There are Two main reasons for the need to transform from three phases to two phases.
1. To give a supply to an existing two phase system from a three phase supply.
2. To supply two phase furnace transformers from a three phase source.

TEC incorporates a special three phase core design which has two windings, the main and teaser, the centre limb is 41% larger in section and left unwound.

The three phase side is connected in a Tee formation, with the main winding wound for the full line volts and centre tapped. This winding is also interleaved so that any magnetic effect of a current entering at the centre point is cancelled out.
The teaser winding is wound for 87% of the line volts with one end connected to the centre point of the main winding.

Each secondary is a simple single phase winding with the volts and amps no different from what would be expected in an ordinary single phase transformer.

 TEC Products