# Calculating Source Impedance for a Transformer for Max Power

A large power transformer has a turns ratio N1/N2 of 1:5. When the primary voltage is 400 V and it is connected to a resistive load, its secondary current is 100 A.

Calculate the required value of source resistance of the supply to give maximum power transfer.

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#### Solution Preview

Turns ratio is N1/N2 = 1/5

Thus, turns ratio squared is {N1/N2}^2 = 1/25

We need to use a combination of the Transformer equations which describe the ratio of primary voltage to secondary voltage V1/V2 and primary current to secondary current I1/I2. Also we need to use the transformation for primary resistance to secondary resistance or impedance ...

#### Solution Summary

Details of a transformer are given including the turns ratio and the primary voltage and secondary current. From the given information the secondary voltage is determined and hence the secondary impedance determined from Ohms law. The primary impedance is thus determined from known equations using the turns ratio for the purely resistive case