Polarity

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 E_i 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 E_i 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.