08-16-2017, 08:42 PM
[attachment=15020]
A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the page link is magnetic and mechanical
OPERATION OF RELAY
A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core, an iron yoke, which provides a low reluctance path for magnetic flux, a movable iron armature, and a set, or sets, of contacts; two in the relay pictured. The armature is hinged to the yoke and mechanically linked to a moving contact or contacts. When an electric current is passed through the coil, the resulting magnetic field attracts the armature, and the consequent movement of the movable contact or 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 vice versa if the contacts were open. 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.
MOTOR PROTECTION
For motor protection following relays are used-
1. Over current relay
2. Earth fault relay
3. Single phasing relay
4. Stall protection relay(Lock rotor protection)
5. Over load relay
TRANSFORMER PROTECTION
For transformer protection following relays are used-
1. Differential relay
2. Over current relay
3. Earth fault relay
4. Temperature relay
5. Bucholz relay