The ratio test ensures that the transformer windings have the proper turns to produce the voltages required. The ratio is a measure of the voltage applied to the primary terminals to the voltage measured at the secondary terminals.

During the test, a known voltage is applied to one winding while the other winding is left open. By measuring the voltage on both windings, you can calculate the turns ratio.

It's a crucial test in ensuring the transformer operates within its specified voltage transformation range, helping identify any faults or discrepancies in the winding turns which could affect performance.