The windings of three single-phase transformers can be wound on a common core. The advantages are:
- One 3-phase transformer is cheaper than the three single-phase transformers.
- It has slightly better efficiency and regulation.
- A 3-phase transformer takes less floor space.
On the other hand, from the point of view of standby, or same capacity, it is economical to have 3 single-phase transformers plus one spare rather than two 3-phase transformers one of which in a spare. However, in large central stations 3-phase transformers are often advantageous.
- Three-phase transformers are much more difficult and costly to repair than are single-phase units.
- When failure does occur and it becomes necessary to substitute a replacement unit to maintain service, the cost of spare is much greater that it would be were a single-phase transformer to be used as a replacement in a three-transformer bank.
- There is a difficulty in transporting a heavier three-phase transformer compared with the moving of each of the three single-phase transformers.
Two general kinds of three-phase transformers are recognized, similar to single-phase transformers, depending upon the relative arrangements of windings and cores. These are the core type and the shell type.
Three-phase core type transformer. Fig. 72 shows three core-type transformers placed together so that they have a common path for the return magnetic circuit. Although the windows should be entirely filled by primary and secondary coils on each of the legs, only primary coils are shown on the outside legs. This simplifies the diagram, which it in no way changes the actual theory that follows, since the primary coils set up the flux. If the three transformers are identical in all respects, a balanced three-phase system of voltages will produce three fluxes in the cores which have the same maximum value, but differ in time phase by 1200. In the common leg of the three cores of Fig. 73, the three fluxes add, and the net flux is therefore always zero. The common leg may then be eliminated, with a subsequent saving in core material and size of transformer. A single polyphase transformer would be of impractical construction if it were the same as Fig. 72 with the centre leg omitted. Instead, the core-type polyphase transformer is manufactured so that it looks like that plane. This causes the magnetic reluctance of coil M to differ somewhat from that of L and N. This produces a slight unbalance in the three magnetising currents, but the effect is not serious, especially under load.