Open Circuit Test and Short Circuit Tests are two important tests which are carried out on a Transformer to determine its equivalent circuit parameters, Voltage Regulation and Efficiency. Though theses parameters can also be found using the physical dimension of Core and Winding detail but using Open Circuit and Short Circuit Tests are quite easy and simple.

The circuit diagram for Open Circuit Test is shown below. As clear from the figure below, Voltmeter, Ammeter and Wattmeter are connected in LV side of the Transformer and HV side is left open. Rated LV voltage is applied to the LV side of the Transformer and the reading of Voltmeter, Ammeter & Wattmeter is noted for further analysis.

As the Transformer Secondary i.e. HV side is kept open therefore Transformer will only take excitation current to set up magnetic flux in the core. Therefore Ammeter A will read Excitation Current Ie which is around 2-6% of the full load current. As Ie is very less therefore, Primary leakage impedance drop is negligible and we can say that applied voltage V1 to LV side is equal to the voltage induced in the Primary winding i.e. E1. Therefore, the equivalent circuit when referred to Primary side reduces to as shown in figure below.

In input power as metered by Wattmeter consists of two components, one is Core Loss and another Ohmic Loss. The exciting current being 2 to 6% of the full load current, the ohmic loss in the primary (I_{e}^{2}r_{1}) varies from 0.04 to 0.36% of full load Primary ohmic loss. In view of this, the ohmic loss in Primary is negligible when compared to the normal core loss which is being directly proportional to square of applied Voltage. Therefore wattmeter reading can directly be taken as the core loss in the Transformer. Thus we see that Open Circuit Test gives us Core Loss of Transformer. The phasor diagram for Open Circuit Test of Transformer is shown below.

Let,

V_{1} = Applied Rated Voltage on Primary

I_{e} = Excitation Current

P_{c} = Core Loss

The Core Loss, P_{c}= V_{1}I_{e}CosƟ

So, no load power factor CosƟ = P_{c} / (V_{1}I_{e})

Also from phasor diagram,

I_{c} = I_{e}CosƟ

I_{m} =I_{e}SinƟ

But P_{c} = V_{1}I_{c}

So, I_{c} = P_{c}/ V_{1}

Therefore,

Core Loss Resistance R_{c}= V_{1} / I_{c}

= V_{1}/ I_{e}CosƟ

= V_{1}^{2}/ V_{1}I_{e}CosƟ

= V_{1}^{2}/ P_{c}

Now,

Magnetizing Reactance X_{m }= V_{1} / I_{m}

= V_{1} / I_{e}SinƟ

Here R_{c }and X_{m}are values referred to LV side. Sometimes a voltmeter is placed in the open HV side of the Transformer to measure the Secondary voltage to get turn ratio.

Thus, Open Circuit Test gives us the following information:

- Transformer Core Loss at rated voltage and frequency.
- Transformer Shunt Branch Parameters like Rc and Xm.
- Turn Ratio of the Transformer.

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