298 ELEMENTS OF ELECTRICAL ENGINEERING

matically, the three-phase transformer can be represented by
Kg. 168, shell type, and Fig. 169, core type.

124. While in its magnetic and electrical characteristics
there is no essential difference between the single-phase shell-
type and the single-phase core-type transformer, there is a
material difference in the three-phase transformer. In the shell
type, Fig- 168, a short circuit of one of the three phases does not
affect the magnetic and thus the electric circuit of the other
two phases, in the core type Fig. 169, however, a short circuit of
one of the three phases short circuits the magnetic return of
the other two phases, and so acts as a partial electrical short
circuit of these two other phases.

In shell-type transformers, Fig. 168, a triple harmonic of
flux can exist, but not in the core type, Fig. 169. In the three-

FIG. 170.—Shell type three-phase transformer.

phase system, the three voltages, currents, etc., are displaced
in phase from each other by 120°. Their third harmonics
therefore are displaced in phase from each other by 3 X 120°,
that is, by 360°, or in other words, are in phase with each other.
In Fig. 169, such triple frequency fluxes in the three cores would
have no magnetic return, except by leakage through the air,
that is, cannot exist, except in negligible intensity, and there-
fore the core type of three-phase transformer cannot give any
serious triple frequency voltage. In the shell type Fig. 168,
however, the three triple frequency fluxes, being in phase with
each other, produce a triple frequency single-phase flux through a
closed magnetic circuit. Where the circuit conditions and
connections are such as to give a triple harmonic—as with YY
connection—the shell-type three-phase transformer may produce
triple frequency voltages, resulting from the triple frequency