Figure 31 illustrates a hardwired forward/reverse motor circuit with electrical
and push button interlockings. Figure 32 shows the simplified wiring
diagram for this motor. 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 (M1 and M2) are not
required in the PLC program because the sealing circuits can be programmed
using the internal contacts from the motor outputs. Low-voltage protection
can be implemented using the overload contact input so that, if an overload
occurs, the motor circuit will turn off. However, after the overload condition
passes, the operator must push the forward or reverse push button again to
restart the motor.
Figure 32. Forward/reverse motor wiring diagram.
For simplicity, the PLC implementation of the circuit in Figure 31 includes
all of the elements in the hardwired diagram, even though the additional
starter contacts (normally closed R and F in the hardwired circuit) are not
required, since the push button interlocking accomplishes the same task. In
the hardwired circuit, this redundant interlock is performed as a backup
Figure 33 shows the field devices that will be connected to the PLC. 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
Figure 33. Real inputs and outputs to the PLC.
devices—the forward and reverse starters and their respective interlocking
auxiliary contacts—have addresses 030 and 032. The forward and reverse
pilot light indicators have address 031 and 033, respectively. Additionally,
the overload light indicators have addresses 034 and 035, indicating that the
overload condition occurred during either forward or reverse motor operation.
The addresses for the auxiliary contact interlocking using the R and F
contacts are the output addresses of the forward and reverse starters (030 and
032). The ladder circuit that latches the overload condition (forward or
reverse) must be programmed before the circuits that drive the forward and
reverse 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
starter contacts will be open and continuity will not exist.
Table 12 shows the real I/O address assignment for this circuit. Figure 34
shows the PLC implementation, which follows the same logic as the
hardwired circuit and adds additional overload contact interlockings. Note
that the motor circuit also uses the overload input, which will shut down the
motor. The normally closed overload contacts are programmed as normally
open in the logic driving the motor starter outputs. The forward and reverse
motor commands will operate normally if no overload condition exists
because the overload contacts will provide continuity. However, if an
overload occurs, the contacts in the PLC program will open and the motor
circuit will turn OFF. The overload indicator pilot lights (OL Fault Fwd and
OL Fault Rev) use latch/unlatch instructions to latch whether the overload
occurred in the forward or reverse operation. Again, the latching occurs
before the forward and reverse motor starter circuits, which will turn off due
Table 12. I/O address assignment.
Figure 34. PLC implementation of the circuit in Figure 31.
to the overload. An additional normally open acknowledge overload reset
push button, which is connected to the input module, allows the operator to
reset the overload indicators. Thus, the overload indicators will remain
latched, even if the physical overloads cool off and return to their normally
closed states, until the operator acknowledges the condition and resets it.
Figure 35 illustrates the motor wiring diagram of the forward/reverse motor
circuit and the output connections from the PLC. Note that the auxiliary
contacts M1 and M2 are not connected. In this wiring diagram, both the
forward and reverse coils have their returns connected to L2 and not to the
overload contacts. The overload contacts are connected to L1 on one side and
to the PLC’s input module on the other (input 003). In the event of an overload,
both motor starter output coils will be dropped from the circuit because the
PLC’s output to both starters will be OFF.
Figure 35. Forward/reverse motor wiring diagram.