CORK INSTITUTE OF TECHNOLOGY INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ Autumn Examinations 2012 Module Title: Electrotechnology for Mech L7 Module Code: ELEC7007 School: School of Mechanical, Electrical and Process Engineering Programme Title: Bachelor of Engineering (Honours) in Sustainable Energy Year 3 Bachelor of Engineering in Mechanical Engineering Year 3 Programme Code: ESENT_8_Y3 EMECH_7_Y3 External Examiner(s): Internal Examiner(s): Mr M. McKeon, Dr E. Ahearne, Mr P. Kenny, Mr C. Buckley Mr Sean McShera Instructions: Attempt Four Questions Duration: 2 Hours Sitting: Autumn 2012 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 to limit the voltage drop. b) A three phase 15kW 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 (60m high) and is supplied from a substation located 40m from the turbine The motor efficiency is 0.82 (82%) The motor power factor is 0.85 lagging The system voltage is 400V, 3-phase 50Hz 15kW 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. d) Explain why load balancing is important to the consumer in an industrial environment. Q2. A 3-phase load is supplied from the secondary of a star connected transformer with a line voltage of 400V. Each phase of the load has an inductive reactance of 10Ω and a resistance of 24Ω. 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)
Q3. a) An installation for a commercial building has the following loads Lighting 20kW @ 0.95 lagging Heating 10kW @ unity Small Power 50kVA @ 0.7 lagging Supply Voltage 400V, 3-Phase & Neutral at Frequency 50Hz i) Calculate the total kw, kvar, kva, pf and total supply current ii) Calculate the new supply current if the power factor is raised to 0.95 lagging. (18 marks) b) Explain why sometimes there are currents in the neural conductor in a 3-phase, 4-wire installation. (3 marks) c) Describe the benefits of improving the power factor to both Industrial and Commercial customers. Q4. a) A coil of resistance 220Ω and inductance 500mH is connected in series with a 350µF capacitor across a 230V, 50Hz supply. Calculate the following: i) Inductive Reactance, X L ii) Capacitive Reactance, X C iii) Impedance, Z iv) Total Current, I T v) Power Factor, pf vi) Circuit Power, W (18 marks) b) Calculate the value of the Capacitor required producing resonance across the Resistor and Inductor. (3 marks) c) Using a diagram, explain resonance and its effects when applied to an a.c. series circuit containing resistance, inductance and capacitance.
Q5. (a) A 3-phase, 10kW, 400V, 50Hz, 8-pole, delta connected squirrel cage induction motor has a full-load efficiency of 89%, power factor of 0.85, and running speed of 735 r.p.m. Calculate for full-load conditions, the motors: (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
ETCI National Rules for Electrical Installations