PLC Electrical Wiring Diagram

Putting 24Vdc on one wire will cause the press to advance, putting 24Vdc on the second wire will cause it to retract. A large hydraulic pump that requires 220Vac rated at 20A is driving the press, this should be running as long as the press is on. The press is outfitted with three push buttons, one is a NC stop button, the other is a NO manual retract button, and the third is a NO start automatic cycle button. There are limit switches at the top and bottom of the press travels that must also be connected.

The input and output cards were both selected to be 24Vdc so that they may share a single 24Vdc power supply. In this case the solenoid valve was wired directly to the out- put card, while the hydraulic pump was connected indirectly using a relay (only the coil is shown for simplicity). Primarily, this decision was made because the hydraulic pump requires more current than any PLC can handle, but a relay would be relatively easy to purchase and install for that load. The input switches, all are connected to the same sup ply and to the inputs.

When a controls cabinet is constructed and designed ladder diagrams are used to document the wiring. A basic wiring diagram would be supplied with AC power (120Vac or 220Vac) on the left and right rails. The lines of these diagrams are numbered, and these numbers are typically used to number wires when building the electrical system. A master disconnect for the power to the entire system is the switch before line 010. A fuse is used when the disconnecting to limit the maximum current drawn by the system. Line 020 of the diagram is used to control power to the outputs of the system. The stop button is closed, while the start button is normally open. The branch and output of the rung are CR1, which is a master control relay. The PLC receives power on line 30 of the diagram.

The inputs to the PLC are all AC, and are shown on lines 040 to 070. Notice that Input I: 0/0 is a set of contacts on the MCR CR1. The three other inputs are a normally open push button (line 050), a limit switch (060) and a normally closed push button (070). After line 080 the MCR CR1 can apply power to the outputs. These power the relay out- puts of the PLC to control a red indicator light (040), a green indicator light (050), a solenoid (060), and another relay (080). The relay on line 080 switches a relay that turn on another device drill station.

In the wiring diagram the choice of a normally open start button and a normally close stop button are intentional. Consider line 020 in the wiring diagram. If the stop but- ton is pushed it will open the switch, and power will not be able to flow to the control relay and output power will shut off. If the stop button is damaged, say by a wire falling off, the power will also be lost and the system will shut down - safely. If the stop button used was open and this happened the system would continue to operate while the stop button was unable to shut down the power. Now consider the start button. If the button was damaged, say a wire was disconnected, it would be unable to start the system, thus leaving the system unstarted and safe. In summary, all buttons that stop a system should be normally closed, while all buttons that start a system should be normally open.