Assumptions and final structure of series motor equations were presented. In the description, a magnetization curve, which significantly affects properties of the motor, was taken into account. Nonlinearly connected with magnetization curve is present in a induction voltage drop and in a rotation voltage, as well. Quantitive examinations were performed for a series motor PBSOa12x type. A share of current dependent induction voltage drop components was examined. It was assumed that additional, so-called “negative inductance”, which is a product of a current and partial, current derivative of inductance, can be omitted. This conclusion can be regarded as general. It refers to a typical motor taking into account construction and magnetic materials. So-called incremental equations of a motor were formulated, which describe it behaviour in a small deviation from an equilibrium point. Using a precise model, a simulation of a motor start fed from a DC controlled rectifier was performed. The simulated system was a closed one with a current limitation. The feeding voltage was an input signal.