It is about the wiring diagram for a three-phase motor and its corresponding three-wire control circuit, where the auxiliary contacts of the starter seal the start push button. First determine which control devices will be part of the PLC I/O system. In this circuit, the start and stop push buttons (inputs) and the starter coil (output) will be part of the PLC system. The starter coil’s auxiliary contacts will not be part of the system because an internal will be used to seal the coil, resulting in fewer connections and less wiring.
The devices must be programmed in the same logic sequences as they are in the hardwired circuit to program the PLC. Therefore, the stop push button will be programmed as an examine-ON instruction in series with the start push button, which is also programmed as an examine-ON instruction. This circuit will drive output 1, which control the starter. If the start button is pressed, output 1 will turn ON, sealing the start push button and turning the motor ON through the starter. The motor will turn OFF if the stop push button is pressed. Note that the stop push button is wired as normally closed to the input module. The starter coil’s overloads are wired in series with the coil.
The PLC is connected to power line L1 and L2 in a PLC wiring diagram. The field inputs are connected to L1 on one side and to the module on the other. The return, or common, connection from the input module goes to L2. The module output receives its power for switching the load from L1. The terminal output 1 is connected in series with the starter overloads and its coil, which go to L2. The output module also connects directly to L2 for proper operation. The PLC output module is wired directly to the starter coil in the motor control circuit’s wiring diagram.
Although the three phase motor has a three-wire of control circuit, its corresponding PLC control circuit has only two wires. The configuration of two-wire is similar to a three-wire configuration because it provides low voltage release, however it does not provide low voltage protection.
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