Industrial Electronics and Control Exam for Mechanical Engineering Students, Exams of Electronics

Download Industrial Electronics and Control Exam for Mechanical Engineering Students and more Exams Electronics in PDF only on Docsity! Cork Institute of Technology Bachelor of Engineering in Mechanical Engineering - Award Bachelor of Engineering in Manufacturing Engineering - Award (NFQ Level 7) Summer 2006 Industrial Electronics and Control (Time: 3 Hours) INSTRUCTIONS: Answer Five Questions, at least Two from each section. All formula and calculations must be shown. Examiners: Mr. R. Daly Mr. M. Cychowski Mr. N. Mulcahy Mr. J. Connolly Mr. R. Simpson Section A (Industrial Electronics) Please Use Answer Book A Q1. The following single-phase loads are connected across a 3-phase, 4-wire, 400V supply: (i) Between Brown and Neutral 20 kW at 0.84 p.f. lagging; (ii) Between Red and Neutral 15 kW at 0.86 p.f. lagging; (iii) Between Yellow and Neutral 18 kW at 0.80 p.f. lagging. (a) Draw a circuit diagram showing the loads above as seen by a 3-phase, 4-wire supply and calculate the current in each phase and its phase angle. (6 marks) (b) Draw a fully labelled vector diagram showing the supply voltages and the phase currents drawn to scale. (6 marks) (c) Calculate mathematically, or determine by means of a vector diagram, the current in the neutral conductor (8 marks) Q2. (a) Explain the terms; synchronous-speed, slip-speed, and slip of a 3-phase induction motor. (6 marks) (b) Calculate for a 6-pole, 50 Hz motor with a full-load slip of 2.5%: (i) The synchronous speed in R.P.M; (ii) The full-load speed in R.P.M. (4 Marks) (c) The motor in (a) has a full-load rating of 18.5kW at 400 V. If the full load efficiency and power factor are 90.2% and 0.77 respectively, calculate: (i) Input power; (ii) Supply current; (iii) Full-load torque. (10 marks) 2 Q3. (a) Detail the number of counters and timers contained within the Mitsubishi FXo30MR-ES PLC? (4 marks) (b) Give an example address of a General and Retentive Memory within a Mitsubishi FXo30MR-ES PLC. (2 marks) (c) A forward and Reverse Direct On-Line Circuit is to be built. Two contactors (KM1 and KM2) connected to outputs Y12 and Y13 on a Mitsubishi FXo30MR-ES PLC are used to start a motor in either direction. Three push buttons PB1, PB2 and PB3 connected to inputs X10, X11 and X12 respectively and are used to control the motor. PB1 and PB2 are Normally Open (NO) contacts and Push Button PB3 is a Normally Closed (NC) Contact. Also connected to the PLC are three indicator lamps, L1, L2 and L3 and these are connected to X1, X2 and X11. A thermal overload is used to provide over-current protection and is connected to input X17. The contact on the thermal overload is of a NO type. The circuit will operate as follow: • Contactor KM1 will start the motor in forward direction and indicator light L1 will light when PB1 is operated; • Contactor KM2 will start the motor in reverse direction and indicator light L2 will light when PB2 is operated; • PB3 when operated will stop the output currently on; Operation of the Thermal Overload will cause the motor to stop operating and a lamp L3 to illuminate. The thermal overload must be manually reset before the motor can be restarted. • The outputs for both contactors must not be on simultaneously. Devise an instruction list via the use of a ladder diagram to carry out the control outlined above. Show network titles and any other information that makes the program clear. (14 marks) Q4. (a) Explain by examples, the effect of low power-factor on the supply system and the need to improve the power factor of an installation. (4 marks) (b) An industrial installation with a total load of 1.2MW and a power-factor of 0.78 is to have a six-step automatic power factor correction unit installed. Determine or calculate the capacitive kVAr required to improve the power to the required 0.95. Suggest a suitable arrangement for switching in the capacitors given that capacitors are available in 30, 60, 100, 150, and 200 kVAr ratings. (Assume that the load never falls below 40% of total load value). (10 marks) (c) A charge for low power factor will be applied as follows: For each 0.01 the power factor falls below 0.95, there will be an additional charge of 2.5% per kW of max demand. Given that the max demand for the billing period is 850kW and the cost per kW of max demand is €15, calculate the extra cost for low power factor, where correction equipment is not installed. (6 marks)