Detection of contact position from coil current in electromagnetic

Abstract

An electromagnetic switch for opening and closing a set of contacts has a microprocessor which monitors coil current to determine the state of the contacts. The switch relies on the phenomenon that the inductance of the switch coil changes significantly between the open position of the contacts when the switch armature carrying the moveable contact is spaced by an air gap from the core of the electromagnet, and the closed position in which the armature abuts the core. For an AC energized coil, which is self-limiting in that the increase in inductance from closing of the contacts reduces the current to the holding value automatically, a coil current of less than about one/half of the nominal open contact value is an indication that the contacts are closed. For a DC coil, where the coil current is regulated by a current regulator to a closing value and a holding value, the coil current is measured and then the current regulator is turned off for an interval during which time the current is recirculated through a fly back diode connected across the coil. After a period of time, which in the preferred embodiment is one cold, open time constant of the coil, coil current is measured again. If the second value of coil current is less than about one/half of the initial value, the contacts are open, otherwise the contacts are closed.