Control and operating condition monitoring of dual series switch contactors for electric motor or other electrical load

Abstract

A circuit for controlling at least two power switches that are series connected between a source of electrical power and an electrical load, for energizing the load in response to a signal at a control input. The circuit includes a programmed controller having an input as the control input and a voltage sensor circuit connected to sense the voltage between the power switches and apply a voltage signal to a controller input. It may also have a sensor for sensing the current through the switches and applying a current signal to the controller. The controller inputs the voltage between the switches or the current through the switches to provide a sensed first value. It compares the sensed first value to an expected first value that exists if both switches are turned off. One of the two switches is turned on if the sensed first value is equal to the expected first value. If the first switch was turned on, the voltage at the node between the switches or the current through the switches is sensed to provide a sensed second value. The sensed second value is compared to an expected second value that exists if the first switch is turned on and the second switch is turned off. If the sensed second value is equal to the expected second value, the second switch is turned on. At any of these stages, if a sensed value is unequal to the expected value, turning on of the switches is aborted and both switches are turned off.