The PLC’s AC Motor Drive Interface

PLC implementation of the VS drive

Speed control of AC motors with variable speeds (VS) drives is a common PLC application. Figure 39 illustrates an operator station that can be used to control a VS motor manually. This station’s programmable controller implementation will allow for automatic motor speed control via an analog interface. The output voltage can be varied (0-10 VDC) … Read more

Reduced-Voltage-Start Motor Control using the PLC

Real inputs and outputs to the PLC

Figure 36 illustrates the control circuit and wiring diagram of a 65% tapped, autotransformer, reduced-voltage-start motor control circuit. The reduced-voltage start reduces the motor’s inrush current (locked-rotor current), to only 42% at full speed. The timer should be set at 5.3 seconds in this example. The instantaneous contacts in lines 2 and 3, must also … Read more

Forward/Reverse Motor Interlocking use the PLC

Real inputs and outputs to the PLC

Figure 31 shows a hardwired forward/reverse circuit with push button and electrical interlockings. The simplified wiring diagram of this motor is shown in Figure 32. Figure 31: Hardwired forward/reverse motor circuit. This circuit must be implemented using the overload contacts in the PLC. They are used to monitor the existence of overload conditions. Auxiliary starter … Read more

Simple Start/Stop Motor Circuit use the PLC

PLC wiring diagram of a three-phase motor

Figure 25 shows the wiring diagram of a three-phase motor and its three-wire control circuit. The starter push button is sealed by the auxiliary contacts. This circuit can be converted into a PLC program by first identifying which control devices will make up the PLC I/O systems. These are the items circled in Figure 26. … Read more