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Cork Institute of Technology
Bachelor of Engineering in Mechanical Engineering - Award
Bachelor of Engineering in Manufacturing Engineering - Award
(NFQ Level 7)
Summer 2007
Electro-Technology and Control
(Time: 3 Hours)
INSTRUCTIONS: Answer Five Questions, at least Two from each section. All formula and calculations must be shown.
Examiners: Mr. G. Geaney Mr. N. Mulcahy Mr. M. Murray Mr. J. Connolly Dr. P. Delassus
Section A ( Electro-technology )
Please Use Answer Book A
Question 1
The following single-phase 400V loads are connected across a 3-phase, 3-wire, 400V supply: (i) Between Brown and Red lines 15 kW at 0.88 p.f. lagging; (ii) Between Red and Yellow lines 11 kW at 1.0 p.f. lagging; (iii) Between Yellow and Brown lines 17 kW at 0.87 p.f. lagging.
(a) Draw a circuit diagram showing the loads above as seen by a 3-phase supply and calculate each phase current and its phase angle. 6 marks
(b) Draw a vector diagram showing the supply voltages and the phase currents drawn to scale. 6 marks
(c) Calculate, or determine by means of a vector diagram, the current in each supply line. 8 marks
Question 2
(a) A 3-phase, 14kW, 400V, 50Hz, 6-pole, delta connected squirrel cage induction motor has a full-load; efficiency of 87%, power factor of 0.82, and running speed of 940 r.p.m. Calculate for full-load conditions, the motor’s: (i) Input power; (ii) Supply current; (iii) Phase (winding) current; (iv) Full-load torque. 12marks
(b) Explain the terms; synchronous-speed, slip-speed, and slip of a 3-phase motor. Calculate for the motor in (a): (i) synchronous speed; (ii) % slip; 4 Marks
(c) Star-Delta starting is a method of induction motor starting that is used to reduce starting currents. Show how the starting current is reduced to 1/3 using thus method of starting. 4 marks
Question 3
(a) The Mitsubishi FXo30MR-ES PLC contains 512 Memories of two general types, General and Retentive Memories. Detail the difference between these two memories. (4 marks)
(b) Give an example address of both types of memories detailed above. (2 marks)
(c) Two contactors (KM1 and KM2) connected to outputs Y12 and Y13 on a Mitsubishi FXo30MR-ES PLC are used to start a motor by means of three push-buttons; PB1, PB2, and PB3 connected to inputs X10, X11 and X12 respectively. Also connected to the PLC are two indicator lamps, L1and L2 and these are connected to Y1and Y2. The circuit will operate as follow:
- When PB1 is operated, contactor KM1 will energise and start the motor in forward direction. Indicator light L1 must also light to indicate motor running forward direction. In the event of power loss the motor will re-energise upon the resumption of power.
- When PB2 is operated, contactor KM2 will energise and start the motor in reverse direction. Indicator light L2 will light to indicate motor running in reverse direction. In the event of power loss the motor will not re-energise upon the resumption of power.
- PB3 when operated will stop the output currently on;
- The outputs for both contactors must not be on simultaneously.
StartForw ard
F1Fuse ControlCircuit
Stop
F
KM
OverloadTrip Lamp
KM
StartReverse
KA1N/O KA2N/O
KM1N/C Contact
KM2N/C Contact
KA1N/O
KA
KA2N/O
InternalMemory 1
InternalMemory 2 KA
Run LampForw ards Run LampReverse
Devise an instruction list to carry out the control outlined above. Show network titles and any other information that makes the program clear. (14 marks)
Q6. (a) With the aid of a Karnaugh map reduce the following switching function to its
simplest form:
G 1 ( A , B , C , D )= ABCD + ABCD + ABCD + ABCD + ABCD + AB^ CD
[5 marks]
(b) A thermocouple gives an output of 0.5 mV/ºC. Its output passes through an analogue-to-digital converter. Temperatures from 0ºC to 400ºC are required to be measured with a resolution of 0.1ºC? What word length is required for the A/D converter? [5 marks]
(c) Explain the operation of the following circuit; use a timing diagram and truth table as appropriate.
[10 marks]
Q7. (a) Explain what is meant by the term I/O bus networks. [5 marks]
(b) List FIVE advantages of I/O bus networks. (^) [5 marks]
(c) Describe FOUR features of a SCADA Interface. [10 marks]
