If you are finding the operating principle of relay then you are at right place. here we know in details what is a relay, relay definition and how relay operates?. so let discuss in detail about the operating principle of the relay.
Relay definition: A relay is an automatic device which senses an abnormal condition in an electric circuit and closes its contacts. These contacts in turn close the circuit breaker trip coil circuit, thereby it opens the circuit breaker and the faulty part of the electric circuit is disconnected from the rest of the healthy circuit.
Operating principle of Relay
Basically, there are two different operating principles of relays: (i) electromagnetic attraction, and (ii) electromagnetic induction.
In the electromagnetic attraction type of relays, the operation is obtained by virtue of an armature being attracted to the poles of an electromagnet or a plunger being drawn into a solenoid. These relays can be operated by both d.c. as well as a.c. quantities. With d.c. the torque developed is constant and if this force exceeds a predetermined value the relay operates.
In the case of a.c. quantity the force is given by
This shows that the force consists of two components, one the constant, independent of time, whereas the other is a function of time and pulsates at double the supply frequency. The total deflecting force, therefore, pulsates at double the frequency. Since the restraining force is constant the net force is a pulsating one which means that the relay armature vibrates at double the power supply frequency. These vibrations will lead to sparking between the contacts and the relay will soon be damaged.
To overcome this difficulty in a.c. an electromagnet, the two fluxes producing the force are displaced in time phase so that the resultant deflecting force is always positive and constant.
This phase displacement can be achieved either by providing two windings on the electromagnet having a phase shifting network or by putting shading ring on the poles of the magnet as shown in Fig. 14.2. However, the shading ring or coil method is more simple and is widely used.
Induction Relays: The induction relays operate based on the electromagnetic induction principle. Therefore, these relays can be used only on a.c. circuits and not on d.c. circuits. Depending upon the type of rotor being used, these relays are categorized as (i) induction disc type, and (ii) induction cup type of relays.
In case of induction disc type of relays, the disc is the moving element on which the moving contact of a relay is fixed whereas in case of induction cup the contact is fixed with the cup.
There are two structures available under the induction disc type of relay: (i) the shaded pole structure, and (ii) the watthour meter structure.
Shaded Pole Structures:
As shown in Fig. 14.2, the disc is placed between the shaded and unshaded poles of the relay. The relay consists of an operating coil which is fed by the current proportional to the system current. The air gap flux produced by this flux is split into two out-of-phase components by a shading ring made of copper that encircles part of the pole face of each pole at the air gap. The disc is normally made of aluminum so as to have low inertia and, therefore, requires less deflecting torque for its motion. Sometimes, instead of shading ring, shading coils are used which can be short-circuited by the contact of some other relay. Unless the contacts of the other relay are closed, the shading coil remains open and hence no torque can be developed. Such torque control is employed where a directional feature is required which will be described later.
some other definition related to relay :
Relay: A relay is an automatic device which senses an abnormal condition in an electric
circuit and closes its contacts. These contacts in turn close the circuit breaker trip coil circuit, thereby it opens the circuit breaker and the faulty part of the electric circuit is disconnected from the rest of the healthy circuit.
Pick up Level: The value of the actuating quantity (current or voltage) which is on the threshold (border) above which the relay operates.
Reset Level: The value of current or voltage below which a relay opens its contacts and
comes to original position.
Operating Time: The time which elapses between the instant when the actuating quantity
exceeds the pick-up value to the instant when the relay contacts close.
Reset Time: The time which elapses between the instant when the actuating quantity
becomes less than the reset value to the instant when the relay contact returns to its normal position.
Primary Relays: The relays which are connected directly in the circuit to be protected.
Secondary Relays: The relays which are connected in the circuit to be protected through
current and potential transformers.
Auxiliary Relays: Relays which operate in response to the opening or closing of its
operating circuit to assist another relay in the performance of its function. This relay may be instantaneous or may have a time delay.
Reach: A distance relay operates whenever the impedance seen by the relay is less than
a prespecified value. This impedance or the corresponding distance is known as the reach of the relay.
Underreach: The tendency of the relay to restrain at the set value or the impedance or
impedance lower than the set value is known as underreach.
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