Three Phase to One Phase.—This transformation may be accomplished by the use of two transformers connected as in fig. 2,175 in which one end of one primary winding is connected to the middle of the other primary winding and the second end of the first primary winding at a point giving 86.6 per cent. of that winding as shown. The two secondary windings are joined in series.
Three Phase to Two Phase.—The three phase system is universally used for long distance transmission, because it requires less copper than either the single or two phase systems. For distribution, however, the two phase system presents certain advantages, thus, it becomes desirable at the distribution centers to change from three phase to two phase. This may be done in several ways.
Ques. Describe the Scott connection.
Fig. 2,176.—The Scott connection for transforming from three phase to two phase. In this method one of the primary wires B of the .866 ratio transformer is connected to the middle of the other primary as at C, the ends of which are connected to two of the three phase wires. The other phase wire is connected at D, the point giving the .866 ratio. The secondary wires are connected as shown.
Ans. Two transformers are used, one having a 10:1 ratio, and the other, a ½√3:1, that is, an 8.66:1 ratio. The connections are arranged as in fig. 2,176.
It is customary to employ standard transformers having the ratios 10:1, and 9:1.
Ques. What names are given to the two transformers?
Ans. The one having the 10:1 ratio is called the main transformer, and the other with the 8.66:1 ratio, the teaser transformer.