The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 75 wound, while in the single-phase transformer the middle leg, which is the core, has twice the section of the outside legs, which here serve as shell. Shell transformers for polyphase currents have also been built as in Fig. 39 (two-phase) and Fig. 40 (three- phase). There are two windings on each core, a primary and a secondary winding, since a transformer is built to transform an alternating current from one voltage to another. Since, however, the primary is not always the high voltage side, or the secondary always the low voltage side, we will here speak of the high-voltage and the low-voltage windings. In step-up trans- formers, the fine winding, or high-voltage winding, is the secondary, while the thick wire winding, or low- voltage winding, is the primary. In step-down trans- formers the reverse is the case. There are two different ways of arranging these windings. In one arrangement, one wind- ing lies around the other, over the whole length of the core. The two are separated by an insulating layer. In the second arrangement, the winding is laid in a row of coils on the core, alternately primary and secon- dary. The former method is represented in Figs. 32 and 33, and the latter in Fig. 41. It is hardly possible