The voltage ratio is defined as the ratio of the Primary voltage of the transformer to the secondary voltage.
In transformers with small transformation ratios, the Turns ratio is usually equal to the Voltage ratio. However, for transformers with higher transformation ratio, the voltage ratio may be different from the Turns Ratio. This is known as Ratio Error.
Ratio error can be caused by a number of factors such as leakage inductance, copper losses within the transformer and inter winding capacitance.
The Ratio Error is detected by a ratio test. A simple ratio test involves applying a fixed voltage on the primary and measuring the voltage induced in the secondary.