The paper presents results of an experimental investigation performed under controlled laboratory conditions, in order to identify real causes of variation in the rotational speed of an axial fan that can be noticed after a certain period following the start of operation. The theoretical background important for understanding the observed phenomenon is given and the hypothesis is proposed which states that the temperature rise in motor windings and a consequential rise of their resistances are responsible for such specific behavior of the axial fan. The experiment was performed on a standard test rig with a small industrial axial fan, driven by a three-phase induction motor with the rated power of 1.5 kW. The experimental procedure is described in details, measured and calculated results are presented and appropriate conclusions are given. The obtained experimental results confirm the proposed hypothesis.