1. A method of manufacturing a shell-type transformer, the method comprising:
stacking a plurality of electromagnetic steel plates and forming an E-shaped stacked body having an E shape as seen in a stacking direction of the plurality of electromagnetic steel plates, the E-shaped stacked body including a lower yoke iron core and three leg iron cores that are vertical to the lower yoke iron core;
pressing the E-shaped stacked body for integration from both sides in the stacking direction, and forming an E-shaped iron core;
rotating the E-shaped iron core such that the lower yoke iron core is located on a lower side;
placing the E-shaped iron core so as to be housed in a tank main body in a state where the lower yoke iron core is located on the lower side;
installing a coil in the E-shaped iron core inside the tank main body such that a leg iron core located at a middle position among the three leg iron cores is inserted through the coil;
installing an upper yoke iron core in the E-shaped iron core having the coil installed therein, and forming an iron core assembly; and
attaching a cover portion to the tank main body to form a tank, and placing the iron core assembly and the coil so as to be enclosed in the tank.
2. The method of manufacturing a shell-type transformer according to claim 1, wherein
in the rotating the E-shaped iron core,
two shaft members cylindrically formed each are coaxially coupled to a position along a side edge of a corresponding one of two leg iron cores on both sides among the three leg iron cores as seen in the stacking direction, and
the E-shaped iron core is rotated about the two shaft members each

serving as a rotation axis.
3. The method of manufacturing a shell-type transformer according to claim 2,
wherein
the E-shaped iron core includes a retaining plate made of a nonmagnetic material and disposed at a middle position in the stacking direction,
the retaining plate has two protruding portions attachably and detachably coupled to the two shaft members, and
in the rotating the E-shaped iron core, each of the two shaft members is coupled to a corresponding one of the two protruding portions in a one-to-one relation.
4. The method of manufacturing a shell-type transformer according to claim 2,
wherein
the E-shaped iron core includes a retaining plate made of a nonmagnetic material and disposed at a middle position in the stacking direction, in the rotating the E-shaped iron core,
two coupling members configured to be attachable and detachable are mounted to the retaining plate, and
each of the two shaft members is coupled to a corresponding one of the two coupling members in a one-to-one relation.
5. The method of manufacturing a shell-type transformer according to any one of claims 1 to 4, wherein
in the forming an E-shaped iron core, the E-shaped stacked body is pressed with a band for integration.
6. The method of manufacturing a shell-type transformer according to claim 2, wherein
in the rotating the E-shaped iron core,

the E-shaped iron core is held by two holding plates, each of which is disposed at a corresponding one of both ends in the stacking direction,
two coupling members configured to be attachable and detachable for coupling the two holding plates are attached to the two holding plates, and
each of the two shaft members is coupled to a corresponding one of the two coupling members in a one-to-one relation.
7. The method of manufacturing a shell-type transformer according to claim 6, wherein the two holding plates are fastened with a fastening member so as to be fixed to the E-shaped stacked body.
8. A shell-type transformer comprising:
a tank;
an iron core assembly including a lower yoke iron core, an upper yoke iron core and three leg iron cores that are formed by a plurality of electromagnetic steel plates stacked on one another and that are connected to one another, the iron core assembly being enclosed in the tank; and
a coil enclosed in the tank and disposed such that a leg iron core located at a middle position among the three leg iron cores is inserted through the coil, wherein the tank is formed of
a tank main body in which the iron core assembly is housed, and a cover portion attached to the tank main body.
9. The shell-type transformer according to claim 8, wherein
the iron core assembly includes a retaining plate made of a nonmagnetic material and disposed at a middle position in a stacking direction of the plurality of electromagnetic steel plates.
10. The shell-type transformer according to claim 9, wherein

the retaining plate includes at least one protruding portion at a position along a side edge of each of two leg iron cores on both sides among the three leg iron cores as seen in the stacking direction.
11. The shell-type transformer according to any one of claims 8 to 10, wherein the iron core assembly includes a band for binding each of the lower yoke iron core and the three leg iron cores.