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Q5. a) What is back emf? Derive the relation for the back emf and the supplied voltage in terms of armature resistance (6)

b) The earth-lamps on a main switchboard comprise two 240 V 60 W lamps connected in the usual manner. The potential difference at the busbars is 220 V. Damage by sea water occurs to a distribution cable so that the insulation resistance to the earth is reduced to 16 ohms and 6 ohms for +ve and -ve cables respectively. Find by calculation:

a) Which of the two lamps burns the brighter:

b) The additional load on the generators occasioned by the fault. The resistance of the cables and the ship’s structure may be neglected, and that of the lamps taken as constant at the value corresponding to the 60 W rating. (10)