Relays : Electrical Seminar Topic

 Relays

        A Relay is an electromechanical device that is actuated by an electric current.All relays contain a sensing unit,control coil and the current may be AC or DC. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Relays were used extensively in telephone exchanges and early computers to perform logical operations.

        Different type of relay that can 0perate on high power required to directly control the loads are called as contactor. Solid-state relays control the power circuits with no any  moving parts, instead of that it uses a semiconductor device to perform switching operation. Relays are used to protect electrical circuits from overload or faults. In modern electric power systems, these functions are performed by digital instruments which are called Protective Relays.

        A simple electromagnetic relay consists of a wire wrapped coil around a soft iron core, an iron yoke which has low reluctance path to the magnetic flux, a movable iron armature and one or more sets of contacts. The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is de energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open.

        When an electric current is passed through the coil it generates a magnetic field that activates the armature, and thus the movement of the movable contacts either makes or breaks a connection with a fixed contact. If the set of contacts was closed when the relay was de energized, then the movement opens the contacts and breaks the connection and when set of contacts was opened, the relay was energized, the movement closes the contact and makes the connection. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly.