Figure 25 shows the wiring diagram of a three-phase motor and its three-wire control circuit. The starter push button is sealed by the auxiliary contacts. This circuit can be converted into a PLC program by first identifying which control devices will make up the PLC I/O systems. These are the items circled in Figure 26. The circuit will include the inputs and outputs for the start and stop buttons and the starter coil. Because the starter coil’s auxiliary contacts are not part of the system, an internal will seal it. This will result in less wiring and fewer connections.
Figure 25: (a) Wiring diagram and (b) relay control circuit for a three-phase motor.
Figure 26: Real inputs and outputs to the PLC.
Table 11 lists the I/O address assignment. It uses the same addressing scheme that the circuit diagram uses (that is, inputs: addresses 000, 001, output: address 030).
The PLC must be programmed in the exact same logic sequence as the hardwired circuit. (See Figure 27). The stop push button will therefore be programmed in parallel with the start button as an examineON instruction.
Figure 27: PLC implementation of the circuit in Figure 25.
This circuit will drive output 0, which controls the starter. When the start button is pressed output 030 will turn on, sealing the start button and turning the starter ON. The motor will turn off if the stop button is pressed. The stop push button can be wired as normal to the input module. The starter coil’s overloads can be wired in series to the coil.
A PLC wiring diagram shows the PLC connected to the power lines L1 (see Figure 28). The module’s field inputs are connected to L1 and the module to the other. The return connection to the input module is L2. L1 is where the output module receives power to switch the load. The output terminal 030 is connected to the starter coil and its overloads. They go to L2.
Figure 28: PLC wiring diagram of a three-phase motor.
For proper operation, the output module can also be connected directly to L2. The wiring diagram for the motor control circuit (see Figure 29) shows that the PLC output module connects directly to the starter coil.
Figure 29: Motor control circuit’s wiring diagram.
Even though the 3-phase motor has a 3-wire control circuit, its corresponding PLC control circuit has only 2 wires. This 2-wire configuration is similar to a 3-wire configuration because it provides low-voltage release; but it does not provide low-voltage protection. In Figure 29, the starter’s seal-in contacts (which is labeled as 3—| |—2) are shown as unconnected because they are not used. If the overload opens while the motor is running, the motor will stop, but the circuit will remain ON. Immediately the overloads cool off, and the overload contacts close, the motor will start again at once.
This situation could be undesirable, depending on the application. This could be a problem with a motor, or someone might be trying to fix it. The motor could then suddenly start up again. This can be avoided by making the auxiliary contact an input, and using its address as the start button seal. (See Figure 30). This configuration means that if overloads are opened while the motor is running the coil will turn off, and their auxiliary contacts will interrupt the circuit in PLC.
Figure 30: Two-wire circuit configured as a three-wire circuit.
