Electromagnetic Relay (Controlling Different Supplies )

January 1, 2023

This post is all about Electromagnetic relay In which we will explain its construction, working & Types. But first we need to know why do we need electromagnetic relay ? Why should we use it ?

REASONS, WHY ARE WE USING ELECTROMAGNETIC RELAY ?

1. Different power supplies can get controlled through Electromagnetic relay

In Electrical power system, there are two supplies AC(Alternating Current) & DC(Direct Current).

Alternating Current – The magnitude of AC continuously changes its direction periodically with respect to time That’s why the given name is alternating. AC supply has both the components phase & magnitude. AC supplies having same magnitude can not be identical if phase shifts are different, that’s why polyphase system exist in AC.

Direct Current – The magnitude of Direct current remains at fixed level that’s why known as direct current. There is no phase shift in DC; only magnitude exist.

Whether it is AC or DC; We can not mix the power supplies with each other. We can not mix AC supplies with different phase shifts & also the DC supplies with different magnitude can not be mixed. Each supply has its own flow path or flow circuit. To control another power supply with some another power supply we can use electromagnetic relay.

2. To Isolate one system from another

The output signal of Programmable controllers like PLC’s are soft in nature means these controllers produces output with having a low current rating with load nature resistive in type; mostly. We can’t connect heavy load directly with the outputs of programmable controllers.

With electromagnetic relay, We can control the load indirectly; controller operates electromagnetic relay while relay further controls load output.

3. To control multiple outputs with single input

With electromagnetic relay we can control multiple outputs simultaneously at a same time with only one single input.

ELECTROMAGNETIC RELAY

Electromagnetic relay consists of an electromagnet, NO & NC contacts & mechanical spring mechanism. Due to the spring tension, the contacts remain positioned at its normal state. When an electromagnet get energized, These contacts changes their state, Further when electromagnet get de-energized, the contacts get back to their normal state due to mechanical spring tension.

8 Pins Electromagnetic Relay by OMRON
Coil 24VDC | Contact Rating 230VAC 10Amps(NO),5Amps(NC)-Resistive

WORKING OF ELECTROMAGNETIC RELAY

In Electromagnetic Relay, there are two types of contacts NO & NC. NO stands for normally open while NC stands for normally closed.

NO Contact– At normal state, NO contact remains open While at forced state NO contact get closed. BELL switch is the best example of NO contact.

Normally the switch remains open While when we press the switch; contact get closed & bell starts ringing. When we release the switch; contact get back open again & bell stops ringing.

Symbol of NO contact

NC Contact– At normal state, NC contact remains closed while at forced state NC contact get opened.

Refrigerator door’s light switch is best example of NC contact.

When we open the refrigerator door, the light starts glowing because switch is closed at its normal state. When we close the door, switch get pressed & then contact get open which results light get OFF.

Symbol of NC contact

When an electromagnet get energized, the contacts get attracted towards the electromagnet Which results electrical contacts get changed. The relay is designed is such a way that all the present contacts get attracted as a single unit . When electromagnet get deenergized, all the contacts pushed back to their normal state because of the pull of mechanical spring arrangement.

Electromagnetic relay Diagram shown below contains a changeover contact. Changeover contact is the combination of both NO & NC contacts with one common contact.

At normal state, common contact(3) is connected with NC contact(4) due to the pull of mechanical spring.

When electromagnet get energized(Forced state), common contact(3) get break with NC(4) contact & get shifted towards NO(5) contact.
When electromagnet get deenergized(Normal state), common contact(3) get break with NO(5) contact & get shifted back to NC(3) contact.

Schematic Diagram of electromagnetic relay when coil is de-energized

SELECTION OF ELECTROMAGNETIC RELAY

Electromagnetic relay can contains 4,5,8 & 14 terminals in which 2 supply terminals are fixed for electromagnet & Rest of the terminals are used as electrical contacts.

The entire selection of electromagnetic relay is depend on our requirements!

What is the type of coil’s supply AC or DC & What is the Supply voltage ?
Which type of contacts do we need NO,NC OR Changeover relay ?
How much contacts do we need ? OR How much changeover relay we need ?
What is the current & voltage rating of the contacts?

TYPES OF ELECTROMAGNETIC RELAY

The electromagnetic relays can be of different sizes & designs but The basic function of electromagnetic relay remains the same.

Next, we are going to represents different types of relay which are available to us.

14 Pins Electromagnetic Relay

14 Pins Electromagnetic Relay by salzer
Coil voltage 230VAC | Contact Rating-240VAC/24VDC, 5Amps

LED as an indication for energized state
14 Pins Electromagnetic Relay by salzer

Terminals of electromagnetic Relay
14 Pins Electromagnetic Relay by salzer

De-energized State of Electromagnetic Relay
14 Pins Electromagnetic Relay by salzer

Energized state of electromagnetic relay
14 Pins Electromagnetic Relay by salzer

8 Pins Electromagnetic Relay

8 Pins Electromagnetic Relay by OMRON
Coil 24VDC | Contact Rating 230VAC 10Amps(NO),5Amps(NC)-Resistive

8 Pins Electromagnetic Relay by OMRON
Coil voltage 230VAC | Contact Rating 230VAC/24VDC,5Amps

PCB Mounted Electromagnetic Relay

PCB Mounted Electromagnetic Relay |
Coil voltage 12VDC | Contact Rating 250VAC/28VDC,7Amps

Base of PCB Mounted Electromagnetic relay having 1 changeover relay

Relay Base

We have seen above multi pin electromagnetic relays. Relays are being fitted on relay bases for proper connections. Every relay has its own relay base.

14 Pins Electromagnetic Relay with 14 pins relay base

8 Pins Electromagnetic relay with 8 Pins relay base OMRON

DRAWBACKS OF ELECTROMAGNETIC RELAY

IF compare with Solid State relays; Electromagnetic relays are slow in speed.
After long use the contacts of the relay get carbonized which further don’t work precisely.
Different type of Relays has their own base; if we need to change the relay sometime, Relay must be according to the base.

CONCLUSION

Electromagnetic relays are similar to Contactors which are having a electromagnet, mechanical spring mechanism & electrical contacts. But it is different from contactor because it is designed for small current ratings.
There are different types of electromagnetic relays but basic function remains the same in ALL. The entire selection of electromagnetic relay depend upon the following parameters.