DieselShip's Online Content Library

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.

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.

Q4. What is the meant by "excitation" in an alternator? With the help of a neat diagram of brushless alternator labelling all the important parts. explain how the excitation is achieved in a brushless alternator.

Q5. With reference to alkaline batteries used on board ship.

A. Describe the operation of a battery cell and state the material used;

B. Describe how the cells are mounted to form a battery;

C. State the advantages and disadvantages compared with lead-acid batteries.

Q2. What is soft starting of an Induction motor? Describe with a circuit using thyristors used for soft starting. Discuss its advantages and dis-advantages.

Q6. A. What are factors on which the speed of a motor depends? Discuss them for series and shunt motors. (6)

b) Three equal resistors are connected to a three-phase system, if one resistor is removed find the reduction in load if they are connected in (a) Star, (b) Delta. (10)

Q7. A. Derive an expression for the e.m.f induced in an a.c. generator. (6)

B. A 220 V, d.c. shunt motor has an armature resistance of 0.5 ohm and an armature current of 40 A on full load. Determine the reduction in flux necessary for a 50 per cent reduction in speed. The torque for both conditions can be assumed to remain constant. (10)

Q8. a) How do the leakage fluxes effect the operation of a transformer? How are they minimized? (6)
B. A 440 load of 400 KW at 0.8 (lagging) power factor is jointly supplied by two alternator A and B. The KW load on A is 150 KW and the KVAr load on B is 150 KVAr (lagging). Determine the KW load on B, the KVAr load on A, the power factor of operation on each machine and the current loading of each machine.  (10)

Q9. a) What is the operational impedance of an R.C. Circuit? Describe its usefulness. (6)
B. A ring-main, 900m long, is supplied at a point A at a p.d. of 220V. At a point B, 240m from A, a load of 45A is drawn from the main, and at a point C, 580m from A, measured in the some direction, a load of 78A is taken from the main. If the resistance of the main (lead and return) is 0.25 ohm per kilometre, calculate the current which will flow in each direction round the main from the supply point A and the potential difference across the main, at the load where it is lowest. (10)

Q10. A. What is back emf? Derive the relation for the back emf and the supplied voltage in terms of armature resistance. (6)

B. Find the synchronous impedance reactance of an alternator in which a given field current produces an armature current of 200 A on short circuit and a generated e.m.f. of 50V on open circuit. The armature resistance is 0.1 ohm. To what induced voltage must the alternator be excited if it is to deliver a load of 100A at a p.f of 0.8 lagging, with a terminal voltage of 200V. (10)