Instead of using three number of single phase transformers, a three phase transformer can
be constructed as a single unit. The advantage of a single unit of 3-phase transformer is that
the cost is much less compared to a bank of single phase transformers. In fact all large
capacity transformers are a single unit of three phase transformer.









To understand, how it is constructed let us refer to figure. Here three, single phase
transformers are so placed that they share a common central limb. The primary and the
secondary windings of each phase are placed on the three outer limbs and appropriately
marked. If the primary windings are connected to a balanced 3-phase supply (after
connecting the windings in say star), the fluxes Ο†
A
(t), Ο†B(t), Ο†B(t)
will be produced in the cores differing in time phase mutually by 120°. The return path of
these fluxes are through the central limb of the core structure. In other words the central
limb carries sum of these three fluxes. Since instantaneous sum of the fluxes,
Ο†
A
(t)+ Ο†B(t) + Ο†c
(t) =0 , no flux lines will exist in the central limb at any time. As such the
central common core material can be totally removed without affecting the working of the
transformer. Immediately we see that considerable saving of the core material takes place if
a 3-phase transformer is constructed as a single unit. The structure however requires more
floor area as the three outer limbs protrudes outwardly in three different directions.
A further simplification of the structure can be obtained by bringing the limbs in the same
plane as shown in the figure. But what do we sacrifice when we go for this simplified
structure? In core structure of figure, we note that the reluctance seen by the three fluxes
are same, Hence magnetizing current will be equal in all the three phases. In the simplified
core structure of figure, reluctance encountered by the flux Ο†Bis different from the
reluctance encountered by fluxes Ο†B(t) , Ο†c
(t).
Hence the magnetizing currents or the no
load currents drawn will remain slightly unbalanced. This degree of unbalanced for no load
current has practically no influence on the performance of the loaded transformer.
Transformer having this type of core structure is called the core type transformer.
For bank of three single phase transformers cost about 15% more than that of 3‐phase unit
transformer. Also, they occupy more space. But the spare capacity cost will be less and single
phase units are easier to transport.