TEET-1400-03 Electro Technology Past Paper PDF

Candidate’s Name: National ID: Ports and Maritime Organization Seafarers' Affairs Directorate Exam Code: TEET-1400-03 Subject: Electro Technol...

Candidate’s Name: National ID: Ports and Maritime Organization Seafarers' Affairs Directorate Exam Code: TEET-1400-03 Subject: Electro Technology Date: 19/08/1400 Rank: 3rd Engineer officer (KW>3000) Time Allowed: 2.5 Hours (Pass mark: 55) Q1 Sketch the Synchronizing with three lamps and describe the condition necessary for paralleling two generators. Answer: 17 marks To achieve smooth manual synchronizing, the incomer must be brought up to speed to obtain approximately the same frequency that is shown on the busbar frequency meter, e.g. 60Hz. The incoming generator voltage is set by its AVR to be equal to the busbar voltage. Fine tuning of the speed can now be observed on the synchro scope or synchronizing lamps. The incomer is adjusted so that the synchro scope indicator rotates slowly clockwise (fast direction) at about 4 seconds per indicator revolution. The circuit breaker should be closed as the indicator approaches the 12 o'clock (in- phase) position. The breaker closing between 5-to and 5-past the 12 o’clock synchro scope position is satisfactory as long as the pointer rotation is fairly slow. Q2 Sketch and describe, Thermal overload relay (Bimetal strip) to protect 3-ph. LV induction motor. And state that, at what value of current should the over current relay (OCR) be set? 16 marks Answer: Most LV motors are protected by less expensive thermal OCRs. Inverse time thermal OCRs usually work with bimetal strips, as shown in Figure below. The strips are heated  by the motor current and bend depending on the temperature. If the motor takes an overload current, the strips operate and open the incorporated normally-closed (NC) contact 95-96which, in turn, trips out the line contactor to stop the motor. Thermal overload relay action (single-phase is shown) To protect a modern continuous maximum rating (CMR) motor, the thermal OCR should be set at the full load current (FLC) rating of the motor. This will ensure that tripping will not occur within two hours at 105% FLC. At 120% FLC, tripping will occur within two hours Q3 a) By regard of sequential control circuit, explain the PLC device? b) Name the basic parts of every PLC device? c) Describe, comparisons of relay logic and PLC control circuit. Answer: a) A programmable logic controller (PLC) or programmable controller is an industrial digital computer which has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, or robotic devices, or any activity that requires high reliability, ease of programming and process fault diagnosis. b) Regular programmable logic controller consists of: 5 marks 1) a processor unit (CPU) which interprets inputs, executes the control program 6 marks stored in memory and sends output signals, 6 marks 2) a power supply unit which converts AC voltage to DC, 3) a memory unit storing data from inputs and program to be executed by the processor, 4) an input and output interfaces, where the controller receives and sends data from/to external devices, 5) a communications interface to receive and transmit data on communication networks from/to remote PLCs. 6) PLCs require programming device which is used to develop and later download the created program into the memory of the controller. c) PLCs are much more reliable and are way easier to troubleshoot. The mechanical functions of relays simply wear out over time.  Plus, the wiring required to operate a relay system is much more complicated than a PLC system. This can also cause higher costs to install and upgrade due to extra labor costs. Q4 What are the important points to replace the fuse protection devices? Explain, Undervoltage Protection device? Answer: Important points to note about fuses: 1) In the event of a fuse blowing, the cause of the fault must be located and repaired before the fuse link is replaced. 2) The replacement fuse link must be of the correct current rating, grade and type. Usually this means the replacement fuse link is identical to the blown fuse link. 3) Replace all three fuses in a three-phase supply, even if only one is found blown after a fault. The others may be seriously weakened, which makes them unreliable for future use. 8 marks Undervoltage Protection 8 marks An undervoltage release mechanism (UVR) is fitted to all generator breakers and some main feeder circuit breakers. Its main function is to trip the breaker when a severe voltage dip (around 50%) occurs. This is achieved by releasing the mechanical latch (which keeps the main contacts closed) to trigger the trip mechanism that opens the breaker main contacts and disconnects the load from the power source. The UVR on a generator circuit breaker prevents it being closed when the generator voltage is very low or absent, and therefore prevents closure of the dead generator's circuit breaker. Q5 A 220V, single phase alternator supplies the following loads: (a) 20KW at unity power factor for lighting and heating. (b) An induction motor taking 82.9KW at a power factor of 0.8 lagging. 17 marks (c) A synchronous motor taking 50KVA at a power factor of 0.5 leading. (d) Find the total KVA, current and the power factor of the combined load. Q6 A brass rod of cross section 1000mm2 is formed into a closed ring of mean diameter 300mm. It is wound uniformly with a coil of 500 turns. If a magnetizing current of 5A flows in the coil, calculate: (a) The magnetizing forces. (b) The flux density. (c) The total flux. μo=4π10-7 H/m 17 marks Answer: Good Luck