The no-load test is designed to determine the variable ratio K of the three-phase transformer, the no-load current I0 and the no-load loss P0.

The operation steps are as follows: the high voltage side windings are in an open circuit, and the low voltage side windings apply the rated voltage of the prescribed frequency, and the U1, U20, I0 and P0 are measured and recorded. The K and other values are calculated according to the measured data.

The test of short circuit is to find short circuit impedance and copper consumption of PK.

The autotransformer increases the voltage gradually from zero and pays attention to the ammeter. When the readout of the ammeter reaches the rated current of the high voltage side, it stops setting up, and records the Ik value of the short circuit current, pK value of the short circuit loss power and Uk value of the short circuit voltage . The parameters are calculated according to the measured data. According to the technical standard, the resistance value of the winding should be converted to the referenced temperature of its insulation grade. The referenced temperature of the A, B and E insulation is 75℃. So it is necessary to record the temperature of the winding during the test.