4/24/05 BAE 30231

Motor ControlPLCs and Ladder Logic

An Introduction

4/24/05 BAE 30232Hard-wired Motor

Control Circuitry

Thermal protection not shown

ControlVoltageReturn

Three PhaseLine

Main Contacts

Auxillary

Contacts

Contactor Coil

M

MotorContactor START

STOP

ControlVoltageSource

L1 L2 L3

4/24/05 BAE 30233Manufacturers wiring

diagram

4/24/05 BAE 30234Definitions - 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.

4/24/05 BAE 30235Definitions - 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 VoltageSource

Control VoltageReturn

ContactCoil

4/24/05 BAE 30236Motor 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

ControlVoltageReturn

ControlVoltageSource

Stop Dryer

Dryer

Start Unload Unload ConveyorStop Unload

Unld. Conv.

Contacts Loads

Contact associated

with load

4/24/05 BAE 30237Allen-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

4/24/05 BAE 30238Demonstration

• Picosoft Demonstrator

4/24/05 BAE 30239Time Delay

• Time delayed start of conveyor with All STOP

Start Dryer Dryer

ControlVoltageReturn

ControlVoltageSource

STOP

Dryer

Time Delay Unload ConveyorSTOP

Unld. Conv.

Time Delay

4/24/05 BAE 302310stop and start delay

• Time delayed start and time delayed stop

Start Dryer Dryer

ControlVoltageReturn

ControlVoltageSource

STOP

Dryer

Start TimeDelay

Unload Conveyor

STOP

Unld. Conv.

Start Time Delay

STOP TimeDelay

STOP TimeDelay

UnloadConveyor

Relay 1Relay 1