International Journal of Automation and Power Engineering, 2012, 1: 141-145 - 141 - Published Online September 2012 www.ijape.org Voltage Sag Immunizer for AC Contactors Dr. P. Suresh Babu 1 , Dr. S.V. Jayaram Kumar 2 , Mrs. P. Rohini Krishna Chaitanya 3 1 Assistant Professor, EE Department, National Institute of Technology, Warangal, INDIA-506004, e-mail: drsureshperli@nitw.ac.in 2 Professor, EEE department, JNTUH College of Engineering, Hyderabad, INDIA-500 082, e-mail: svjkumar10@rediffmail.com 3 Keywords: AC Contactor; Power Quality; Voltage Sags; Immunity. 1. Introduction Power quality disturbances create major problems in continuous process industries. Momentary interruptions, voltage sags and dropouts are some of the most serious power quality problems faced by industrial customers [1-4]. Power grid disturbances, 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 [5, 6]. Most of these disturbances (92-98%) are voltage sags that occur due to lightning strikes, accidents, squirrels or equipment failure on the transmission and distribution grid feeding the plant. Voltage sags (also known as “voltage dips”) are short-duration reductions in voltage magnitude. Their duration is typically between a few cycles of the power system frequency and a few seconds. Most of them are of short duration (1-6 cycles), corresponding to the clearing time of upstream utility protective equipment. Due to industrial equipment reacts to voltage sags in a variety of ways, there is not a unique solution to protect them. In some cases, the best solution is to protect the entire plant from voltage sag with a device such as a dynamic voltage restorer [7]. In other cases it is better and even more economical to identify particularly susceptible components and protect them alone. Adhoc Lecturer, EE Department, National Institute of Technology, Warangal, INDIA-506004, e-mail: prk_chaitanya@yahoo.com (Abstract) 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. An AC contactor is an electrically controlled switch that uses a solenoid to close one or more pairs of electrical contacts when an appropriate AC voltage is applied to the coil of the solenoid. The solenoid consists of an electromagnet that attracts a moveable bar. The electric contacts are attached to the movable bar or armature that is spring loaded so as to cause the armature to move away from the electromagnet when there is no electric signal on the coil. When an appropriate voltage is applied to the coil, the magnetic flux crossing the electromagnet and the armature provides an attractive force that overcomes the spring tension, moves the armature and closes the contacts. AC contactors are often used to provide a remote switch between electric motors and the power grid that supplies them. Although electric motors can ride through some voltage sags [8], AC contactors are extremely sensitive to this type of disturbances [9]. This weakness of AC contactors can cause the unnecessary interruption of industrial process resulting in important economical losses. Many devices that are able to mitigate such problem have therefore been proposed. Most of these provide a momentary ridethrough capability by maintaining the AC contactor in the closed position, by means of various procedures, so that the electric motor or the electric load remains connected to the grid when a power disturbance occurs in the power supply. Different devices with AC/DC conversion are known to regulate the voltage applied to the coil of an electromagnet and specially that of a contactor in order to get it closed in a controlled manner to reduce the bounce of the contacts, and to energize and de-energize the contactor coil quickly [10,11]. Power supplies with AC/DC conversion have been proposed, including a capacitor connected in parallel with a maintenance coil added to the contactor, in order to provide a slight delay in the separation of the contacts in front any power disturbance of the network [12]. It has been described a method and an apparatus to improve the characteristics of devices with AC solenoid during disturbances in the quality of supply. This device is configured to superimpose a DC component to the AC signal provided by the AC source and give to the output, at