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PLC Programming Questions & Answers – 1

A PLC has been programming to control the starting and stopping of a three-phase electric motor. Shown here is a partial wiring diagram and offline PLC program display for the system:

PLC Programming Questions

Identify the problem(s) in this PLC program, and modify it so that it will work as it should.

Answer :

The OL_switch instruction in the PLC program needs to be changed to NO (Normally-Open). Since the real-world OL switch contact is supposed to be in the closed state when everything is okay, and this means the third input channel should be energized when all is well, a NO contact instruction is required so that the “1” bit state will maintain that instruction in the “colored” state and permit the coil instruction to receive color (virtual power) to run the motor.

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