The term φm is called the magnetic loading and I1T1 is called the electric loading.

**Volt / turn equation **

Rating of the transformer per phase
kVA / ph = V_{1}I_{1} x 10^{-3} = E_{1}I_{1}
x 10^{-3}

= 4.44 φm f T_{1} I_{1} x 10^{-3}

The term φ_{m} is called the
magnetic loading and I_{1}T_{1} is called the electric loading.
The required kVA can be obtained by selecting a higher value of φ_{m}
and a lesser of I_{1}T_{1} or vice-versa.

As the magnetic loading increases, flux
density and hence the core loss increases and the

efficiency of operation decreases.
Similarly as the electric loading increases, number of turns, resistance and
hence the copper loss increases. This leads to reduced efficiency of operation.
It is clear that there is no advantage by the selection of higher values of I_{1}T_{1}
or φ_{m}. For an economical design they must be selected in certain
proportion. Thus in practice

Where K = 4.44 f x 10^{3} x
K_{t} is another constant and
kVA is the rated output of the transformer. The constant K depends on the type
of transformer-single or three phase, core or shell type, power or distribution
type, type of factory organization etc.,

**Emperical
values of K :**

( 1.0 to 1.2) for single phase shell type

1.3 for three-phase shell type (power)

(0.75 to 0.85) for single phase core type

(0.6 to 0.7) for three phase core type (power)

0.45 for three-phase core type (distribution)

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Design of Electrical Machines : Transformers : Volt / turn equation - Design of Transformers |

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