The PLC implementation of this circuit should include the use of the overload contacts to monitor the occurrence of an overload condition. The auxiliary starter contacts are not required in the PLC program because the sealing circuits can be programmed using the internal contacts from the motor outputs. The protection of low voltage can be implemented using the overload contact input if an overload occurs, the motor circuit will turn off. However, after the overload condition passes, the operator must push the reverse or forward push button again to restart the motor.
The PLC implementation of the circuit includes all of the elements in the hardwired diagram, even though the additional starter contacts are not required, since the push button interlocking accomplishes the same task. This redundant interlock is performed as a backup interlocking procedure in the hardwired circuit.
The stop push button has address 000, while the normally open sides of the forward and reverse push buttons have addresses 001 and 002 respectively. The overload contacts are connected to the input module at address 003. The output devices, the reverse and forward starters and their respective interlocking auxiliary contacts, have addresses 030 and 032. The reverse and forward pilot light indicators have address 031 and 033 respectively. In addition, the overload light indicators have addresses 034 and 035, indicating that the overload condition occurred during either reverse or forward motor operation.
The addresses for the interlocking of auxiliary contact using the R and F contacts are the addresses output of the reverse and forward starters. The ladder circuit that latches the overload condition must be programmed before the circuits that drive the reverse and forward starters as we will explain shortly. Otherwise, the PLC program will never recognize the overload signal because the starter will be turned off in the circuit during the same scan when the overload occurs. If the latching circuit is after the motor starter circuit, the latch will never occur because the contacts starter will be open and continuity will not exist.