Electrotechnology Exams, Cork Institute of Technology, 2011/2012, Exams of Electrical Engineering

Download Electrotechnology Exams, Cork Institute of Technology, 2011/2012 and more Exams Electrical Engineering in PDF only on Docsity! CORK INSTITUTE OF TECHNOLOGY INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ Semester 1 Examinations 2011/2012 Module Title: Electrotechnology for Mech L7 Module Code: ELEC7007 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_Y3 EMECH_7_Y3 EMANF_7_Y3 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 415V. Each phase of the load has an inductive reactance of 15Ω 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) ETCI — National Rules for Electrical Installations EFG ‘Tapte AS2-F4: XLPE/EPR INSULATED CABLES WITH COPPER CONDUCTORS Continuous current-carrying capacities in amperes, Conductor temperature 90°C Reference ambient temperature 30°C Installation methods E, F, G of Table A52-D Installation method Multi-core cables Single-core cables oe Reid tesa hae Three loaded conductors, flat | conductors | conductors ae conductors | Touching ; Spaced ——, 2 | trefoil Horizontal | Vertical area of i | 1 | fel fe) ) je) iF) eS i I ! [ = Method E | Method | Method F | Method | Method F | MethodG | Method G 1 2 3 4 5 6 7 8 1s 26 2B - 2 ~ 2 = 25 36 32 - = = “ = 4 49 42 e & = és e 6 68 54 “ = = es z 10 86 75 = - ss = EB 16 us 100 - - - - - 25 149 127 161 135 141 152 161 35 185 158 200 169 176 226 201 50 225 192 242 207 216 - 246" 70 289 246 310 268 279 353 318 95 352 298 377 328 342 430 389 120 410 346 437 383 400 500 454 150 473 399 504 444 464 577 527 185 542 456 375 510 533 661 605 240 641 536 679 607 634 781 719 300 741 621 783 703 736 902 833 400 “ s 940 62a 368 1.065 1 008 500 - = 1083 946 998 1253 1169 630 cs ss 1254 1036 List 144 1362 Note: D, is the external diameter of the cable ETCI — National Rules for Electrical Installations TABLE A52-J3 Voltage drop mV / ampere / metre for XLPE insulated multi core cables with copper conductors Conductor operating temperature 90°C Installation methods A B CE ‘Conductor Methods A & B Methods C & E size mm? 1 phase 3 phase 1 phase 3 phase 15 31 27 31 27 25 19 16 19 16 4 12 10 12 10 6 79 68 79 68 10 47 40 4.0 16 29 26 25 > 25 19 1.68 1.60 35 135 1s 1s 50 1.05 0.90 0.87 70 0.75 0.65 0.62 95 0.58 0.50 0.46 120 0.48 0.42 0.38 150 0.43 0.37 0.32 185 0.39 0.32 0.28 240 0.33 0.29 0.24 300 031 0.27 0.20 TABLE A52-J4 Voltage drop mV / ampere / metre for XLPE — insulated single core cables with copper conductors Conductor operating temperature 90°C Installation reference methods F and G 7 3 phase Conductor 1 phase Method F 3 phase feaian? size mm Method F Sai oa Method G 50 1.00 0.87 0.88 0.90 70 0.71 0.62 0.65 0.70 95 0.55 0.47 052 0.53 120 0.45 0.39 0.44 051 150 0.38 0.33 0.39 0.45 185 0.33 0.28 0.34 0.41 240 0.28 0.24 0.30 0.37 300 0.25 0.21 0.20 0.34