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How to control three DC motors using PLC so that two other motors' speed is based on the first motor?

Hi,

First motor should produce speed feedback (tachometer, encoder...) to your PLC. Other two motors are then controlled using this value.

tn

You could control all three motors from one DC drive, or if they are controlled by three separate drives, let the PLC send the same speed reference/command to all three drives. You also could attach a tachometer or encoder to the first motor, and input the speed signal from that motor to your PLC, then use that speed feedback as a speed command to the other two drives. This really depends on the details of the application, what the motors are driving, response required, etc.