The mathematical model of the three-phase, synchronous phase modifier has been developed. This model has been represented as a system of six (salient-pole machine) and eight (nonsalient-pole machine) differential equations for flux-linkages and angular velocity determination. The differential equations for the flux-linkages of the stator, amortisseur and excitation windings have been expressed in synchronously rotational rectangular direct and quadrature coordinates. Both the rotational and transformation electromotive forces have been taken into consideration in the stator differential equations. The additional resistance for excitation current control has been taken into consideration in the differential equation of the excitation winding. The currents in both axes have been presented in matrix form. The curves of the synchronous phase modifier angular velocity; field and stator currents during induction starting have been calculated and illustrated.