Open Circuit Test of Single Phase Transformer

open circuit test of single phase transformer
The set up for open circuit test of single phase transformer is shown in the figure. This test is carried on the transformer to determine its iron losses.
In this test generally, AC voltage is applied on low voltage side and the secondary is kept open.
The primary reason of performing open circuit test on the low voltage side is that it draws significantly large no load current for convenient reading. Also, it requires only low voltage supply which is easily available and safe to perform the test.

This test is performed to determine the iron losses, Ro and Xo of transformer.

Procedure for Open Circuit Test Transformer


  • Connect the circuit as shown in the figure.
  • Keep the autotransformer at its minimum output voltage position.
  • Switch on the power supply and adjust the autotransformer to get the rated supply voltage.
  • Now note down the current and power shown by the ammeter and wattmeter respectively. Let these are Io and Wo.

  • The wattmeter reads the no load input power to the transformer. The no load current of the transformer is very small as compared to full load current (about 3 to 5% of the full load value) and hence the copper loss in the winding connected to the supply is small. As the high voltage winding is kept open therefore the copper loss in that winding is zero. Therefore the total copper loss is very small and can be neglected. Hence the watt meter reading represents the iron losses.
  • i.e Wo = Pi = Iron losses

While performing the open circuit test of a single phase transformer, high voltage winding should not be touched because it may cause a serious electric shock.

Calculation of Parameters

The two parameters which can be calculated from the open circuit test of a transformer are Ro and Xo. They are calculated as follows.
Step 1: Calculate no load power factor (cos φo)
The wattmeter reads the real power input.
Therefore, Wo = VoIocos φo
or cosφo = Wo/VoIo
We can calculate φo from this.

Step 2: Calculate Im and Iw :
Im = Iosin φo
Iw = Iocos φo
Step 3: Calculate Ro and Xo
Ro = Vo/Iw Ω
Xo = Vo/Im Ω
The value of power factor of a transformer at no load is very small. Therefore the watt meter used while performing the open circuit test of a single phase transformer should be able to show accurate readings on small power factors.

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