Voltage Sag Immunizer for AC Contactors

Power grid disturbances, mainly voltage sags, which sometimes last less than a fraction of second, can cause a plant to shut down and interrupt production for considerable periods of time, which leads to loss of production and high costs. AC contactors are a safe, easy and cheap way to control electric loads. AC contactors provide a remote switch between electric motors and the power grid that supplies them. AC contactors are more vulnerable to voltage sags, than the motors they control. Often, the AC contactor is forced to open when a short voltage sag occurs and as a result the motor stops, whereas the motor alone, due to its inertia, could withstand this voltage sag. This article describes an electronic device that enables AC contactors to ride through power quality disturbances. The proposed device is connected to the contactor coil and consists of a power conversion excitation and hold-in circuit, a control circuit, an immunity circuit and a shutdown circuit. It does not disturb contactor operation, is easy to use and can be built from cheap, commercially available components. Experimental tests have demonstrated the effectiveness of the proposed electronic device for improving the immunity of AC contactors during voltage sags.