Why is required to vary the field current in one direction while performing the open circuit test?

Varying the field current in one direction during the open circuit test helps establish a consistent magnetic field in the machine, allowing for accurate measurement of the generated voltage. This method ensures that the relationship between field current and voltage is linear and helps avoid hysteresis effects that could arise from reversing the field current. Additionally, it simplifies the analysis of the machine's performance characteristics, such as the voltage regulation curve.