The ideal transformer may be considered as the limiting case of the linear transformer of Section 14.7. Thus, in (14a) set

that is, the ampere turns of the primary equal the ampere turns of the secondary. A rule can be formulated which extends this result to transformers having more than two windings. A positive sign is applied to an ampere-turn product if the current enters the winding by the dotted terminal; a negative sign is applied if the current leaves by the dotted terminal. The ampere-turn dot rule then states that the algebraic sum of the ampere-turns for a transformer is zero.

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R₁ = R₂ = X₁₁ = X₂2 = 0 (no losses) and then let XM → (infinite core permeability), to obtain = - lim [a² Xy la) (2₁) (jXMla)(Z₁) (jXMla) + ZL in agreement with (16) Ampere-Turn Dot Rule Since a = N₁/N₂ in (15), Z in 12/²₁] = a ²2₁. L N₁I₁ = N₂1₂