The given figure represents a transformer with two windings

The given figure represents a transformer with two windings 1 and 2 wound on the core as shown. By applying a voltage V₁ across winding 1, a voltage V₂ is induced across winding 2. In an idealised condition V₂ would lag V₁ by

Figure shows that two fluxes produced by the windings are opposing each other, this inducing voltages having phase difference of 180 degrees.