Turns Ratio

TRANSFORMER THEORY Transformers Turns Ratio Each winding of a transformer contains a certain number of turns of wire. The turns ratio is defined as the ratio of turns of wire in the primary winding to the number of turns of wire in the secondary winding. Turns ratio can be expressed using Equation (13-1). (13-1) Turns ratio N_P N_S where N_P = number of turns on the primary coil N_S = number of turns on the secondary coil The 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) (Figure 1). Impedance Ratio Maximum power is transferred from one circuit to another through a transformer when the impedances are equal, or matched. A transformer winding constructed with a definite turns ratio can perform an impedance matching function. The turns ratio will establish the proper relationship between the primary and secondary winding impedances. The ratio between the two impedances is referred to as the impedance ratio and is expressed by using Equation (13-2). (13-2) � � � � � � � � N_P N_S 2 Z_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. 0 where N_P = number of turns in the primary N_S = number of turns in the secondary Z_P = impedance of primary Z_S = impedance of secondary ES-13 Page 2 Rev. 0