen to control the operation of a large Industrial Pump. (4 marks)
d) Explain why ‘load balancing’ is important to the consumer in an industrial environment. (4 marks)
Q2. A 3-phase load is supplied from the secondary of a star connected transformer with a line voltage of 415 V. Each phase of the load has an inductive reactance of 1 5 Ω and a resistance of 18Ω. If the load is connected in;
a) Star b) Delta
Determine, for each of the connections i) The potential difference across each phase of the load. ii) The current in each phase of the load. iii) The power taken from the supply. iv) The power factor. v) The current in the transformer secondary windings. (20 marks)
c) With the aid of a circuit diagram, explain how two watt-meters may be used to determine the total power in an unbalanced, 3 phase, 3 wire delta connected load. (5mark)
Q5. (a) A 3-phase, 10kW, 400V, 50Hz, 6-pole, delta connected squirrel cage induction motor has a full-load efficiency of 89%, power factor of 0.85, and running speed of 900 r.p.m. Calculate the following parameters for the motor under full-load conditions: (i) Input power (VA); (ii) Supply Line current; (iii) Phase (winding) current; (iv) Full-load torque. (20 marks)
(b) Explain the terms; synchronous-speed and slip of a 3-phase motor.
Calculate for the motor in (a): (i) synchronous speed (in RPM) (ii) % slip
(5 marks)
ETCI – National Rules for Electrical Installations
