A transformer turns ratio test provides a quick verification of the most fundamental operational characteristic of a transformer – its ability to transform voltage as anticipated. In doing so, the test provides invaluable reassurance to the operator. Open- and short-circuit conditions in transformer main and tap windings may cause the transformer turns ratio to change and therefore this test is at once providing useful diagnostic information.
Engineers have noted, however, that traditional "hand-crank" TTR instruments sometimes give values for turns ratio that are different from those given by modern automated TTR instruments. Indeed, many insist that the traditional instruments provide results that are more dependable. But is this actually true? And why do the instruments produce different results?