Four wire monitor and control for railway points - has three phase motor drive position indicated by current sensing relay circuit

摘要

The four-wire monitor and control system for three-phase drives on railway points installations provides both power supply to the drive motors and supervisory signaling by using only a four-wire connection between points and control centre. DC power supply for the monitor relay and points drive repeater relay is provided by a rectifier coupled via a transformer to a separate AC supply. Two further secondary windings of the same transformer are in series with the neutral line and one of the phases of the three-phase supply to the motor drive. The drive is fitted with limit switches so that when the points reach the correct operating position the currents in the transformer secondaries are in antiphase, causing the repeater relay to drop out.