This formula is valid for single phase voltage ratio
Remember single phase voltage ratio is not equal to three phase voltage ratio
N_{p}
N_{s}

= 
V_{p}
V_{s}

N_{p}
N_{s}

= 
V_{p}
V_{s}

Using the above sketch, If you apply 120 Vac in primary winding, what will be the expected secondary voltage if you know the transformer have 100 coils in primary and 10 coils in the secondary winding. Transformer ratio was given as 10:1 ; Note transformer ratio , letter symbol used in transformer literature to denote it is " a ". It is define as the ratio of number of turns of primary winding conductor divided by the number of turns of secondary winding conductor.
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