Q4. A) With reference to voltage variation profiles caused by load changes imposed on alternating current generators when starting large motors online:
a) Sketch a voltage dip, showing an acceptable recovery time. (2)
b) State salient factors that cause the variation in part (a) (3)
c) Outline salient factors that assist recovery from the deviation shown in part (3)
B) A three-phase induction motor is wound for four poles and is supplied from a 50 Hz System calculate:
a) The synchronous speed
b) The speed of rotor when the slip is 4 per cent.
c) The rotor frequency when the speed of the rotor is 600 r/min (8)
Q5. A) With reference to large electrical transformers on board ships:
a) State where these transformers may be used (2)
b) State a typical efficiency range for a transformer (2)
c) State the protective devices that are fitted (2)
d) Describe the maintenance requirements (2)
B) What are the losses in transformers? Mention the various factors which affect these losses. In a 25KVA, 3300/223 V, Single phase transformer, the iron and full load Cu. Losses are respectively 350 and 400 watts. Calculate the efficiency at half-full load, 0.8 power factor (8)
Q6. A total load of 8000 kW at 0.8 power factor is supplied by two alternators in parallel. One alternator supplies 6000kW at 0.9 power factor. Find the kVA rating of the other alternator and the power factor. (16)
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