The effects of magnetic saturation, hysteresis and eddy currents on rail track impedance

The authors describe an approach used to identify and measure the magnetic properties of rails that cause additional power loss (eddy current and hysteresis) and contribute to rail inductance. Experiments are described from which rail magnetic and electrical data can be obtained. Some simplified theoretical approaches are presented to quantify the results. Finally, some applications are described where traction and track circuit signals can combine to suffer distortion and create interference. It is shown that it is not possible to devise a complete analytical model for series rail track impedance because nonlinear saturation effects produce harmonics which contribute to both the internal inductance and the AC resistance change. Separation of rail inductance into external and internal components, with external inductance depending on rail shape as well as position, is valid only for far-field effects. For near-field effects, such as required for cab signaling receiver studies, more accurate modeling is required. The most important determinator of induced voltage is the rail shape