Theory - 76 :- Relay Principle ,its Types & Applications

A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays.

Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal.

“A relay is a device designed to cause a sudden predicted change in a single

or multiple electrical output circuits when certain conditions are satisfied by

the electrical circuit that contains the relay device”.

 

Symbol

The above figure shows the most commonly used symbol of a relay. A1 and A2 represent its coil and 11, 12 & 14 represent its contacts.

 

 Relay Symbol 

Where do we use relays?

Relays have a wide range of application. You can find them everywhere: home appliances, automobiles, industries, and even in copy machines. In some applications, they are used for switching or control of circuit (as in timer-based lighting control) whereas in others they are used to sense and protect circuits (as in the case of earth fault protection relays). Therefore, it is difficult to specify their area of application area.

 

Principle of operation

They are basically classified into two types based on their working principle as electro-mechanical and solid-state relays. Let us discuss the principle of operation of each one of them in detail.

 

Operation of electromechanical relays

 

Electromechanical relays transfer signals between its contact through a mechanical motion. It consists of two sections: the first is the electromagnet section and the other is the armature and mechanical contacts section. The electromagnet section consists of a set of coil wound over a magnetic core.

 

When an input voltage (almost equal to the rated voltage of the coil) is applied to the coil, it gets magnetized and attracts the armature. The mechanical contacts are attached to the armature. Hence, when the armature is pulled towards the electromagnet, the contact closes. When the input voltage applied to the coil is removed, the armature is brought back to its original position by the spring release.

Operation of Solid-state relays

Solid-state relays are commonly known as SSRs. Unlike electromechanical type, they do not posses any mechanically moving parts. On the other hand, it consists of semiconductor and electronic components within. In solid-state relays, the electromagnetic section is replaced by optocoupler and required driver circuits and the output contact section is replaced by a TRIAC or transistor plus snubber and driver circuits.

 

When the rated voltage is applied to the input section, current flows through the optocoupler. The output of the optocoupler is used to operate the switching circuit of TRIAC or transistor. Switching circuit applies a gate pulse to the TRIAC and the TRIAC starts conducting. Similarly, when the applied input voltage is removed, the optocoupler turns off the TRIAC switching circuit and which, in turn, stops the gate pulse to the TRIAC and the TRAIC stops conducting.

 

In the following sections, we shall discuss in detail the parts and operation of electromechanical and solid-state relays in detail.

 

Parts of an electromechanical relay

A typical electromechanical relay consists of the following components:

 

1. Electromagnetic coil
2. Armature
3. Core
4. Movable contacts
5. Spring return arrangement

Electromagnetic coil

The electromagnetic coil is the most important part of an electromechanical relay. It consists of a set of copper windings over a magnetic core. As you know, the flow of current through the coil produces a magnetic field. Therefore, when voltage is applied to the coil, it becomes an electromagnet and attracts the armature.

 

Armature

An armature is a movable piece of metal, balanced using a pivot.

 

Core

Core is the metallic part over which the coil is wound.

 

Movable contacts and fixed contact

Contacts are the conducting parts inside the relay, that open or close when voltage is applied to its coil. The contact that is attracted by the electromagnet is called movable contact and that is stationary and connected to the terminals are called fixed contacts.

 

Spring arrangement

Spring arrangements are also present in them, such as to bring the armature and the contacts back to the original position when the coil is de-energized.

 

Parts of a solid-state relay

As discussed earlier, solid-state relays do not have any movable parts within. In order to explain their internal parts, here, we have split it to the following sections:

 

• Input circuit section
• Electrical isolation
• Driver circuitry
• Output section

1. Input Circuit Section                                            The input circuit consists of diodes transistors gates and resistors required to drive the optocoupler.
2. Electrical Isolation                                                  Unlike electromagnetic relays, the input and output sections of an SSR do not have any physical contacts. Galvanic separation is provided between them using optocouplers. 
3. Driver Circuit                                                        Driver circuits consist of components required to turn on the TRIAC or transistors or thyristors in the output circuit. The o/p of the optocoupler is      conditioned and the gate pulse needed to trigger the transistor is generated.                         
4. Output Section                                                          The output section consists of semiconductor devices such as transistor or TRIAC or thyristor as an alternative to relay contacts.

Application

Relays have a wide range of applications starting from washing machines at homes to the telecommunication systems at the International space station, they can be found everywhere. The following are a few key applications:

1. They are used in electronic circuits and home appliances for isolating low voltage or DC circuits from high voltage AC circuits.
2. They are the backbone of industrial process automation systems. They are used in combination with PLCs for process control. They are one of the key components in an automation cabinet.
3. Used for signaling and control in railway networks.
4. In motor control circuits for motor switching, protection as well as control.
5. In substations and power distribution centers for sensing various faults and operating the circuit breaker.

Relay selection considerations

The following factors must be considered while selecting a relay for any application.

• Nominal Voltage: The voltage at which the coil is designed to operate.
• Rated Power: The power consumed by the coil at normal room temperature.
• Contact Rating: The current carrying capacity and voltage rating of their contacts
• Contact Mechanism: The number of contacts required and the contact configuration (NO/NC/changeover).
• Environmental Protection: the degree of sealing required, meaning, whether the external casing of relay is necessary or not?
• Insulation Resistance: Insulation resistance between any two sets of contacts and that between the contacts and the coil.