SIGNALLING RELAYS

 SIGNALLING RELAYS

A relay is an electromagnetic device, which is used to convey information from one

circuit to another circuit through a set of contact i.e. front or back contact.

Constructional and electrically, relays may be divided into DC and AC relays, because

the means by which the electrical energy in the coil is converted in to mechanical Energy in

order to move the contacts are fundamentally different.

 In DC type, the contacts are carried on an armature, forming part of a magnetic circuit in which a field is set up by the current flowing in he coils. 

In AC types, the contacts are attached by a link mechanism to a metal sector, disc or cylinder in which currents are induced by the alternating magnetic field produced by the currents in the coils.

Every endeavor has been made to explain the action of each type of relay in the simplest possible manner.

Relays are sophisticated switch gears used for remote control and succession control of

various electrical equipment. In present days they are widely used because they are capable of

protecting the controlled equipment from cross feeding and overloading even as they cater for

speedy operations.

Most of the relays in present day signaling are electromagnetic devices, although some

of the relays control circuits through electronic components like diode/transistors/ Integrated

Chips etc

Railway signaling relays are unique in that:

(a) They operate on low voltage and current

(b) They are more articulate as, according to their special features, they can work under

restrictive conditions and in any specified manner. Virtually they can cater for all

situations while contributing to speed and accuracy in operations

CLASSIFICATION OF SIGNALLING RELAYS:

(a) According to the method of their mounting or fixture, they are classified as:

(i) Shelf type: Relays, which are loosely kept on shelves.

(ii) Plug in type: Relays, which are plugged into a pre- wired plug boards.

(b) According to their connection and usage, they are classified as:

(i) Track relays: Relay, which is directly connected to the track, to detect the

presence of vehicle.

(ii) Line Relays: Other than track relay all are line relays. Relays connected to the

selection circuit.

(c) According to their vitality or importance in ensuring train working safety, they are

classified as:

(i) Vital Relays: All relays used for traffic control such as signal, point, controls,

track detection etc.

(ii) Non-vital Relays: Relays, which operate control aids and accessories like

warnings, buzzers, Indications etc.

(d) According to their special provisions to ensure reliability of their contacts, they are

classified as:

(i) Proved type: are those whose normalization after each operation shall be

proved in circuit controlled by their contacts. Contacts in which both the springs

have metal surfaces on their tips. They may get fused due to high sparking

current across them during operation. These may prevent relay normalization

and causes unsafe condition in traffic control. To avoid this, proving of relay

normalization after each operation is necessary.

(ii) Non - proved type: Need not to be proved to have been normalized after each

operation as their contacts have at least one non-fusible contact (carbon contact)

(e) According to their feed source, relays are broadly classified as:

(i) DC relays: The relay, which requires DC power supply for its operations are,

called DC relays. Among the DC relays.

o DC neutral relays: This relay closes the same set of contacts on

energization, with Normal polarity or Reverse polarity supply.

o Polar Relays: This relay closes different set of contacts when energized with

Reverse polarity supply. They may or may not have contact to close when deenergized.

(ii) AC Relay: AC Induction motor track relays. Time element relays, flashing

indication control etc.

(iii) Electronic Relays: DC relays with electronic components in them are called

electronic relays

DC Neutral Relay 

Each Relay has usually one or two coils with a hollow center to accommodate a core.

The coils are made up of a large numbers of turns of small gauge soft drawn copper wire. The

two coils can be connected in series or parallel according to the requirement of relay resistance.

The ends of the coils are terminated on binding post to which the control wires are connected.

Each coil is placed around a core of specially selected Iron or steel having high permeability and

low retentivity. The core should be susceptible to magnetism and at the same time should have

little residual magnetism. The cores are connected at the top by a yoke to complete the

magnetism coupling between two ends of coils. The bottom of each core is equipped with a

large steel or Iron block known as pole piece or face.

A flat piece of Iron or steel called armature is supported by brackets, which are securely

fastened to the pole piece. The armature, yoke, and the pole pieces are also made of specially

selected iron or steel of the same quality as the core. The armature carries the metallic spring

contacts, which are insulated from it.

The circuit through the coils of the relay is closed. It sets up a magnetic flux through the

core, yoke and the armature. The flux passing between the armature and pole faces causes the

armatures get attracted to the pole faces and armature picks up and closes front contacts.

When the circuit is opened the magnetic flux collapses and the armature drops away by gravity

from the pole faces, the front contacts break and back contacts close. The front and back

contacts of the relay can be utilised to make or break other circuits. Two stop pins of nonmagnetic material are fixed either on the armature or pole faces so that the armature cannot

come in contact with the pole faces. It is essential to maintain a small air-gap between the

armature and pole faces so that low value of residual magnetism may not retain the armature in

picked up position and causes the relay to fail to drop away with a break in its control circuit.

