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.
Q3. Explain the matching of an induction electric motor to a pump required for main circulating duty, with the aid of pump characteristic and torque/ship diagrams.
Q3. A. What are the characteristics of PN junction diode? Point out its specifications. Also point out the significance of dynamic and static resistances.
B. Discuss different methods of speed control of a D.C. series motor by adjusting field ampere turns.
Q7. Discuss the criteria of the classification of marine high voltage for A-C. and D.C. Systems. Sketch a Ships high voltage distribution system and explain its features. Discuss the various methods of testing the insulation of HV system. Mention the significance of PI Test, why 3 terminals insulation testers are used in HV insulation measurements?
Q9. A. Explain the term power factor correction; B. State advantages of PF correction; C. Explain, with the aid of a circuit diagram, how power factor correction can be effected using capacitors in a 3-phase circuit; D. state one method, other than the use of capacitors, by which power factor correction can be effected.
Q6. A. Describe the normal criteria used for setting thermal protection relays and its advantage compared to magnetic types.
B. The low-voltage release of an a.c. motor-starter consists of a solenoid into which an iron plunger is drawn against a spring. The resistance of the solenoid is 35 ohm. When connected to a 220 V, 50 Hz, a.c. supply the current taken is at first 2A, and when the plunger is drawn into the “full-in” position the current falls to 0.7 A. Calculate the inductance of the solenoid for both positions of the plunger, and the maximum value of flux-linkages in weber-turns for the “full-in” position of the plunger.
Q7. A) State briefly, the meaning of the expressions ‘star-connected’ as applied to three-phase a.c. practice. What is the ratio of the maximum line voltage to the maximum phase voltage in each case. (6)
B) Determine the line current taken by a 440V, three-phase, star-connected motor having an output of 45kW at 0.88(lagging) power factor and an efficiency of 93 per cent. (10)
Q8. A. What are semiconductor devices? What are its advantages over thermionic devices?
What are semiconductor devices?
B. A 20kVA, 2000/220V, single-phase transformer has a primary resistance of 2.1Ω and a secondary resistance of 0.026Ω. The corresponding leakage reactance’s are 2.5Ω and 0.03Ω. Estimate the regulation at full load under power-factor conditions of.
(a) Unity;
(b) 0.5 (lagging) and
(c) 0.5 (leading).
Q8. A. Electric motors contain a stationary member as well as a rotating member. For each of the following machines, identify in which part of the motor three field winding and the armature winding are located: three phase induction motor, three phase synchronous motor, d.c. motor. B. An 18.65 KW,4-pole,50HZ, 3 phase induction motor has friction and windage losses of 2.5 percent of the output. The full load slip is 4% compute for full load (a) the rotor Cu loss (b) the rotor input (c) the shaft torque (d) the gross electromagnetic torque.
Q10. A) Define torque and explain how it is developed in a D.C. Motor. Drive from first principles an expression for the torque developed in a D.C. Motor. (6)
(b) A diode valve, having the following characteristic is connected in series with a resistor of 10,000 ohm to a 240 V d.c. supply. If a resistor of 40,000 ohms is connected between the anode and cathode, determine the current through the diode.
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