Figure 31 shows a hardwired forward/reverse circuit with push button and electrical interlockings. The simplified wiring diagram of this motor is shown in Figure 32.
Figure 31: Hardwired forward/reverse motor circuit.
This circuit must be implemented using the overload contacts in the PLC. They are used to monitor the existence of overload conditions. Auxiliary starter contacts M1 and M2 are not necessary in the PLC program. Sealing circuits can be programmed using internal contacts from motor outputs. The overload contact input can be used to provide low-voltage protection so that the motor circuit will shut down in the event of overload. To restart the motor, however, the operator must press the forward or reverse button once the overload condition has passed.
Figure 32: Forward/reverse motor wiring diagram.
The circuit shown in Figure 31 is a PLC implementation. It does not include the starter contacts R and F, which are normally closed in the hardwired circuit. However, push button interlocking does the same job. This redundant interlock acts as an additional interlocking mechanism in the hardwired circuit.
Figure 33 illustrates the field devices that will connect to the PLC. Address 000 is the stop button, while addresses 001 and 022 are the normally open sides for the forward and reverse buttons.
Figure 33: Real inputs and outputs to the PLC.
At address 003, the overload contacts are connected with the input module. Addresses 030 and 032 are the output devices, which include forward and reverse starters as well as their interlocking auxiliary contacts. The forward and reverse pilot lights have addresses 031 and 033, respectively. The overload light indicators also have addresses 034 or 035, which indicate that an overload condition occurred while either the forward or reverse motor were operating.
Addresses for the auxiliary contacts interlocking using R and F contacts are output addresses of forward and reverse starters (030, 032). As we will see, the ladder circuit that latches an overload condition (forward/reverse) must be programmed prior to the circuits driving the forward or reverse starters (030 and 032). The overload signal will not be recognized by the PLC program if the starter is turned off in the circuit at the same time as the overload. The latching circuit must be placed after the motor starter circuit. This is because the starter contacts are open and there will not be continuity.
Table 12 shows the actual I/O address assignment of this circuit. Figure 34 illustrates the PLC implementation. It follows the same logic and has additional overload contact interlockings. The overload input will also be used by the motor circuit, which will turn off the motor. Programming the motor starter outputs’ logic to open the normally closed overload contacts is done as normal.
Figure 34: PLC implementation of the circuit in Figure 31.
If there is no overload condition, the forward and reverse motor commands will work normally. The overload contacts will ensure continuity. If overload occurs, however, the contacts in PLC program will be opened and the motor circuit will turn off. The overload indicator pilot light (OL Fault Fwd & OL Fault Rev) uses latch/unlatch instructions for latching whether overload occurred in forward or reverse.
The latching takes place before the forward and reverse motor starter circuits. These will shut off due to overload. The operator can reset the overload indicators by pressing the normal open acknowledge overload reset button that is attached to the input module. The overload indicators will still be locked, even after the physical overloads cool down and return to normal closed states until the operator acknowledges that the condition has been rectified and resets them.
Figure 35 shows the motor wiring diagram for the forward/reverse circuit and the output connections to the PLC. The auxiliary contacts M1 & M2 are not connected. The wiring diagram shows that both the forward coil and reverse coils have the returns connected to L2 but not the overload contacts. The overload contacts are connected on one side to L1 and the PLC’s input (input 033) on the other. The overload condition will cause both motor starter output coils to be removed from the circuit. This is because both starters are connected to the PLC’s input module (input 003).
Figure 35: Forward/reverse motor wiring diagram.
