For transformer turns ratio measurement by precisely measuring the voltage across the unloaded transformer windings at fixed or different tap positions.
The purpose of a transformer is to convert power from one system voltage to another. This voltage relationship, or voltage ratio, is determined by the number of turns on the high voltage coil and the number of turns on the low voltage coil of the transformer. The ANSI specification states that the measured ratios must have a maximum variation of plus or minus 0.5% compared to specified ratios.
The ratio is measured by using a transformer turns ratio meter (commonly called a TTR) which applies a low voltage to the transformer under test. A comparison is made between the transformer under test and an adjustable variable ratio transformer in the ratio meter, when the two ratios’ are equal; a balance is obtained on the detector. A high exciting current during the ratio test may indicate a shorted turn.