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Q3. A. Give a brief outline of the care maintenance that should be given to the stator and rotor of an A.C. generator;

B. Explain what is likely to occur if the driving power of one A.C. generator suddenly fails when two generators are running in parallel. What safety devices are usually provided for such events?

Q3. Compare methods of obtaining speed regulation of three-phase induction motor generally used in tankers by means of:

A. Rotor resistance; B. Cascade system; C. Pole-changing.

Give examples where each system may be employed with advantage.

Q4. A. Explain how the efficiency and regulation of a transformer can be assessed by open circuit and short circuit tests?

B. What is meant by equivalent resistance?

Q4. With reference to a 3 speed a.c. cage motor driven cargo winch:

A. Sketch a circuit diagram for a pole change motor;

B. Describe how speed change and braking are achieved.

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. 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.

Q7. A. What is leakage flux as it applies to the iron-core transformer? How is it taken into account in the analysis of the transformer? (6)

B. The following results were obtained on a 50 KVA transformer: open circuit test-primary voltage, 3300 V; secondary voltage, 415 V; primary power, 430 W. Short circuit test-primary voltage, 124 V; primary current, 15.3 A; primary power, 525 W; secondary current, full load value. Calculate:

(i) The efficiencies at full load and at half load for 0.7 power factor

(ii) The Voltage regulations for power factor 0.7 (i) Lagging, (ii) Leading

(iii) The secondary terminal voltages corresponding to (i) and (ii). (10)

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)

Q9. A. Show how the power that is transferred across the air gap of the three-phase induction motor is represented. Explain the terms. What portion of this is useful power? B. The primary and secondary windings of a 500 Kva transformer have resistance of 0.42 and 0.0019 respectively. The primary and secondary voltages are 11 000 V and 415 V respectively and the core loss is 2.9 Kw, assuming the power factor of the load to be 0.8. Calculate the efficiency on (i) Full load; (ii) Half load;

Q10. A 100 kW, 460 V shunt generator was run as a motor on no load at its rated voltage and speed. The total current taken was 9.8 A, including a shunt current of 2.7 A. The resistance of the armature circuit at normal working temperature was 0.11  Calculate the efficiencies at i. Full load; ii. Half load. (16)