Q1. With respect to the High Voltage power systems installation, explain the different types of circuit breaker that are used, comparing them on merits and de-merits. Describe the theory of arc phenomenon and the mechanism fitted to mitigate the arc.
Q2. Explain the methods used to control the speed of a 3 Phase induction motors. Draw and Explain a Variable Frequency Drive used for optimization of energy efficiency of auxiliary machineries on board vessels.
Q3. Diesel electric propulsion is now being chosen as the power plant for an increasingly wide variety of vessels.
(a) Sketch a simple layout of such an installation.
(b) Explain the advantages of selecting such a plant.
Q4. With reference to preferential tripping in a marine electrical distribution system.
A. With the aid of a sketch, describe a typical arrangement to provide three stages of tripping an instantaneous protection against short circuit.
B. State why this protection is required;
Q5. With reference to the condition monitoring of electrical machinery:
A. State TWO important parameters that may be recorded;
B. Explain how the parameters are measured and what defects may be revealed.
Q6. A. What are the characteristics of PN junction diode? Point out its specifications. Also point out the significance of dynamic and static resistances.
B. Draw the circuit of Half-wave rectifier and its output waveform. A diode whose internal resistance is 20 is to supply power to 1000 load from 110 V (RMS) source. Calculate
(i) peak load current, (ii) DC load current, (iii) AC load current.
Q7. A. Describe the no-load saturation characteristic of a d.c. generator.
B. A d.c. motor takes an armature current of 110 A at 480 V. The resistance of the armature circuit is 0.2 . The machine has six poles and the armature is lap-connected with 864 conductors. The flux per pole is 0.05 Wb. Calculate;
(i) The speed;
(ii) The gross torque developed by the armature.
Q8. A. What is a commutator? Discuss its rectifying action in detail.
B. Calculate the e.m.f. generated by a 4-pole, wave wound armature having 40 slots with 18 conductors per slot when driven at 1000 r.p.m. The flux per pole is 0.015 wb.
Q9. A. Discuss different methods of speed control of a d.c. series motor by adjusting field ampere turns.
B. A 230 V, d.c. shunt motor runs at 1000 r.p.m and takes 5 amperes. The armature resistance of the motor is 0.025𝛀 and shunt field resistance is 230𝛀. Calculate the drop in speed when the motor is loaded and takes the line current of 41 amperes. Neglect armature reaction.
Q10. A. Why is a synchronous motor not self-starting? What are the various ways in which it can be started?
B. A 500V, single phase synchronous motor gives a net output mechanical power of 7.46kW and operates at 0.9 power factor lagging. Its effective resistance is 0.8 . If the iron and friction losses are 500 w and excitation losses are 800w, calculate the armature current and the commercial efficiency.
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