DieselShip's Online Content Library

Q1. A. Explain the meaning of the term power factor correction. (16)

B. State TWO advantages of power factor correction.

C. Explain, with the aid of a circuit diagram, how power factor correction can be effected in a three phase circuit using capacitors.

D. Explain one method other than the use of capacitors by means of which power factor correction may be effected.

Q2. A. Describe with the aid of a simple sketch the arrangement of the three-phase winding of an alternator showing the neutral point. (6)

B. Explain why for most ships the neutral point is insulated. (5)

C. Explain why in some installation the neutral point is Earthed? (5)

Q3. A. State the necessary conditions required prior to the synchronizing of electrical alternators. (4)

B. Describe the type of cumulative damage that may be caused when alternators are incorrectly synchronized. (4)

C. Explain how the damage referred to in (b) can be avoided / reduced. (4)

D. For two alternators operating in parallel state the consequences of: (4)

(i) Reduced torque from the prime mover of one machine.

(ii) Reduced excitation on one machine.

Q4. A. What is intrinsic electric safety? (6)

B. Can live maintenance be done on intrinsically safe circuits? (5)

C. Describe intrinsically safe equipment used on board ship. (5)

Q5. A. (i) Sketch a diagrammatic arrangement of a static or self-excited alternator. (5)

(ii) Describe the operation of the self-excited alternator. (5)

B. State why the voltage dip is less in the self-excited alternator than in brushless or conventional alternators. (6)

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

B. A shunt motor supplied at 230 V runs at 900 rpm. When the armature current is 30 A, the resistance of the armature circuit is 0.4 𝛀, calculate the resistance required in series with the armature circuit to reduce the speed to 500 rpm. Assume that the armature current is 25 Amps. (10)

Q7. Two three phase 415 V alternators supply a ship’s load comprising:

(1) Lighting totalling 800 kW at unity power factor; and

(2) Motors totalling 1700 kW at power factor 0.7 lag.

One alternator supplies 1400 kVA at power factor 0.75 lag.

(a) Calculate each of the following for the other alternator:

(i) The kVA output; (ii) The power factor; (iii) The line output current. (16)

Q8. A. With reference to an A.C. generator used in marine practice derive an expression for the frequency of the generated e.m.f. in terms of the speed of the machine and the number of poles. (6)

B. A 200V, long-shunt compound-wound generator has a full-load output of 20kW. The various resistances are as follows: armature (including brush contact) 0.15 ohm, series field 0.025 ohm, interpole field 0.028 ohm, shunt field (including the field-regulator resistance) 115 ohm. The iron losses at full load are 780W, and the friction and windage losses 590W. Calculate the efficiency at full load. (10)

Q9. A. What are the factors which determine the synchronous speed of a motor? (6)

B. A 72 KVA transformer supplies a heating and lighting load of 12 KW at unity power factor and a motor load of 70 kVA at 0.766 (lagging) power factor; Calculate the minimum rating of the power-factor improvement capacitors which must be connected in the circuit the ensure that the transformer does not become overloaded. (10)

Q10. (A) Compare the effectiveness of a current limiting circuit breaker with that of a HRC fuse. (6)

(B) Evaluate for a frequency of 15 kHz, the amplification and the phase difference between input and output signals of a voltage amplifier using a triode having an amplification factor of 48 and a mutual conductance of 1.2 mA/V with an anode-load resistant of 160 k𝛀. The output p.d. is fed by a coupling capacitor of negligible reactance to a subsequent circuit of resistance 480 k𝛀 and the total shunt capacitance is 90 µF. (10)