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  Current Position:Home > PLC Fundamental > Content
Simple start / stop motor circuit
Source:  Date: 2011-07-04

Figure 25 shows the wiring diagram for a three-phase motor and its corresponding
three-wire control circuit, where the auxiliary contacts of the starter
seal the start push button. To convert this circuit into a PLC program, first
determine which control devices will be part of the PLC I/O system; these are
the circled items in Figure 26. In this circuit, the start and stop push buttons
(inputs) and the starter coil (output) will be part of the PLC system. The starter
coil’s auxiliary contacts will not be part of the system because an internal will
be used to seal the coil, resulting in less wiring and fewer connections.

Table 11 shows the I/O address assignment, which uses the same addressing
scheme as the circuit diagram (i.e., inputs: addresses 000 and 001, output:
address 030).
To program the PLC, the devices must be programmed in the same logic
sequence as they are in the hardwired circuit (see Figure 27). Therefore, the
stop push button will be programmed as an examine-ON instruction

(a normally open PLC contact) in series with the start push button, which is
also programmed as an examine-ON instruction. This circuit will drive output
030, which controls the starter. If the start push button is pressed, output 030
will turn ON, sealing the start push button and turning the motor ON through
the starter. If the stop push button is pressed, the motor will turn OFF. Note
that the stop push button is wired as normally closed to the input module. Also,
the starter coil’s overloads are wired in series with the coil.
In a PLC wiring diagram, the PLC is connected to power lines L1 and L2
(see Figure 28). The field inputs are connected to L1 on one side and to the
module on the other. The common, or return, connection from the input
module goes to L2. The output module receives its power for switching the
load from L1. Output terminal 030 is connected in series with the starter coil
and its overloads, which go to L2. The output module also directly connects
to L2 for proper operation. Note that, in the motor control circuit’s wiring
diagram (see Figure 29), the PLC output module is wired directly to the
starter coil.
Although the three-phase motor has a three-wire control circuit, its corresponding
PLC control circuit has only two wires. This two-wire configuration
is similar to a three-wire configuration because it provides low-voltage
release; however, it does not provide low-voltage protection. Referring to

Figure 29, the starter’s seal-in contacts (labeled as 3—| |—2) are not used and
are shown as unconnected. If the motor is running and the overloads open,
the motor will stop, but the circuit will still be ON. Once the overloads cool
off and the overload contacts close, the motor will start again immediately.
Depending on the application, this situation may not be desirable. For
example, someone may be troubleshooting the motor stoppage and the motor
may suddenly restart. Making the auxiliary contact an input and using its
address to seal the start push button can avoid this situation by making the
two-wire circuit act as a three-wire circuit (see Figure 30). In this configuration,
if the overloads open while the motor is running, the coil will turn off
and their auxiliary contacts will break the circuit in the PLC.

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