Basic Control System unit4

E3145/4/1INTRODUCTION TO RELAY AND CONTACTOR

INTRODUCTION TO RELAY AND CONTACTOR

OBJECTIVES

General Objective : To know and to apply the concept of relay and contactor.

Specific Objectives : At the end of the unit you will be able to :

State the definition of relay.

Draw the symbol and diagram of relay .

Explain the type of relays.

State the definition of contactor.

Draw the symbol and diagram of contactor .

Compare between relay and contactor.

UNIT4


4.0 EXPLANATION OF RELAY AND CONTACTOR

The representation of relays and contactor in the electrical

circuit diagram is identical, as is their operating principle.

i. Relays are used to switch relatively small output and currents;

ii. Contactors to switch relatively large output and currents.

4.1 DEFINITION OF RELAY

Relays are electromagnetically actuated switches. They consist of a housing with electromagnet and movable contacts. An

INPUTINPUT

A relay is an electromagnetic switch. A small current flowingthrough a coil in the relay creates a magnetic field that pulls one switch contact against or away from another.


electromagnetic field is created when a voltage is applied to the coil of the electromagnet. This results in attraction of the movable armature

to the coil core. The armature actuates the contact assembly. The contact assembly can open or close a specific number of contacts by mechanical means. If the flow of current through the coil is interrupted, a spring returns the armature to its original position.

Relay can be used for various regulating, control and monitoring functions:i. as interfaces between control circuits and load circuits,ii. for signal multiplication,iii. for separation of direct current and alternating current circuits,iv. for delaying, generating and converting signals andv. for linking information.

4.2 THE SYMBOL AND DIAGRAM OF RELAY

Figure 4.1: Symbol of Relay (Type SPDT-Single Pole Double Throw)

(Source : FESTO DIDACTIC, Electro-hydraulics – Learning System for Automation & Communication)

1

2

4

A2 A1

coil


Figure 4.2: Symbol of Relay (Type DPDT-Double Pole Double Throw)(Source : FESTO DIDACTIC, Electro-hydraulics – Learning System for Automation &

Communication)

1

5

2

4

6

8

coil

A2 A1


Functions of Relays Numbers

Number of Relays Description

1 2 Normally closed contact

3 4 Normally open contact

5 6 Normally closed contact, time delay

7 8 Normally open contact, time delay

1 2 4 Changeover contact

5 6 8 Changeover contact, time delay

A1 A2 The coil terminals (common)

Table 4.1 : Functions of Relays Numbers(Source : FESTO DIDACTIC, Electro-hydraulics – Learning System for Automation &

Communication)


Figure 4.3 : Diagram of relay(Source : FESTO DIDACTIC, Electro-hydraulics – Learning System for Automation &

Communication)

4.3 TYPES OF RELAYS

Relay contacts are either normally open (NO) or normally closed (NC), The term “normally” refers to the state in which the coil is not energized. Relays can have many independent contacts, some NO and others NC, and each contact can be used in a different circuit for a different task. When the coil is energized, all NO contacts belonging to that relay close, whereas all NC contacts open.

In most relays with multiple contacts, these are arranged in the double-throw (also called the “changeover” or “transfer”) configuration, as


shown in figure 4.1 and 4.2 above, where an NO and NC contact have a common pole. Such relays are designated as:

i. SPDT (Single Pole Double Throw) refer to symbol figure 4.1

ii. DPDT (Double Pole Double Throw) refer to symbol figure 4.2

The word pole refers to the number of sets of contacts and the word throw refers to the number of positions or combinations ( open or close ) the contacts can have.

Refer to the two types of relays above (Figure 4.1), single pole (SP) is a single contact (common) where a normally open (NO) and normally closed (NC) relay have a common pole.

While a double pole (DP) means that each set of contacts has a common terminal ( 1 and 5 ) connected to both an open and a closed set of contacts (Figure 4.2). When the coil is energized, the circuit between common and NO is closed, and when the coil is de-energized the circuit between common and NC is closed.


Activity 4A

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

4.1 Explain the definition of relay.

4.2 Draw the symbol of relay.


Feedback To Activity 4A

4.1 A relay is an electromagnetic switch. A small current flowing

through a coil in the relay creates a magnetic field that pulls one

switch contact against or away from another.

4.2

Symbol of Relay (Type SPDT-Single Pole Double Throw)

1

2

4

A2 A1coil


Symbol of Relay (Type DPDT-Double Pole Double Throw)

1

5

2

4

6

8

coil

A2 A1


4.4 EXPLANATION OF CONTACTOR

Contactors work on the same basic principle as relays. The typical features of contactor are:

i. double- break ( 2 break points per contact),ii. positive-action contacts and iii. closed arcing chambers (spark arresting chambers).

