TTR stands for Transformer Turns Ratio. The function of a transformer is to transform power from one voltage level to another. The ratio test ensures that the transformer windings have the proper turns to produce the voltages required. For example a 480V primary to 240V secondary will have a ratio of 2:1 with a TTR value of 2.
The Standards state that when rated voltage is applied to one winding of the transformer all the other rated voltages at no load shall be correct within one half of one percent of the nameplates readings. It also states that all tap voltages shall be correct to the nearest turn if the volts per turn exceed the one half of one percent of the desired voltage. The ratio test verifies that these conditions are met.
The calculated ratios for a transformer with a rated primary phase voltage of 12470 with 2.5% taps above and below rated voltage and a rated secondary phase voltage of 480 are shown below. The ratio limits given permit a tolerance of one half of one percent from the rated voltage.
Ratio Limits (calculated by dividing the primary voltage by the secondary voltage):
|Voltage||Ratio||Upper Limit||Lower Limit|