The paper deals with significance of the starting generator (SG) driving torque modeling in the studies of synchronous starting of synchronous machines (SSSM). The detailed mathematical model of SSSM in reversible pumped storage plant (RPSP) "Bajina Bašta" is derived. Because it is very difficult to include the detailed model of the SG turbine in the system model, function of SG driving torque is modeled approximately. The most frequent case in SSSM studies is its approximation with a time linear function, until it reaches the value that should provide rotating of both machine rotors with synchronous velocities, which is also used in the paper Afterwards, three different cases are considered: constant, linear and polynomial approximation in terms of SG rotor velocity. Numerical results obtained by applying developed models to a particular case of synchronous starting are compared with the corresponding experimental results. In this way, it is shown that the application of the most frequently used SG driving torque modeling leads to the erroneous results. The advantages of the polynomial approximation suggested in the paper and its validity is demonstrated.