In dc machine why one is required to vary the field current in only one direction while performing the open circuit test in dc machine?

In a DC machine, the open circuit test is performed to determine the no-load characteristics, such as the magnetization curve. Varying the field current in only one direction ensures that the magnetic field strength increases consistently, allowing for a clear relationship between field current and generated voltage. This prevents any confusion or complications that may arise from negative or reverse magnetic fields, which could lead to inaccuracies in the test results. Thus, maintaining a unidirectional field current helps in obtaining reliable data for analysis.