dq transformation using phasor quantities
Park-vector \${\over i (t)}\$ represents the magnetomotive force of the three-phase winding system denoted by  \$\alpha \beta \$ or d, q instead of x, y.

For 3 phase currents a,b,c including the 120 angle , a denotes the real axis.



Park transforms can also be done by Fourier Sereis for steady state.

Ref