A contactor possesses several contact elements, normally between 4 and 10. There are also different types of contactors with various combinations of normally closed contacts, normally open contacts, changeover contacts, delayed normally closed contacts etc. The contacts are divided into main contact elements and auxiliary contacts (control contacts)

i. Outputs of 4 – 30kW are switched via main contact elements.

ii. The auxiliary contacts can be used to simultaneously switch further control functions or logic operations.

iii. Contactors which only switch auxiliary contacts (control contacts) are called contactor relays (control contactors).

iv. For the purpose of classification, contactors with main contact elements for power switching are called power contactors (main contactors).

INPUTINPUT


4.5 THE SYMBOL AND DIAGRAM OF CONTACTOR

Figure 4.4 : Symbol of Contactor.

(Source : FESTO DIDACTIC, Electro-hydraulics – Learning System for Automation & Communication)

14 13


Figure 4.5 : Diagram of Contactor(Source : FESTO DIDACTIC, Electro-hydraulics – Learning System for Automation &

Communication)


4.6 COMPARISON BETWEEN RELAY AND CONTACTOR

Relay Contactor

Relays possess a clapper-type armature and are characterized by single contact separation

Contactors possess a lifting armature and are characterized by double contact separation.

Relays are used to switch relatively small outputs and currents.

Contactor are used to switch relatively large outputs and currents.

Table 4.2 : Comparison between Relay and Contactor(Source : FESTO DIDACTIC, Electro-hydraulics – Learning System for Automation &

Communication)


Activity 4B

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

4.3 Draw the symbol of contactor.

4.4 State the typical features of contactor.


Feedback To Activity 4B

4.3

Figure : Symbol of Contactor

4.4 The typical features of contactor are:i. double- break ( 2 break points per contact),ii. positive-action contacts and iii. closed arcing chambers (spark arresting chambers).

14 13


KEY FACTS

1. (i) Relays possess a clapper-type armature and are

characterized by single contact separation.

(ii) Relays are used to switch relatively small outputs and

currents.

2. (i) Contactors possess a lifting armature and are characterized

by double contact separation.

(ii) Contactors are used to switch relatively large outputs and

currents


SELF-ASSESSMENT

You are approaching success. Try all the questions in this self-assessment section and check your answers with those given in the Feedback on Self-Assessment given on the next page. If you face any problems, discuss it with your lecturer. Good luck.

Q4-1 (a) Identify the application of relay.

(b) Draw the diagram of relay.

(c) Explain the function of normally open (NO) and normally closed (NC) relay contacts.

(d) State the strengths and weaknesses of relay.

Q4-2 (a) Give the definition of coil in contactor.

(b) Identify the comparison between relay and contactor.


Feedback To Self-Assessment

Have you tried the questions????? If “YES”, check your answers now.

Q4-1 (a) Relay can be used for various regulating, control and monitoring functions:

i. as interfaces between control circuits and load circuits,ii. for signal multiplication,iii. for separation of direct current and alternating current circuits,iv. for delaying, generating and converting signals and

for linking information.

(b)


Diagram of relay

Q4-3 Relay contacts are either normally open (NO) or normally closed (NC), The term “normally” refers to the state in which the coil is not energized. Relays can have many independent contacts, some NO and others NC, and each contact can be used in a different circuit for a different task. When the coil is energized, all NO contacts belonging to that relay close, whereas all NC contacts open.

Q4-4

Strengths Weaknesses

Easy to change out Contact wear

Durable Moving parts (springs) limit live expectancy from 1-3 million cycles

Easy to test Require more power

Available with plug-in base

Miniature relays available


Immune to noise

The strengths and weaknesses of Relay

Q4-5 The coil is manufactured in a rectangular shape with two square holes in it. This holes provide a space for the magnet yoke to pro-armature move to the closed position.

Q4-6

Relay Contactor

Relays possess a clapper-type armature and are characterized by single contact separation

Contactors possess a lifting armature and are characterized by double contact separation.

Relays are used to switch relatively small outputs and currents.

Contactors are used to switch relatively large outputs and currents.

Comparison between Relay and Contactor





.






Basic Control System unit4

Education

explanation of relay

concept of relay

definition of relay

diagram of relay figure

closed nc relay

contactor introduction

types of relays relay

diagram of relay source