Port and Maritime Organization

examination and certification directorate Exam Code : SEET

Subject: Electro-technology Date: Rank : Second Engineer Officer ((kw>3000) Time allowed: 2.5 Hrs

Pass Mark : 50

Q1) On no load, a 1 ph, 440/110 V transformer takes 5 A at a power factor of 0.6 lagging on full load the transformer supplies 65 A at a power factor of 0.6 lagging.

Determine: a) - The primary full load current. (8M) b) - The supply power factor at full load. (4M) c) - The full load efficiency of the transformer (4M)

Q2) a - Explain the reasons why an automatic voltage regulator is used with a ships a.c. generator. (4M) b - Draw a block diagram for an A.V.R, labeling each block and explain each function. (8M) c - GIVE a brief account of the events taking place in the generator A.V.R immediately after a large load is switched off. (4M)

Q3) Referring to hazardous zones on board tankers, answer to the following questions:

a - What are the sources of ignition (4M) b - Define a hazardous area by classifying it into various zones, giving an example for each. (4M) c - List the type of protections with each one symbol, then among them explain increased safety protection. (4M) d - Show the marking of an item for explosion protected apparatus. (4M) Page 1 of 2

Q4) The p.d. across the base-emitter of the silicon transistor shown in fig Q2 is 0.6v And the out put voltage Vo is 4v. Neglecting the small base current, determine;

i) The p.d. across the bias resistors R1 and R2. (4 marks) ii) The p.d. across the collector-emitter. (4 marks) iii) The value of the load resistor Rc. (4 marks) iv) The power used by load resistor. (4 marks)

10k R1 Rc b 12V Vo 2 k R2 140 RE Fig Q2

Q5) a-State the advantages and disadvantages of the wound rotor induction motor starting

method relative to Direct on line switching. (7M)

b-Sketch a circuit diagram showing :star rotor / slip rings / starting resistor connections for a three phase wound rotor induction motor. (3M) c-State the relationship between rotor resistance and reactance in order to achieve maximum / torque conditions, when at starting and when at running. (3M)

d-A 3PH-wound rotor induction motor has a rotor induced emf of 200volts,60HZ at stand still .Calculate the rotor emf and frequency at a slip of 0.04. (3M)

Q6) An oil cooled 50 KVA, 3PH, Transformer, when on full load, has an efficiency of 90%, feeding fully resistive load. Under these conditions, the copper loss is twice the iron loss which produces a temperature rise of 48 degrees(Celsius) . overload the transformer by 25% at same type load and calculate:

1) The final steady temperature rise of the oil. (6M) 2) The new efficiency. (6M) 3) Why all power transformers are rated based on apparent power (KVA). (4M)

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PORTS AND MARITIME ORGANIZATION EXAMINATION AND CERTIFICATION DIRECTORATE

Exams Code: SEET

Subject:Electrotechnology Date:Rank : 2nd Engineer Officer Time allowed: 2 Hrs

Pass mark : 50

Q.1) For the circuit shown in Fig.1, calculate each of the following: (a) The value of the currents I1 , I2 and I3 :

(12M) (b) The power supplied by the battery;

(2M) (c) The p.d. across the 7 resistor.

(2M) I1 I2 3 4 10 12V Fig. 1 I3 7 8

Q.2) A 3-ph , 440V , 60 Hz, 8-pole cage induction motor power a 7 KW loads at a speed of 14.4 rev/s and operates at a power factor of 0.8 lagging. The stator and rotational losses are 0.6 KW and 0.4 KW respectively. Calculate each of the following:

(a) The slip; (6M)

(b) The frequency of the rotor emf; (2M)

(c) The input power to the motor; (6M)

(d) The motor line current. (2M)

Q.3)

a) Draw the control and power circuit diagram of a star- delta starter, for 440 V, 3-phase induction motor. (12M) b) Explain the circuit operation in response to stop and start commands. (8 M)

Q.4) An instrument magnetic circuit in in the form of a steel toroid with a radial air gap,the mean circumference of the toriod is 1.0 m and its cross sectional area is 4.0 x 10 3 m2 . The air gap is 1mm long. The toroid is wound with a coil of 1500 turns, the relative permeability of the steel is 100 and the permeability of free space(0) is 4 x 10-7. Calculate the current required to produce a flux of 9.0mWbs in the air gap. (16M)

Q.5) Describe with the aid of a circuit diagram the operation of a self-excited brushless

generator. (16M) Q.6) With regard to electrical machinery on board ships, explain what is meant by;

(a) Drip proof; (4M) (b) Water proof (4M) (c) Submersible enclosure (4M)

State any particular disadvantages of totally enclosed d.c. machine. (4M)