TRANSFORMER THEORY TransformersTurnsRatioEach winding of a transformer contains a certain number of turns of wire. The turns ratio isdefined as the ratio of turns of wire in the primary winding to the number of turns of wire in thesecondary winding. Turns ratio can be expressed using Equation (13-1).(13-1)Turns ratioN_{P}N_{S}whereN_{P} = number of turns on the primary coilN_{S} = number of turns on the secondary coilThe coil of a transformer that is energized from an AC source is called the primary winding(coil), and the coil that delivers this AC to the load is called the secondary winding (coil) (Figure1).ImpedanceRatioMaximum power is transferred from one circuit to another through a transformer when theimpedances are equal, or matched. A transformer winding constructed with a definite turns ratiocan perform an impedance matching function. The turns ratio will establish the properrelationship between the primary and secondary winding impedances. The ratio between the twoimpedances is referred to as the impedance ratio and is expressed by using Equation (13-2).(13-2)N_{P}N_{S}2Z_{P}Z_{S}Another way to express the impedance ratio is to take the square root of both sides of Equation(13-2). This puts the ratio in terms of the turns ratio, which is always given for a transformer.0whereN_{P} = number of turns in the primaryN_{S} = number of turns in the secondaryZ_{P} = impedance of primaryZ_{S} = impedance of secondaryES-13 Page 2 Rev. 0

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