Unit 4

Symbols

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

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

Symbols• Push buttons

Symbols (cont’d.)• Switch symbols

– Represent common control-sensing devices

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

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

Symbols (cont’d.)

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

Sensing Devices (cont’d.)

Sensing Devices (cont’d.)

Sensing Devices (cont’d.)

Explanation of Common Symbols

Explanation of Common Symbols (cont’d.)