National Electrical Manufacturers Association (NEMA) defines PLC as a digital electronic device that uses a programmable memory to store instructions and to implement specific functions such as logic, sequence, timing, counting, and arithmetic operations to control machines and processes. ● The PLC is an assembly of solid-state digital logic elements designed to make logical decisions … Read more
This is an example of modernization for a machine control system. It will go from hardwired relay logic, to PLC programmed logic. With the exception of the ones that the controller can execute (e.g. timers and control relays), the field devices will not change. Modernizing this machine’s control system will bring many benefits: * A … Read more
This example (see Figure 43) will show how to implement a control program. It detects the position and waits for 0.5 seconds before filling the bottle with water until the photosensor detects that it is full. Figure 43: Bottle-filling system. The control program will wait for 0.7 seconds after the bottle has been filled before … Read more
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
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
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
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
This relay replacement example uses the PLC implementation for the electromechanical circuit in Figure 23. Figure 23: Electromechanical relay circuit. Hardware timer TMR1 requires instantaneous contacts in the first rung, which are used for latching the rung. If instantaneous TMR1 contacts were implemented using a PLC timedelay contact, then PB1 should be pushed until the … Read more
Modernization is the process of moving a machine’s or process’s control over from traditional relay logic to a programmable controller. The control logic is housed in traditional hardwired relay panels. They can be difficult to maintain, including contact chatter and contact welding. The machine’s performance can be improved by switching to a PLC. It also … Read more
Special programming is required for certain PLC circuits or input connections. Programming normally closed input devices is one example. It is important to remember that how a device behaves in a control program affects its programming. Normally Closed Devices A normally closed input device can be programmed so that it acts as either a normally … Read more
The user must decide which devices will be disconnected from the controller when assigning inputs and outputs. These elements will not be wired to the controller (electromagnetic control logic). These elements often include devices such as hydraulic pumps and compressors that are rarely turned off after start. For safety reasons, components such as master start … Read more