The Diseases Of Electrical Machinery

62 THE DISEASES OF feature which may give rise to faults of insulation as well as to other faults. Slip-rings can indeed be easily insulated from one another, and from the shaft. A fault is, however, more likely to appear there than on the fixed terminals of the continuous-current machine. Sparking at the slip-rings may be due to two causes. It may either be due to a periodically occurring break in the magnet-winding; or, more usually, it results from bad material for the brushes or slip-rings, care- less handling, untruth or dirtiness of the rings. At any rate, all faults occurring in the exciting winding of an alternator are easily found. Moreover, the large number of the poles presents the advantage that in the case of a breakdown of one pole, one may without fear cut out one pole, connecting together the preceding and following poles. Then, by increasing somewhat the magnetising current, the correct voltage can be still maintained. This results in an irregularity in the strength of the various poles, which is however of no importance, because sparking, such as would occur with a continuous-current machine under the same circumstances, is impossible. Exciter.—The continuous current for the field is usually supplied by a direct-coupled exciter, which usually possesses a single shunt winding, but some- times a series winding ^Iso. With this machine the same faults may occur that we considered at the be- ginning, so that it appears useless to add anything more here.