Models 5E, 5F, 5G & 5H Electric Switch Machines: Single ... - Alstom

Theory of Operation2.5.9.2. Overload RelayThe Overload Relay, Figure 2–37, operates to release the energized contactor, which inturn opens the motor circuit whenever an overload is encountered. The operating coil isconnected in multiple with external resistance wire RT. Normally, most of the operatingcurrent flows through the resistance wire and therefore, there is not sufficient currentflowing in the operating coil to operate the relay.High current, such as the motor would draw if there were an overload, heats resistancewire RT and increases its resistance. This effect causes sufficient current to flowthrough operating coil 0 to operate the relay. Once operated, the armature is held up bya stick coil until the control circuit is either interrupted or reversed.A metal slug and washers slow operation of the relay so that it will not operate inresponse to the momentary surges of current that occur when starting or reversingthe motor. When the overload relay operates, it removes energy from contactors C1 andC2.Figure 2-37. Overload RelayP1293, Rev. Jun/07 2–36 Alstom Signaling Inc.