Effect on Open Circuit Test result, if rated Voltage and Frequency is doubled?
Since the ratio of the frequency to the voltage remains constant, the magnetizing current will remain constant. The magnetizing current is responsible for the reactive component of the total current. The eddy current losses are proportional to the square of the frequency and the square of the flux variation. Therefore the eddy current losses will increase even though the magnetizing current does not increase. The hysteresis losses are proportional to the frequency, so they will also increase. The increase in iron losses will cause the real component of the current to increase. Thus the total current and the power factor will increase.

Example

An open-circuit test performed at on a transformer at 240 V and 50 Hz. The measured results are 5 A and 200 W. The power factor is calculated to be 0.167. Neglecting the leakage reactance and winding resistance, the equivalent circuit consists of a resistor representing the iron losses in parallel with an inductor representing the magnetizing branch. The value of the resistance is calculated to be 288 ohms and the current in the resistance is 0.833 amps.

In contemplating an open-circuit test at 480 V and 100 Hz, it is assumed that the current and level of magnetic flux will be unchanged since the ratio of voltage to frequency is unchanged. Since the losses are partly proportional to frequency and partly proportional the frequency squared, assume the losses are a little more than double at 100 Hz, say 500 W. That would make the resistance 460.8 ohms and the current 1.041 amps. Since the only change in current is the increase in the current in the resistor, the total current must increase. Since the reactive volt-amperes double due to the doubling of the voltage, and the power more than doubles, the power factor must increase.

It is important to keep in mind that the resistor that represents the core losses is not a real resistor. The value of the resistor is whatever it needs to be to account for the power loses due to hysteresis and eddy-currents in the iron.