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# Basic Control System unit4

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### Basic Control System unit4

1. 1. E3145/4/1 INTRODUCTION 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
2. 2. E3145/4/2 INTRODUCTION TO RELAY AND CONTACTOR 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 flowing through a coil in the relay creates a magnetic field that pulls one switch contact against or away from another.
3. 3. E3145/4/3 INTRODUCTION TO RELAY AND CONTACTOR 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 and v. 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
4. 4. E3145/4/4 INTRODUCTION TO RELAY AND CONTACTOR 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
5. 5. E3145/4/5 INTRODUCTION TO RELAY AND CONTACTOR 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)
6. 6. E3145/4/6 INTRODUCTION TO RELAY AND CONTACTOR 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:
7. 7. E3145/4/7 INTRODUCTION TO RELAY AND CONTACTOR 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.
8. 8. E3145/4/8 INTRODUCTION TO RELAY AND CONTACTOR 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.
9. 9. E3145/4/9 INTRODUCTION TO RELAY AND CONTACTOR 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 A1 coil
10. 10. E3145/4/10 INTRODUCTION TO RELAY AND CONTACTOR Symbol of Relay (Type DPDT-Double Pole Double Throw) 1 5 2 4 6 8 coil A2 A1
11. 11. E3145/4/11 INTRODUCTION TO RELAY AND CONTACTOR 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
12. 12. E3145/4/12 INTRODUCTION TO RELAY AND CONTACTOR 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
13. 13. E3145/4/13 INTRODUCTION TO RELAY AND CONTACTOR Figure 4.5 : Diagram of Contactor (Source : FESTO DIDACTIC, Electro-hydraulics – Learning System for Automation & Communication)
14. 14. E3145/4/14 INTRODUCTION TO RELAY AND CONTACTOR 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)
15. 15. E3145/4/15 INTRODUCTION TO RELAY AND CONTACTOR 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.
16. 16. E3145/4/16 INTRODUCTION TO RELAY AND 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
17. 17. E3145/4/17 INTRODUCTION TO RELAY AND CONTACTOR 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
18. 18. E3145/4/18 INTRODUCTION TO RELAY AND CONTACTOR 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.
19. 19. E3145/4/19 INTRODUCTION TO 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
20. 20. E3145/4/20 INTRODUCTION TO RELAY AND CONTACTOR 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.
21. 21. E3145/4/21 INTRODUCTION TO RELAY AND CONTACTOR 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
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