2. Three-wire motor control Logic with
Fault Monitoring , Pilot light, and
Flasher Circuit
I have build a ½ Second Pilot Flasher and Motor Starter Fault Monitoring Logic, this
can be used for a basic three-wire motor control circuit. In this example I will show
from the start of the rung how this circuit operate.
I used a dual air compressor application to write a PLC program.
I installed a Siemens Keyed Selector Switch on the Main electrical control panel, Keyed
selector switch will be program on the first rung (rung 00) it will control the MCR
rung.
I installed two Balluff sensor these sensors are used for safety if somebody open the
electrical door panel or the air chamber cover on the tank for the compressor it will
stop the compressor system .

3. Program Logic
The Balluff sensor will be program on rung 1 (rung 001) this will control the internal
bit relay.
Rung 002 will be program to a Internal safety bit, motor OL, and motor start push
button to control a Internal start output bit.
Rung 003 will be program to a Internal start bit, pressure switch 1, and pressure
switch 2 to control the motor starter coil.
Rung 004 will be program to a dual compressors switch to control JSR.
The start and stop on rung 003 will control the motor, the start push button also be
used to reset a motor fault if for some reason one should happen. The start and stop
push buttons will also have a pilot light to indicate when the motor is running, as well
as when a fault occurs.

4. Sequence of Operation
Ladder Logic 2 is a basic three-wire motor circuit for controlling the motor output. A
start and stop push button in Rung 002, also and N.C. contact label “ Motor fault/
Service this is in series with the motor internal start bit.
The N.C contact is used to turn off the output to the motor starter if a fault happen to
occur.
Rung 5 is the Motor Starter Fault Monitoring Logic, if a failure occur and N.O. auxiliary
contact is mounted on the starter must be wired to an input of the PLC ( Overload
Contact) this input as well as the PLC output to the starter, this is the monitoring and
fault detection circuit. ( Motor Starter Fault Monitoring Logic)