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Open circuit test or No load Test of Transformer

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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, Iw= I0 cosΦ0,

∴ X0 = V1/ Iµ and R0 = V1 /Iw Thus,

Z0=(R02+ X02)

The admittance is the inverse of impedance. Therefore,

Y0= 1/Z0

The conductance G0 can be calculated as,


Hence the exciting susceptance ,

B0=(Y02- G02)

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

Ans: As no load current is very small so the no load copper loss is very small in comparison with the iron losses so the copper losses are neglected in open circuit test.

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

Ans:High voltage side is kept open due to suitability of using metering arrangement in the low voltage side rather than high voltage side.

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