Selected Electrical Question And Answer for MEO Class 2 (7)

In large DC generators where the current is in excess of 1000A, it is necessary to take steps to avoid magnetising the shaft :

           

            (a)        explain why this is so, and                                                                  (5m)

            (b)        briefly describe how this is accomplished.                                         (5m)

Answer :-

(a)                In large generators where the current exceed 1000A, it is necessary to take steps to avoid magnetising the shaft as this could lead to the penetration of stray current accumulated at the bearing, sufficient to puncture the oil film with consequent pitting on the bearing and journal faces. This can lead to bearing failure in due course and possibility of sludging of lubricating oil.

(b)               In DC generator, the compounding coils are connected in series and in effect constitutes one complete turn around the shaft.

The commutating poles are similarly connected in series and act as if it is one complete turn around the shaft. By arranging for the current in the two sets of connection to in opposite directions to each other around the shaft, the current neutralise each of then, avoiding magnetising of the shaft.