GENERAL USAGE

 DC Neutral line relays are most commonly used for Railway Signalling controls and

detection. Among them, plug-in type relays are preferred in larger installations for space

considerations. Shelf type relays are also in use, mostly in wayside stations.

There are many DC Neutral line relays in use with special features such as

(i) Delayed operation

(ii) Biased DC control

(iii) DC control unaffected by AC interference currents

(iv) Getting latched in operated condition till further feeding, and others.

 Usage of DC polar relays is mostly in conjunction with block instruments that control traffic

between stations.

 AC line relays are almost extinct in installations of British Signalling practice. They are,

however, used for time control operations, flashing indication control and such other special

purposes in installations with Siemens signalling practice widely.

 Track relays are used according to the type of track detection circuits chosen for a given

location and context. While most of the track circuits are still of the DC working type requiring

DC neutral track relays with them, the prospects of their being replaced with Electronic track

circuits directly feeding DC line relays, in future, are great.

 AC Track Circuits are used in DC Traction area, as conventional DC Track Circuits are not

suitable there. AC Track relays are used with them, almost all, of the induction motor type.

 In the British practice of signalling which was first introduced on Indian Railways, nonproved type relays with carbon to metal switching contacts are generally used for vital controls.

They facilitate simple circuit designs. But with the advent of German Practice, introduced

by M/s Siemens later, proved type relays with all 'metal to metal' contacts are widely accepted in

spite of complications in circuit design caused by them. A recent introduction is that of the same

type relays made by M/s Integra control. However, for some time now, the appreciable features

of both the practices are getting incorporated together in the indigenous designs of signalling by

railwaymen. With this, the usage of all types of relays anywhere can be found without straightjacketed segration of relay types.

CHARACTERSTIC OF ELECTRO-MAGNETIC RELAY:

The following are the characteristic of electro-magnetic relays. A brief study of them helps in

understanding the choice of their components and designs features.

1) Force of attraction

2) Effect of air gap.

3) Effect of Hysterisis

4) Transient condition. 

Force of attraction:

In any electro-magnetic system, the force of attraction is given by.

F α B2

 a

 Where: B - is the flux density

 a - is the cross sectional area of the particular part of the magnetic circuit.

In the case of a DC neutral Relay, B is proportional to the current, that is flowing in the

coil surrounding the electro-magnet and thus the force of the attraction is directly proportional to

the square of the current. This square relationship has its own advantage especially in the case

of DC track relay, in that a small reduction in the current will have a great effect on the working

of the relay. Also for a given change of current, the make and the break will be quicker with

lesser possibility of arcing.

Effect of air gap:

Curve ‘A’ is magnetisation curve for the iron and is all practical purpose a straight line up

to the saturation point. Curve ‘B’ is the magnetisation curve for the open-air gap, which is a straight line through out because permeability µ for air is 1. Curve ‘C’ is the resultant

magnetisation curve of the whole magnetic circuit of the relay and for a given force is the sum of

the amp-turns for the iron part and the amp-turns for the air gap.

When the front contacts are open, the force required to pick up the armature is shown on

curve ‘C’ to be F1 but after the armature has operated, it will be separated from the core by stop

pins. In this position the amp-turns required to maintain the armature is less, as indicated by the

dotted line from 1 on curve C to 2 on curve F. But actually the current in the coil is unaltered,

the force on the armature is greater than required, as indicated at 3 on curve F. Part of this

extra force is used to flex the front contacts sufficiently to give good contact pressure, when it is

in energised position.

The difference between the pick-up and the drop-away current should be as small as

practicable in track relay to ensure good shunting characteristics. This is achieved firstly by the

choice of good quality relay iron and secondly by having a small air gap between armature and

core. If the air gap is not available, then the residual magnetism fluxes might cause the

armature to be retained when the supply is disconnected. For this reason, residual pins are

provided to ensure a definite minimum air gap in the energised position. 

Effect of Hysterisis:

Hysterisis is the property by which the flux produced lags behind the current. In the deenergized condition there will be small residual flux in the core. When the voltage is applied to

the coils, the current in rising to its steady value first causes the flux to rise from 1 to 2 along the

curve. At this point the flux density will be sufficient to attract the armature and reduce the air

gap, the flux then raise to 3 and continue to 4 which corresponds to the steady current in the

coils. When the voltage is disconnected, the current in falling caused the flux to fall from 4 to 5

along the curve. At this point the flux density will fall below the value required to maintain the

armature, which will release, thus increasing the air gap and reducing the flux to 6. Finally the

flux will decrease from 6 to 1 where the current will again be zero.

The relay core is made of material having high permeability and low retentivity. As mentioned in

the IRS specification, Electromagnet iron may be in the form of a

(a) Bar of silicon steel

(b) Best Yorkshire wrought iron

(c) Swedish charcoal iron

(d) Electrical steel sheets

This reduces the difference between pick up value and Drop away value. By selecting

good quality core material, Percentage release and sensitivity of the relay will be improved