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)

Q6. A. Explain what is meant by the terms wave form, frequency and average value. (6)

B. A moving coil ammeter, a thermal ammeter and a rectifier are connected in series with a resistor across a 110 V sinusoidal a.c. supply. The circuit has a resistance of 50 𝛀 to current in one direction and, due to the rectifier, an infinite resistance to current in the reverse direction. Calculate:

(i) The readings on the ammeters.

(ii) The form and peak factors of the current wave. (10)

Q7. A. What is leakage flux as it applies to the iron-core transformer? How is it taken into account in the analysis of the transformer? (6)

B. The following results were obtained on a 50 KVA transformer: open circuit test-primary voltage, 3300 V; secondary voltage, 415 V; primary power, 430 W. Short circuit test-primary voltage, 124 V; primary current, 15.3 A; primary power, 525 W; secondary current, full load value. Calculate:

(i) The efficiencies at full load and at half load for 0.7 power factor

(ii) The Voltage regulations for power factor 0.7 (i) Lagging, (ii) Leading

(iii) The secondary terminal voltages corresponding to (i) and (ii). (10)

Q8. A. What is Silicon controlled rectifier (SCR)? How is the breakover voltage of the SCR defined? (6)

B. A d.c. motor takes an armature current of 110 A at 480 V. The resistance of the armature circuit is 0.2𝛀. The machine has six poles and the armature is lap-connected with 864 conductors. The flux per pole is 0.05 Wb. Calculate: (a) The speed; (b) The gross torque developed by the armature. (10)

Q9. A. Show how the power that is transferred across the air gap of the three-phase induction motor is represented. Explain the terms. What portion of this is useful power? B. The primary and secondary windings of a 500 Kva transformer have resistance of 0.42 and 0.0019 respectively. The primary and secondary voltages are 11 000 V and 415 V respectively and the core loss is 2.9 Kw, assuming the power factor of the load to be 0.8. Calculate the efficiency on (i) Full load; (ii) Half load;

Q10. A 100 kW, 460 V shunt generator was run as a motor on no load at its rated voltage and speed. The total current taken was 9.8 A, including a shunt current of 2.7 A. The resistance of the armature circuit at normal working temperature was 0.11  Calculate the efficiencies at i. Full load; ii. Half load. (16)