Programmable Controllers: Theory and Implementation

Introduction to PLC Programming and Implementation—from relay logic to PLC logicFigure 29, the starter’s seal-in contacts (labeled as 3—| |—2) are not used andare 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 cooloff and the overload contacts close, the motor will start again immediately.Depending on the application, this situation may not be desirable. Forexample, someone may be troubleshooting the motor stoppage and the motormay suddenly restart. Making the auxiliary contact an input and using itsaddress to seal the start push button can avoid this situation by making thetwo-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 offand their auxiliary contacts will break the circuit in the PLC.L1L2Stop Start MStop 000 001 030M OL000030StartM002L1L2M001002Figure 30. Two-wire circuit configured as a three-wire circuit.FORWARD/REVERSE MOTOR INTERLOCKINGFigure 31 illustrates a hardwired forward/reverse motor circuit with electricaland push button interlockings. Figure 32 shows the simplified wiringdiagram for this motor. The PLC implementation of this circuit shouldL1ForStop Rev R M1FAll OLsL2M1For PL1FM2RM2Rev PL2Figure 31. Hardwired forward/reverse motor circuit.Industrial Text & Video Company www.industrialtext.com 1-800-752-839833