Motor Control PLCs and Ladder Logic

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Motor ControlPLCs and Ladder Logic

• An Introduction

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Hard-wired Motor Control Circuitry
Thermal protection not shown
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Manufacturers wiring diagram
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Definitions - 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.

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Definitions - 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

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Motor 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

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Allen-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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Demonstration

• Picosoft Demonstrator

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Time Delay

• Time delayed start of conveyor with All STOP

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stop and start delay

• Time delayed start and time delayed stop