Connection diagram:

The test points out the iron losses of the machine: it is carried out both to obtain the parallel elements of the equivalent circuit, and to be able to obtain some of the elements that are necessary for the calculation of the conventional efficiency of the transformer.
The reference quantity is the voltage that has to be therefore suitably varied from a value equal to 1.1 ÷ 1.2 times the rated voltage until zero.

The quantities to be measured are: Voltage Current I1, I2, I3 Power WA, WB

The quantities to be calculated are: No load power Power factor No load current

From the diagrams, in correspondence of the voltage rated value, we obtain respectively:

  I₀,    P₀    and  cosØ₀, 

 that are necessary for the calculation of the parallel parameters of the equivalent diagram: