Inductive Interference in Audio-Frequency Rapid Transit Signalling Systems Due to Chopper Propulsion Control

Use of chopper-controlled propulsion systems on rail rapid transit vehicles can cause inductive interference with audio-frequency signalling systems. Inductive interference is caused by the time-varying magnetic flux lines, emanating from propulsion equipment on the vehicle, passing through the rail-axle loop under the vehicle. It is evidenced by the observation of abnormally high levels of rail-to-rail voltage observed at locations under the vehicle. The passage of a vehicle over a fixed location induces a transient interference voltage from rail to rail which can have measurable harmonic content throughout the audio-frequency spectrum. The induced voltages can be coupled into audio-frequency track circuit apparatus, and can disrupt the normal operation of such equipment. A physical model for calculating interference levels observed is developed in this report, results of field observations are given, and methods for alleviating the problem are discussed.