The performance ( efficiency, regulation, losses ) of a transformer is best judged if the following four parameters of its equivalent circuit are known :
 The equivalent resistance R_{01} is referred to as primary, and the equivalent resistance R_{02} is referred to as secondary.
 The equivalent leakage reactance X_{01} is referred to as primary, and the equivalent leakage reactance X_{02} is referred to as secondary.
 The coreloss resistance R_{o} ( parallel circuit ).
 The magnetizing reactance X_{o} ( parallel circuit ).
These four parameters or constants of a transformer equivalent circuit can be determined by the following two tests :
(a) Opencircuit test, and
(b) Shortcircuit test.
These tests are very simple and cheap to perform because we can find the required information without actually loading the transformer.
Open Circuit Test :
The aim of the test is to determine the ironloss or coreloss and noload current I_{o} which is helpful in finding R_{o} and X_{o}. The amount of power wasted in the core of a transformer can be obtained by this test.
In this test one winding usually, high voltage winding is left open and the other ( L.V. side ) is connected to normal voltage supply as shown in the figure. Since normal voltage is applied to the primary, the normal flux will be set up in the core hence normal coreloss will occur which are measured by the wattmeter.
As no current flows in the open secondary, the noload primary current I_{o} is small ( usually 3 to 10 of fullload current ), the primary copper loss is negligibly small and nil in secondary. Hence, the O.C. test gives core loss alone practically ( i.e., wattmeter reading ) and is the same for all loads.
Sometimes, a high resistance voltmeter is connected across secondary to read secondary induced e.m.f. which helps to find the transformation ratio K.
The transformer vector diagram is the same as the noload condition of a transformer as shown in the figure. If W_{o} is the wattmeter reading.
Therefore,
Since noload current I_{o} is very small therefore pressure coils of the wattmeter and voltmeter should be connected such that the current taken by them should not flow through the current coil of the wattmeter.
Separation of Hysteresis and Eddy Current Loss :
The core loss or iron loss consists of two parts
(i) Hysteresis loss
W_{h} = P B_{m}^{1.6} f watt / m^{3}
(ii) Eddy current loss
W_{e} = Q B_{m}^{2} f^{2} watt / m^{3}
Where P, Q are two constants, B_{m} = maximum flux density (T)
 f = frequency (Hz)
The total iron loss is given by,
W_{i} = W_{h} + W_{e}
= P B_{m}^{1.6} f + Q B_{m}^{2} f^{2}
The observation table to perform an opencircuit on the transformer is shown below. Where noload current can be calculated from the ammeter, total power loss ( i.e., iron loss in core ) is calculated through wattmeter, and the voltage applied is known by the voltmeter.
S.No.

Primary Rated Voltage V_{1} (volts)

Noload Current I_{o} (amp)

Wattmeter Readings (iron loss) W_{o} (watt) 
Secondary Induced Voltage V_{2} (volts)

Readings oberved from the test

Why Open Circuit Test is Performed on L.V. Side only ?
The main thing is that an open circuit of a transformer is performed on any side of the transformer i.e., either on H.V. ( high voltage ) or L.V. ( low voltage ). But there are some benefits while performing the O.C. test on L.V. side of the transformer.
The aim of this test is to determine the noload losses i.e., core loss at rated voltage. The term rated voltage means we have to apply the rated value of the voltage on the L.V. side. This rated value of voltage is smaller ( 440V, 11kV ) when compared to the H.V. side ( 33kV, 132kV, and above ).
So when we prefer H.V. side to test it becomes harder, costlier, and care must be taken while performing the test. Because the equipment ( measuring instruments ) used for tests i.e., to apply high voltages about 33kV, 132kV, and above becomes costlier. And precautions must be taken with proper insulation to the body to avoid shocks that may lead to death.
The turns on the L.V. side of the transformer are less than the turns present on the H.V. side. Hence the power loss ( i.e., due to copper loss ) is less almost negligible on the L.V. side of the transformer when it is on noload.
As we know that the open circuit test is to determine the core loss. Thus when a rated voltage is applied on the L.V. side power loss occurred is very small. Therefore, the wattmeter reads only the core loss component of the transformer.