A relay, in the general sense of the word, is any apparatus which serves to actuate or control the regime of a high power system by the action of a relatively small power on the relay. An electric relay is a mechanism which actuates auxiliary electric circuits or mechanical devices, when electrical factors act on it. The relay mechanism consists of a sensing member and an operating member; the former “perceives” the action of the electrical factors and actuates the operating member, which performs some operation.
Relays are widely used for protection against circuit overload, keying remote switching, reversing, control etc. The uses of relays in industrial control circuits are numerous and it will not be possible to describe all of them here. It can generally be said, however, that the relay is used in any industrial circuit in which a large amount of load power is to be controlled by a small amount of control power.
Basic Construction of Electromagnetic Relays:
Figure 2.25 (a) shows the basic construction of a typical electromagnetic relay. The electromagnetic relay essentially consists of a coil to which a voltage is applied, a core upon which the coil is wound, relay contacts, and a movable steel armature which is held against one of the contacts by means of the spring. Before the voltage is applied to the coil the relay is said to be in its normal or de-energized position. Its action depends on the interaction of the magnetic field set up by the coil carrying the current and the movable steel armature.
Operation of Electromagnetic Relays:
In the relay shown in Fig. 2.25(a) any external circuit connected between terminals 1 and 3 is in a closed position at this instant and the external circuit connected between terminals 1 and 2 is in an open position. When the voltage is applied to the coil, the relay is energized and the magnetic field set up around the coil makes the armature to be attracted toward it. Contact 4 breaks with terminal 3 but makes with terminal 2, and any external (controlled) circuit connected between terminals 1 and 2 is now completed. The relay remains in the position till the coil remains energized.
The schematic interpretation of this basic relay is illustrated in Fig. 2.25 (b). Terminals 2 and 3 are the fixed contacts, and terminal 1 is connected to the movable armature (in some relays the armature remains isolated from all contacts). The contacts of the basic relay acts as single pole double throw (SPDT) switch. Although switch contacts are manually controlled, but relay contacts are remotely controlled.
Relay contacts are made of metals and alloys of low resistance in order to prevent overheating. In addition, the metal used in the manufacture of the contacts must be able to resist both the welding action that occurs when the contacts close and the arcing that forms when the contacts open.
The shapes of the contacts should be such as to resist the pity action. Some common shapes are shown in Fig. 2.25 (c). Metal used in the manufacture of relay contacts include silver, tungsten, palladium and certain alloys. The current at which any given pair of contacts will interrupt successfully (without arcing) is generally smaller for dc than ac voltages, and is smaller at higher voltages.
Such relays have a shading coil placed over a portion of the pole face at the air gap to reduce vibration and chattering of the contacts. The shading coil (a single turn of wire fastened over approximately half of the pole face area) shifts the phase of the flux in the shaded portion of the pole face. This prevents the total force of the flux (taken over the entire pole) from decreasing to zero at any point in the cycle.
As a result, chattering is minimized. In certain ac relays, a capacitor is connected in series with the relay coil, producing a frequency-sensitive device. The relay is then energized only when the applied ac signal is at or near the resonant frequency of the coil-capacitor combination.