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4/24/05 BAE 3023 1

Motor Control

PLCs and Ladder Logic

An Introduction

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4/24/05 BAE 3023 2Hard-wired Motor

Control Circuitry

Thermal protection not shown

Control

Voltage

Return

Three Phase

Line

Main Contacts

Auxillary

Contacts

Contactor Coil

M

Motor

Contactor START

STOP

Control

Voltage

Source

L1 L2 L3

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4/24/05 BAE 3023 3Manufacturers wiring

diagram

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4/24/05 BAE 3023 4Definitions - PLC

PLC: (Programmable Logic Controller) An industrial

control device developed to accept contact closure

inputs and to provide outputs that drive relay coils. The device may be re-programmed with code relating

inputs to outputs

Modern PLCs may allow analog input/output and may

support high level language functions (C, C++, BASIC)

Programming logic includes simple logic, timing functions

counting functions, sequences, and may include complex

algorithms (eg. PID)

Examples:Allen-Bradley, Schneider-Modicon, GE, Moeller,

etc.

http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/1239756/2165075/http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/1239756/2165075/http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/1239756/2165075/http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/1239756/2165075/

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4/24/05 BAE 3023 5Definitions - Ladder

Logic

Ladder Logic: Graphical language for describing

output of an electrical switching system as a

function of its inputs. Also known as relay ladder logic or RLL

Commonly used in PLC coding

Primarily used to relate logical inputs (switch closures) to

relay coil outputs. (Start switch used to energize a motor

contactor relay)

Commonly used to document control equipment in process

plants

Example:

START MOTOR

Control Voltage

Source

Control Voltage

ReturnContact

Coil

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4/24/05 BAE 3023 6Motor Control Ladder

Logic

Note:

Power circuit is not shown, only control wiring

Left side ladder is source, right is sink

Many rungs or circuits can be shown

Start Dryer Dryer

Control

Voltage

Return

Control

Voltage

Source

Stop Dryer

Dryer

Start Unload Unload Conveyor Stop Unload

Unld. Conv.

Contacts Loads

Contact

associate

d

withload

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4/24/05 BAE 3023 7Allen-Bradley Pico PLC

Circuit elements

I Input contact closures

Q Output contact closures (drives coil)

T Timer device

C Counter device

M Auxillary relay H Time of day

A Analog input

D Display on screen

R,S Expansion devices

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4/24/05 BAE 3023 8Demonstration

Picosoft Demonstrator

http://c/Program%20Files/PicoSoft%203.0/PicoSoft.exehttp://c/Program%20Files/PicoSoft%203.0/PicoSoft.exehttp://c/Program%20Files/PicoSoft%203.0/PicoSoft.exe

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4/24/05 BAE 3023 9Time Delay

Time delayed start of conveyor with All STOP

Start Dryer Dryer

ControlVoltage

Return

ControlVoltage

Source

STOP

Dryer

Time Delay Unload Conveyor STOP

Unld. Conv.

Time Delay

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4/24/05 BAE 3023 10stop and start delay

Time delayed start and time delayed stop

Start Dryer Dryer

Control

VoltageReturn

Control

VoltageSource

STOP

Dryer

Start TimeDelay

Unload Conveyor

STOP

Unld. Conv.

Start Time Delay

STOP TimeDelay

STOP TimeDelay

UnloadConveyor

Relay 1Relay 1