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Index

Open circuit test or No load Test of Transformer

The Open circuit test of the transformer is one of the type test of transformer by which core losses of the transformer are determined. In this test normal voltage is applied on the low voltage side at rated frequency and high voltage side is open condition that means there is no load in the transformer. At that time the reading of wattmeter connected on the low voltage side gives the no load loss or core losses of the transformer and no load impedance of transformer.

For measuring the Open circuit test, the high voltage winding is left open .A wattmeter, a voltmeter and an ammeter are connected in the another winding i.e low voltage winding of the transformer. The connection of Wattmeter, voltmeter and ammeter are shown in the figure. Now normal voltage applied to the low voltage winding and then recorded the reading of wattmeter, voltmeter & ammeter which are connected to the low voltage winding.

1. The Wattmeter measures the iron loss (consisting of the hysteresis loss and the eddy current loss ) of the transformer because the cu- loss is negligibly small in low voltage winding and nil in the high voltage winding under no load condition.
2. The Ammeter measures the no load current I₀ which is very small ( 2 to 10 % of rated load current).
3. The voltmeter measures the normal voltage which is applied in the low voltage winding.

Let W is the wattmeter reading and V₁ is the applied voltage and I₀ is the ammeter reading ,Then

W = V₁I₀Cos Φ₀

∴ cosΦ₀= W/V₁I₀

And Iµ = is the magnetizing component of no load current, Iw is the core loss component of no load current, from the vector diagram of no-load transformer,

Iµ = I₀ sin Φ₀, I_w= I₀ cosΦ₀,

∴ X₀ = V₁/ Iµ and R₀ = V₁ /I_w Thus,

Z₀=√(R₀²+ X₀²)

The admittance is the inverse of impedance. Therefore,

Y₀= 1/Z₀

The conductance G₀ can be calculated as,

G₀=W/V₁²

Hence the exciting susceptance ,

B₀=√(Y₀²- G₀²)

Q-1: Why are copper losses neglected in open circuit test ?

Q-2:Why is kept open high voltage side of transformer during measuring the open circuit test?

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