1. Unit 4
Symbols

2. Objectives After studying this unit, you should be able to:
Identify common electrical symbols used in motor control diagrams
Use electrical symbols in drawing schematic and wiring diagrams
Draw and define supplementary contact symbols

3. Introduction Pictorial shorthand
Shows how pieces of electrical equipment are connected in a circuit
No actual standard
Vary from one manufacturer to another, or one country to another
Most standard set used in U.S. is NEMA symbols

4. Symbols
Push buttons

5. Symbols (cont’d.) Switch symbols
Represent common control-sensing devices

6. Symbols (cont’d.)
Switch symbols (cont’d.)

7. Symbols (cont’d.) Basic schematics
Show components in electrical sequence
Without regard for physical location
Show components in de-energized or off state
Any contact that has the same label or number as a coil is controlled by that coil
When a coil energizes, all contacts controlled by it change position

8. Symbols (cont’d.)

9. Sensing Devices
Control systems depend on devices to know circuit conditions
Limit switches
Float, pressure, flow, and temperature switches
Coils
Timed contacts
Contact symbols
Other

10. Sensing Devices (cont’d.)

11. Sensing Devices (cont’d.)

12. Sensing Devices (cont’d.)

14. Explanation of Common Symbols

15. Explanation of Common Symbols (cont’d.)