The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 3i are wrongly connected. It is in this event only neces- sary to change the leads to the coils ; that is, to reverse the direction of the current in the exciting winding. There is also always a possibility that when the excit- ing coils have been removed from the poles during repairs, and have been afterwards replaced, an incorrect connection may have been made, so that the two poles, instead of being one north and the other south, are both alike, perhaps both north or both south. This fault may not be at once perceived. It is generally necessary to know the direction of winding of the magnet coils. This can usually be ascertained by ex- amining the upper layer of the winding. Polarity.—That pole is a north-pole around which the current in the exciting coil flows in a counter clockwise direction, as viewed from a point opposite the pole face. This rule is correct only when looking towards the bored-out pole-surface. Vice versa, that Fig. 12. pole is a south-pole where the circulation of the current is in the clockwise direction. Here, also, one is sup- posed to be viewing the pole-face from a point opposite it. Figs. 11 and 12 explain these rules.