The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 61 Field Winding.—If we now pass on to the faults in the field system of modern alternators, it is obvious that they are the same as those in continuous-current machines. Almost all modern alternators have a stationary and external armature which surrounds a revolving magnet-wheel. The latter consists of a number of outwardly-pointing poles fixed upon the periphery of a massive wheel. The field-magnet winding of an alternator differs from that of a con- tinuous-current macliine in three points : (i) the mag- nets are rotating; (2) as a result of this, the current must be supplied to the magnetising coils by means of slip-rings ; (3) the alternator is not self-exciting, but is excited by current from a special continuous- current machine called the exciter. Considering these points, one after another, it is clear that by reason of the first the magnet winding is subjected to a mechanical strain, which does not occur with continuous-current machines. Under simi- lar conditions, therefore, breakdowns in the insulation would happen oftener were it not tliat the danger is lessened by another cause. If there were the same heavy mass of copper on each pole, as there is in the case of a continuous-current machine, the above would be correct. Alternators have, however, always more numerous poles, and therefore a smaller weight of copper per pole, for machines of the same output. The necessity of using two slip-rings, and the brush gear belonging thereto, brings into the field system a