DieselShip's Online Content Library

Q1. With reference to a three-phase shipboard electrical distribution system: (16)

a) Enumerate the advantages of an insulated neutral system.

b) Enumerate the disadvantages of an insulated neutral system.

c) Describe how the earthed neutral system is Earthed.

d) Compare the use of an insulated neutral system as opposed to the use of an Earthed neutral System with regard to the risk of electric shock from either system.

Q2. a) i. Describe the characteristics of a D.C motor. (8)

ii. Explain the advantages of such a motor for deck machinery.

b) Describe with the aid of a sketch a control system for the motor in (A). (8)

Q3. List at least two factors that cause deterioration of the frequency response of a transistor amplifier. Explain how each factor affects the performance of the amplifier and the portion of the frequency range where it is effective. (16)

Q4. Differentiate with the aid of simple sketches between two of the following types of electronic circuits.

A. Rectifier circuit (6)

B. Amplifier circuit (5)

C. Oscillator circuit (5)

Q5. a) Describe the circuit breaker for an a.c. generator using a sketch to show how arcing is controlled. (6)

b) Explain the sequence of events that might occur if the breaker opens on a short circuit and state the check you would require following such event. (5)

c) Give a safe procedure to follow should a main circuit breaker fail to open under fault Condition. (5)

Q8. A. What is meant by negative and positive feed backs? Explain the characteristics of negative feedback; B. Compare the series and parallel resonance circuits. Find the frequency at which the following circuit resonates.

Q10.  (a). Describe the no-load saturation characteristic of a d.c. generator.

(b). A 4-pole machine running at 1500 r.p.m. has an armature with 80 slots and 6 conductors per pole. The flux per pole is 6 x 106 lines. Determine the terminal e.m.f. of d.c. generator if the coils are lap connected. If the current per conductor is 100 A, determine the electrical power.

Q8. A. What is slip? What are the factors on which the torque or an induction motor depends? B. A 220-V, 3-  4-pole, 50Hz, Y-connected induction motor is rated 3.73 KW The equivalent circuit parameters are: R1=0.45 , X1=0.8 , R2=0.4 , B0=1/30ohm. The stator core loss is 50W and rotational loss is 150W. For a slip of 0.04, find (i) input current (ii) p.f. (iii) air-gap power (iv) mechanical power (v) electro-magnetic torque (vi) output power and (vii) efficiency?

Q10. A. How does change to frequency affect the operation of the transformer? What makes this ratio different from the ratio of transformer? B. A 550 KVA, 50 Hz single phase transformer has 1875 and 75 turns in the primary and secondary windings respectively. If the secondary voltage is 220 V, calculate (i)Primary voltage; (ii)Primary and secondary currents; iii. Maximum value of flux.

Q7. A. Which of the following three motors has the poorest speed regulation: shunt motor, series Motor or cumulative compound motor? Explain.

B. A 440V shunt motor takes an armature current of 30A at 700 rev/min. The armature resistance is 0.7ohm. If the flux is suddenly reduced by 20 per cent, to what value will the armature current rise momentarily? Assuming unchanged resisting torque to motion, what will be the new steady values of speed and armature current? Sketch graphs showing armature current and speed as functions of time during the transition from initial to final, steady-state conditions.