Programmable Controllers: Theory and Implementation

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Introduction to PLC Programming and Implementation—from relay logic to PLC logicL1Stop000L2Stop000Start001Trap1000L1L2Trap1000Start001Trap1000TMRPR: 4000 = 53AR: 4001TB = 0.1TMR1001Trap1000TMR1001S1031M1030030M1OLTrap1000M1030TMR1001S1031031S1Trap1000M1030S1031S2032032S2S2032Figure 38. PLC implementation of the circuit in Figure 36.normally open (closed when the overloads are closed and the motor isrunning) and placed in series with contact 1000 in line 3 of the PLC program.If the overloads open, the circuit will lose continuity and M1 will turn OFF.AC MOTOR DRIVE INTERFACEA common PLC application is the speed control of AC motors with variablespeed (VS) drives. The diagram in Figure 39 shows an operator station usedto manually control a VS drive. The programmable controller implementationof this station will provide automatic motor speed control through ananalog interface by varying the analog output voltage (0 to 10 VDC) to thedrive.The operator station consists of a speed potentiometer (speed regulator), aforward/reverse direction selector, a run/jog switch, and start and stop pushbuttons. The PLC program will contain all of these inputs except thepotentiometer, which will be replaced by an analog output. The required inputfield devices (i.e., start push button, stop push button, jog/run, and forward/reverse) will be added to the application and connected to input modules,rather than using the operator station’s components. The PLC program willcontain the logic to start, stop, and interlock the forward/reverse commands.Industrial Text & Video Company www.industrialtext.com 1-800-752-839840
Introduction to PLC Programming and Implementation—from relay logic to PLC logicSpeedPotentiometerTB-112Adjust+12 VTo Speed RegulatorForward34ForwardCommon (Controller)Reverse5ReverseChassisGround6CR1StartRun7CR1StopOLJog8Spare115VACOLOL9SpareField10DriveFigure 39. Operator station for a variable speed drive.Table 16 shows the I/O address assignment table for this example, whileFigure 40 illustrates the connection diagram from the PLC to the VS drive’sterminal block (TB-1). The connection uses a contact output interface toswitch the forward/reverse signal, since the common must be switched. Toactivate the drive, terminal TB-1-6 must receive 115 VAC to turn ON theinternal relay CR1. The drive terminal block TB-1-8 supplies power to thePLC’s L1 connection to turn the drive ON. The output of the module (CR1)is connected to terminal TB-1-6. The drive’s 115 VAC signal is used tocontrol the motor speed so that the signal is in the same circuit as the drive,avoiding the possibility of having different commons (L2) in the drive (thestart/stop common is not the same as the controller’s common). In thisconfiguration, the motor’s overload contacts are wired to terminals TB-1-9and TB-1-10, which are the drive’s power (L1) connection and the outputinterface’s L1 connection. If an overload occurs, the drive will turn OFFIndustrial Text & Video Company www.industrialtext.com 1-800-752-839841
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Programmable Controllers: Theory and Implementation

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