Q1. A. Sketch a main engine shaft driven generator arrangement with an electronic system for frequency correction. B. Describe the operation of the generator arrangement sketched in (A).
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;
Q9. Explain What is meant by, and the significance of, four of the following terms.
I. Voltage Stabilization,
II. Filter choke;
III. Impedance,
IV. Rectification,
V. Grid bias voltage.
Q4. Explain the meaning of “p” and “n” type semi-conductor materials and give a brief description of the mechanism by which current passes through them.
Q5. With reference to electronic control systems A. Draw a simple block diagram for temperature control B. Describe each component shown in the diagram in (a).
Q6. A. Describe with the aid of a sketch, an isolator for a 3 phase 440V, 20-amp electric supply List the safety features of the isolator described in the sketch. 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 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.
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)
Q8. A. List the factors that determine the starting torque of the three-phase induction motor. How does this torque generally compare with the value of the rated torque? B. A three-phase induction motor is will for four poles and is supplied from a 50 Hz system Calculate: i) the synchronous speed; ii) the speed of the rotor when the slip is 4 per cent; iii) the rotor frequency when the speed of the rotor is 600 r/min
Q9. A. Describe the situations in which hand regulation might be desirable for operation of generators. B. Three conductors fitted side by side in the stator of a salient-pole alternator. Each generates maximum voltage of 200V (sinusoidal). The angle subtended at the centre of the stator between adjacent conductors is 20 electrical degrees. If the three conductors are connected in series, find (i) the r.m.s. value of the effective voltage and (ii) the ‘breadth factor’ Using the theory that is the basis of this problem, give one reason why three-phase current has been introduced.
Q8. A. Sketch a simplified circuit of a reverse power relay explain why there is a time delay incorporated before the reverse power relay operates; B. A twelve-pole, three-phase, delta-connected alternator runs at 600 rev/min and supplies a balanced star-connected load. Each phase of the load is a coil of resistance 35 ohm and inductive reactance 25 ohms. The line terminal voltage of the alternator is 440V. Determine A. frequency of supply, B. current in each coil, C. current in each phase of the alternator, D. total power supplied to the load.
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