The paper presents a particular problem with determining the permissible boundaries of the working area for two-zone asynchronous traction drive with high dynamic requirements, designed to work in a very broad range of motor rotational speed. A result of the procedure of maximizing the internal motor torque, based on an analysis of equations for drive steady-state model, is the family of nonlinear characteristics of the main magnetic flux values desired by the control system, expressed as a function of shaft rotational speed and parameterized with the selected values of the overhead line voltage. On the border between two areas - the first one, at low rotation of the motor shaft, when the magnetic flux remains constant, and the second one, when the magnetic flux is significantly weakened with the increase in shaft speed, these characteristics have asignificant discontinuity in the first derivative, ie. "edge". It can cause quite often unwanted oscillations in the driver control system operation during the dynamic transitions of a drive working point from one zone to another. The paper contains a proposal to reduce this phenomenon by applying a suitable technique of spline functions to smooth the characteristics of the magnetic flux reference values.