Programmable Controllers: Theory and Implementation

Introduction to PLC Programming and Implementation—from relay logic to PLC logicto the overload. An additional normally open acknowledge overload resetpush button, which is connected to the input module, allows the operator toreset the overload indicators. Thus, the overload indicators will remainlatched, even if the physical overloads cool off and return to their normallyclosed states, until the operator acknowledges the condition and resets it.Figure 35 illustrates the motor wiring diagram of the forward/reverse motorcircuit and the output connections from the PLC. Note that the auxiliarycontacts M1 and M2 are not connected. In this wiring diagram, both theforward and reverse coils have their returns connected to L2 and not to theoverload contacts. The overload contacts are connected to L1 on one side andto 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 thePLC’s output to both starters will be OFF.L1 L2 L3FWDREVL1L1L2L1M1M23232FROLTo PLCInput 003MFigure 35. Forward/reverse motor wiring diagram.REDUCED-VOLTAGE-START MOTOR CONTROLFigure 36 illustrates the control circuit and wiring diagram of a 65% tapped,autotransformer, reduced-voltage-start motor control circuit. This reducedvoltagestart minimizes the inrush current at the start of the motor (lockedrotorcurrent) to 42% of that at full speed. In this example, the timer must beset to 5.3 seconds. Also, the instantaneous contacts from the timer in lines 2and 3 must be trapped.Industrial Text & Video Company www.industrialtext.com 1-800-752-839837