Programmable Logic Controller (PLC) Questions and Answers – 12

Allen-Bradley MicroLogix 1000  Programmable Logic Controller (PLC) 

Suppose we have an Allen-Bradley MicroLogix 1000 controller connected to a pair of pushbutton switches and contactor controlling power to an electric motor as shown in this illustration:

Allen-Bradley PLC Circuit

This motor control system has a problem, though: the motor refuses to start when the “Start” pushbutton is pressed. Examine the “live” display of the ladder logic program inside this Allen-Bradley PLC to determine what the problem is:

motor control system

Identify at least two causes that could account for all you see here.

Answers :

  • Contactor coil failed open
  • Wire connecting contactor coil to O:0/2 failed open
  • Wire connecting VAC-VDC terminal to DC COM terminal failed open
  • Wire connecting input switch “commons” to contactor coil failed open
  • Output channel O:0/2 defective on the PLC
  • 24 VDC power supply in the PLC is insufficient to power the contactor’s coil

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Credits :  Tony R. Kuphaldt – under CC BY 1.0

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