A Relays is automatic device which senses an abnormal condition of electrical circuit and closes its contacts. These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit.
Classification of Relays
According to the relays operating characteristics, the relay may be classified as follows:-
1) Solenoid type: In this type of relay, an iron core or plunger is moved into the solenoid coil.
2) Attracted armature type: This type of relay is connected in a circuit and this type of relay operate when the current of the circuit increase beyond the normal. The iron armature of the relay is attracted by the magnetic force which is produced by the non-desired current in the relay.
3) Electromagnetic type: In this type of relay, the coil of the relay moves in an electromagnetic field i.e. the coil as moving component.
4) Moving coil type: These relays have a coil which moves in a magnetic field which provided by the permanent magnet.
5) Induction type: In this type of relay, a metal disc or cylinder is placed in an alternating magnetic field. Hence, a rotating torque is produced in the disc or cylinder by the interaction of eddy current and the alternating field.
6) Direction or reverse current type: The relay is actuated only when the direction of the current is reversed or the phase angle of the current takes up a phase displacement more than the desired value.
7) Directional or reverse power type: When the applied voltage and current attains a certain specified phase displacement, at that time this type of relay is actuated.
8) Under voltage under current relay: This type of relay is actuated when rated voltage current falls below the specified value.
9) Over current & over voltage relay: This type of relay is actuated when rated voltage current rises above a certain specified value.
10) Thermal relay: This type of relay is actuated when the heat produced by the current in the relay element.
11) Different relay: This type of relay operates when the different in magnetic or phase of current on the voltage of relay.
12) Distance Relay: The principle of operation of this type of relay is dependent on voltage to the current ratio or is dependent on the current by the relay element.
Time of operation of the relay can also control, the i.e time between the instant of relay operation and the instant of relay operation of its tripping contacts can be controlled. In this point of view, the relays can be classified as follows:-
a) Instantaneous Relay: In this type of relays a definite time intervenes between relay operation and operation of tripping contacts.
b) Definite time Relay: In this type of relays a definite time intervenes between relay operation and operation of the tripping contracts.
c) Inverse time relay: In this type relay, the time elapsed between the relay operation and operation of the tripping contact is inversely proportional to the magnitude of fault current.
d) Inverse –define-minimum time lag: In this case, the operation of the relay is inversely proportioned to the value of current time of the relay tends to a definite minimum time.
In this time lag relay, a permanent magnet has arranged a position that the relay motor cuts the flux between the poles of this magnet, such a magnet is called the “drag magnet”. The mechanical device such as bellows, oil das pots etc.. is used as a delay element.
In a dash pot arrangement, a piston in an oil dashpot is connected to the lower end of the solenoid plunger is pulled, the piston in the coil retards plunger motion, thus provides the necessary delay in the relay operation.