Ratio Conventions

The ratio of the voltage (5:1) is equal to the turns ratio (400:80). Sometimes, instead of specific values, you are given a turns or voltage ratio. However, there are two conventions regarding the notation of Turns Ratio. The American version of Turns Ratio convention is:

Whereas the British version of Turns Ratio convention is:

The effect of having different notations is minimal, providing you know which one is being used whenever a turns ratio is being provided in any question.

If a turn ratio is given as 6:1 (British notation), you can assume a number of turns for the primary and compute the secondary number of turns (10:60, 6:36, 5:30, etc.), or if American notation of 6:1 were used, , you can assume a number of turns for the primary and compute the secondary number of turns (60:10, 36:6, 30:5, etc.).

The transformer in each of the above problems has fewer turns in the secondary than in the primary. As a result, there is less voltage across the secondary than across the primary. A transformer in which the voltage across the secondary is less than the voltage across the primary is called a step-down transformer. The ratio of a four-to-one stepdown transformer is written as 4:1 in American notation, or 1:4 in British notation. A transformer that has fewer turns in the primary than in the secondary will produce a greater voltage across the secondary than the voltage applied to the primary. A transformer in which the voltage across the secondary is greater than the voltage applied to the primary is called a step-up transformer. The ratio of a one-to-four step-up transformer should be written as 1:4 in American notation or 4:1 in British notation. Notice in the two ratios that the value of the primary winding is always stated first for American notation, and stated last in British notation.