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Let turns from equation 2 be substituted in equation 5. Then we have

N1/I1 = N2I2 [11]

or the primary and secondary ampere-turns are equal and opposite. This equality holds for only the load component of I1; that is, exciting current has been regarded as negligibly small. If there is a direct current in the load, but not in the primary, or vice versa, equation 11 is true for only the a-c components.

Fig. 7. Transformer polarity

A 1:1 turns ratio transformer is shown diagramatically in Fig. 7. Impressed voltage is E1, and primary current is I1. Induced voltage Ei is slightly less than E1, and is the same in magnitude and direction for both windings. Secondary current I2 flows in the opposite direction to I1. Instantaneous polarities are indicated by + and - signs. That is, when E1 reaches positive maximum so do Ei and E2. Dots are conventionally used to indicate terminals of the same polarity; dots in the circuit symbol at the right of Fig. 7 are used to indicate the same winding directions as in the left-hand figure.

Last Update: 2011-01-24